ALBERT

Notes: 1. Ophrydium versatile is a symbiotic ciliate which forms gelatinous colonies up to several centimetres in diameter in transparent temperate lakes. The ciliates are evenly spaced at the colony surface and constitute a small proportion of the surface area (7%) and volume (3.1%) of the colony, but a large proportion of organic carbon (74%) and nitrogen content (82%) (exemplified for 1 cm3 colonies). The majority of the colony volume is formed by the jelly. The biomass proportion of ciliates scales inversely with colony size, following the decline of surface area to colony volume. The largest colonies found in Danish lakes in early summer contain almost 1 million ciliates, and assuming they derive from a single ciliate undergoing exponential division, they need twenty generations and, presumably, almost a year to reach maximum size.2. The ciliates contain numerous symbiotic zoochlorellae that constitute about 10% of ciliate volume and more than half of the carbon content. Zoochlorellae dominate oxygen metabolism of the assemblage, resulting in low light compensation points, a large diel photosynthetic surplus, and a marked dependence on light for sustained growth and ciliate metabolism. Estimated gross photosynthesis (7ng C ciliate−1 day−1) of Ophnrydium from shallow, clear waters in June greatly exceeded the estimated carbon contained in filtered bacteria and small algae (1.9ng C cilicate−1 day−1). Nitrogen and phosphorus content of the prey, however, may provide the main nutrient source consistent with the correspondence between mass-specific rates of nutrient uptake and measured relative growth rates (average 0.067 day−1, generation time 10 days).3. The large Ophrydium colonies require increased allocation of photosynthetic carbohydrates with increasing colony size to maintain the jelly. The large colonies tend to become gas-filled, floating, mechanically destroyed and their ciliate inhabitants abandon them as swarmers. Colony formation, however, should offer protection against predators which may be more important for the natural abundance than the costs of growing in a colony.

Notes: 1. We examined changes in position and growth of Callitriche cophocarpa patches in a shallow section of a small Danish stream during the main expansion of plant cover between April and June. Mean upstream growth of patches was only 7.5% of downstream growth. The mean growth rate was 1,02cm day−1 in length and 0.55 cm day−1 in width and the growth rates were not significantly dependent on patch size. For patches with an area (A) above 100cm2, the growth rates in patch area (dA/dt) was proportional to patch circumference and thus √Aand the relative growth rate (dA/dt A−1) was inversely proportional to √A.The smallest patches (〈100cm2), however, expanded less than expected because of their combined tendency to grow more slowly in patch length and width than larger patches.2. The expansion of plant cover will be much more rapid in many medium-sized patches compared to few large patches of a similar combined area, because internal selflimitation by light and space is partially relieved in smaller patches. We anticipate that recruitment and mortality of new-formed patches are critical steps preceding areal expansion. Quantification of these processes are needed fully to evaluate patch dynamics and space occupation.

Notes: 1. Aquatic plant stands are flexible, mesh-like open structures that undergo modification in shape and experience a cascade of declining flow velocities and micro-scale Reynolds numbers with increasing distance into the stands. It is not possible to define or measure the frontal area of this open flexible plant structure. Total wetted area was used as a reference area for drag because it can be measured with high accuracy and it was the most suitable measure of plant size impeding the flow and absorbing light for photosynthetic production. What is important is that it is made absolutely clear which type of reference area that is used.2. Numerous important relationships in biology are open to discussion because of differences in dimensions between variables. Relating dimensionless drag coefficients to dimensionless Reynolds numbers resolve such discussions, but defining Reynolds numbers cause other problems. Relating drag coefficients to macro-scale Reynolds numbers would result in exactly the same form of relationship as to water velocity because macro-scale Reynolds numbers changed in direct proportion to water velocity in the experiments, while kinematic viscosity and characteristic length within species remained constant.

