ALBERT

Description: Biovolume is commonly used as a size descriptor in the study of phytoplankton ecology. Usually, biovolume is not measured directly but is obtained from a standardized set of geometric models based on linear dimensions measured by light microscopy. This commonly used method allows visualization and measurement in two dimensions (2D) yielding no information at all on the third dimension of phytoplankton cells. Inaccurate biovolume assessment resulting from geometric approximation leads to erroneous interpretation of eco-physiological processes and morpho-functional traits. Here, we use confocal microscopy coupled with an image analysis system (NIS Elements AR software, Nikon) to determine directly shape and biovolume by means a 3D reconstruction of Coscinodiscus cf . granii specimens. We evaluate the accuracy of current methods by comparing the results obtained using geometric models with direct biovolume and shape. We find that calculation of biovolume by approximation to geometric models (cylindrical and complex shapes) leads to a significant overestimation with respect to direct volume. We also propose a data-driven formula for calculating the biovolume of Coscinodiscus cf. granii specimens based on diameter or other linear measurements.

Description: The phytoplankton colour index (PCI) of the Continuous Plankton Recorder (CPR) survey is an in situ measure of ocean colour, which is considered a proxy of the phytoplankton biomass. PCI has been extensively used to describe the major spatiotemporal patterns of phytoplankton in the North Atlantic Ocean and North Sea since 1931. Regardless of its wide application, the lack of an adequate evaluation to test the PCI's quantitative nature is an important limitation. To address this concern, a field trial over the main production season has been undertaken to assess the numerical values assigned by previous investigations for each category of the greenness of the PCI. CPRs were towed across the English Channel from Roscoff to Plymouth consecutively for each of 8 months producing 76 standard CPR samples, each representing 10 nautical miles of tow. The results of this experiment test and update the PCI methodology, and confirm the validity of this long-term in situ ocean colour data set. In addition, using a 60-year time series of the PCI of the western English Channel, a comparison is made between the previous and the current revised experimental calculations of PCI.

Description: We investigated the selective predation of mixotrophic and heterotrophic flagellates (MF and HF) on different heterotrophic prokaryote phylotypes (HPP; Bacteria + Archaea ) living in natural assemblages from oligotrophic Antarctic lakes. In situ prey preference was analyzed for the first time on different mixotrophic taxa ( Pseudopedinella sp., Ochromonas -like cells, Chrysophyceae 〉5 µm). The relative abundances of seven different HPP hybridized by CARD-FISH (catalyzed reporter deposition-fluorescent in situ hybridization) in natural community were compared with the proportions of hybridized cells inside digestive vacuoles. Our results showed some general trends to selectivity over some HPP. Alphaproteobacteria and Betaproteobacteria were the most abundant groups, and strikingly, a negative selection trend was detected in most samples by all bacterivorous protists. In contrast, for Actinobacteria a positive selection trend was observed in most samples, whereas Bacteroidetes seemed to be randomly preyed upon. Interestingly, similar prey preferences were observed in all bacterivorous flagellates. Our results suggest that phylogenetic affiliation determines part of the process of prey selection by protists in these lakes. Nevertheless, other features, such as cell size, morphology and the presence of the S-layer, might also significantly contribute to prey selectivity on the HPP.

Description: The invasive ctenophore Mnemiopsis leidyi , which now may be established in the NW Mediterranean, is a voracious predator of zooplankton and ichthyoplankton. Pelagia noctiluca , an abundant scyphomedusa there, eats other gelatinous species. We measured predation, digestion and escape when different sizes of medusae fed on ctenophores. Clearance rates increased with predator size and ingestion rates increased with prey concentration. Digestion times were longer when medusae were smaller than the ctenophores. All large ctenophores escaped from the medusae, but small ctenophores were ingested completely. Pelagia noctiluca is potentially an important predator of M. leidyi that may help control populations of this ctenophore.

