ALBERT

Notes: 1. We assessed the impacts of deforestation on the energy base of headwater food webs in seven headwater streams in the Upper Chattahoochee basin, GA, U.S.A where percentage forest in catchments ranged from 82 to 96%. We measured terrestrial organic matter standing crop and determined consumer (crayfish and insectivorous fish) dependence on terrestrial versus aquatic energy sources via gut content and stable isotope analyses.2. Standing crop of coarse particulate organic matter (CPOM) declined with deforestation at large scales (i.e. catchment deforestation and riparian deforestation at the entire stream network scale). Terrestrial plant matter, the dominant component of crayfish guts, declined in crayfish guts with reductions in CPOM standing crop and with deforestation.3. Crayfish and insectivorous fish δ13C showed enrichment trends with deforestation, indicating isotopic divergence from CPOM, the most 13C-depleted basal resource, with reductions in catchment and riparian forest cover. Crayfish δ13C also exhibited enrichment with decreased instream CPOM standing crop.4. A concentration-dependent mixing model was used to calculate the relative dependence of crayfish and fish on terrestrial versus aquatic basal resources. Results suggested that both allochthonous CPOM and autochthonous production were important basal resources. Consumer dependence on CPOM decreased with reductions in canopy cover.5. Our data suggest the importance of forest cover to headwater food webs at multiple scales, and that relatively low levels of riparian deforestation along headwater streams can lead to reductions in stream food web dependence on terrestrial subsidies.

Notes: 1. Bacterioplankton production was measured in the water columns of two ultra-oligotrophic, freshwater Antarctic lakes (Crooked Lake and Lake Druzhby) during an annual cycle. In both lakes bacterial production, measured by the incorporation of [3H] thymidine, continued in winter and showed a cycle over the year. The range of production was between 0 and 479 ng C L−1 h−1 in Crooked Lake and 0–354 ng L−1 h−1 in Lake Druzhby.2. Abundance and mean cell volume both varied, producing marked changes in biomass during the year, with highest biomass occurring in the winter and early spring. Biomass showed similar seasonal trends in both lakes.3. For most of the year inorganic forms of nitrogen and phosphorus were detectable in the water columns of the lakes and were unlikely to have limited bacterial production. Dissolved organic carbon (DOC) was below 3000 μg L−1. Dissolved amino acids and carbohydrates contributed 5–25% of the DOC pool in Crooked Lake and 5–64% in Lake Druzhby. Dissolved carbohydrates were consistently low, suggesting that this may have been the preferred carbon substrate for bacterioplankton.4. Aggregate associated bacteria had higher mean cell volume, abundances and production than freely suspended bacteria in Lake Druzhby, while in Crooked Lake aggregate associated bacteria consistently had higher mean cell volumes than free bacteria, but abundance and production were on occasion higher in free bacteria compared with aggregate associated communities.5. The data indicated that production is limited by continuous low temperatures and the limited availability of suitable DOC substrate. However, the bacterioplankton functions year round, responding to factors other than temperature.

Notes: 1. Previous studies compared the functional responses to their prey, and both intraspecific and interspecific interference, in mature larvae of Dinocras cephalotes, Perla bipunctata, Isoperla grammatica and Perlodes microcephalus. The present study examines switching by larvae of the same species presented with different proportions of two contrasting prey types; larvae of Baetis rhodani and Chironomus sp. In each experiment, 200 prey were arranged in nine different combinations of the two prey types (20 : 180, 40 : 160, 60 : 140, 80 : 120, 100 : 100, 120 : 80, 140 : 60, 160 : 40, 180 : 20). Prey were replaced as they were eaten. A model predicted the functional response in the absence of switching and provided a null hypothesis against which any tendency for switching could be tested.2. No evidence for prey switching by Dinocras and Perla was obtained, both species showing a slight preference for Baetis over Chironomus. Prey switching occurred in Isoperla and Perlodes. As the relative abundance of one prey type increased in relation to the alternative, the proportion eaten of the former prey changed from less to more than expected from its availability, the relationship being described by an S-shaped curve. Isoperla and Perlodes switched to a preference for Baetis when its percentage of the total available prey exceeded 57 and 42%, respectively. Equivalent values for Chironomus were 43 and 58% for Isoperla and Perlodes, respectively. Switching was strongest in Perlodes.3. Non-switching in Dinocras and Perla was related to their feeding strategy, both species being more successful when using a non-selective ambush strategy at dusk and dawn rather than a search strategy during the night. Both Isoperla and Perlodes used a search strategy. The smaller Isoperla fed chiefly at dusk and dawn, and preferred Chironomus larvae, whereas most of the larger Perlodes fed continuously from dusk to dawn and preferred Baetis larvae.

Notes: 1. Daphnia pulex and Daphnia middendorffiana are commonly found in the Toolik Lake region of arctic Alaska. These two species are very similar morphologically, although their natural distributions differ markedly: D. pulex is restricted to shallow ponds, while D. middendorffiana is widely distributed and found in a variety of ponds and lakes. We compared the reproductive capabilities of D. pulex and D. middendorffiana grown under similar resource conditions and in the absence of the invertebrate predator Heterocope septentrionalis. In situ life table and mesocosm experiments were conducted in Toolik Lake and Dam Pond, habitats that have historically contained natural populations of D. middendorffiana, but never D. pulex.2. Daphnia pulex exhibited a significantly higher net growth rate than D. middendorffiana in both life table and mesocosm experiments although D. pulex has never been found in either Toolik Lake or Dam Pond. Daphnia middendorffiana exhibited a negative net growth rate in Dam Pond, which had lower resource levels then Toolik Lake. Therefore, the smaller D. pulex appears to have a lower food threshold concentration than the larger D. middendorffiana.3. Our results indicate that D. pulex is a superior resource competitor in the Toolik Lake region. These results combined with distributional patterns suggest that the restricted distribution of D. pulex in these arctic lakes and ponds cannot be explained by resource competition alone. We suggest that in the presence of H. septentrionalis, predation is an important factor structuring arctic zooplankton communities in the Toolik Lake region.

Notes: 1. We investigated the diversity and distribution of freshwater mussels at 40 sites in an agricultural catchment, the River Raisin in south-eastern Michigan, to relate mussel assemblages and individual taxa to reach and catchment-scale variables. Unionids were surveyed by timed searches in 100-m reaches, and in-stream and riparian habitat were quantified as well as flow, water chemistry and channel morphology. Land use/cover and surficial geology were determined for site subcatchments and riparian buffers.2. Some 21 mussel species were found overall; richness ranged from 0 to 12 living species per site. From the upper to middle to lower catchment, the number of individuals, number of species, Shannon–Weaver diversity and relative abundance of intolerant unionids all declined significantly.3. Four groupings based on overall mussel diversity and abundance were significantly related to reach-scale habitat variables. The richest mussel assemblages were associated with sites with higher overall habitat quality, greater flow stability, less fine substratum, and lower specific conductance.4. Stepwise multiple regressions revealed that the distribution and abundance of the total mussel assemblage, as well as the most common species, could be predicted from a combination of reach- and catchment-scale variables (R2 = 0.63 for total mussels, R2 = 0.51–0.86 for individual species).5. Flow stability, substratum composition and overall reach habitat quality were the most commonly identified reach-scale variables, and measures of surficial geology were the most effective catchment-scale variables. The spatial pattern of geology is likely to be responsible for the diversity gradient from the upper to the lower catchment.6. Prior studies, attempting to explain mussel distributions from local habitat features alone, have found relatively weak relationships. By employing a combination of reach- and catchment-scale habitat variables, this study was able to account for a substantial amount of the spatial variability in mussel distributions.

Notes: 1. Wind-induced sediment resuspension can affect planktonic primary productivity by influencing light penetration and nutrient availability, and by contributing meroplankton (algae resuspended from the lake bed) to the water column. We established relationships between sediment resuspension, light and nutrient availability to phytoplankton in a shallow lake on four occasions.2. The effects of additions of surficial sediments and nutrients on the productivity of phytoplankton communities were measured in 300 mL gas-tight bottles attached to rotating plankton wheels and exposed to a light gradient, in 24 h incubations at in situ temperatures.3. While sediment resuspension always increased primary productivity, resuspension released phytoplankton from nutrient limitation in only two of the four experiments because the amount of available nitrogen and phosphorus entrained from the sediments was small compared with typical baseline levels in the water column. In contrast, chlorophyll a entrainment was substantial compared with baseline water column concentrations and the contribution of meroplankton to primary production was important at times, especially when seasonal irradiance in the lake was high.4. Comparison of the in situ light climate with the threshold of light-limitation of the phytoplankton indicated that phytoplankton in the lake were only likely to be light-limited at times of extreme turbidity (e.g. 〉200 nephelometric turbidity units), particularly when these occur in winter. Therefore, resuspension influenced phytoplankton production mainly via effects on available nutrients and by entraining algae. The importance of each of these varied in time.5. The partitioning of primary productivity between the water column and sediments in shallow lakes greatly influences the outcome of resuspension events for water column primary productivity.

Notes: 1. Two types of artificial neural networks procedures were used to define and predict diatom assemblage structures in Luxembourg streams using environmental data.2. Self-organising maps (SOM) were used to classify samples according to their diatom composition, and multilayer perceptron with a backpropagation learning algorithm (BPN) was used to predict these assemblages using environmental characteristics of each sample as input and spatial coordinates (X and Y) of the cell centres of the SOM map identified as diatom assemblages as output. Classical methods (correspondence analysis and clustering analysis) were then used to identify the relations between diatom assemblages and the SOM cell number. A canonical correspondence analysis was also used to define the relationship between these assemblages and the environmental conditions.3. The diatom-SOM training set resulted in 12 representative assemblages (12 clusters) having different species compositions. Comparison of observed and estimated sample positions on the SOM map were used to evaluate the performance of the BPN (correlation coefficients were 0.93 for X and 0.94 for Y). Mean square errors of 12 cells varied from 0.47 to 1.77 and the proportion of well predicted samples ranged from 37.5 to 92.9%. This study showed the high predictability of diatom assemblages using physical and chemical parameters for a small number of river types within a restricted geographical area.

Notes: 1. Zebra mussels aggregate to form dense colonies where, depending on the flow rate, individuals in different vertical locations within the colony may experience restricted food availability.2. Using 32P-labelled Chlamydomonas angulosa, we found ingestion rates of individual mussels located at the surface to exceed those in the bottom of a 6 cm thick colony by up to 75%.3. Higher velocities (10 and 20 cm s−1) increased algal delivery to the colony's middle layer (2–4 cm depth), subsequently increasing ingestion rates to equal those in the surface layer, while increasing ingestion only for the smallest mussels in the bottom (4–6 cm).4. At all vertical locations within the colonies, smaller mussels showed higher ingestion rates per unit mass than larger mussels, particularly at higher flow rates.

Notes: 1. Groundwater nitrate contamination has become a worldwide problem as increasing amounts of nitrogen fertilisers are used in agriculture. Alluvial groundwater is uniquely juxtaposed between soils and streams. Hydrological connections among these subsystems regulate nutrient cycling.2. We measured denitrification using an in situ acetylene-block assay in a nitrate-contaminated portion of the Garonne River catchment along a gradient of surface water–ground water mixing during high (snowmelt) and low flow.3. During high flow (mid-April to early June) the water table rose an average of 35 cm and river water penetrated the subsurface to a great extent in monitoring wells. Denitrification rates averaged 5.40 μgN2O L−1 min−1 during the high flow period, nearly double the average rate (2.91 μgN2O L−1 min−1) measured during base flow. This was driven by a strong increase in denitrification in groundwater under native riparian vegetation. Nitrate concentrations were significantly lower during high flow compared with base flow. Riparian patches had higher dissolved organic carbon concentrations that were more aromatic compared with the gravel bar patch closest to the river.4. Multiple linear regression showed that the rate of denitrification was best predicted by the concentration of low molecular weight organic acids. These molecules are probably derived from decomposition of soil organic matter and are an important energy source for anaerobic respiratory processes like denitrification. The second best predictor was per cent surface water, reflecting higher denitrification rates during spring when hydrological connection between surface water and ground water was greatest.5. Our results indicate that, while denitrification rates in Garonne River alluvium were spatially and temporally variable, denitrification was a significant NO3 sink during transport from the NO3-contaminated floodplain to the river. DOC availability and river–floodplain connectivity were important factors influencing observed spatial and temporal patterns.

Notes: 1. To determine the consequences of dispersal and gene flow for temporary pond water mites (Hydrachnida), we compared distributional, genetic and morphological characters in the closely related species Arrenurus angustilimbatus and A. rufopyriformis. The former has larvae that parasitise and disperse on adult mosquitoes, whereas larvae of the latter forego any association with hosts.2. Allometrically adjusted egg size and gonopore size were found to be useful characters for distinguishing between females of the two species.3. Arrenurus angustilimbatus possesses a broader and more continuous geographic distribution than its ‘direct developing’ counterpart. Allozyme heterozygosity was higher and population differentiation lower in A. angustilimbatus. In addition, populations of A. rufopyriformis were morphologically divergent, whereas populations of A. angustilimbatus were not. Isolation by distance analyses on both genetic and morphological characters indicated that the results were not biased by different sampling regimes for the two species.4. These results demonstrate the importance of mosquito parasitism for maintaining ecological and genetic linkages between A. angustilimbatus populations. More broadly, we hypothesise that insect-mediated dispersal has contributed to the ecological and evolutionary success of water mites, because the Hydrachnida lack other obvious adaptations for dispersing in space or time.

Notes: 1. The photosynthetic efficiencies of the mixotrophic ciliate Ophrydium naumanni and the autotrophic dinoflagellate Gymnodinium paradoxum were investigated using laboratory and field experiments in Lake Moreno Oeste (41°5′S and 71°33′W, 758 m a.s.l.), in the Nahuel Huapi System (North Patagonia, Argentina).2. The effect of different underwater light intensities on net primary production (NPP) was assessed during one summer. Additionally, laboratory experiments were carried out to obtain photosynthesis-irradiance response curves for each species.3. Ophrydium naumanni and G. paradoxum dominated the metalimnetic (30 m depth) deep chlorophyll maximum (DCM) in the lake.4. Despite these deep higher abundances, the cell-specific production of both species was higher at 10 m than at 30 m (DCM) depth. In addition, at 5 m depth, NPP was reduced by PAR + UV-A radiation.5. Both species exhibited a positive NPP at very low irradiance but the mixotrophic ciliate was more efficient in exploiting the DCM irradiance level both in situ and at comparable light intensities in laboratory experiments. Light acclimatised O. naumanni showed a higher NPP at lower irradiances and photoinhibition at medium and high irradiances.6. Under the strong wind-driven turbulence commonly found in Patagonian lakes, organisms cannot select their position in the epilimnetic water column and will be dragged to potentially harmful UV radiation levels. Thus, metalimnetic DCM colonisation by these two species represents a tradeoff between higher survival and lower cell-specific NPP.

Notes: 1. Lake managers suspect that taste and odour-causing algal blooms are increasing in frequency and intensity, although long-term monitoring records are scarce, and a number of critical scientific and management questions remain unanswered.2. In nutrient-poor lakes and reservoirs, these events are caused primarily by sporadic outbreaks of some chrysophyte algae, which leave identifiable markers in lake sediments. We examine the siliceous remains of these organisms in more than fifty boreal lakes at broad temporal and spatial scales.3. Colonial scaled chrysophytes, including the taste and odour-causing Synura petersenii, have increased markedly in more that 90% of the lakes examined since pre-industrial times.4. Detailed stratigraphic analyses of two lakes show a rise in the abundance of colonial taxa in the 1930s to 1950s, with a sharp increase over the past two decades.5. An examination of biogenic silica and biological ratios in Crosson Lake, Ontario, Canada, indicate that these changes represent true increases in the absolute abundance of colonial chrysophytes.6. Rapid increases over the past two decades indicate that these trends are the result of one or more anthropogenic stressors that are operating at a broad, regional scale.