Notes: 1. Lobelia dortmanna is a common representative of the small isoetid plants dominating the vegetation in nutrient-poor lakes in Europe and North America. Because of large permeable root surfaces and continuous air lacunae Lobelia exchanges the majority of O2 and CO2 during photosynthesis across the roots. This leads to profound diel pulses of O2 and CO2 in sandy sediments with low microbial O2 consumption rates. The ready radial root loss of O2 may, however, make Lobelia very susceptible to more reducing sediments. Therefore, we grew Lobelia for 6 months on natural and organically enriched sandy sediments to test how: (i) root oxygenation influenced degradation of organic matter and depth profiles of N and C; (ii) Lobelia and microbial O2 consumption rates influenced pool size and depth penetration of O2 in the sediments; and (iii) sediment enrichment influenced growth and mineral nutrition of Lobelia.2. Naturally low-organic sediments (0.32% DW) accumulated organic C and N during the experiment as a result of growth of Lobelia and surface micro-algae. In contrast, surface layers of enriched sediments (0.58, 0.87 and 2.46% DW) lost organic C and N because of enhanced mineralisation rates because of oxygen availability. In deeper layers of enriched sediments no significant differences in organic C and N pools were found between plant-covered and plant-free sediments probably because faster organic degradation because of root oxygenation was balanced by release of organic matter from the plants and because short roots with dense Fe-Mn coatings in the most enriched sediments constrained O2 release.3. Depth-integrated O2 pools were much higher in light than darkness, higher in plant-covered than plant-free sediments and higher in sandy than in organically enriched sediments. All sediments had a primary O2 maximum 1–2 mm below the sediment surface in light because of photosynthesis of micro-algae. Plant-covered sediments of low organic content (0.32 and 0.58% DW) also had a secondary deep maximum (2–4 cm) because of higher O2 release from Lobelia roots than microbial O2 consumption. Nitrification occurred here resulting in depletion of NH〈inlineGraphic alt="inline image" href="urn:x-wiley:00465070:FWB1382:FWB_1382_mu1" location="equation/FWB_1382_mu1.gif"/〉 and accumulation of NO〈inlineGraphic alt="inline image" href="urn:x-wiley:00465070:FWB1382:FWB_1382_mu2" location="equation/FWB_1382_mu2.gif"/〉. In low organic sediments, oxygen pools increased with higher plant biomass both in light and darkness. The deep O2 and NO3 maxima disappeared in high organic sediments of greater O2 consumption rates and smaller O2 release rates.4. Lobelia was stressed by increasing O2 consumption rate of the sediments. Plant weight and leaf number declined twofold and maximum root length declined fourfold suggesting severe problems maintaining sufficient axial O2 transport to the root tips because of rapid radial O2 loss. Despite markedly higher nutrient concentrations in the enriched sediments, leaf-N declined twofold and leaf-P declined fourfold to growth-limiting levels. These responses can be explained by constrains on mycorrhisal activity, root metabolism and vascular transport because of O2 depletion. Management efforts to stop the decline and ensure the recovery of the isoetid vegetation should therefore focus on improving water quality as well as sediment suitability for growth.

Notes: 〈list xml:id="l1" style="custom"〉1 Invertebrate herbivory on the submerged macrophyte Potamogeton perfoliatus was studied in a Danish lowland stream during the main growth seasons of summer 1991 and 1992. Young apical leaves escaped consumption probably because of their location and dense packing, but herbivory loss increased linearly with age and exposure time of older leaves. Nitrogen content was relatively high in both young (4.61% DW) and old leaves (3.65% DW) but a food preference experiment showed that young leaves were preferred by the main herbivore, the trichopteran Anabolia nervosa.2 The percentage of standing plant biomass lost to herbivory (the apparent loss) between sampling periods increased from early May to a mid-June maximum (24.8% in 1991, 4.2% in 1992), and subsequently declined to zero within a month. The averages for the growth seasons were 10.5% in 1991 and 2.0% in 1992. Yet, the proportions of annual plant production harvested by herbivores were low and almost the same (1.3 and 1.8%) because consumption was low when plant production peaked in late summer.Consumption was almost the same early in the two years, but plant growth dynamics differed markedly and was the main factor determining apparent herbivory loss.3 Despite heavy damage early in the summer, P. Perfoliatus contributed a minor fraction (1–5%) of the trichopteran diet. This fraction appeared to be restricted by the low macrophyte biomass during early summer.4 The results emphasize that apparent herbivory loss does not estimate the harvested proportion of plant production, and that plant growth dynamics should be analysed to attain precise estimates of herbivory rates.