Description: Allometric scaling rules have been classically used in plankton ecology to describe how the maximum ingestion rate ( I max ) under steady-state conditions changes with the body size of the consumer. Empirical and theoretical concerns, however, motivate a more accurate and mechanistic description of size–ingestion relations. Here, I propose to relate I max to the digestive surface area, which expresses the capacity in preprocessing and digesting food items. This surface area depends on both the body size and the optimal prey size of the consumer. The allometry in I max , hence, includes a second major variable which describes different feeding modes within a consumer size class. Species with a small optimal-prey-to-predator-size ratio and, thus, a small "internal" surface-to-volume ratio, as is typical for filter feeders, have large intra-body transport lengths and lower I max than raptorial-feeding species of the same body size. Digestive surface scaling tries to mechanistically accommodate feeding ecology, physiology and geometry. It does not explicitly resolve further possible factors affecting maximal ingestion such as nutritional quality. Still, digestive surface scaling explains the variability in published data compilations better than classical approaches when applied to the entire plankton size range. This is corroborated in further applications where the theory precisely fits anomalously steep scaling relations reported for heterotrophic nanoflagellates, ctenophores and a scyphomedusa. By introducing feeding mode and related morphological diversity into the size-dependency in ingestion rates, digestive surface scaling can be expected to improve the accuracy of size-based plankton models.

Description: The photosynthetic capacity of phytoplankton is frequently limited by the availability of nutrients. To explore the mechanism of the effect of nutrient availability on photosynthesis, batch cultures of a marine diatom Skeletonema costatum (Greville) Cleve were carried out under different nitrate (N: 0–220.75 µM) and phosphate (P: 0–9.075 µM) concentrations. Changes of photosynthetic characters, including the photosynthetic rate (measured by O 2 evolution) and the rbcL mRNA content (encoding the large subunit of the Calvin cycle enzyme, RuBisCO), were both studied. Within the range tested, both the photosynthetic rate and rbcL transcript levels correlated significantly with N and P concentrations in the medium. Additionally, the photosynthetic rates and rbcL transcript levels were both growth dependent. Significant correlations were also found between the abundance of rbcL mRNA and the photosynthetic rate ( R 2 = 0.800), rbcL and growth rate ( R 2 = 0.855), the photosynthetic rate and growth rate ( R 2 = 0.815). These results are consistent with N and P nutrition-regulating photosynthesis from the transcriptional levels of rbcL . The rbcL transcript level has the potential to be a good marker of algal growth and primary productivity.

Description: We suggest that podocysts are responsible for bloom versus non-bloom years of the giant jellyfish Nemopilema nomurai . They were capable of dormancy for at least 6 years. Their excystment was very rare (ca. 1%) in well-aerated seawater at constant temperature (19°C), but increased significantly under exposure to abnormally high temperatures (≥27°C), low salinities (8–24), hypoxia (1.0 mg O 2 L –1 ) and burial in organic-rich mud. Their behavior (e.g. maintaining dormancy or mass excystment) may influence medusa population sizes in any given year.

Description: Molecular probes were developed for the dinoflagellate genus Azadinium to discriminate among three taxa difficult to differentiate by light microscopy. This genus contains azaspiracid toxin-producing Azadinium spinosum , but also non-toxigenic species. Quantitative polymerase chain reaction (qPCR) and fluorescence in situ hybridization (FISH) assays were applied to cultured isolates and Azadinium -spiked field plankton. Molecular methods were highly specific and sensitive in the unambiguous detection of Azadinium , and thus are valuable for routine plankton, biogeographic and phylogenetic investigations.

Description: The in situ reproductive activity of the copepods Centropages ponticus and Temora stylifera was investigated in autumn 2006 in the Bizerte Channel in relation to abiotic factors and phytoplankton composition. Egg production rate (EPR), hatching success (HS) and nauplii survival rate of both species were measured from samples taken daily over 45 days from September 28 to November 11 and then weekly over 8 supplementary weeks. Over this period, C. ponticus and T. stylifera had mean EPRs of 13.7 ± 0.3 and 35.2 ± 3.8 eggs f –1 day –1 , and mean HS of 24.4 ± 1.6 and 50.4 ± 5.5%, respectively. EPR and HS remained fairly stable for C. ponticus throughout the study period compared with T. stylifera in which reproductive traits fluctuated considerably. Changes in EPR and HS for both species resulted from environmental parameter variations and in particular, from phytoplankton composition, with slight decreasing EPR and HS observed after the important phytoplankton blooms. Centropages ponticus , a lagoon species, was able to withstand high turbidity, whereas T. stylifera , a marine species, had low EPR and HS under turbid conditions. The low HS recorded for C. ponticus , in particular in December, may be due to production of resting eggs.