Notes: 1. To improve mechanistic understanding of plankton responses to eutrophication, a mesocosm experiment was performed in the shallow littoral zone of a south Swedish lake, in which nutrient and fish gradients were crossed in a fully factorial design.2. Food chain theory accurately predicted total biomass development of both phyto- and zooplankton. However, separating zooplankton and algae into finer taxonomic groups revealed a variety of responses to both nutrient and fish gradients.3. That both nutrients and fish are important for phytoplankton dynamics was seen more clearly when viewing each algal group separately, than drawing conclusions only from broad system variables such as chlorophyll a concentration or total phytoplankton biovolume.4. In some taxa, physiological constraints (e.g. sensitivity to high pH and low concentrations of free CO2) and differences in competitive ability may be more important for the biomass development than fish predation, grazing by herbivorous zooplankton, and nutrient availability.5. We conclude that food chain theory accurately predicted responses in system variables, such as total zooplankton or algal biomass, which are shaped by the dynamics of certain strong interactors (‘keystone species’), such as large cladocerans, cyanobacteria and edible algae (〈50 μm), whereas responses at finer taxonomic levels cannot be predicted from current theory.

Notes: 1. Mesocosm experiments were carried out to examine the relative importance of top down (fish predation) and bottom up (nutrient addition) controls on phytoplankton abundance in a small shallow lake, Little Mere, U.K., in 1998 and 1999. These experiments were part of a series at six sites across Europe.2. In the 1998 experiment, top-down processes (through grazing of large Cladocera) were important in determining phytoplankton biomass. The lack of plant refugia for zooplankton was probably important in causing an increasing chlorophyll a concentration even at intermediate fish density. Little Mere normally has abundant macrophytes but they failed to develop substantially during both years. Bottom-up control was not important in 1998, most probably because of high background nutrient concentrations, as a result of nutrient release from the sediments.3. In 1999 neither top-down nor bottom-up processes were significant in determining phytoplankton biomass. Large cladoceran grazers were absent even in the fish-free enclosures, probably because dominance of cyanobacteria and high phytoplankton biomass made feeding conditions unsuitable. As in 1998, bottom-up control of phytoplankton was not important, owing to background nutrient concentrations that were even higher in 1999 than in 1998, perhaps because of the warmer, sunnier weather.4. The differing outcomes of the two experiments in the same lake with similar experimental designs highlight the importance of starting conditions. These conditions in turn depended on overall weather conditions prior to the experiments.

Notes: 1. The ciliate Euplotes octocarinatus responds to the presence of Stylonychia mytilus with a morphological transformation rendering its ingestion more difficult. Predation on Euplotes is reduced when an alternative prey, the flagellate Chlorogonium elongatum, is available. Lower predation is accompanied by reduced induction in Euplotes.2. We quantified the motility of both ciliates in the presence and absence of Chlorogonium to test the hypothesis that the differential response in Euplotes is a consequence of behavioural changes affecting the encounter rate of the prey and the predator.3. The results indicate that S. mytilus uses different feeding strategies for each prey. It is a filter feeder when small food particles (Chlorogonium) are abundant, however, it becomes a raptorial feeder when large prey with an escape capability (Euplotes) is available and small alternative prey are absent.4. As the mobility of filter feeding Stylonychia is strongly limited, the presence of the flagellate may indirectly affect the defence level in Euplotes by reducing the frequency of contacts with the predator.5. An experiment with Chaetogaster diastrophus, a more specialised predator not affected by the presence of Chlorogonium, as well as direct manipulation of the encounter rate by changing the surface area available for the crawling ciliates, supported the encounter rate hypothesis.

Notes: 1. Previous work has indicated, at least in two river systems, that some Characiformes species migrate from nutrient-poor rivers to spawn in nutrient-rich rivers. In the present work, larval and juvenile fish were surveyed to index spawning activity in order to determine whether this spawning pattern is repeated in nine major tributaries of the Amazon basin.2. Adult Mylossoma aureum, M. duriventre, Colossoma macropomum, Anodus elongatus, Triportheus elongatus, Brycon cephalus, Semaprochilodus insignis, S. taeniurus and Prochilodus nigricans were recorded in nutrient-poor and nutrient-rich rivers. However, larval and juvenile individuals of these species were found in nutrient-rich rivers only, indicating that spawning activity was restricted to that river type.3. Concentration of suspended solids in the river was correlated with total ichthyoplankton density and related to species composition of juvenile characiform assemblages.4. Our findings reinforce the hypothesis that nutrient-rich rivers and associated floodplains function as spawning and nursery grounds, and suggest that they function as source habitats for these species in the Amazon Basin.

Notes: 1. The relationship between morphology and site-occupancy provides opportunities to infer differences in dispersal and flight ability, but empirical data for aquatic insects is limited.2. In this study, 17 species of caddisflies from 10 families were collected from springs, streams and lakes, and total body mass, relative thorax mass, relative wing area (wing loading), and the aspect ratio of the fore and hind wings (combined) were measured.3. Partial least-squares regression analysis of two independent distributional data sets produced significant models within which total body mass, relative thorax mass and wing loading were positively associated with site-occupancy, whereas aspect ratio was negatively associated with site-occupancy.4. These results suggest that the faunal composition of streams is influenced by species dispersal abilities.

Notes: 1. The Daphnia pulex-pulicaria species complex has been proposed as an example of rapid ecological speciation, associated with divergence along the gradient of waterbody size from temporary ponds to deep, stratified lakes. However, this divergence is incomplete, and thus represents an opportunity to study ecological divergence as it is occurring.2. Dynamics of twelve populations of Daphnia in the pulex-pulicaria species complex were monitored over 1 year. Six temporary pond populations and six permanent lake populations were compared to evaluate demographic differences that may contribute to ecological divergence in this complex.3. Pond populations experienced greater changes in density, which were reflected in more extreme growth rates, higher birth rates and higher mortality rates than those of lake populations.4. Mitochondrial DNA was isolated from up to three clones from each population, the D-loop of the control region was sequenced, and a phylogenetic tree was constructed. This tree revealed two strongly supported clades. The clades were not congruent with habitat type and nominal status, indicating that interhabitat gene flow occurs easily and that the nominal taxa are incompletely diverged.5. Published reports of genetic life history differences in the D. pulex-pulicaria complex are consistent with the demographic differences reported here. This suggests that ecological differences between the habitats are selectively maintaining trait differences despite the possibility for genetic exchange. Thus, these taxa may be at the inception of ecological speciation.

Notes: 1. We investigated the occurrence (presence or absence) of stream-dwelling Dolly Varden (Salmonidae: Salvelinus malma) in the 78 tributaries of the Sorachi River system, Hokkaido, Japan to examine evidence for metapopulation structure.2. We hypothesised (i) that the occurrence of Dolly Varden would be less likely in small tributaries because of the higher extinction risk and (ii) that occurrence would be less likely in isolated tributaries because of reduced rates of dispersal and colonisation. In addition to these two general predictions, we hypothesised (iii) that the occurrence would be more likely in spring-fed tributaries, because Dolly Varden prefer cold water as present in these systems.3. When all the tributaries were included in a multiple logistic regression analysis, habitat size and habitat type (spring-fed versus non-spring-fed tributaries) were selected for the best model, whereas isolation was not important. However, when tributaries in the lower Sorachi basin (an area likely influenced by human activities) were excluded from the analysis, the effect of isolation became significant.4. Our results indicate there is a potential metapopulation structure of the Dolly Varden in the Sorachi River basin. Furthermore, it is possible that metapopulation dynamics follow a clinal pattern. In the upper and middle reaches of the Sorachi River basin, an equilibrium between extinction and colonisation of Dolly Varden populations is a possibility. In the lower portion of the basin, isolation appeared to be less important, and extinction rate may be driving patterns of occurrence toward systematic extinction of local populations.5. Our results illustrate the possibility of spatial variation in the processes influencing occurrence of Dolly Varden within a single river basin, and the potential influences of habitat alteration on extinction and colonisation processes.

Notes: 1. Lakes formed in mining pits often contain high concentrations of dissolved ferric iron and sulphate (e.g. 2 and 16 mmol L−1, respectively) and the pH is buffered between 2.5 and 3.5. Efforts to neutralise their water are based on the stimulation of lake internal, bacterial iron- and sulphate reduction. Electron donors may be supplied by organic carbon compounds or indirectly by enhancement of primary production. Here, we investigated the function of mixotrophic algae, which can potentially supplement or deplete the organic carbon pool, in the carbon metabolism and alkalinity budget of an acidic mining lake.2. Two weeks after organic substrates had been added in a large in situ mesocosm of 30 m diameter, a bloom of Chlamydomonas occurred, reaching a biovolume of 80 mm3 L−1. Growth experiments using filtered lake water showed that the alga reduced the overall dissolved organic carbon (DOC) concentration despite significant photosynthetic activity. However, when Chlamydomonas were grown together with natural bacterioplankton, net DOC consumption did not increase.3. Uptake experiments using [14C]-glucose indicated that bacteria dominated glucose uptake and remineralisation. Therefore, the DOC leached in the water column was processed mainly by planktonic bacteria. Leached DOC must be regarded as loss, not transferred by larger organisms to the sediment, where reduction processes take place.4. From phytoplankton biomass and production 2 years after fertilisation we estimated that pelagic photosynthesis does not supply an electron donor capacity capable of reducing more than 2% of actual stock of acidity per year. We estimated that only the benthic primary production was in a range to compensate for ongoing inputs of iron and sulphate.

Notes: 1. Invertebrate communities at 187 least impacted streams in the Manawatu–Wanganui region of New Zealand were sampled between February and May 2000 to investigate the relative influence of catchment and reach scale environmental influences on community structure.2. Of the 138 biological (fish and periphyton), local habitat and catchment scale descriptors used to examine invertebrate community patterns, alkalinity and conductivity were the most consistently influential predictors.3. Of the 52 geographical information system (GIS)-derived catchment variables (catchment geology, catchment land use, rainfall and topography) only per cent catchment in pasture, indigenous forest, coastal sand, crushed argillite and wind blown sand were associated with any measures of the invertebrate communities.4. Grouping of communities based on GIS data in general, did not generate distinct community types. Groupings based on river catchment, conductivity and alkalinity however, did produce distinct communities.5. Streams with very low alkalinity were dominated by Ephemeroptera, Plecoptera and Trichoptera that were gradually replaced by Mollusca, Crustacea and Chironomidae as alkalinity increases.6. Habitat characteristics measured at the scale of the reach were more closely linked with measures of invertebrate community structure than any GIS derived variables or river classifications.

Notes: 1. Acid-base status has major effects on diatoms, but there is little information on their short-term response to changing acidity. This is despite the use of diatoms as bioindicators in streams where acid episodes are important during rainstorms (hours to days) or snowmelt (days to weeks). In the Llyn Brianne experimental catchments (Wales, UK), we attempted to mimic the effects of short-term acidification by (i) reciprocally transplanting diatoms between two streams of contrasting acidity and (ii) using acid-diffusing substrates.2. Diatom diversity decreased rapidly on substrata transplanted from the circumneutral into the acidic stream, and increased in the reciprocal transplantation. Changes in dominant taxa occurred within three days in the acidic stream because of the rapid growth of Eunotia exigua, and by nine days in the circumneutral stream because of the proliferation of Achnanthidium minutissimum. Transplants were near indistinguishable from ambient assemblages by day 12.3. There were no effects of enclosures on assemblage composition, but diatoms responded more rapidly to altered chemistry in enclosures with coarse mesh (26 × 50 mm) than finer mesh (320 μm).4. Chemical diffusing substrates comprised terracotta tiles attached to dosing reservoirs that created locally acid (using H2SO4) or metal-rich conditions (using MnSO4) in the circumneutral stream over 26 days. Diatom responses were compared with reference substrates dosed with deionised or circumneutral stream water, and we also assessed whether effects were moderated by macroinvertebrate grazers.5. Surface pH was lower by 1–2 pH units on acid-dosed substrates than on reference tiles or in surrounding streamwater. Grazed assemblages on acid-dosed substrates differed significantly from ungrazed reference assemblages, acquiring significantly greater relative abundance of Eunotia spp. However, the magnitude of response was less than in the between-stream transplantations either because (i) metal exposure and base cation concentrations differed between the transplants and dosing substrates or (ii) diatom response to reduced pH on the diffusing substrates was restricted by the scarcity of acidobiontic diatoms in the circumneutral stream. Similar filter, founder or dominance effects might also affect diatom responses to real acid episodes.6. These data show that diatom assemblages can respond rapidly and directly to changes in acid-base status, but short-term acidification might affect diatoms more rapidly than subsequent recovery. Because the experimental methods used were imperfect representations of episodic effects, diatom response to real acid events requires further field evaluation.

Notes: 1. This study investigated how the size of crevices might affect their effectiveness as refuges for diatom-dominated algal assemblages from the grazing minnow Campostoma anomalum.2. Crevice size was manipulated by making fired-clay substrates, using moulds to produce eight substrates with pits from 1.17 to 22.0 mm diameter. Non-pitted clay-stones and limestone were also tested. Cages were used to control the access of Campostoma to arrays of the 10 different algal-colonised substrates. The grazing treatments were: open and grazed, caged and ungrazed, and a grazed cage control. The experiment was replicated in eight large outdoor tanks. After 3 weeks, substrates were brushed and chlorophyll a concentrations of the removed algae and the algae remaining in pits were measured.3. The experiment was field-validated by exposing arrays of substrates to grazing Campostoma in five pools of a limestone stream.4. The clay-stone and limestone substrates accrued similar algal biomass and assemblages.5. Smaller crevices provided more protection against grazing than larger crevices. Specifically, pits with openings smaller than 2 mm protected the enclosed algal assemblages in both the tank and field experiments. Larger pits provided less protection and pits over 7 mm in diameter were heavily grazed and may even be preferentially grazed by Campostoma.6. None of the tested pit sizes were protective against larval chironomid grazers in the tank experiment, demonstrating that differences in the grazer size influence the effectiveness of crevice refuges.

Notes: 1. We used fossil diapausing eggs extracted from 210Pb-dated sediment cores to reconstruct historical changes in the Daphnia community of Lake Naivasha, a climate-sensitive lake in Kenya which over the past 200 years has experienced a series of well-documented natural and anthropogenic environmental changes.2. Contiguous sampling and analysis of four cores yielded ephippial capsules of eight Daphnia species. Only two of these had been recorded previously in live collections from Lake Naivasha, and one species is new to science. The four more common species (Daphnia barbata, D. laevis, D. magna, and D. pulex) show striking differences in abundance patterns and population dynamics through time. Four other species (D. lumholtzi, D. curvirostris, D. longispina s.l., and Daphnia sp. nov. type Limuru.) appear to have been present only occasionally. Nevertheless, between 1895 and 1915 seven species of Daphnia inhabited Lake Naivasha simultaneously.3. Despite considerable natural environmental change associated with climate-driven lake-level fluctuations, the Daphnia community of Lake Naivasha has been severely affected by human activities over the past century, especially the introduction of exotic fishes and water-quality changes because of agricultural soil erosion. The recent reappearance of large-bodied Daphnia species (D. magna, D. barbata, D. lumholtzi, Daphnia sp. nov. type Limuru) after 20–110 years of absence can be explained by their release from fish predation, following a dramatic increase in turbidity caused by excess clastic sediment input from eroded catchment soils. The small-bodied species D. laevis has fared less well recently, presumably because the benefit of lowered predation pressure is counteracted by more pronounced negative effects of increased turbidity on this species and loss of submerged macrophyte beds which formerly served as predation refuge.4. Our results suggest that, despite considerable environmental instability and the absence of specialised zooplanktivores, top-down control of fish on large zooplankton is important in Lake Naivasha. Predation pressure from fish has led to clear-cut shifts in local Daphnia species composition, but failed to drive the larger taxa to extinction.