Notes: 1. We analysed photosynthetic rates and inorganic carbon use of thirty-five vascular macrophyte species collected submerged in eight nutrient- and CO2-rich Danish lowland streams. The species were classified in four groups as mainly terrestrial, homophyllous and heterophyllous amphibious and truly submerged. These groups represent plant species differently adapted to life in water.2. Photosynthetic rates measured in water increased in the gradual transition from mainly terrestrial, through amphibious to truly submerged species. Species normally in contact with air adapted to submergence by increasing the photosynthetic rate at limiting CO2. Photosynthetic rates of submerged parts of heterophyllous amphibious species were close to those of submerged species. Submerged species with thin or finely dissected leaves had the highest photosynthetic rates, probably because of low diffusional resistance to uptake of nutrients and gases.3. In contrast to submerged species, terrestrial and amphibious species were unable to use HCO3−. Extensive oversaturation with CO2 in the streams allows, however, many amphibious species to photosynthesize well under water, based on CO2-use alone. Amphibious CO2-users, with very few structural adaptations to life under water, can therefore be as dominant in the submerged vegetation of lowland streams as HCO3−-using water plants. Moreover, the streams provide open space for colonization from the dense vegetation ashore.4. Among the 1265 Danish herbaceous species no less than seventy-five terrestrial species occasionally grow submerged, forty-five species are amphibious, and fifty-one are true water plants. These numbers suggest that adaptation to permanent or temporary submergence is an ongoing process involving many species and that the land-water interface does not represent as difficult a barrier as often believed.

Notes: SUMMARY. 1. We examined the abundance and oxygen metabolism of epiphytic organisms on the dominant macrophyte, Potamogeton pectinatus, in headwaters of the eutrophic River Suså. Microbenthic algae were abundant in the stream during spring and macrophytes during summer.2. The low macrophyte biomass in spring supported a dense epiphyte cover whereas the high macrophyte biomass during summer had a thin epiphyte cover of 10–100-fold lower abundance per unit area of macrophyte surface. The epiphyte community was dominated by microalgae in spring and by heterotrophs, probably bacteria, during summer. This seasonal shift was shown by pronounced reductions of the chlorophyll a content (from 2–3% to 0.1–0.7% of organic DW), the gross photosynthetic rate (from 20–85 to 3–15 mg O2, g-1 organic DW h−1) and the ratio of gross photosynthesis to dark respiration in the epiphyte community (from 5–18 to 1). The reduced contributions of epiphytic microalgae correlated with reduced light availability during summer.3. Both the density and the photosynthetic activity of epiphytic algae were low on a stream area basis relative to those of microbenthic algae and macrophytes. Rapid variations in water velocity and extensive light attenuation in water and macrophyte stands probably constrained the development of epiphytic algae. The epiphyte community was more important in overall stream respiration, contributing c. 10% to total summer respiration and c. 20% to summer respiration within the predominantly heterotrophic microbial communities on sediments and macrophyte surfaces.