Description: Long-term data sets from 1979 to 2011 were analysed for trends in spring diatom development. As irregular phytoplankton sampling cannot record phytoplankton blooms completely, diatom growth during spring was estimated by silicate consumption. The spring diatom biomass decreased sharply at the end of the 1980s in the Baltic Proper but not in the neighbouring western Baltic area. The diatoms, mainly Thalassiosira spp. and Chaetoceros spp., reappeared after a series of cold winters. A strong negative correlation between the minimum water temperature and the magnitude of the diatom growth in the southern Baltic Proper confirms that diatom growth has decreased after mild winters, when motile phytoplankton, such as dinoflagellates and Mesodinium rubrum , may form blooms instead of diatoms. Silicate limitation did not occur in the Baltic Proper. Decreased convective mixing after mild winters may hamper diatom growth (stratification hypothesis), but this effect could not be proved. Our study supports the hypothesis that increased selective grazing pressure after mild winters may control diatom spring blooms (feeding hypothesis).

Description: Blooms of Karenia brevis , the major bloom-forming dinoflagellate in the Gulf of Mexico, are thought to originate in the eastern Gulf. Single-cell polymerase chain reaction and five microsatellites were used to obtain genotypes for 〉1800 cells from 38 samples collected from six bloom events. A consistent pattern of genetic divergence between blooms from Florida and Texas was not detected, which supports the hypothesis of a common origin for blooms of K. brevis in the Gulf of Mexico.

Description: The cold-water dinoflagellate Biecheleria baltica has increasingly dominated the phytoplankton spring bloom in the Baltic Sea during the past years. Life cycle transitions between bloom forming cells and resting cysts are assumed to regulate the bloom dynamics of this species. We investigate the seasonal cycle and succession of Biecheleria baltica's life cycle stages using a numerical model with four different stages, vegetative cells, gametes, resting cysts and germinating cells. The transitions among the stages are functions of environmental conditions and endogenous factors. Coupled to a water column model, the model is able to represent the seasonal cycle of Biecheleria baltica with two blooms in spring. The first bloom can be explained by germination of resting cysts in winter, the second by growth of vegetative cells. Sensitivity experiments indicate that temperature is an important factor regulating the composition of Biecheleria baltica life cycle stages; increased or decreased temperature leads to fewer growing cells and more resting cysts during spring. Our newly developed life cycle model can be used to study in more detail cyst formation, cyst distribution and consequences for biogeochemical cycling in the past and future.

Description: Many copepod species rely on pheromone cues to find partners. Some parasitic and benthic copepod males are able to distinguish between females of different reproductive states. Here, we demonstrate that the swimming activity and velocity of males of a pelagic copepod, Oithona davisae , increases in the presence of virgin when compared with mated females and that the cue is waterborne. The ability to distinguish between virgin and mated females may reduce male mortality during mate search and the cost related to mating behaviour (precopula) in both sexes. We estimate that at realistic population densities the ability of males to distinguish between virgin and mated females saves them several hours per day of dangerous and energetically expensive fast female tracking.

Description: Deployment of a Dual-frequency IDentification SONar (DIDSON) revealed the seasonal variations in spatial distribution and abundance of medusae of the common jellyfish, Aurelia aurita s.l. (detectable bell diameters: ca. ≥5 cm) in a shallow (max. depth: ~6 m) Japanese brackish-water lake between October 2008 and October 2009. The medusae were usually in patchy aggregations, more prominently so during warm seasons. Integration of the DIDSON-detected medusae with smaller ones estimated from the bell diameter composition in net hauls enabled us to study their seasonal population dynamics. Abundance was highest in June, due to recruitment, and declined to a relatively stable level in August and thereafter. The population carbon biomass was highest in October due to somatic growth of individual medusae. The annual mean biomass (63.8 mg C m –3 ) was much higher than that of mesozooplankton (26.0 mg C m –3 ), suggesting a substantial top-down impact. The DIDSON is an effective tool for studies of the spatial distribution and population dynamics of large jellyfish species like A. aurita . Concomitant net sampling to determine the bell diameter composition is required to offset the geometric limitations of this instrument.