Notes: 1. Shallow lake ecosystems are normally dominated by submerged and emergent plants. Biological stabilising mechanisms help preserve this dominance. The systems may switch to dominance by phytoplankton, however, with loss of submerged plants. This process usually takes place against a background of increasing nutrient loadings but also requires additional switch mechanisms, which damage the plants or interfere with their stabilising mechanisms.2. The extent to which the details or even major features of this general model may change with geographical location are not clear. Manipulation of the fish community (biomanipulation) has often been used to clear the water of algae and restore the aquatic plants in northerly locations, but it is again not clear whether this is equally appropriate at lower latitudes.3. Eleven parallel experiments (collectively the International Mesocosm Experiment, IME) were carried out in six lakes in Finland, Sweden, England, the Netherlands and Spain in 1998 and 1999 to investigate the between-year and large-scale spatial variation in relationships between nutrient loading and zooplanktivorous fish on submerged plant and plankton communities in shallow lakes.4. Comparability of experiments in different locations was achieved to a high degree. Cross-laboratory comparisons of chemical analyses revealed some systematic differences between laboratories. These are unlikely to lead to major misinterpretations.5. Nutrient addition, overall, had its greatest effect on water chemistry then substantial effects on phytoplankton and zooplankton. Fish addition had its major effect on zooplankton and did not systematically change the water chemistry. There was no trend in the relative importance of fish effects with latitude, but nutrient addition affected more variables with decreasing latitude.6. The relative importance of top-down and bottom-up influences on the plankton differed in different locations and between years at the same location. The outcome of the experiments in different years was more predictable with decreasing latitude and this was attributed to more variable weather at higher latitudes that created more variable starting conditions for the experiments.

Notes: 1. Laboratory experiments were conducted to investigate the effect of small-scale turbulent motion on the growth and respiration of bacteria in an oscillating grid apparatus. The experiments were performed under a range of energy dissipation levels similar to those occurring in freshwater systems.2. The results showed that small-scale turbulent motion does have an effect on bacterial growth and respiration. A higher gradient in the dissolved oxygen time series, higher 5-day biochemical oxygen demand values, increased bacterial abundance, increased bacterial specific respiration, higher bacterial growth rate and increased nutrient uptake were all observed when the energy dissipation rate in the water column was increased.3. This has implications for traditional laboratory procedures that are used to characterise bacterial metabolic rates under stagnant fluid-flow conditions, such as biochemical oxygen demand (BOD), which would be influenced by the effects of the small-scale fluid motion inherent in aquatic environments. According to our results, BOD values in natural systems experiencing fluid motion would be higher than traditional bottle-derived rates.

Notes: 1. Isotopic signatures (δ15N and δ13C) from young-of-the-year (YOY) yellow perch (Perca flavescens) were collected over the initial 4 month summer growing period from three separate and distinctive sites in northern Alberta, Canada. Data were analysed to test the hypotheses that there are within- and among-population differences in the patterns of isotopic δ15N and δ13C change over the growing season, and that observed isotopic dilution and/or enrichment patterns were influenced by site-specific physical and chemical factors.2. Increases in δ15N relative to spawned egg masses were observed in immediate posthatch (emergent) YOY and attributed to enrichment associated with the assimilation of yolk during embryonic development.3. Posthatch dilution of YOY δ15N signatures associated with ontogenetic dietary shifts from yolk to exogenous feeding and zooplanktivory to benthivory occurred at all sites and was associated at most sites with a concomitant increase in δ13C.4. The rate and pattern of δ15N dilution and δ13C enrichment observed for the study populations varied between and within sites and depended on maternal trophic status and timing of ontogenetic dietary shifts, as determined by prey availability and site-specific biogeochemical factors.5. Comparisons of isotopic dilution patterns among species, using results from this study and literature-derived values, indicated that dilution rates and patterns are species dependent and may vary in relation to key life-history events.6. Seasonal and spatial isotopic variability among populations and between species complicates field sampling. In particular, the connectivity to site-specific conditions found here suggests that for locally resident juvenile fishes, spatial, as well as temporal variability must be included in isotopic sampling programmes designed to characterise littoral zone foodweb relationships.

Notes: 1. The indirect effects of predators on lower trophic levels have been studied without much attention to phenotypically plastic traits of key food web components. Phenotypic plasticity among species creates phenotypic diversity over a changing environmental landscape.2. We measured the indirect effects of planktivorous larval walleye (Stizostedion vitreum) on phytoplankton biomass through their effects on the dominant herbivore species, Daphnia pulicaria and D. mendotae.3. Fish had no effect on phytoplankton biomass or overall Daphnia density. We observed a compensatory response to predation by functionally comparable species within a trophic level in the form of shifting dominance and coexistence of Daphnia species. We hypothesized that this phenotypically plastic response to predation decoupled a potential trophic cascade in this freshwater pelagic system. Daphnia pulicaria density decreased over time with fish predation, but D. mendotae density increased over time with fish predation.4. Phenotypically plastic life history trait shifts and reproductive rates differed between species in fishless and fish enclosures, accounting for population trends. Daphnia pulicaria were also proportionally higher in walleye larvae stomachs than in the enclosures, indicating that walleye preferred to feed on D. pulcaria over D. mendotae. The resultant shift in dominance may partially explain the overall benign effect of fish on grazers and supports the hypothesis that trophic level diversity can decouple a trophic cascade.

Notes: 1. We investigated the impacts of saltcedar invasion on organic matter dynamics in a spring-fed stream (Jackrabbit Spring) in the Mojave Desert of southern Nevada, U.S.A., by experimentally manipulating saltcedar abundance.2. Saltcedar heavily shaded Jackrabbit Spring and shifted the dominant organic matter inputs from autochthonous production that was available throughout the year to allochthonous saltcedar leaf litter that was strongly pulsed in the autumn. Specifically, reaches dominated by saltcedar had allochthonous litter inputs of 299 g ash free dry mass (AFDM) m−2 year−1, macrophyte production of 15 g AFDM m−2 year−1 and algal production of 400 g AFDM m−2 year−1, while reaches dominated by native riparian vegetation or where saltcedar had been experimentally removed had allochthonous litter inputs of 7–34 g AFDM m−2 year−1, macrophyte production of 118–425 g AFDM m−2 year−1 and algal production of 640–900 g AFDM m−2 year−1.3. A leaf litter breakdown study indicated that saltcedar also altered decomposition in Jackrabbit Spring, mainly through its influence on litter quality rather than by altering the environment for decomposition. Decomposition rates for saltcedar were lower than for ash (Fraxinus velutina), the dominant native allochthonous litter type, but faster than for bulrush (Scirpus americanus), the dominant macrophyte in this system.

Notes: 1. Several studies have shown that wind-induced water movements have an important effect on the spatial distribution of crustacean zooplankton. However, few attempts have been made to quantify the effect of physical processes on these broad-scale patterns. Much of our understanding of this spatial structure has been based on the results of isolated surveys, which do not capture the dynamic nature of the pelagic environment.2. In this study, we have used a combination of high-speed sampling (at a spatial resolution of 240 m) and spatial data analysis to quantify the factors influencing the horizontal spatial structure of the Daphnia galeata population in Windermere.3. The results show that lake-wide circulation patterns, as indicated by water temperature, account for 29–47% of the basin-scale spatial variance in D. galeata abundance. However, these patterns are highly dynamic and change in response to the prevailing weather. This lack of temporal persistence means that the results of single-survey sampling campaigns must be interpreted with caution.

Notes: 1. Thirty-six years of winter meteorological and limnological measurements from four lakes in the English Lake District are analysed and related to variations in the North Atlantic Oscillation (NAO). Winter weather conditions were strongly influenced by the NAO with mild, wet winters being associated with strongly positive values of the NAO index (NAOI).2. Lake surface and bottom temperatures were strongly positively correlated with the NAOI, with the highest correlations being recorded in the shallower lakes.3. Variations in the NAOI also had a significant effect on the winter concentration of nitrate. In all the lakes, there was a significant negative correlation between the NAOI and the detrended winter concentration of nitrate. The key driving variable was the local air temperature, which appeared to limit the quantity of nitrate reaching the lake by increasing the amount assimilated in the surrounding catchment in mild winters.4. Dissolved reactive phosphorus (DRP) concentrations were not significantly correlated with the NAOI in the two larger basins but significant positive correlations were recorded in the two smaller lakes. The key driving variable was the local rainfall with higher DRP concentrations being recorded after heavy rain in the lakes with a short retention time.5. The NAOI-related changes in rainfall also influenced the phytoplankton. In wet winters the concentration of chlorophyll in the two smaller lakes with the shortest retention time was lower and the spring growth of Asterionella formosa was delayed in the smallest lake.6. These differential responses demonstrate how the large-scale effects associated with the NAO can be ‘filtered’ by the physical characteristics of a particular site.

Notes: 1. Cannibalism has often been suggested as an important mechanism to reach the necessary developmental stage and size before a critical time horizon is reached, but this role has been largely unexplored. We studied effects of cannibalism on the life history of the damselfly Lestes viridis under combinations of a time constraint (by manipulating the perceived time available in the growth season) and a biotic constraint (density).2. Larvae had a faster development and growth rate when reared at high time stress (late photoperiod). They also had a higher growth rate and mass at emergence when cannibalism occurred (density 2 and 4). Cannibalism occurred earlier at higher density. Accelerated life history responses (faster development and growth rate) and a higher mass at emergence were dependent upon the timing of cannibalism. Responses were more pronounced or only present if cannibalism occurred early in the larval period.3. Our data suggest that cannibalism may not only act as a lifeboat mechanism by enabling cannibals to survive detrimental ecological conditions, but may also act as a compensatory mechanism to keep life history variables near-optimal at life history transitions, even under sub-optimal conditions.

Notes: 1. Urbanisation is an important cause of eutrophication in waters draining urban areas. We determined whether benthic algal biomass in small streams draining urban areas was explained primarily by small-scale factors (benthic light, substratum type and nutrient concentrations) within a stream, or by catchment-scale variables that incorporate the interacting multiple impacts of urbanisation (i.e. variables that describe urban density and the intensity of drainage or septic tank systems).2. Benthic algal biomass was assessed as chlorophyll a density (chl a) in 16 streams spanning a rural–urban gradient, with both a wide range of urban density and of piped stormwater infrastructure intensity on the eastern fringe of metropolitan Melbourne, Australia. The gradient of urban density among streams was broadly correlated with catchment imperviousness, drainage connection (proportion of impervious areas connected to streams by stormwater pipes), altitude, longitude and median phosphorus concentration. Catchment area, septic tank density, median nitrogen concentration, benthic light (photosynthetically active radiation) and substratum type were not strongly correlated with the urban gradient.3. Variation in benthic light and substratum type within streams explained a relatively small amount of variation in log chl a (3–11 and 1–13%, respectively) compared with between-site variation (39–54%).4. Median chl a was positively correlated with catchment urbanisation, with a large proportion of variance explained jointly (as determined by hierarchical partitioning) by those variables correlated with urban density. Independent of this correlation, the contributions of drainage connection and altitude to the explained variance in chl a were significant.5. The direct connection of impervious surfaces to streams by stormwater pipes is hypothesised as the main determinant of algal biomass in these streams through its effect on the supply of phosphorus, possibly in interaction with stormwater-related impacts on grazing fauna. Management of benthic algal biomass in streams of urbanised catchments is likely to be most effective through the application of stormwater management approaches that reduce drainage connection.

Notes: 1. River plankton are often assumed to come from upstream lakes, but the factors controlling the movement of plankton between lakes and rivers into outflow streams are unclear. We tested the possibility that the physical structure of the littoral zone near the lake outlet (depth, presence of macrophytes) and diurnal differences in plankton composition at the lake surface influence the movement of plankton from the lake into the stream and determine their persistence downstream.2. Zooplankton and phytoplankton biomass, community composition and mean body size were compared between two deep lakes without macrophytes at the lake edge and two shallow lakes with macrophytes at the lake edge. Samples were collected day and night on three dates, in the lake centre, in the littoral zone adjacent to the lake outlet, at the outlet and at two sites downstream in Algonquin Park, Ontario, Canada.3. The morphology of lake edges clearly affects the movement of lake zooplankton into outlet streams. Outlets draining deeper littoral zones had higher zooplankton biomass than shallow littoral outlets (P 〈 0.0001), but these differences disappeared within 50 m downstream of the lake. There was no difference in mean zooplankton body size among lake outlets or between littoral and outlet samples. However, shallow littoral zones were dominated by cyclopoid copepods and deeper littoral zones were dominated by Bosmina longirostris. In contrast, phytoplankton biomass entering the outlet was similar to that found within the lake and did not vary with lake outlet morphology. These effects were consistent across several sampling weeks and were not affected by surface zooplankton biomass changes associated with diurnal vertical migration in the lake centre.4. A comparison with published river zooplankton data suggests that zooplankton are rapidly eliminated from shallow outlet streams (≤1 m deep) but persist in most deeper outlet rivers (≥2 m deep). Because the depth of an outlet river determines downstream zooplankton community development, the contribution of lakes to river plankton communities may be influenced by the location of each lake within the drainage basin. These findings suggest that lake and outflow physical structure influences connection strength between spatially successive habitats.

Notes: 1. Perturbation pathways affecting interactions between feeding habitat, food supplies and diet of a lotic avian predator, blue duck (Hymenolaimus malacorhynchos Gemlin), were tracked in a New Zealand river following substantial inputs of sediment from a volcanic eruption. Sediment impacts were separated temporally into two distinct phases: (i) deposition of fine ash from volcanic fall-out, and (ii) pulsed releases of volcanic sand and gravel retained by an upstream dam.2. Levels of interstitial suspendable inorganic sediments increased by several orders of magnitude following ash inputs to the river, but returned to low levels within 3 months. Flushing of volcanic sand and gravel retained by the dam resulted in sediment deposition upstream of a large island where coarse material lodged firmly between larger benthic substratum elements in blue duck feeding habitat.3. Changes in algal biomass appeared to largely reflect seasonal and hydrological influences, but the percentage inorganic content of periphyton increased significantly during both posteruption phases.4. Diversity and abundance of blue duck food supplies on boulders and on the riverbed declined following the eruption, whereas significant impacts on biomass were only detected in benthic habitats which were more susceptible to sediment deposition. However, percent composition of the benthic and boulder invertebrate food resources appeared largely unaffected by the eruption.5. There was no apparent association between the eruption and the composition or electivity (E*) of blue duck diet, but there was a significant increase in the percent inorganic content of faeces, suggesting a reduction in the quality of food rations.6. Changes in food quantity and quality associated with deposition of volcanic sediments were linked to an observed decline in blue duck population density and fecundity following the eruption. An interacting continuum of adaptive responses to perturbation intensity and impact duration is proposed for blue duck, and potentially other long-lived, riverine birds, that enable them to cope with disturbances in highly variable and heterogenous lotic environments.

Notes: 1. The application of functional ecology models to aquatic plants often relies on morphological and life-history traits which may reflect, in part, the phenotypic plasticity displayed by aquatic plants. The present study was designed to evaluate the use of physiological traits, such as nutrition patterns, to describe aquatic plant strategies along a gradient of increasing resource availablity.2. Taking phosphorus (P) as an example, nutrition-use efficiencies were evaluated in five species, through the P-content in plant tissues, the variations in P-content according to nutrient availability and the perenniality of P-storage. Plasticity in P-storage was also investigated in Ranunculus peltatus, a morphologically highly plastic species.3. In 2001, P-content was analysed in Callitriche hamulata, C. obtusangula, C. platycarpa, Elodea nuttallii and R. peltatus tissue samples. These five species were sampled at nine different sites in streams along an increasing resource gradient in the Northern Vosges Biosphere Reserve (NE France). Variations of P-content in the roots, stems and dissected and floating leaves of R. peltatus were also studied.4. Only C. platycarpa and R. peltatus were found to occur in low nutrient availability conditions. Callitriche hamulata, C. obtusangula and E. nuttallii were restricted to mesotrophic and eutrophic sites. The highest nutrient-use efficiency was found for E. nuttallii which was able to adapt its P-storage to varying resource availabilities. Ranunculus peltatus was able to store high concentrations of P, but its P-integration within the vegetative structure was less efficient under eutrophic conditions. Callitriche spp. appeared to have relatively low nutrient-use efficiencies, although C. obtusangula displayed a high P-content. While P was stored preferentially in roots in R. peltatus populations occurring in nutrient-rich sites, there was no particular P-storage organ for populations from nutrient-poor sites.5. On the basis of P-usage, R. peltatus and E. nuttallii presented competitor traits, C. hamulata and C. platycarpa displayed stress-tolerant nutrient signatures and for C. obtusangula, ruderal or competitor characteristics dominated. The use of physiological traits, such as nutrition patterns, may provide valuable, complementary information about aquatic plant strategies, independent from the influence of morphological trait plasticity often displayed by these plants.