Notes: 1. Temperature, organic carbon and oxygen consumption were measured over a year at 13 sites in four lowlands streams within the same region in North Zealand, Denmark with the objectives of determining: (i) spatial and seasonal differences between open streams, forest streams and streams with or without lakes, (ii) factors influencing the temperature dependence of oxygen consumption rate, (iii) consequences of higher temperature and organic content in lake outlets on oxygen consumption rate, and (iv) possible consequences of forecasted global warming on degradation of organic matter.2. High concentrations of easily degradable dissolved (DOC) and particulate organic carbon (POC) were found in open streams downstream of plankton-rich lakes, while high concentrations of recalcitrant DOC were found in a forest brook draining a forest swamp. Concentrations of predominantly recalcitrant POC and DOC were low in a groundwater-fed forest spring. Overall, DOC concentration was two to 18 times higher than POC concentrations.3. Oxygen consumption rate at 20 °C was higher during summer than winter, higher in open than shaded streams and higher in lake outlets than inlets. Rate was closely related to concentrations of chlorophyll and POC but not to DOC. The ratio of oxygen consumption rate to total organic concentrations (DOC + POC), serving as a measure of organic degradability, was highest downstream of lakes, intermediate in open streams and lowest in forest streams.4. Temperature coefficients describing the exponential increase of oxygen consumption rate between 4 and 20 °C averaged 0.121 °C−1 (Q10 of 3.35) in 70 measurements and showed no significant variations between seasons and stream sites or correlations with ambient temperature and organic content.5. Oxygen consumption rate was enhanced downstream of lakes during summer because of higher temperature and, more significantly, greater concentrations of degradable organic carbon. Oxygen consumption rates were up to seven times higher in the stream with three impoundments than in a neighbouring unshaded stream and 21 times higher than in the groundwater-fed forest spring.6. A regional climate model has calculated a dramatic 4–5 °C rise in air temperature over Denmark by 2070–2100. If this is realised, unshaded streams are estimated to become 2–3 °C warmer in summer and winter and 5–7 °C warmer in spring and, thereby, increase oxygen consumption rates at ambient temperature by 30–40% and 80–130%, respectively. Faster consumption of organic matter and dissolved oxygen downstream of point sources should increase the likelihood of oxygen stress of the stream biota and lead to the export of less organic matter but more mineralised nutrients to the coastal waters.

Notes: 1. Invertebrate herbivory was studied in twenty-eight populations of the submerged macrophyte Potamogeton perfoliatus in Danish streams and lakes in mid-June. All populations but one experienced invertebrate herbivory and loss ranged from 0 to 11.9% of leaf area among populations. Loss generally increased with leaf age towards the base of the plants, and young apical leaves were rarely damaged.2. Herbivory loss was significantly higher in streams (mean 5.0%) than in lakes (mean 2.2%), but varied greatly among populations within the same stream or lake and was not correlated to physico-chemical site characteristics, size or density of plant population, or leaf N and P content. High levels of invertebrate herbivory were therefore not associated with certain types of streams or lakes.3. High herbivore biomass relative to abundance of plants was conducive to high loss. In streams, the biomass of the trichopteran Anabolia nervosa accounted for 50% of the variability in loss. No single species appeared to be equally important in lakes, although loss was correlated to the biomass of the chrysomelid beetle Macroplea appendiculata. Obligate herbivores, such as lepidopteran larvae, apparently exerted little damage on P. perfoliatus, and leaf mining and channelization from specialist feeders were negligible. It is concluded that shredders acting as facultative herbivores were the most important invertebrate herbivores on P. perfoliatus in Danish freshwaters.

Notes: SUMMARY. The distribution and quantitative development of aquatic macrophytes have been studied in oligotrophic Lake Kalgaard, Denmark. The vegetation is dominated by isoetid species, which are widely distributed (about 40% of the lake bottom) compared to emergent and floating-leaved macrophytes (about 4%). Littorella uniflora dominates at depths of 0–2 m and Isoetes lacustris from 2.0 to 4.5m. Within the colonization area the mean midsummer biomass of Littorella is 112g organic dry weight m−2 and that of Isoetes, 66 gm−2. The total biomass of these two species constitutes 99% of the biomass of submerged macrophytes.The perennial Littorella shows only small seasonal biomass variations. The vegetational biomass, the above-ground fraction of the biomass, and the weight of individual plants all increased with the organic content of the sediment at water depths from 0 to 0.75 m. At the same time the interstitial concentrations of carbon dioxide, extractable inorganic nitrogen, and exchangeable inorganic phosphorus increased, thus supporting the hypothesis that an increasing organic content of the sediments at this low level creates a physiologically richer medium for the plants.The isoetid growth form is discussed in relation to the chemical environment of oligotrophic, softwater lakes.