Description: This study investigated to what extent natural predation or net feeding contribute to the presence of pre-flexion fish larvae in the stomachs of more developed larvae in ichthyoplankton samples from the Paraná River. A digestion trial and two sampling experiments were conducted. First, pre-flexion larvae were offered to post-flexion larvae and prey digestion stages were analyzed hourly. Second, we evaluated the number of prey retained in the mouth and contained in the stomachs of predators from 1, 5 and 10 min samples taken with a single-standard plankton net (300-μm mesh size) and with two standard and 1600-μm mesh size nets simultaneously deployed. At time zero, most of the prey were in a fresh condition. Digested prey accounted for ~40% of the prey after 1 h and were dominant after 6 h digestion. Stomachs of predators captured with the standard net contained significantly more fresh and bitten prey in longer- than in short-duration samples, whereas no prey were observed in samples collected with the larger mesh size net. Our results show intra-net predation in the 300-μm mesh net and imply that most prey were eaten in the net.

Description: Diel vertical migration (DVM) of zooplankton is commonly considered an adaptation for feeding in food-rich and warm surface waters at night and avoiding visual predators during the day. However, the critical assessment of migration patterns frequently suggests that: (i) zooplankton may leave deeper waters with rich, deep-chlorophyll layers and move into the epilimnion where food resources are lower and/or (ii) the night-time increase in epilimnetic plankton abundance is not matched by a density decrease in deeper strata. To study these discrepancies, we measured DVM of zooplankton in a 1.3-ha Spanish karst lake (Laguna del Tejo) where the phytoplankton in the deep chlorophyll layer had been labelled with the isotopic tracer 15 N. There were only limited changes in the vertical distribution patterns of rotifers and copepods over the diel cycle, but night-time biomass (mg L –1 ) of the cladoceran, Diaphanosoma brachyurum in the water column of the oxic zone increased 168% in the central area of the lake but there was not a parallel reduction in their density in the metalimnion or hypolimnion. Additionally, the isotopic signatures of the zooplankton in the epilimnion stayed constant from day to night, suggesting that there was little vertical migration. The results suggest that horizontal movements rather than vertical migration explain the increasing abundance of zooplankton in the central epilimnion of Laguna del Tejo during the night and that low food resources in this layer provide no incentive for DVM.

Description: The diatom Thalassiosira minima was first recorded in the Bahía Blanca Estuary in 1992. In 1992–1993 it exhibited a broad seasonal occurrence. A recent survey (2006–2007) showed a seasonal appearance restricted mainly to summer together with a greater relative abundance within the phytoplankton. A close connection was found with warmer, more saline and highly turbid conditions experienced in recent summers in the estuary. Whether these changes will impact the estuary trophic dynamics remains an open question.

Description: In a tropical reservoir, the cyanobacteria Microcystis aeruginosa and Cylindrospermopsis raciborskii are the dominant species, with changes in dominance throughout the year. Since allelopathy has been suggested as a factor that could promote or stabilize harmful algal blooms, we investigated potential allelopathic effects of C. raciborskii on M. aeruginosa. Microcystis aeruginosa was exposed to exudates of a C. raciborskii monoculture and exudates of mixed cultures of both species. Significant growth inhibition of M. aeruginosa was observed only when it was exposed to exudates from the mixed culture with high proportion of C. raciborskii . This result suggests that the production of growth inhibitors seems to depend on stress (competition) and on the density of the producer species. In contrast to the control, M. aeruginosa formed colonies when exposed to filtrates of mixed cultures. As far as we know, this is the first report on colony induction resulting from the interaction between cyanobacteria. Our results suggest that compounds produced by C. raciborskii may induce defense mechanisms in M. aeruginosa . Our results also indicate specificity in this interaction, since another strain of M. aeruginosa showed different responses.