Notes: 1. A 2-year study was carried out on the roles of nutrients and fish in determining the plankton communities of a shallow lake in north-west Spain. Outcomes were different each year depending on the initial conditions, especially of macrophyte biomass. In 1998 estimated initial ‘per cent water volume inhabited’ (PVI) by submerged macrophytes was about 35%. Phytoplankton biomass estimated as chlorophyll a was strongly controlled by fish, whereas effects of nutrient enrichment were not significant. In 1999 estimated PVI was 80%, no fish effect was observed on phytoplankton biomass, but nutrients had significant effects. Water temperatures were higher in 1998 than in 1999.2. In the 1998 experiment, cladoceran populations were controlled by fish and cyanobacteria were the dominant phytoplankton group. There were no differences between effects of low (4 g fresh mass m−2) and high (20 g fresh mass m−2) fish density on total zooplankton biomass, but zooplankton biomass was higher in the absence of fish. With the high plant density in 1999, fish failed to control any group of the zooplankton community.3. Total biovolume of phytoplankton strongly decreased with increased nutrient concentrations in 1998, although chlorophyll a concentrations did not significantly change. At higher nutrient concentrations, flagellate algae became more abundant with likely growth rates that could have overcompensated cladoceran feeding rates. This change in phytoplankton community composition may have been because of increases in the DIN : SRP ratio. Both chlorophyll a concentration and total phytoplankton biovolume increased significantly with nutrients in the 1999 experiment.4. A strong decline of submerged macrophytes was observed in both years as nutrients increased, resulting in shading by periphyton. This shading effect could account for the plant decline despite lower water turbidity at the very high nutrient levels in 1998.

Notes: 1. The response of major food-web constituents to combinations of nutrient addition and zooplanktivorous fish abundance was tested during two subsequent years in the shallow charophyte-dominated lake Naardermeer in the Netherlands, using in situ enclosures.2. Treatment effects differed sharply between study years. In 1998, when the summer temperature was low (17–21 °C), high algal biomass only developed at high nutrient levels in the presence of fish, but with no major effect on Chara biomass. In 1999, when the summer temperature was relatively high (20–25 °C), algal blooms occurred at high nutrient levels regardless of fish abundance, and were associated with a drastic decline in Chara biomass.3. Differences between years in temperature and initial zooplankton composition and biomass were likely to contribute to the varying relative importance of top-down and bottom-up effects in these enclosure experiments.4. The results suggest that when nutrient loads are increased towards levels where the macrophyte-dominated state is being destabilised, a ‘switch’ is more likely to occur in warm summers.

Notes: 1. Responses of zooplankton to nutrient enrichment and fish predation were studied in 1998 and 1999 by carrying out parallel mesocosm experiments in six lakes across Europe.2. Zooplankton community structure, biomass and responses to nutrient and fish manipulation showed geographical and year-to-year differences. Fish had a greater influence than nutrients in regulating zooplankton biomass and especially the relative abundances of different functional groups of zooplankton. When fish reduced the biomass of large crustaceans, there was a complementary increase in the biomasses of smaller crustacean species and rotifers.3. High abundance of submerged macrophytes provided refuge for zooplankton against fish predation but this refuge effect differed notably in magnitude among sites.4. Large crustacean grazers (Daphnia, Diaphanosoma, Sida and Simocephalus) were crucial in controlling algal biomass, while smaller crustacean grazers and rotifers were of minor importance. Large grazers were able to control phytoplankton biomass even under hypereutrophic conditions (up to 1600 μg TP L−1) when grazer biomass was high (〉80–90 μg dry mass L−1) or accounted for 〉30% of the grazer community.5. The littoral zooplankton community was less resistant to change following nutrient enrichment in southern Spain, at high temperatures (close to 30 °C), than at lower temperatures (17–23 °C) characterising the other sites. This lower resistance was because of a greater importance of nutrients than zooplankton in controlling algal biomass.6. Apart from the reduced role of large crustacean grazers at the lowest latitude, no consistent geographical patterns were observed in the responses of zooplankton communities to nutrient and fish manipulation.

Notes: 1. High biomass of macrophytes is considered important in the maintenance of a clear-water state in shallow eutrophic lakes. Therefore, rehabilitation and protection of aquatic vegetation is crucial to the management of shallow lakes.2. We conducted field mesocosm experiments in 1998 and 1999 to study community responses in the plant-dominated littoral zone of a lake to nutrient enrichment at different fish densities. We aimed to find the threshold fish biomass for the different nutrient enrichment levels below which large herbivorous zooplankton escapes control by fish. The experiments took place in the littoral of Lake Vesijärvi in southern Finland and were part of a series of parallel studies carried out jointly at six sites across Europe.3. In 1998, when macrophyte growth was poor, a clear-water state with low phytoplankton biomass occurred only in unenriched mesocosms without fish or with low fish biomass (4 g fresh mass m−2). Both nutrient enrichment and high fish biomass (20 g fresh mass m−2) provoked a turbid water state with high planktonic and periphytic algal biomass. The zooplankton community was dominated by rotifers and failed to control the biomass of algae in nutrient enriched mesocosms. The littoral community thus had low buffer capacity against nutrient enrichment.4. In 1999, macrophytes, especially free-floating Lemna trisulca L., grew well and the zooplankton community was dominated by filter-feeding cladocerans. The buffer capacity of the littoral community against nutrient enrichment was high; a clear-water state with low phytoplankton biomass prevailed even under the highest nutrient enrichment. High grazing rates by cladocerans, together with reduced light penetration into the water caused by L. trisulca, were apparently the main mechanisms behind the low algal biomass.5. Effects of fish manipulations were less pronounced than effects of nutrient enrichment. In 1999, clearance rates of cladocerans were similar in fish-free and low-fish treatments but decreased in the high-fish treatment. This suggests that the threshold fish biomass was between the low- and high-fish treatments. In 1998, such a threshold was found only between fish-free and low-fish treatments.6. The pronounced difference in the observed responses to nutrient enrichment and fish additions in two successive years suggests that under similar nutrient conditions and fish feeding pressure either clear or turbid water may result depending on the initial community structure and on weather.

Notes: 1. Macrobrachium hainanense is a predatory palaemonid shrimp (total length 〉7 cm) that can be abundant [density 3–5 m−2; biomass 484–606 mg ash-free dry mass (AFDM) m−2] in forest streams in Hong Kong, China. This study investigated the growth and production of M. hainanense during 2001 and 2002 in pools of two forested streams (one third- and one fourth-order).2. The growth of tagged individuals was recorded in situ and compared with that of tagged and untagged shrimps in laboratory tanks. Field and laboratory estimates yielded similar growth rates of 0.7 mm carapace length (CL) per month, and instantaneous growth rate was 0.004 g AFDM g−1 day−1. Tagging did not affect growth in the laboratory. Cohort analysis of field populations produced similar estimates of growth to that of tagged individuals, and the growth of M. hainanense was generally slower than has been reported for other Macrobrachium species. Mass-specific growth rate of M. hainanense in the field varied with size and was two to five times higher in small individuals (〈10 mm CL). In addition, growth rate varied with season and was 40% lower in the dry season when temperature was at the annual minimum.3. Males grew bigger than females (36 versus 25 mm CL). The minimum lifespan of M. hainanense in the field, calculated from size-specific growth rates, ranged from 29.3 months (females) to 47.6 months (males). Male lifespan derived from cohort analysis was estimated as 48 and 46 months in the two streams. Females reached maturity in 17–18 months (at 15–17 mm CL) while males matured at 24–26 months (at 18–22 mm CL). Females bred twice (at 2 and 3 years of age) while males probably bred three times (at 2, 3 and 4 years) in both streams.4. Macrobrachium hainanense production in the fourth-order stream, calculated by the size-frequency method, was 900 and 1096 mg AFDM m−2 year−1 (for 2001 and 2002, respectively) with a production/biomass (P/B) of 2.1–2.3 year−1. In the third-order stream, production was 987 and 1304 mg AFDM m−2 year−1 (for 2001 and 2002, respectively) with a P/B of 1.7–2.1 year−1. Production estimates based on the instantaneous growth method were half of those obtained by the size-frequency method.5. Although M. hainanense production at the third-order stream exceeded that in the fourth-order, growth rates showed the opposite pattern and were 0.31–0.43 mm CL month−1 and 0.56–0.65 mm CL month−1 in the third- and fourth-order streams, respectively. Greater mortality in the latter may account for low production at a site where growth rate was high.6. Production of M. hainanense in both streams was lower during 2001 when rainfall was higher. This may reflect the influence of spates associated with monsoonal rains, which could have reduced M. hainanense production through spate-induced mortality or by reducing the abundance of prey. This study provides the first in situ estimate of secondary production by a non-commercial Macrobrachium species in Asia or elsewhere. It involved a whole-pool approach to sampling that allowed the estimation of production and population parameters on a realistic scale.

Notes: 1. Results are analysed from 11 experiments in which effects of fish addition and nutrient loading on shallow lakes were studied in mesocosms. The experiments, five in 1998, six in 1999, were carried out in six lakes, distributed from Finland to southern Spain, according to a standard protocol.2. Effects of the treatments on 29 standard chemical, phytoplankton and zooplankton variables are examined to assess the relative importance of bottom-up (nutrient enrichment) and top-down (fish predation) effects. For each year, the experiments in different locations are treated as replicates in a meta-analysis. Results of individual experiments are then compared in terms of the patterns of significant influences of nutrient addition and fish predation with these overall results (the baseline), and between years in the same location.3. The overall meta-analysis gave consistent results across the 2 years, with nutrient loading influencing all of the chemical variables, and on average 31% of primary producer and 39% of zooplankton variables. In contrast, fish influenced none of the chemical variables, 11% of the primary producer and 44% of the zooplankton variables. Nutrient effects on the system were thus about three times greater than fish effects, although fish effects were not inconsiderable.4. The relative importance of nutrients and fish in individual experiments often differed between years at the same location and effects deviated to varying degrees from the baseline. These deviations were treated as measures of consistency (predictability) of conclusions in repeat experiments. Consistency increased southwards and this is interpreted as a consequence of more variable annual weather northwards.5. The influence of nutrient loading was greater southwards and this was probably manifested through naturally greater annual macrophyte abundance in warmer locations in consequence of the longer plant growing-season. There was no trend in the relative importance of fish effects with latitude but this may partly be an artefact of the simple fish community used. These findings suggest that nutrient control should be a greater priority than biomanipulation in the restoration of eutrophicated shallow lakes in warm temperate regions.6. Starting conditions affected the outcome of experiments. High initial concentrations of total phosphorus and planktonic chlorophyll a concentration (created by local conditions prior to the experiment) led to de-emphasis of the importance of nutrient loading in the experiment.

Notes: 1. Nutrient and fish manipulations in mesocosms were carried out on food-web interactions in a Mediterranean shallow lake in south-east Spain. Nutrients controlled biomass of phytoplankton and periphyton, while zooplankton, regulated by planktivorous fish, influenced the relative percentages of the dominant phytoplankton species.2. Phytoplankton species diversity decreased with increasing nutrient concentration and planktivorous fish density. Cyanobacteria grew well in both turbid and clear-water states.3. Planktivorous fish increased concentrations of soluble reactive phosphorus (SRP). Larger zooplankters (mostly Ceriodaphnia and copepods) were significantly reduced when fish were present, whereas rotifers increased, after fish removal of cyclopoid predators and other filter feeders (cladocerans, nauplii). The greatest biomass and diversity of zooplankton was found at intermediate nutrient levels, in mesocosms without fish and in the presence of macrophytes.4. Water level decrease improved underwater light conditions and favoured macrophyte persistence. Submerged macrophytes (Chara spp.) outcompeted algae up to an experimental nutrient loading equivalent to added concentrations of 0.06 mg L−1 PO4-P and 0.6 mg L−1 NO3-N, above which an exponential increase in periphyton biomass and algal turbidity caused characean biomass to decline.5. Declining water levels during summer favoured plant-associated rotifer species and chroococcal cyanobacteria. High densities of chroococcal cyanobacteria were related to intermediate nutrient enrichment and the presence of small zooplankton taxa, while filamentous cyanobacteria were relatively more abundant in fishless mesocosms, in which Crustacea were more abundant, and favoured by dim underwater light.6. Benthic macroinvertebrates increased significantly at intermediate nutrient levels but there was no relationship with planktivorous fish density.7. The thresholds of nutrient loading and in-lake P required to avoid a turbid state and maintain submerged macrophytes were lower than those reported from temperate shallow lakes. Mediterranean shallow lakes may remain turbid with little control of zooplankton on algal biomass, as observed in tropical and subtropical lakes. Nutrient loading control and macrophyte conservation appear to be especially important in these systems to maintain high water quality.

Notes: 1. Leptodora is a key species in many temperate freshwater systems, but so far its role in the food web could not be properly evaluated because detailed information about its secondary production was lacking. As we wanted to estimate the secondary production of Leptodora, we measured its development and growth rates in the laboratory.2. Employing improved methods to estimate growth and instar durations, we cultured Leptodora kindtii in the laboratory at four constant temperatures (15, 17.5, 20 and 25 °C). Growth in length and development times of eggs and instar stages were assessed.3. Growth rates at 15, 17.5 and 20 °C were similar, but at 25 °C growth was distinctly faster. At 17.5 °C we observed seven juvenile instar stages before the first adult instar stage was reached.

Notes: 1. The general notion is that negative effects of vertebrate herbivores on water plants, which play a key role in freshwaters, prevail, and that positive feedbacks of herbivores on plants are insignificant.2. The most likely systems to find such positive feedbacks are those in which herbivores exert strong feeding pressures on plants during part of the year. Previous theoretical work has suggested that compensatory production occurs when migratory Bewick's swans forage on tubers of fennel pondweed. As a corollary, the swans can exploit the tubers down to a level that maximises their tuber yield.3. In order to test these hypotheses, I measured pondweed tuber biomass on three occasions per year (just before and after foraging, and just before tuber sprouting) in three consecutive years. The 17 sampling sites in the Lauwersmeer (the Netherlands) were classified according to their silt content and water depth. Within four silt-depth classes, I predicted for each year tuber biomass production and, from that, the optimum foraging threshold that would result in the maximum tuber biomass yield.4. Water depth did not affect tuber production, and silt content only did in one of the 3 years. In accordance with overcompensation predictions, tuber production was higher at plots with moderate foraging pressures than at plots with little or no grazing. However, the winter and summer conditions following the swan foraging had large unpredictable effects on tuber mortality and production.5. These results indicate that overcompensation by fennel pondweed occurs and that Bewick's swans are generally able to profit from it, albeit without fine-tuning of the foraging threshold to the yield.

Notes: 1. We examined the influence of hydrologic seasonality on temporal variation of planktonic bacterial production (BP) in relatively undisturbed lowland rivers of the middle Orinoco basin, Venezuela. We sampled two clearwater and two blackwater rivers over 2 years for dissolved organic carbon (DOC), chlorophyll, phosphorus and bacterial abundance to determine their relationship to temporal variation in BP.2. Dissolved organic carbon concentration was greater in blackwater (543–664 μm) than in clearwater rivers (184–240 μm), and was generally higher during periods of rising and high water compared with low water. Chlorophyll concentration peaked (3 μg L−1) during the first year of study when discharge was lowest, particularly in blackwater rivers. Soluble reactive phosphorus (SRP) was very low in the study rivers (〈3.8 μg L−1) and concentration increased during low water.3. Average BP was higher in clearwater (0.20–0.26 μg C L−1 h−1) than in blackwater rivers (0.14–0.17 μg C L−1 h−1), although mean bacterial abundance was similar among rivers (0.6–0.8 × 106 cells mL−1).4. Periods of higher chlorophyll a concentration (low water) or flushing of terrestrial organic material (rising water) were accompanied by higher BP, while low BP was observed during the period of high water.5. Interannual variation in BP was influenced by variations in discharge related to El Niño Southern Oscillation events.6. Seasonal variation in BP in the study rivers and other tropical systems was relatively small compared with seasonal variation in temperate rivers and lakes. In addition to the low seasonal variation of temperature in the tropics, low overall human disturbance could result in less variation in the inputs of nutrients and carbon to the study rivers compared with more disturbed temperate systems.

Notes: 1. Fine-scale physical and chemical gradients and deep photosynthetic microbial populations were assessed to provide an initial characterisation of a small, thermally stratified reservoir (Cross Reservoir, Kansas, U.S.A.) and its deep chlorophyll maxima (DCM). Factors were identified that may affect vertical positioning of subepilimnetic photosynthetic sulphur bacteria (PSB) in lakes.2. Results indicate that Cross Reservoir is a mesotrophic, dimictic lake with large subepilimnetic chlorophyll maxima containing dense layers of PSB. Characteristics of the deep PSB community of Cross Reservoir strongly correlate with both light and nutrient gradients.3. The deep bacterial community mostly contained single-celled and aggregating green sulphur bacteria, specifically free-living Chlorobium limicola and the conspicuous motile ectosymbiotic consortium known as ‘Chlorochromatium aggregatum’. The bacteria were within the anaerobic hypolimnion, beneath a metalimnetic plate of Cryptomonas spp. and within very low sulphide and light conditions [mean of 67 μgS L−1 and 0.05% photosynthetically active radiation (PAR)]. Pigment concentrations and fluorescence trends indicate that the bacteria made up a larger proportion of the DCM biomass than did phytoplankton in 1996.4. Cross Reservoir shares characteristics with natural lakes world-wide that also include a deep PSB community containing dense layers of ‘C. aggregatum’. Correlation analyses indicate that PSB community positioning and density are related to light, sulphide supply, redox potentials and pH. A 2-factor principal components analysis (PCA) and other data trends supported these interpretations and indicated that PSB are sensitive to the thermal stability of the water column, are nitrogen limited and regulated more by sulphide or sulphide to light ratios than local levels of light. The sensitivity of these deep photosynthetic bacteria to environmental gradients, and their significance to some aquatic systems, demonstrate their potential as indicators of environmental disturbance.

Notes: 1. Macrobrachium hainanense is a large predatory palaemonid shrimp, present at high densities in pools of low-order forested streams in Hong Kong. The present study investigated the impacts of M. hainanense on benthic community structure and functions in pools of two streams: Tai Po Kau Forest Stream and Tai Shing Stream.2. Repeated whole-pool experiments involving shrimp density manipulations (removal, control and addition) were conducted in both streams between October 2000 and April 2002, and included a wet (May to September) and two dry (October to April) seasons. The three objectives of the study were to determine the effects of M. hainanense predation on benthic macroinvertebrate abundance and species richness, rates of leaf litter breakdown because of effects on detritivores, and periphyton standing stocks by way of an effect on herbivores.3. Wet season results showed consistent reductions in benthos densities and species richness following heavy rainfall, irrespective of shrimp manipulation. These results suggested that spate-induced disturbances might override biotic effects and play a dominant role in structuring benthic communities in stream pools in Hong Kong.4. No significant, reproducible effects on any of the response variables measured in either stream were found during the dry season. Litter breakdown was reduced in the absence of shrimps during one experiment only, suggesting it might be a type I error. These results signified no effect of shrimp removal on benthic communities, or on the functional processes of litter breakdown, or on periphyton accumulation. The large scale of the experimental units (8–40 m2), refuge availability, and the presence of benthic predatory fishes that cropped excess prey made available by removal of M. hainanense, may have contributed to the lack of any effect, despite the abundance of the predatory shrimps.

Notes: 1. Cryptic species diversity poses evolutionary questions about its origin and maintenance, and ecological questions about the coexistence of seemingly identical species.2. We examine patterns of morphological and life history differentiation in three sympatric cryptic species of freshwater amphipods within the Hyalella azteca species complex. These species are separated by extensive molecular evolution, but appear similar in phenotype. Species were collected from the littoral zone of a small kettle lake in Michigan, U.S.A., and identified to species by molecular genetic methods.3. Two of the species were similar, differing only in female size, whereas the third was larger in body size and had larger clutches of smaller eggs than the other two. There were differences between the species also in pleon spine length and antennal segment number.4. An analysis of allozyme variation among the cryptic species in three lakes suggests that the species are reproductively isolated within lakes.5. We suggest that phenotypic similarity of these species is maintained by size-selective predation by fish. The small, but statistically significant, differences in body size may form the axis for a tradeoff between resource exploitation and predator avoidance, a condition that can foster coexistence of phenotypically similar species.

Notes: 1. Headwater streams of the Hubbard Brook Experimental Forest (HBEF) are typically characterised by a periphyton assemblage of low biomass and diversity. However, periphyton blooms have been observed following catchment deforestation experiments and occasionally during the annual spring thaw before canopy leaf-out.2. There is pronounced seasonal variation in both nutrient and light availability in HBEF streams. Stream water nitrogen (N) concentrations and light levels are higher before canopy leaf-out and after leaf senescence and are lower during the growing season. Periphyton accrual rates also change seasonally; they are highest in spring prior to leaf-out and significantly lower during summer and in autumn.3. Periphyton biomass rarely responded positively to in-situ experimental enrichment with nitrogen or phosphorus. In the summer, nutrient enrichment overall had no effect on periphyton biomass, while outside the growing season N enrichment had inhibitory effects on periphyton.4. Despite these experimental results, surveys of ambient chlorophyll a concentrations in streams across the HBEF demonstrated no relationship between streamwater dissolved inorganic N or P concentrations and benthic chlorophyll a.5. Our results suggest that HBEF periphyton communities are not closely regulated by nutrient availability, even during periods of high light availability. The inhibitory effects of nutrient enrichment outside the growing season are interesting, but further research is necessary to elucidate the mechanisms driving these responses.

Notes: 1. Molecular approaches have increasingly revealed hidden genetic structure within ecologically important species, leading to the creation of sibling species whose ecological relevance is often unclear. A prime example is Daphnia galeata mendotae, which was split into D. dentifera and D. mendotae based on differences at two allozyme loci.2. In a set of lake populations in Michigan USA, we test the geographical and temporal consistency of the genetic structure underlying this species split. We also test the morphological relevance of this molecular variation and its ecological significance in lakes. In essence, we ask: does recognition of these new species provide valuable information for plankton ecologists?3. We found that D. dentifera and D. mendotae represent morphologically and ecologically distinct forms that are distributed among lakes in non-random fashion, which were remarkably stable over 6 years. Key differences between the species concern their body and head shape, vertical habitat use within lakes and distribution among lakes of different size. We hypothesise that these differences represent specialisation to habitats that differ in risk of invertebrate predation.4. Reproductive barriers alone are insufficient to explain the pattern of genetic structure; in some lakes complete introgression is apparent. However, parent species and hybrids exhibit a stable co-existence in many lakes, which suggests that ecological specialisation reinforces divergence within this taxon.

Notes: 1. Three groups of laboratory experiments clarified the role of nematodes as a potential food resource for the triclad Dugesia gonocephala. The first group measured the functional response of adult D. gonocephala feeding on juvenile or adult Caenorhabditis elegans. The feeding rates of D. gonocephala on adult and juvenile C. elegans followed a type II functional response. The maximum number of adult nematodes and juvenile nematodes eaten by a single D. gonocephala individual within 3 h was 94 and 197 nematodes, respectively.2. A second group of microcosm experiments investigated the effect of D. gonocephala on the density and the vertical distribution of a nematode community in fine sand. The following treatments were performed: (i) microcosms with 400 nematodes and (ii) microcosms with 400 nematodes and one D. gonocephala. After 5 days, nematodes as a group, as well as the dominant species Tobrilus pellucidus and Trischistoma monohystera, showed no significant difference in vertical patterns between the treatments with and without D. gonocephala.3. The third group of experiments determined whether grain size of the sediment (sand, fine gravel and coarse gravel) altered the ability of D. gonocephala to consume adult C. elegans. Sand and fine gravel reduced the predation effectiveness of D. gonocephala by 100%, whereas the predator consumed nematodes in coarse gravel (19 nematodes within 3 h).

Notes: 1. A method based on hierarchical clustering and Bayesian probabilities is used to identify phytoplankton assemblages and analyse their pattern of occurrence and temporal coherence in three deep, peri-alpine lakes. The hierarchical properties of the method allowed ranking by order of importance of the effects of changes related to climate and to human activity on the phytoplankton structure.2. The three deep, peri-alpine lakes (the Lower Zurich, Upper Zurich and Walen lakes) investigated in this study have been monitored since 1972. During that period they have undergone oligotrophication as a result of management programmes and they have been subject to similar meteorological effects that have led to higher water temperatures since 1988.3. The phytoplankton assemblages of the most eutrophic lake (Lower Zurich) differ strongly from those observed in the two meso-oligotrophic lakes. Local environmental conditions appear to be the main factor responsible for species composition and change in climate characterised by the warmer water temperatures observed since 1988 have had a major impact on the winter composition of the lower basin of Lake Zurich by promoting Planktothrix rubescens.4. Some phytoplankton assemblages are found in all the lakes. Their patterns of occurrence display strong synchrony at the annual and/or inter-annual scales. However, temporal coherence between the lakes sometimes also involves different assemblages.5. The reduction in phosphorus had a great influence on long-term changes in composition. In all three lakes, decreases in phosphorus are associated with a community characterised by some mixotrophic species or species adapted to low nutrient concentrations or sensitive to transparency. In the Lower Lake Zurich the decrease in phosphorus has also led to the development of species adapted to low light intensities.6. Seasonal meteorological forcing has also induced synchronous changes, but the same assemblages are not necessarily involved, because the pool of the well-placed candidate taxa that may develop is determined by the local environmental conditions, and mainly by phosphorus concentrations. In the most eutrophic lake, the seasonal pattern is characterised by a succession of more stages. However, the seasonal assembly dynamics involve the succession of species sharing common selective advantages that make them relatively stronger under these nutrient and light conditions.

Notes: 1. Selective grazing of adults and larvae of the zebra mussel (Dreissena polymorpha) on phytoplankton and detritus from both laboratory cultures and natural seston was quantified using flow cytometry.2. Mean clearance rate of adult zebra mussels was higher on a mixture of the green alga Scenedesmus and the cyanobacterium Microcystis than when Scenedesmus was offered as single food, suggesting selective feeding by the mussels.3. Feeding on lake seston both adults and larvae showed a higher clearance rate on phytoplankton than on detritus particles, suggesting that zebra mussels select for phytoplankton. Furthermore, it was noted that adults preferred seston particles in the 0–1 and 30–100 μm size ranges.4. In our study, zebra mussels did not discriminate against cyanobacteria, and our results indicate that they may even ingest them preferentially.

Notes: 1. Vertical transport of nutrients in sedimenting faecal material is greatly reduced by coprophageous organisms. Unfortunately, nearly all work on faecal production, sedimentation and coprophagy has dealt with copepods in marine ecosystems. Here, we report the first evidence of coprophagy in freshwater zooplankton from oligotrophic and eutrophic lakes. We used 14C-labelled algae and faecal material to estimate the rates of algal clearance and coprophagy.2. Measured feeding rates per individual on faecal material were similar (Daphnia pulex, D. rosea, Leptodiaptomus tyrelli) or even higher (D. lumholtzi) than filtering rates on phytoplankton. This finding does not necessarily implicate active selection of faeces over algae because: (i) we did not use the same food concentrations for faeces and algae, and (ii) grazers of slightly different sizes were used in each test.3. Weight-specific clearance rates of L. tyrelli and Holopedium gibberum on faecal matter (0.084–0.089 mL μg−1 h−1) were higher than in the daphniids (0.026 mL μg−1 h−1).4. The data indicate that coprophagy in freshwater ecosystems is an important mechanism of nutrient recycling, and this process should be taken into account when studying nutrient fluxes within lakes and reservoirs.

Notes: 1. Environmental stress may have indirect positive effects on population size through modification of food-web interactions, despite having negative effects on individuals. Here we evaluate the individual- and population-level effects of acidification on crayfish (Cambarus bartonii) in headwater streams of the Allegheny Plateau (PA, U.S.A.) with field experiments and survey data. Median baseflow pH of 24 study reaches in nine streams varied from 4.4 to 7.4, with substantial variation found both among and within streams.2. Two bioassays were conducted to evaluate the relationship between stream pH and crayfish growth rates. Growth rates were always higher in circumneutral reaches than in acidic reaches. Crayfish originating in acidic water grew less when transplanted into neutral water than did crayfish originating in neutral water, providing some evidence for a cost of acclimation to acidity.3. Stream surveys showed that fish were less abundant and crayfish more abundant in acidified streams than in circumneutral streams. Crayfish density was sixfold higher in reaches with the lowest pH relative to circumneutral reaches. Large crayfish made up a higher proportion of crayfish populations at sites with high fish biomass, consistent with the hypothesis that fish predation on small individuals may be limiting crayfish population size at these sites.4. Although individual crayfish suffered lower growth in acidified streams, increased acidity appeared to cause an increase in crayfish population size and shifts in size structure, possibly by relieving predation pressure by fish.

Notes: 1. In situ experiments were conducted using specialised incubation devices to grow biofilms under varying light regimes and grazing intensities (by excluding fish and large-sized zooplankton, 〉2 mm) both within and between two sites in Lake Saint-Pierre.2. Biofilms growing under greater in situ UVR and light exposures found in the south water mass were characterised by a greater biomass and nutrient content, but their total fatty acid (FA) contents and ratios of elemental nutrients were not significantly different from the north. There was a relatively greater abundance of chlorophytes and cyanobacteria in the south water mass, along with a greater proportion of low nutritional quality saturated fatty acids (SAFA). Conversely, biofilms growing in the north had a greater relative abundance of diatoms, as well as greater eicosapentaneoic acid (20:5ω3) and docosahexaneoic acid (22:6ω3) concentrations (two FAs implicated in the physiological competency of grazers).3. The prevailing community structures created differences in terms of nutritional status of the biofilms for benthic grazers and their predators at the two sites. The biofilms from the southern site were characterised by greater food quantity at the expense of quality, while biofilms from the northern site contained less food of a better quality. Despite this, the nutritional regime in the south supported a greater productivity at higher trophic levels. The secondary treatments (light and grazing by fish and macro-invertebrates) had lesser effects on food quality.

Notes: 1. Persistence of zebra mussel populations in river systems probably depends upon the presence of upriver sites capable of hosting self-recruiting adult populations that act as sources of larvae. In this paper we examine the importance of Lake Pepin, a natural riverine lake in the Upper Mississippi River, as an upriver source of larvae to the downstream populations of zebra mussels.2. Field studies and modelling suggest that Lake Pepin plays a major role in maintaining zebra mussel populations in the Upper Mississippi River. Long water residence times in Lake Pepin allow for self-recruitment under the right hydraulic conditions. Larval abundance was low to absent upstream of the lake but increased dramatically downriver in all 3 years of the study. Travel time estimates in the Upper Mississippi River show that newly fertilised larvae drifting out of Lake Pepin can contribute substantially to the major downstream peak in larval abundance. In contrast, backwater and other off-channel sites are unlikely to drive main-channel abundance patterns. Larval abundances in off-channel sites were less than or equal to those in the main channel.3. A key factor in assessing the importance of Lake Pepin as a source population was the abundance of early stage, unshelled larvae. Studies that consider only abundances of older shelled stages (visible by cross polarised lighting) may yield misleading results. Results of this study suggest that efforts to control zebra mussels in the Upper Mississippi River should focus on controlling adult populations within Lake Pepin and reducing or eliminating larvae exiting the lake.

Notes: 1. Movements between a stream reach and two adjacent lentic macrohabitats, a beaver pond and a lake, were followed for the Appalachian crayfish and two fish species, brook charr and brown bullhead, over an 85-d period from early June to late August, and were analysed in relation to water level, maximum water temperature, photoperiod length, lunar luminosity, and age, by use of time-series regressions.2. Brook charr showed strong net immigration to the stream reach for underyearling (age class 0+) fish but net emigration for 1+ fish. Both immigration and emigration were positively related to water level and temperature; migratory responses to temperature were age-specific.3. Brown bullhead used the stream primarily as a corridor for downstream migration from the beaver pond to the lake. As with brook charr, water level and temperature had a positive effect on movement and responses were stronger in younger individuals.4. Crayfish emigrated from the stream during the summer. Movements were positively related to increases in water level and temperature, with responses of 1+ crayfish much stronger than those of older individuals.5. The results indicate that the stream tributary played different roles for brook charr (nursery), brown bullhead (dispersal corridor) and Appalachian crayfish (early summer refuge), and suggest that variation in water level or temperature resulting from climatic change or local anthropogenic activities might modify connectivity between macrohabitats, particularly for younger individuals.