Description: The biomass of Calanus spp. in the Norwegian Atlantic Current and the West Spitsbergen Current (2001–2009) was statistically related to a combination of salinity, temperature, water depth and sea ice concentration. The aim of this study was to identify the significance of these environmental factors for controlling the species-specific biomass distribution and the nature of such relationships. Calanus finmarchicus dominated the entire area and its biomass was mainly related to salinity. Calanus glacialis was mainly found along the northwest shelf of the Barents Sea, but its biomass was also related to the sea ice concentration and temperature with a critical threshold ~6°C, above which the presence and biomass of C. glacialis decreased. The Calanus hyperboreus biomass was related to all environmental factors, which characterized by a confined spatial distribution to water associated with the Greenland Sea Gyre. We conclude that C. hyperboreus is an expatriate to the study area. The Calanus biomass distribution will not change greatly in the study area with an expected increase of water temperature by 2°C, whereas the critical temperature threshold for C. glacialis will be exceeded with temperature increases of ~4°C with a likely disappearance of this cold water species from the north-west shelf of the Barents Sea.

Description: Spatial and temporal variations in the abundance and biomass of heterotrophic protists and of their grazing impact were investigated during five cruises between July 2006 and February 2009 in the continental shelf waters of the northern East China Sea (ECS). Strongly patchy distributions were observed on all cruises, generally with a higher biomass in the western areas affected by the Changjiang River discharge. An opposite pattern was observed in February when the Kuroshio onshore transport is greatest, with a greater biomass in the eastern area. Small heterotrophic dinoflagellates (〈20 µm) were most abundant numerically, whereas ciliates contributed the most to the biomass, accounting for 28–58% of the total heterotrophic protist biomass. Small heterotrophic dinoflagellates were more strongly correlated with phytoplankton biomass than were other types of protists. The total protist biomass was often most strongly related to amounts of particulate organic carbon of non-phytoplankton origin, suggesting that their abundance distribution often depended on prey other than phytoplankton, such as heterotrophic bacteria. Heterotrophic protists consumed 30.1–91.5% of Chl a production (mean 68.2%), with grazing rates increasing with the phytoplankton biomass. The results suggest that heterotrophic protists were the major consumers of primary production, and that their grazing is one of the most important losses affecting the phytoplankton biomass in the northern ECS.

Description: Larger egg size and body size at lower temperature are widely observed in zooplankton and other groups. Theoretically, considering the increased cost of producing larger eggs, females would produce larger and fewer eggs only if the newborn from such eggs perform better under certain environmental constraints, such as low temperature. In temperate lakes, low water temperature is a fairly reliable cue of future declining dietary conditions for herbivorous zooplankters. Under these conditions, females that produce more resistant offspring are likely to be selected. Nevertheless, it is not efficient for them to do this at high temperature, when the dietary conditions are usually more abundant. In this study, the adaptive significance of temperature-induced plasticity in a planktonic rotifer, Brachionus calyciflorus , was investigated. We found that: first, the egg size and body size were negatively related to temperature; second, neonates from larger eggs induced by low a temperature of 13°C were more resistant to starvation at three different experimental temperatures; third, life table experiments showed that offspring from larger eggs had a higher population growth rate and shorter juvenile period than those from smaller ones at low temperature, and vice versa, which suggests that this may be attributed to some form of anticipatory maternal effect. These differences in population growth rates were mainly due to higher fertility during the first 1 or 2 days after maturation. All of these findings support the view that temperature-induced egg size plasticity has adaptive significance in rotifers.

Description: Ingestion of the large pelagic ciliates Stentor araucanus and S. amethystinus by the cyclopoid copepod Mesocyclops araucanus was independent of light conditions and copepod sex, but rates were twice as high on S. araucanus as on S. amethystinus . Copepods consumed 44–183% of their biomass daily. Absorption efficiency was 5–40%, while 20–30% of the ingested food was found in the faeces. In field samples, 32–93% of Mesocyclops had ingested Stentor, indicating the importance of this food source.