Notes: 1. Our objective was to measure the effects of bioturbation and predation on the physical characteristics and biogeochemical processes in river sediments.2. We investigated the impacts of tubificid worms tested separately and together with an omnivore (Gammarus pulex), which does feed on tubificids, on sediment distribution, water flux, sediment organic carbon, biofilm biomass and microbial activities, and the concentrations of dissolved oxygen, dissolved organic carbon, PO〈inlineGraphic alt="inline image" href="urn:x-wiley:00465070:FWB1233:FWB_1233_mu1" location="equation/FWB_1233_mu1.gif"/〉, NO〈inlineGraphic alt="inline image" href="urn:x-wiley:00465070:FWB1233:FWB_1233_mu2" location="equation/FWB_1233_mu2.gif"/〉, NO〈inlineGraphic alt="inline image" href="urn:x-wiley:00465070:FWB1233:FWB_1233_mu3" location="equation/FWB_1233_mu3.gif"/〉 and NH〈inlineGraphic alt="inline image" href="urn:x-wiley:00465070:FWB1233:FWB_1233_mu4" location="equation/FWB_1233_mu4.gif"/〉 in slow filtration sand–gravel columns. We hypothesised that gammarids, which exploit the top 2–3 cm of the sediment, would modify the impact of worms at the sediment surface.3. In experiments both with and without gammarids, bioturbation by the tubificids modified both the distribution of surface particles in the sediment column and water flux. In addition, microbial aerobic (oxygen consumption) and anaerobic (denitrification and fermentative decomposition of organic matter) processes in the sediment were stimulated in the presence of tubificid worms. However, G. pulex did not affect either the density or bioturbation activity of the tubificid worms.4. Bioturbation by the benthos can be a major process in river habitats, contributing to the retention of organic matter in sediment dynamics. The presence of at least one predator had no effect on bioturbation in sediments. In such systems, physical heterogeneity may be sufficient for tubificids to escape from generalist predators, though more specialised ones might have more effect.

Notes: 1. We studied seasonal changes in the carbon and nitrogen stable isotope ratios of larval Chironomus anthracinus and C. plumosus from the profundal sediments of four contrasting lakes.2. Pronounced seasonal changes in both δ13C and δ15N values were evident in chironomid larvae of both species from two summer-stratified, eutrophic lakes: Esthwaite Water and Wyresdale Park. Changes were most marked in the larvae of C. plumosus and in larvae from greater depths. In contrast, neither C. anthracinus in summer-stratified but mesotrophic Schöhsee, nor C. plumosus in polymictic Großer Binnensee, showed marked seasonality in larval stable isotope ratios.3. The particularly strong 13C-depletion of larvae from the stratified, eutrophic lakes is attributed to a significant contribution of methane-derived carbon to their diets. Feeding by larvae on isotopically light methanotrophic bacteria appears to occur mainly when autumn overturn of the water column restores oxygenated conditions to the sediment surface. At this time both δ13C and δ15N values of larvae decreased sharply.4. Changes in the mean stable isotope ratio of the larval populations can also occur when larger, more isotopically light, larvae pupate and emigrate from the population to hatch as imagos. This effect can induce seasonal changes in larval isotope values even in lakes in which there is no evidence of a significant involvement of methane-derived carbon in their diets. Variations in emergence patterns between species and between lakes may generate differences in the seasonal pattern of change in stable isotope ratios in larval populations.5. Our results emphasise the importance of adequate seasonal sampling if stable isotope ratios are to be used as biomarkers to study the role of key groups, such as chironomid larvae, in the trophic structure of lakes.

Notes: 1. The sources of nitrogen for phytoplankton were determined for a bloom-prone lake as a means of assessing the hypothesis that cyanobacteria dominate in eutrophic lakes because of their ability to fix nitrogen when the nitrogen : phosphorous (N : P) supply ratio is low and nitrogen a limiting resource.2. Nitrogen fixation rates, estimated through acetylene reduction with 15N calibration, were compared with 15N-tracer estimates of ammonium and nitrate uptake monthly during the ice-free season of 1999. In addition, the natural N stable isotope composition of phytoplankton, nitrate and ammonium were measured biweekly and the contribution of N2 to the phytoplankton signature estimated with a mixing model.3. Although cyanobacteria made up 81–98% of phytoplankton biomass during summer and autumn, both assays suggested minimal N acquisition through fixation (〈9% for the in-situ incubations; 〈2% for stable isotope analysis). Phytoplankton acquired N primarily as ammonium (82–98%), and secondarily as nitrate (15–18% in spring and autumn, but 〈5% in summer). Heterocyst densities of 〈3 per 100 fixer cells confirmed low reliance on fixation.4. The lake showed symptoms of both light and nitrogen limitation. Cyanobacteria may have dominated by monopolizing benthic sources of ammonium, or by forming surface scums that shaded other algae.

Notes: 1. We compared fungal biomass, production and microbial respiration associated with decomposing leaves in one softwater stream (Payne Creek) and one hardwater stream (Lindsey Spring Branch).2. Both streams received similar annual leaf litter fall (478–492 g m−2), but Lindsey Spring Branch had higher average monthly standing crop of leaf litter (69 ± 24 g m−2; mean ± SE) than Payne Creek (39 ± 9 g m−2).3. Leaves sampled from Lindsey Spring Branch contained a higher mean concentration of fungal biomass (71 ± 11 mg g−1) than those from Payne Creek (54 ± 8 mg g−1). Maximum spore concentrations in the water of Lindsay Spring Branch were also higher than those in Payne Creek. These results agreed with litterbag studies of red maple (Acer rubrum) leaves, which decomposed faster (decay rate of 0.014 versus 0.004 day−1), exhibited higher maximum fungal biomass and had higher rates of fungal sporulation in Lindsey Spring Branch than in Payne Creek.4. Rates of fungal production and respiration per g leaf were similar in the two streams, although rates of fungal production and respiration per square metre were higher in Lindsey Spring Branch than in Payne Creek because of the differences in leaf litter standing crop.5. Annual fungal production was 16 ± 6 g m−2 (mean ± 95% CI) in Payne Creek and 46 ± 25 g m−2 in Lindsey Spring Branch. Measurements were taken through the autumn of 2 years to obtain an indication of inter-year variability. Fungal production during October to January of the 2 years varied between 3 and 6 g m−2 in Payne Creek and 7–27 g m−2 in Lindsey Spring Branch.6. Partial organic matter budgets constructed for both streams indicated that 3 ± 1% of leaf litter fall went into fungal production and 7 ± 2% was lost as respiration in Payne Creek. In Lindsey Spring Branch, fungal production accounted for 10 ± 5% of leaf litter fall and microbial respiration for 13 ± 9%.

Notes: 1. Herbivorous zooplankton face considerable temporal and spatial variation in food quality, to which they respond by adapting their life histories. Zooplankton may even take up mineral nutrients directly, and use these to counter the effects of algal nutrient limitation (mineral compensation). This study examined the life history of the rotifer Brachionus calyciflorus fed phosphorus-, and nitrogen-limited Scenedesmus obliquus (Chlorophyta), and investigated whether B. calyciflorus was capable of mineral compensation.2. Both phosphorus- and nitrogen-limited algae gave similar life history responses: somatic growth and reproduction were reduced, whereas lifespan remained unaffected.3. No evidence was found for mineral compensation in B. calyciflorus in relation to detrimental life history effects, so mineral compensation does not seem to be relevant for this species under field conditions.4. The similarity in life history responses of B. calyciflorus and the low levels of ω-3 PUFAs in both phosphorus- and nitrogen-depleted algae suggest that ω-3 PUFAs were limiting to B. calyciflorus, although other (bio)chemicals or mineral nutrients may also have been important.5. No trade-off was observed between life span and reproduction during algal nutrient limitation. Reduced population growth rates of B. calyciflorus were caused by shorter reproductive periods.

Notes: 1. Habitat complexity is thought to exert a significant influence on ecological communities, but its operation under variable natural conditions is not well understood, particularly in freshwater. To elucidate the role of habitat complexity, in particular the fractal structure of surface irregularity, in a stream system, field colonisation experiments were conducted at three times of year (summer, winter and spring) using natural substrates with different levels of fractal dimension in a small coastal mountain stream of southern Japan.2. In the winter experiment, comparison was also made between the standard (control) treatment and the resource-preconditioning treatment whereby experimental plates were conditioned in the natural stream environment to allow the accumulation of potential food resources (algae and detritus) for 1 month prior to the experiment.3. Species abundance patterns observed at different times of year showed little systematic variation with levels of habitat complexity but largely followed the patterns expected from, or lying in between, the Random Assortment model and the random fraction model.4. Taxon richness and density increased with habitat complexity in all seasons except for density in spring. Different taxa showed different patterns of change with habitat complexity, which also varied with seasons. Biomass of invertebrates showed no systematic trend with an increase in habitat complexity.5. Chlorophyll-a concentrations tended to be lower in more complex habitats, particularly in summer. In contrast, fine particulate organic matter (FPOM) tended to increase with habitat complexity. However, the relationship between these potential food resources and invertebrate assemblages remain unclear.6. While there were no significant differences in taxon richness and biomass of invertebrates between the resource-preconditioning and the control treatment, density was higher in the former than in the latter. The abundance of relatively large, surface-dwelling animals showed more marked temporal variation over the entire period of colonisation in the resource-preconditioning treatment than in the control treatment.7. Body size of invertebrates tended to decline with fractal complexity, indicating that crevice sizes could affect habitat use by benthic animals of different sizes. In addition, body size was larger in the resource-preconditioning treatment than in the control treatment, suggesting that body size in invertebrate assemblages was controlled by a mixture of factors. Thus, the present study demonstrates that habitat structure affects benthic invertebrate assemblages in a complex manner.

Notes: 1. The impact of burrowing larvae of Ephoron virgo (Ephemeroptera, Polymitarcidae) on sediment microbiology has not been previously investigated because of difficulties in sampling the sediment of large rivers under in situ conditions. Therefore, we conducted experiments in the on-ship Ecological Rhine Station of the University of Cologne (Germany), in which ambient conditions of the River Rhine can be closely mimicked.2. In two consecutive seasons, experimental flow channels were stocked with Ephoron larvae and continuously supplied with water taken directly from the River Rhine. Sediment from the immediate vicinity of Ephoron burrows (i.e. U-shaped cavities reaching 10–80 mm deep into the sediment) and bulk sediment samples were analysed for (i) particulate organic matter content, (ii) microscale in situ distribution of O2, NO〈inlineGraphic alt="inline image" href="urn:x-wiley:00465070:FWB1258:FWB_1258_mu1" location="equation/FWB_1258_mu1.gif"/〉, and NH〈inlineGraphic alt="inline image" href="urn:x-wiley:00465070:FWB1258:FWB_1258_mu2" location="equation/FWB_1258_mu2.gif"/〉, and (iii) potential activities of exoenzymes.3. Sediment surrounding the Ephoron burrows had markedly higher organic matter contents and exoenzyme activities compared with the bulk sediment. Microsensor measurements demonstrated that local O2 and NO〈inlineGraphic alt="inline image" href="urn:x-wiley:00465070:FWB1258:FWB_1258_mu3" location="equation/FWB_1258_mu3.gif"/〉 penetration into the sediment were greatly enhanced by larval ventilation behaviour. Volumetric O2 and NO〈inlineGraphic alt="inline image" href="urn:x-wiley:00465070:FWB1258:FWB_1258_mu4" location="equation/FWB_1258_mu4.gif"/〉 turnover rates that were calculated from steady state concentration profiles measured directly in the burrow lining were considerably higher than at the sediment surface.4. In the sediment of the fast flowing River Rhine Ephoron burrows are preferential sites of organic matter accumulation and dissolved oxidant penetration. Our data suggest that the burrows are surrounded by a highly active microbial community that responds to the inputs from the water column with elevated O2 and NO〈inlineGraphic alt="inline image" href="urn:x-wiley:00465070:FWB1258:FWB_1258_mu5" location="equation/FWB_1258_mu5.gif"/〉 turnover, and release of exoenzymes into the sediment pore water. Especially during periods of mass occurrence, the larvae of E. virgo may thus significantly contribute (i) to the ecological connection between the water column and the sediment and (ii) to biogeochemical processing of organic matter in the riverbed.

Notes: 1. Semi-aquatic birds may be sensitive to altered water quality. While avian species are not used in the bioassessment of streams, they may complement the more common use of benthic macroinvertebrates and fish. We estimated the extent to which water quality can predict attributes of the populations of one common semi-aquatic bird, the American dipper (Cinclus mexicanus).2. First, we estimated dipper presence/absence in relation to water quality as measured by a multimetric assessment index and individual bioassessment metrics. Second, we estimated dipper territory area and reproductive success in response to variation in water quality. We studied the diet, territory area and fecundity of dippers and sampled benthic macroinvertebrates, water chemistry and physical variables at 32 sites with and 17 sites without nesting dippers.3. Dipper presence was only weakly related to chemical, physical and commonly recorded bioassessment metrics such as per cent Ephemeroptera, Plecoptera and Trichoptera (%EPT). Dippers were strongly related to the abundance of their common prey, Drunella and Heptageniidae, which are only a small component of the commonly recorded bioassessment metrics. The variances in territory area and reproductive success were weakly predicted by water quality variables.4. Dipper presence reflected disturbance as measured by their common prey, showing that lower abundance of these stream invertebrates affected this semi-aquatic bird. We suggest dipper presence/absence might be used in multimetric indices of biotic integrity for the bioassessment of streams.

Notes: 1. In a thermally stratified water column with a deep-water algal maximum, Daphnia face a trade-off between food (high fecundity) and temperature (fast development). Recent studies showed that Daphnia populations move up and down the entire water column to take advantage of both, but the proportion of time allocated by individuals to the epilimnion, metalimnion and hypolimnion with their specific food and temperature conditions is not yet known.2. In a system of 1 m deep, vertical perspex tubes, I established three temperature gradients with 2, 5 and 10 °C differences between the surface (epilimnion) and the bottom layer (hypolimnion). Algae were added to the hypolimnion to simulate a deep-water algal maximum.3. The migration behaviour of individual neonate and egg-bearing Daphnia hyalina × galeata was monitored in order to measure the proportions of time the individuals allocated to the different vertical habitats and to assess the frequency of their shifts between epilimnion and hypolimnion.4. Neonates stayed continuously at the surface, taking advantage of the higher temperature, possibly because feeding was less important for them because of egg yolk reserves. In contrast, egg-bearing females spent more time feeding in the hypolimnion when the temperature gradient was weak, but also migrated into the epilimnion to take advantage of the higher temperature. In the steepest temperature gradient, the egg-bearing females either shifted between epilimnion and hypolimnion, or dwelled constantly in the metalimnion with intermediate conditions.

Notes: 1. Many natural ecosystems are heterogeneous at scales ranging from microhabitats to landscapes. Running waters are no exception in this regard, and their environmental heterogeneity is reflected in the distribution and abundance of stream organisms across multiple spatial scales.2. We studied patchiness in benthic macroinvertebrate abundance and functional feeding group (FFG) composition at three spatial scales in a boreal river system. Our sampling design incorporated a set of fully nested scales, with three tributaries, two stream sections (orders) within each tributary, three riffles within each section and ten benthic samples in each riffle.3. According to nested anovas, most of the variation in total macroinvertebrate abundance, abundances of FFGs, and number of taxa was accounted for by the among-riffle and among-sample scales. Such small-scale variability reflected similar patterns of variation in in-stream variables (moss cover, particle size, current velocity and depth). Scraper abundance, however, varied most at the scale of stream sections, probably mirroring variation in canopy cover.4. Tributaries and stream sections within tributaries differed significantly in the structure and FFG composition of the macroinvertebrate assemblages. Furthermore, riffles in headwater (second order) sections were more variable than those in higher order (third order) sections.5. Stream biomonitoring programs should consider this kind of scale-dependent variability in assemblage characteristics because: (i) small-scale variability in abundance suggests that a few replicate samples are not enough to capture macroinvertebrate assemblage variability present at a site, and (ii) riffles from the same stream may support widely differing benthic assemblages.