Description: To assess the effect of Microcystis blooms on the crustacean zooplankton community, we conducted a large-scale monthly survey from 2008 to 2010 in Lake Taihu, China. During the sampling period, Microcystis was the dominant genus in the phytoplankton community (〉79% of total biomass), while the crustacean zooplankton community was dominated by small-sized cladocerans (primarily Bosmina coregoni and Ceriodaphnia cornuta ) and copepods. During the study, plankton exhibited significant spatial heterogeneity. The northern and western regions suffered from harmful Microcystis blooms and had higher cladoceran production. In the eastern region where Microcystis blooms seldom occurred, copepods accounted for a relatively larger proportion. Both stepwise multiple linear regression and redundancy analysis demonstrated that Microcystis was the most important factor controlling the spatial dynamics of zooplankton. Linear regression analysis revealed that both cladocera and copepoda were positively correlated with Microcystis and the response of cladocerans ( R 2 = 0.73 , P 〈 0.001) to Microcystis was stronger than that of copepods ( R 2 = 0.44, P 〈 0.001). Our study suggested that cyanobacterial blooms may change competitive relations of small-sized cladocerans and copepoda, favoring small cladocerans (except larger genera).

Description: The scyphozoan Pelagia noctiluca reproduces by direct development without a benthic stage. Typically, this jellyfish is found offshore with a holoplanktonic lifecycle, vertical migration and feeding behaviours. Frequent outbreaks have been well documented on the Mediterranean shores since the 19th century; however, the offshore distribution of this species remains mostly unknown. In this study, we performed a bimonthly monitoring of P. noctiluca surface density, at high resolution, from a sailboat, along a 35-km coastal to offshore transect in the Ligurian Sea, between February and October 2011. During daylight, P. noctiluca was rarely seen. At night, offshore, P. noctiluca was always present, while within 5 km of the coast, P. noctiluca was rarely observed. Pelagia noctiluca aggregations were most abundant within the Northern Current of the Ligurian Sea. Our findings suggest that P. noctiluca outbreaks observed on Mediterranean shores may result from the transport of the permanent offshore population inshore by specific hydrodynamic conditions.

Description: Fecal pellet flux and size distribution at 500, 1500 and 3200 m depths were measured in sediment trap samples collected by the Oceanic Flux Program time-series off Bermuda, December 2006–November 2007. During the study, three mesoscale eddies passed through: a cyclonic eddy in bloom stage (February through mid-April), a decaying post-bloom mode-water eddy (late April through May), and an anticyclonic eddy (August through October). Variability associated with eddy passage masked any seasonal trends in flux or size distributions. At a depth of 1500 m, the fecal pellet flux ranged from a minimum of 100 pellets m –2 day –1 to a maximum of 500 pellets m –2 day –1 during the cyclonic eddy passage, corresponding to ranges in the fecal pellet mass and the carbon flux of 0.5–1.7 mg m –2 day –1 and 0.07–0.25 mg C m –2 day –1 , respectively. Fecal pellets averaged 7 ± 3% of the organic carbon flux, a minimum estimate as disassociated pellets were not quantifiable. Size distribution shifts indicated small zooplankton and immature stages were more abundant within the cyclonic eddy, whereas larger zooplankton were present within the mode water and anticyclonic eddies. The fecal pellet number, flux and size distributions showed no consistent depth trends and indicated extensive fecal pellet reprocessing within the water column.

Description: The community of bacteria associated with the fecal pellets of planktonic copepods and those free living in surrounding seawater were investigated by denaturing gradient gel electrophoresis (DGGE) during a 10-day laboratory fecal pellet degradation experiment. Generally, fecal pellets containing bacteria were composed of different phylogenetic groups compared with those living in seawater. Bacteria in fecal pellets were dominated by -Proteobacteria and Sulfitobacter ( α-Proteobacteria ), whereas bacteria in seawater had higher species richness and mainly consisted of α-Proteobacteria . Remarkable bacterial community shifts occurred in the first 2 days of the experiment along with the apparent increase in dissolved organic carbon and decrease of dissolved oxygen in the incubation bottles. Throughout the incubation, bacteria that were initially unique to fecal pellets were never found in surrounding seawater, while the communities of bacteria in fecal pellets and seawater became more similar, indicating colonization of bacteria from seawater to fecal pellets during the degradation process. These results suggest that the colonization of free-living bacteria took place rapidly and that they might contribute significantly to the degradation of planktonic copepod fecal pellets.

Description: The diel behavior and feeding ecology of three coexisting species of chaetognaths in two shallow subtropical bays in the northeastern part of Hong Kong were studied in summer 2005 and winter 2005–2006. Average densities of adult Flaccisagitta enflata , Aidanosagitta delicata and Aidanosagitta neglecta ranged from 21 to 48 ind. m –3 in the semi-enclosed Tolo Harbour, but densities of the two Aidanosagitta species were low in Mirs Bay, which is deeper and opens into the South China Sea. Adult F. enflata , the largest of the three species, exhibited the most pronounced diel vertical migration (DVM) and diel feeding rhythm. The smaller juvenile F. enflata and adult A. delicata did not exhibit DVM. Although copepods were the most abundant component of the mesozooplankton, they were not preferred prey of chaetognaths. Predation impacts on copepods were generally 〈0.3% of the standing stock. Larvaceans, on the other hand, were highly preferred, with predation impact as high as 〉30%. As prey abundances in Tolo Harbour could be 〉9 x 10 4 ind. m –3 , there was no evidence of resource partitioning behavior by the coexisting chaetognaths.

Description: The sea ice diatom biomarker IP 25 has been detected and quantified in bulk zooplankton obtained from the Amundsen Gulf (Canada) in 2008. This study represents the first example of the detection of this biomarker in the pelagic food web. Concentrations of IP 25 ranged from ~5 to 15 ng g –1 dry weight with peak values occurring broadly at the same time as those found for this biomarker in sea ice samples determined previously from the same region; a 25–30 day lag between the sea ice bloom and zooplankton IP 25 profiles is interpreted in terms of a predator–prey relationship. IP 25 concentrations in zooplankton declined towards the end of the spring sea ice algal bloom and during the main period of ice melt. At this point, concentrations of n -C 21:6 , a common biomarker of general marine diatoms, increased substantially in the zooplankton, indicative of a switch in feeding patterns. This detection of IP 25 in one of the first trophic levels of the Arctic marine ecosystem has potentially important implications for the investigation of polar food webs and the impacts that changes to sea ice conditions will have on these.

Description: This study aimed at describing changes in the stable isotopic composition of late copepodite stage V (CV) subarctic marine copepods ( Calanus finmarchicus and C. hyperboreus ) during overwintering non-feeding periods. Diapausing stage CVs sampled in deep waters of the Lower St. Lawrence Estuary (Québec, Canada) in late-September 2009 were monitored for 4 months under controlled laboratory conditions. CVs and newly moulted adults were analyzed for 13 C and 15 N signatures as well as lipid, carbon and nitrogen content. Lipids were extracted in half of the samples to compare 13 C of individuals with and without lipids and to evaluate the accuracy of mass balance correction models for 13 C under lipid influence. Lipid content generally decreased with time for both species, which was reflected in an increase of 13 C values of CVs but a constant 13 C in newly moulted adults. Accordingly, lipid extraction resulted in an increase of 13 C in CVs and adults. The mean 13 C signature of lipid-extracted individuals remained constant through the time for CVs of both species and for C. finmarchicus adults. 15 N signatures of individuals increased after lipid extraction, but this did not result in a constant value over time, suggesting that several endogenous metabolic processes affected nitrogen isotopic content. The accuracy of the mass balance model differed between species and stages, suggesting that lipid extraction should always be performed prior to applying mathematical corrections.

Description: The composition of ultraphytoplankton (〈10 µm) sampled in surface waters (1 m depth) was investigated during summer 2010 in the Bay of Marseille (NW Mediterranean) using flow cytometry. In addition to groups of Synechococcus , Prochlorococcus , picoeukaryotes and nanoeukaryotes, an unidentified additional cluster was observed. The particles forming this cluster had a high abundance (〉7 x 10 4 events mL –1 ), with both orange and red fluorescence intensities like Synechococcus , but with a size signal larger than that of 2 µm fluorospheres (beads). These unknown particles were sorted out by flow cytometry and then observed by epifluorescence microscopy: they appeared to be chain-forming microorganisms, just like trichomes of some diazotroph cyanobacteria with one heterocyst, but not unambiguously distal. The chains observed after cell sorting were not straight, but rather folded. To our knowledge, this is the first report of a possible bloom of such free trichomes. The small sample volume available for the delayed analysis and the limited resolution of our photomicrography did not allow for species identification. The brief occurrence of free trichomes in the Bay of Marseille with such a high abundance remains to be explained. High temperature in summer, induced stratification of the water column and nitrate depletion may be related to this event. High-frequency surveys of ultraphytoplankton assemblages at the single cell level appear to be necessary to observe such phenomenon more efficiently and document their dynamics.