Notes: 1. Comparative studies of distinct, but not ecologically isolated, systems such as lakes and streams may improve our understanding of the importance of ecological linkages in aquatic ecosystems.2. In this study we compared the macroinvertebrate benthos of stony habitats in Swedish lakes and streams. Community composition was used to evaluate zoogeographic patterns and functional feeding guilds were used to identify mechanisms potentially affecting such patterns.3. Stream communities were generally more diverse and species-rich and had a higher proportion of grazers, shredders and passive-filter feeders than lakes. Lake communities had a higher proportion of predators and collector-gatherers. Of the 10 most common taxa, only Leptophlebia mayflies, clams (Sphaeriidae) and the isopod Asellus aquaticus were recorded in both lakes and streams.4. Among-site variance in macroinvertebrate communities accounted for by regional-scale variables was low (6.4% for lakes and 10.1% for streams), compared with that by local-scale variables (21% for lakes and 37.6% for streams). For lakes, the among-site variance in macroinvertebrate communities was best explained by habitat-scale characteristics followed by ecosystem, riparian, catchment, geographic position and ecoregion. For streams, the variance in macroinvertebrate communities was best explained by ecosystem characteristics followed by habitat, catchment, riparian, ecoregion and geographic position.5. Conspicuous differences in spatial pattern were revealed between lakes and streams. For lakes, the most unequivocal differences in community composition and function occurred at the transition zone between the mixed forests in the south and the boreal coniferous forests in the north. Surprisingly, streams did not respond as strongly to profound landscape-level differences in climate and vegetation cover.6. The spatial differences noted between macroinvertebrate communities of lakes and streams may be because of differences in retention of detrital matter. Our findings imply that detrital inputs are qualitatively similar, but that the retention and processing of coarse particulate organic matter was presumably higher in lake littoral regions than in stream riffle habitats.7. Although our findings support the conjecture that species distribution is determined fundamentally by conditions prevailing at the local-scale, regional factors such as land use/type and the role of history were important and seemingly act as strong determinants of large-scale patterns in biodiversity.

Notes: 1. The anostracan fairy shrimp Branchinecta gaini inhabits one of the most hostile environments on earth, living in pools and lakes in Antarctica. Between January 2002 and January 2003 temperatures in two pools where B. gaini are extremely abundant on Adelaide Island ranged from −18.6 to −15.7 °C in winter, to 19.4 to 17.1 °C in summer, whilst air temperatures ranged from −34 to 6.3 °C.2. Branchinecta gaini survives winter as cysts, but endures large summer temperature fluctuations as adults. Cysts froze between −24.4 and −25.7 °C. In experiments adults survived 0–10 °C with no mortality for 1 week, 25 °C for nearly 48 h with 50% mortality, and at 32 °C complete mortality occurred in 〈1 h.3. Oxygen consumption (ṀO2) in B. gaini approximately doubled for every 10 °C temperature rise (Q10 = 2.04) up to 20 °C where it reached a peak. Females had, on average 19% higher ṀO2 than males. Females also had greater metabolic scopes, (maximum–minimum ṀO2 across temperatures was ×3.6 for females, ×3.1 for males).4. Ventilation frequency increased linearly with temperature, and did not decline at 25 °C, indicating animals were ‘trying’ progressively harder to supply oxygen to tissues, and oxygen deficiency was the probable cause of death. Females had a higher ventilation frequency than males (8.6–17.1% higher) and they also exhibited greater scope to raise ventilation frequency (×2.4 for females versus ×1.5 for males).5. Great metabolic flexibility allows B. gaini to exploit extreme, highly fluctuating environments, and larger ventilatory and respiratory scopes allow females to survive higher temperatures than males. Because of this flexibility their prospects for coping with physical environmental change are high.

Notes: 1. We studied variation in the composition of fatty acids in the seston of a small freshwater reservoir with changes in phytoplankton composition during four growth seasons. We focused on the dynamics of the ω3 fatty acids because of their potential importance for zooplankton nutrition.2. Total diatoms were related to the 20:5ω3 fatty acid (eicosapentaenoic, EPA) content in seston. Among two dominant diatom genera, Cyclotella was not associated with EPA content. In contrast, there was a significant correlation between Stephanodiscus and the percentage contribution and content of EPA throughout the study. Hence, freshwater diatoms can differ strongly in content of the essential EPA.3. We considered abundant cyanobacteria as a potential source of 18:3ω3 fatty acid (linolenic, ALA) to aquatic food webs. Among four dominant cyanobacteria species, two (Anabaena flos-aquae and Planktothrix agardhii) showed significant correlation with the ALA content of the seston, while the other two (Aphanizomenon flos-aquae and Microcystis aeruginosa) did not.4. Dinophyta had a relatively high level of 22:6ω3 (docosahexaenoic, DHA) for freshwater species and can be also a source of EPA to aquatic food webs.5. Our results show that various species of diatoms as well as cyanobacteria can be of contrasting nutritional value for zooplankton because of their different content of the essential PUFAs. Diatoms, which are low in EPA, could not be considered as a valuable food, while some field populations of cyanobacteria might be valuable sources of essential ALA.

Notes: 1. The global spread of non-native species is a major concern for ecologists, particularly in regards to aquatic systems. Predicting the characteristics of successful invaders has been a goal of invasion biology for decades. Quantitative analysis of species characteristics may allow invasive species profiling and assist the development of risk assessment strategies.2. In the current analysis we developed a data base on fish invasions in catchments throughout California that distinguishes among the establishment, spread and integration stages of the invasion process, and separates social and biological factors related to invasion success.3. Using Akaike's information criteria (AIC), logistic and multiple regression models, we show suites of biological variables, which are important in predicting establishment (parental care and physiological tolerance), spread (life span, distance from nearest native source and trophic status) and abundance (maximum size, physiological tolerance and distance from nearest native source). Two variables indicating human interest in a species (propagule pressure and prior invasion success) are predictors of successful establishment and prior invasion success is a predictor of spread and integration.4. Despite the idiosyncratic nature of the invasion process, our results suggest some assistance in the search for characteristics of fish species that successfully transition between invasion stages.

Notes: 1. Urbanization and its associated contamination could degrade the quality of suspended fine particulate organic matter (SFPM) (20 μm to 1 mm) as a food resource for aquatic insects. SFPM was collected at four sites along the main stem of the Chattahoochee River, which drains metropolitan Atlanta at base and high flow during four seasons.2. Composition of SFPM was estimated using measures conventionally associated with food quality: bacteria, N/C ratio, caloric content, % inorganic, and % lipids, and metal (Cd, Cu, Pb, and Zn) concentration. In SFPM collected during base flow, % inorganic matter, calories, Cu, Pb, and Zn concentrations increased with cumulative permitted wastewater treatment discharge (an indicator of extent of urbanization upstream). In SFPM samples collected during high flow, % diatoms, Cu, Pb and Zn concentrations increased with urbanization.3. A growth assay was used as an integrated and direct measure of SFPM quality as a food resource. The instantaneous growth rate (IGR) of chironomids fed SFPM collected during base flow declined downstream of the city. IGRs of chironomids fed SFPM collected at all sites during high flow were as low as the lowest IGR measured during base flow.4. Insects fed SFPM collected from the Chattahoochee River had IGRs only 20% of those of chironomids fed SFPM collected from the Little Tennessee River, a relatively undisturbed river in North Carolina. The mortality rate of chironomids fed SFPM was not different among sites or rivers. While the decline in SFPM quality in the Chattahoochee River is probably attributable to some aspect of urbanization, the decline was not related to conventional measures of food quality or metal contamination.

Notes: 1. Macroinvertebrates are frequently classified in terms of their tolerance to human disturbance and pollution. These tolerance values have been used effectively to assess the biological condition of running waters.2. Generalised additive models were used to associate the presence and absence of different macroinvertebrate genera with different environmental gradients. The model results were then used to classify each genera as sensitive, intermediately tolerant or tolerant to different stressor gradients as quantified by total phosphorus concentration, sulphate ion concentration, qualitative habitat score and stream pH. The analytical approach provided a means of estimating stressor-specific tolerance classifications while controlling for covarying, natural environmental gradients.3. Computed tolerance classification generally conformed with expectations and provided some capacity for distinguishing between different stressors in test data.

Notes: 1. Experimental data on the maximum growth and food consumption of winter-acclimatised Atlantic salmon (Salmo salar) juveniles from three Norwegian rivers situated at 59 and 70°N were compared with predictions from published models of growth and food consumption of summer-acclimatised fish from the same populations.2. All winter-acclimatised fish maintained positive growth and a substantial energy intake over the whole range of experimental temperature (1–6 °C). This contrasted with predictions from growth models based on summer acclimatised Atlantic salmon, where growth and energy intake ceased at approximately 5 °C.3. Growth and food consumption varied significantly among populations. Winter-acclimatised fish from a Northern population had a higher mass-specific growth rate, higher energy intake and higher growth efficiency than southern populations, which is contrary to predictions from models developed using summer-acclimatised salmon, where fish from the Northern population had the lowest growth efficiency.4. The experiment provides evidence that thermal performance varies seasonally and suggests adaptation to the annual thermal regime.

Notes: 1. Denitrification, net oxygen consumption and net nitrous oxide flux to the atmosphere were measured in three small rivers (discharge approximately 2–27 m3 s−1) at the whole reach scale during Spring and Summer, 2002. Two of these rivers (Iroquois River and Sugar Creek in north-west Indiana – north-east Illinois, U.S.A.) drained agricultural catchments and the other (Millstone River in central New Jersey, U.S.A.) drained a mixed suburban–agricultural catchment.2. Denitrification, oxygen consumption and N2O flux were measured based on net changes in dissolved gas concentrations (N2, O2, and N2O) during riverine transport, correcting for atmospheric exchange. On each date, measurements were made during both light and dark periods.3. Denitrification rates in these rivers ranged from 0.31 to 15.91 mmol N m−2 h−1, and rates within each river reach were consistently higher during the day than during the night. This diurnal pattern could be related to cyclic patterns of nitrification driven by diurnal variations in water column pH and temperature.4. Oxygen consumption ranged from 2.56 to 241 mmol O2 m−2 h−1. In contrast to denitrification, net oxygen consumption was generally higher during the night than during the day.5. River water was consistently supersaturated with N2O, ranging from 102 to 209% saturated. Net flux of N2O to the atmosphere ranged from 0.4 to 60 μmol N m−2 h−1. Net flux of N2O was generally higher at night than during the day. The high flux of N2O from these rivers strengthens the argument that rivers are an important contributor to anthropogenic emissions of this greenhouse gas.

Notes: 1. Spatial relationships between hyporheic invertebrates and subsurface water flow patterns, sediment characteristics, water physicochemical parameters and several possible food sources were compared over three seasons at one site beneath a riffle. Measures of food sources included particulate organic matter (POM), bacterial activity (aerobic respiration, nitrate respiration and mineralisation of organic nitrogen) and microbial abundance.2. Patterns of water flow changed significantly over the 9-month study period, from predominantly upwelling beneath the entire riffle in spring, to distinct differentiation between downwelling and upwelling zones in summer and autumn. Water physicochemical parameters changed accordingly, showing gradually weaker correlations with depth and stronger correlations with zone between spring and autumn.3. Despite these changes, depth remained the strongest predictor of invertebrate richness, density and taxon composition throughout the study period. However, invertebrate distributions were secondarily correlated with water physicochemical parameters, and a minor gradient in invertebrate distributions between downwelling and upwelling zones became stronger from spring to summer.4. The correlations between invertebrates and physicochemical parameters changed in both magnitude and direction with season. In spring, invertebrates showed a negative correlation with surface water infiltration, whereas in summer and autumn, the correlation was positive. Correlations were strongest in summer, when interstitial dissolved oxygen concentrations were lowest.5. No relationships were found between hyporheic invertebrates and POM, microbial abundance or activity. This suggests that at this site, proximity to the streambed surface and physicochemical variables are more important than the abundance of food in controlling invertebrate distributions.

Notes: 1. Long-term monitoring requires repeated visits to a study site, greatly increasing the potential for cumulative visitation effects. For ecological studies in general, and for monitoring in particular, data must be evaluated for confounding artefacts from researcher presence. We compared aquatic communities at long-term sampling plots (nine sites, each with three plots, studied continuously from 6 to 22 years) in the Everglades National Park to previously unsampled reference plots adjacent to them to assess the effects of researcher visitation on the flora and fauna.2. We identified two criteria that are sensitive to local habitat heterogeneity for assessment of visitation impacts. First, the long-term plots must differ from adjacent reference plots by a magnitude that exceeded variation among plots separated by equal or greater distance (i.e. the difference is greater than expected by scaling of community change proportional with distance); and second, multiple reference plots must consistently differ in direction (e.g. greater abundance or less abundance) from adjacent long-term plots. We also tested for increased heterogeneity among samples from long-term plots compared with those not previously visited.3. We found no evidence of researcher effects on fish or macroinvertebrates, and only weak evidence for alteration of emergent plants and periphyton floating mats. Our failure to document visitor impacts may result from either low visitation rate or the dynamic nature of the wetlands studied.

Notes: 1. Stream ecologists have been puzzled by the apparent paradox that invertebrate populations persist in headwater streams despite the high frequency with which individuals drift downstream. To resolve this ‘drift paradox’, directions and distances of both larval and adult movement must be identified. Using over 50 interception traps in combination with results from several mark–capture experiments using 15N as a label, we tested the assumption that interception traps accurately represent the ultimate direction of adult insect flight.2. In several streams in the Hubbard Brook Experimental Forest, 76% of 15N-labelled stoneflies (Leuctra ferruginea) had flown upstream from where they emerged to where they were captured. In contrast, over 60% of stoneflies were flying downstream when captured, i.e. on the upstream side of an interception trap.3. The instantaneous direction, as indicated by the side of the interception trap on which they were captured, indicated the ultimate flight direction for fewer than 1/3 of the individuals captured. Thus, such traps did not accurately reflect the ultimate flight patterns of individuals, as indicated by mark–capture data.4. Conclusions drawn from interception trap counts regarding the direction of movement and the distribution and persistence of populations may need to be re-evaluated. We suggest that better tracking methods, including mass mark–capture studies using stable isotopes, be used to evaluate the potentially complex patterns of adult insect movement and the consequences of that movement for individuals and populations.

Notes: 1. In this paper we test the usefulness of acoustic backscatter measurements from a 614 kHz Acoustic Doppler Current Profiler (ADCP) for the qualitative and quantitative characterisation of zooplankton distributions in lakes. ADCP-based backscatter estimates were compared with frequent net hauls obtained during a calibration experiment in which the acoustic backscatter was strongly dominated by vertical migrating Chaoborus flavicans larvae.2. The correlation between backscatter estimates and the C. flavicans concentration was very good. Vertical swimming speed of larvae, measured directly by the ADCP, was up to a maximum of 5 mm s−1 and agreed very well with the observed vertical movement of the backscatter contour lines. Although the strong backscatter from C. flavicans overwhelmed the signal from the remaining zooplankton, a good correlation between backscatter strength and the total remaining zooplankton concentration, dominated by Cyclops spp., was found for the depth and time intervals where no C. flavicans were present.3. In addition to the calibration experiment, longer-term ADCP measurements from different lakes revealed a strong temporal correlation between the onset of the up- and downward migration of zooplankton and the local sunset and sunrise.4. We conclude that ADCPs can be used to monitor plankton distributions both temporally and spatially. It also seems possible to estimate plankton densities after appropriate calibration.

Notes: 1. Phytoplankton abundance and species composition in Lake Kinneret, Israel, have been monitored at weekly or fortnightly intervals since 1969. This paper summarises the resulting 34-year phytoplankton record with a focus on the last 13 years of new data, and reassesses an earlier conclusion that the lake phytoplankton shows remarkable stability despite a wide range of external pressures.2. The Kinneret phytoplankton record can be split into two major periods. The first, from 1969 till 1993, was a period of distinct stability expressed by a typical annual pattern revolving around a spring bloom of the dinoflagellate Peridinium gatunense that repeated each year. The second period, starting around 1994 and ongoing, is characterised by the loss of the previously predictable annual pattern, with both ‘bloom years’ and ‘no-bloom years’.3. In the second period, deviations from the previous annual pattern include: the absence of the prevailing spring P. gatunense blooms in some years and increased variability in the magnitude of the bloom in others; intensification of winter Aulacoseira granulata blooms; higher summer phytoplankton biomass with replacement of mostly nanoplanktonic, palatable forms by less palatable forms; new appearance and establishment of toxin-producing, nitrogen fixing cyanobacteria in summer; increase in the absolute biomass and percentage contribution of cyanobacteria to total biomass; and fungal epidemics attacking P. gatunense.4. The 34-year record serves to validate Schindler's (1987) assessment that phytoplankton species composition will respond to increased anthropogenic stress before bulk ecosystem parameters.

Notes: 1. We investigated whether Daphnia galeata × hyalina hybrids of Lake Constance and Lake Greifensee show the same pattern of life history parameters as previously reported for D. galeata × cucullata hybrids and whether such a pattern is consistent between Daphnia populations from those two lakes.2. Hybrids in Lake Constance were intermediate in size compared with the parental species. Hybrids in Lake Greifensee were smaller than D. galeata. The intrinsic growth rate (r) of hybrids from Lake Constance was not significantly different from the faster growing parental taxon D. galeata. However, r of hybrids from Lake Greifensee was significantly lower than that of D. galeata.3. The observed juvenile body length differences between the taxa varied with the clutch number. The first clutch juvenile lengths of the three taxa did not differ for Lake Constance. First clutch juveniles of Lake Greifensee D. galeata were smaller than hybrid first clutch juveniles. The third clutch juvenile length did not differ between taxa from Lake Greifensee, but D. galeata juveniles from Lake Constance were bigger than those of D. hyalina.4. The life history pattern found in Lake Constance corresponds to previous findings from other studies. The hybrids in this lake combine the faster population growth of one parental species with a relatively small size. In the case of Lake Greifensee hybrids, the relatively large size of first clutch juveniles and the small size of the adults could be interpreted as dual adaptations to invertebrate and fish predation. We speculate that the lower population growth rate of the hybrids is a trade-off for this twofold protection.

Notes: 1. Describing and understanding patterns in biological diversity along major geographical gradients is an important topic in ecology. Samples collected from a large number of physically and chemically comparable stream sites along a 4000 m gradient of altitude in the Andes of Ecuador served to characterise patterns of family richness of aquatic macroinvertebrates at the scale of the stream site (local) and at that of discrete altitudinal zones.2. Both mean local and zonal family richness decreased by about 50% from sea level to 4000 m a.s.l. Local richness declined linearly, while zonal richness remained constant from sea level up to a threshold altitude of about 1800 m, whereafter it decreased.3. From sea level to 1800 m few families were lost from zonal richness and few were gained. From 1800 to 3800 m the decrease in the number of families was accounted for by a loss of families present in lowland streams, with few new families gained. Hence, there was relatively little turnover of families along the entire gradient.4. The diverging pattern of local and zonal richness was caused by sporadically occurring families inflating zonal richness at mid-altitudes. If the sporadic families were represented by the same species found commonly in the lowlands, then the mid-altitudinal zonal richness would be maintained by a ‘rescue effect’. More probably, however, the sporadically occurring families found at mid-altitudes are each represented by new species replacing each other along the gradient, the families progressively diminishing in species richness and occurrence as the overall temperature tolerance of the family is approached.5. This study demonstrates that spatial scale affects altitudinal patterns in the taxonomic richness of stream invertebrates. It also showed that family-level identification can facilitate interpretation of sources and sinks of biodiversity along geographic gradients.

Notes: 1. Movements and habitat use by Atlantic salmon parr in Catamaran Brook, New Brunswick, were studied using Passive Integrated Transponder technology. The fish were tagged in the summer of 1999, and a portable reading system was used to collect data on individual positions within a riffle-pool sequence in the early winter of 1999. Two major freezing events occurred on November 11–12 (Ice 1) and November 18–19 (Ice 2) that generated significant accumulations of anchor ice in the riffle.2. Individually tagged parr (fork length 8.4–12.6 cm, n = 15) were tracked from 8 to 24 November 1999. Over this period, emigration (40%) was higher from the pool than from the riffle. Of the nine parr that were consistently located, seven parr moved 〈5 m up- or downstream, and two parr moved more than 10 m (maximum 23 m). Parr moved significantly more by night than by day, and diel habitat shifts were more pronounced in the pool with some of the fish moving closer to the bank at night.3. During Ice 2, there was relatively little movement by most of the parr in the riffle beneath anchor ice up to 10 cm in thickness. Water temperature was 0.16 °C above the freezing point beneath anchor ice, suggesting the existence of suitable habitats where salmon parr can avoid supercooling conditions and where they can have access to low velocity shelters. To our knowledge, these are the first data on habitat use by Atlantic salmon parr under anchor ice.

Notes: 1. We used stream fish and decapod spatial occurrence data extracted from a national database and recent surveys with geospatial landuse data, geomorphologic, climatic, and spatial data in a geographical information system (GIS) to model fish and decapod occurrence in the Wellington Region, New Zealand.2. To predict the occurrence of each species at a site from a common set of predictor variables we used a multi-response, artificial neural network (ANN), to produce a single model that predicted the entire fish and decapod assemblage in one procedure.3. The predictions from the ANN using this landscape scale data proved very accurate based on evaluation metrics that are independent of species abundance or probability thresholds. The important variables contributing to the predictions included the latitudinal and elevational position of the site reach, catchment area, average air temperature, the vegetation type, landuse proportions of the catchment, and catchment geology.4. Geospatial data available for the entire regional river network were then used to create a habitat-suitability map for all 14 species over the regional river network using a GIS. This prediction map has many potential uses including: monitoring and predicting temporal changes in fish communities caused by human activities and shifts in climate, identifying areas in need of protection, biodiversity hotspots, and areas suitable for the reintroduction of endangered or rare species.

Notes: 1. Using two- and three-dimensional video recordings, we examined the steps involved in predation that lead to the differential vulnerability of three sympatric rotifer sibling species (Brachionus plicatilis, B. ibericus and B. rotundiformis) to a co-occurring, predatory, calanoid copepod (Arctodiaptomus salinus).2. Brachionus rotundiformis, the smallest prey tested, was the most vulnerable with the highest encounter rate, probability of attack, capture and ingestion, and the lowest handling time.3. Comparison of our results with those of a previous study shows that A. salinus is a more efficient predator than a co-occurring cyclopoid copepod (Diacyclops bicuspidatus odessanus) feeding on these same rotifer species. However, despite its higher capture rates, A. salinus seems to be less selective than D. b. odessanus based on attack distances and prey handling times.4. The differential vulnerability to both calanoid and cyclopoid copepod predation can help explain the coexistence and seasonal succession of these co-occurring rotifer species.

Notes: 1. Temporal and spatial variation in planktonic abundance, biomass and composition were determined in Lake Hoare (McMurdo Dry Valleys, Antarctica) over two summer seasons (1996–97 and 1997–98).2. Phototrophic nanoflagellates (PNAN) dominated planktonic biomass, with a mean monthly biomass ranging between 27.3 and 40.4 μg C L−1. The deep chlorophyll maximum was mainly composed of cryptophytes (〉87% of total PNAN biomass) and varied in depth between 6 and 12 m.3. Maximum bacterial concentration was 11.8 × 105 cells mL−1. Bacterial abundance showed relatively little temporal variation, with the exception of a drop in numbers that occurred in late November of both years studied.4. Ciliates were the most successful heterotrophic protozoan group, with a mean monthly biomass (1.2–3.2 μg C L−1) being typically at least double that of heterotrophic nanoflagellate (HNAN) biomass (0.1–0.7 μg C L−1).5. Microbial processes within this lake appear to be dominated by bottom up control. The relative importance of allochthonous inputs into the lake (from the ice-cover and stream flow) and autochthonous recycling (by microzooplankton regeneration) are considered.6. Results from a horizontal transect indicate that the permanence of the main sample hole may have enhanced planktonic biomass over a relatively small spatial scale.

Notes: 1. The invasion of carp (Cyprinus carpio L.) in Australia illustrates how quickly an introduced fish species can spread and dominate fish communities. This species has become the most abundant large freshwater fish in south-east Australia, now distributed over more than 1 million km2.2. Carp exhibit most of the traits predicted for a successful invasive fish species. In addition, degradation of aquatic environments in south-east Australia has given them a relative advantage over native species.3. Derivation of relative measures of 13 species-specific attributes allowed a quantitative comparison between carp and abundant native fish species across five major Australian drainage divisions. In four of six geographical regions analysed, carp differed clearly from native species in their behaviour, resource use and population dynamics.4. Climate matching was used to predict future range expansion of carp in Australia. All Australian surface waters appear to be climatically suitable for carp.5. This assessment strongly reinforces the need for immediate management of carp in Australia to include targeted control of human-assisted dispersal, such as use of carp as bait by anglers, distribution to new locations by anglers and the use of the ‘Koi’ strain in the aquarium industry.6. Given their historical spread, dispersal mechanisms and ecological requirements, the expansion of carp across most of the remainder of Australia is to be expected.

Notes: 1. To assess the influence of oviposition patterns on distributions of hydrobiosid caddisfly larvae, abundances of three hydrobiosid caddisfly species were estimated in whole reaches with and without suitable oviposition substrata along an upland temperate Australian stream. In addition, within-reach spatial patterns were examined in relation to known oviposition locations as well as flow characteristics that corresponded to oviposition preferences.2. Larval abundances in all samples were low relative to previous estimates of egg inputs into reaches. The presence of suitable oviposition substrata at a reach did not influence the abundances of larvae. For one species, benthic samples taken proximate to oviposition substrata revealed a sharp decline in abundance between first and later instars. Larvae of two taxa exhibited flow-specific habitat preferences mirroring those described as important as cues for oviposition site selection. Previous estimates of egg mass aggregation were also reflected in similarly high levels of larval clumping; however, larval aggregation did not differ between reaches with and without oviposition sites.3. Collectively, our results suggest that a large difference in the supply of potential recruits does not translate into marked differences in larval abundances of hydrobiosids at the reach level but may account for some variation in larval distribution within a reach and between instars. This evidence is consistent with the notion that (i) posthatching dispersal between reaches is substantial and (ii) mortality of larvae, particularly early instars, is high and (iii) within reach habitat preferences change with larval growth.4. If adults are unable to lay eggs at reaches without suitable oviposition substrata, then reaches with oviposition substrata may be crucial as a source of recruits elsewhere. Furthermore, high mortality and/or dispersal among first instars signal this as an important part of the life history. Further data on the mortality and dispersal rates of newly hatched larvae would greatly benefit our understanding of the importance of local births in structuring patterns of abundance in stream invertebrates.

Notes: 1. In the heterogonic life cycle of monogonont rotifers, amictic (female-producing) females develop from two types of eggs: fertilised resting (diapausing) eggs and parthenogenetic subitaneous eggs. Females hatched from resting eggs initiate clonal populations by female parthenogenesis and are called stem females. This study compares females from resting and parthenogenetic eggs that were produced under identical culture conditions and were of similar birth order.2. Newborn stem females had many more lipid droplets in their tissues than similar-sized, newborn females from parthenogenetic eggs. When neonates were stained with Nile Red and viewed under epifluorescent illumination, these droplets were shown to be sites of neutral-lipid storage products.3. Stem females had no posterolateral spines and short anterior spines, while their mothers and offspring in subsequent, parthenogenetic generations typically had long posterolateral spines and elongated anterior spines.4. Newborn stem females survived starvation significantly longer than newborn females from parthenogenetic eggs.5. When females from resting and parthenogenetic eggs were cultured from birth to death at a high food concentration, the reproductive potential (r day−1) of the stem females was significantly higher (0.82–0.88 versus 0.70), primarily because of egg production at an earlier age. The mean lifetime fecundity (Ro) of stem females was significantly greater than that of females from parthenogenetic eggs.6. Extensive lipid reserves should increase the ability of stem females to colonise new habitats. Firstly, compared with females from parthenogenetic eggs, stem females are more likely to experience starvation or food limitation. Resting eggs hatch in response to physical and chemical factors that are not directly related to food availability, and from sediments that may be far from food-rich surface waters. Secondly, when food is abundant, stem females have a greater reproductive potential.

Notes: 1. Density gradients of cladocerans and copepods were generated in an enclosure experiment to compare the impact on the plankton of a filter feeder (Daphnia hyalina × galeata) with that of more selective feeders (calanoid and cyclopoid copepods). The experiment was conducted in situ over 25 days during spring in a mesotrophic lake, Schöhsee, Germany.2. The plankton community was monitored regularly. Daphniids were able to graze on the phytoplankton present, which mainly consisted of small (〈1000 μm3) species, whereas copepods did not show any impact on algae.3. At the end of the experiment, Daphnia and remaining cyclopoid copepods were harvested and sorted manually, prior to analyses for stable isotopes of carbon and nitrogen. Daphniids from mesocosms stocked purely with differing densities of Daphnia showed little variability in stable isotope values, whereas those that thrived in enclosure bags together with copepods exhibited lower δ13C values.4. The change in Daphniaδ13C indicates a change of food sources, modified by the presence of the copepods: the higher the mean abundance of copepods in the enclosures, the more 13C-depleted the daphniids. Increasing abundance of high nucleic acid (HNA) bacteria in the copepod bags may account for the trend in Daphniaδ13C via increased grazing on the bacteria themselves, or via grazing on phytoplankton utilising isotopically light CO2 from respiratory release.5. Cyclopoid copepod stable isotope signatures were related to Daphnia and copepod abundances in copepod bags, suggesting that cyclopoids preyed on the available zooplankton.

Notes: 1. We predicted that zebra mussel, Dreissena polymorpha (Pallas), genetic structure in the Mississippi River would follow a model of invasive species genetics, which predicts low genetic structure among populations of recently established species. This prediction was upheld in our previous genetic study using allozymes, however, one locus yielded anomalous results.2. We employed amplified fragment length polymorphism (AFLP) analysis as a neutral marker to assess the amount of genetic structure within and among populations, and as a test of expected population structure from both invasion genetic theory, and the results from our previous study.3. There was greater spatial differentiation, as measured by Fst, observed using AFLP's than for allozymes (P 〈 0.001). There was no evidence that AFLP variation conformed to an isolation by distance model, and genetic relationships of populations, as measured by AFLP markers, were not similar to those detected in our allozyme survey.4. The lack of concordance between these two genetic marker systems probably reflects their differential responses to drift, migration, and selection occurring during this rapid invasion. Strong population structure is counter to predictions that populations of invasive species will not be differentiated, as with observations based on allozyme markers. Therefore, newly established species may require genetic surveys using multiple marker systems to evaluate population structure.

Notes: 1. We determined the effects of nutrient enrichment on wood decomposition rates and microbial activity during a 3-year study in two headwater streams at Coweeta Hydrologic Laboratory, NC, U.S.A. After a 1-year pretreatment period, one of the streams was continuously enriched with inorganic nutrients (nitrogen and phosphorus) for 2 years while the other stream served as a reference. We determined the effects of enrichment on both wood veneers and sticks, which have similar carbon quality but differ in physical characteristics (e.g. surface area to volume ratios, presence of bark) that potentially affect microbial colonisation and activity.2. Oak wood veneers (0.5 mm thick) were placed in streams monthly and allowed to decompose for approximately 90 days. Nutrient addition stimulated ash-free dry mass loss and increased mean nitrogen content, fungal biomass and microbial respiration on veneers in the treatment stream compared with the reference. The magnitude of the response to enrichment was great, with mass loss 6.1 times, and per cent N, fungal biomass and microbial respiration approximately four times greater in the treatment versus reference stream.3. Decomposition rate and nitrogen content of maple sticks (ca. 1–2 cm diameter) also increased; however, the effect was less pronounced than for veneers. Wood response overall was greater than that determined for leaves in a comparable study, supporting the hypothesis that response to enrichment may be greater for lower quality organic matter (high C : N) than for higher quality (low C : N) substrates.4. Our results show that moderate nutrient enrichment can profoundly affect decomposition rate and microbial activity on wood in streams. Thus, the timing and availability of wood that provides retention, structure, attachment sites and food in stream ecosystems may be affected by nutrient concentrations raised by human activities.