Description: In the rotifer genus Brachionus , the proportion of mictic females in the offspring is significantly related to the population density experienced by their mother. In the present study, we examined whether population densities experienced by mothers could affect the propensity of their offspring to produce mictic females in Brachionus calyciflorus . Results showed that when the mother experienced a high population density, her offspring produced a significantly higher proportion of mictic daughters, 8.5–81.5% higher than those from a low maternal population density, along a gradient of population densities (1 ind. mL –1 to 0.025 ind. mL –1 ). The effect of maternal crowding on the propensity of offspring to produce mictic females was more significant when the offspring faced lower population densities.

Description: The two Cape hake species of the southern Benguela ecosystem, the shallow-water and deep-water hakes Merluccius capensis and M. paradoxus , are economically the most important marine resources in South Africa. Recruitment is a key process in the dynamics of marine organisms, yet very little is known about the early life history of Cape hakes, especially the location of spawning grounds and transport of eggs and larvae. For each species, ichthyoplankton dispersal off South Africa is simulated by coupling oceanographic simulations to an individual-based model in order to track virtual individuals. Results indicate that the most favorable spawning areas for transport to nursery areas are located off the south-western coast and the eastern Agulhas Bank, and highlight partly different drift routes followed by the two ichthyoplankton species off Cape Columbine. Transport from spawning to nursery areas is the highest in austral winter for a spawning depth ranging between 0 and 100 m. These modeling results are in broad agreement with available knowledge on the ecology of Cape hakes. The present work on Cape hakes complements previous modeling studies on anchovy and sardine in the same area. Taken together, these studies underline the correspondence between cross-shore (for hakes) or alongshore (for anchovy and sardine) transport mechanisms and the spawning strategies used by these key species of the southern Benguela ecosystem.

Description: Controlling noxious phytoplankton blooms by biomanipulation largely depends on the presence of large-sized (〉3000 µm) generalist grazers, not common in the tropics. Therefore, we compared the ability of small (≤2000 µm) microcrustaceans Ceriodaphnia dubia, Moina macrocopa, Daphnia pulex, Simocephalus vetulus and Heterocypris incongruens to feed and grow on cyanobacterial diets. We studied their feeding preferences on phytoplankton from natural lake water with a dominance of Microcystis sp., Fragilaria sp. or Planktothrix sp. to which we added Scenedesmus acutus . Also tested were the ability of D. pulex , S. vetulus and H. incongruens to reduce cyanobacterial densities by grazing, and the rate of grazing and demographic responses of S. vetulus and H. incongruens on diets of S. acutus, Microcystis sp. and Planktothrix sp. All species fed on small colonies of Microcystis sp., but not on filamentous Planktothrix sp. or colonial Fragilaria sp. Simocephalus vetulus and H. incongruens were generalists and most capable of reducing cyanobacterial densities. Demographic variables of S. vetulus , but not H. incongruens , were significantly lower on the cyanobacterial diet; for instance, the population growth rates were between 0.07–0.2 day –1 on cyanobacterial diets, but 0.31 on S. acutus while for H. incongruens they ranged between 0.07 and 0.08 day –1 , regardless of whether the diet was a cyanobacteria or S. acutus . Our study warrants further tests on the ostracod H. incongruens in order to test its efficacy in reducing cyanobacterial densities in shallow tropical ponds.

Description: Embryogenesis (single cell to twitching stage) and early larval stages (nauplius to calyptopis 3) internal morphology of the sac-spawning species Nyctiphanes simplex was investigated to explore its temporal association with gonad development and molt development cycles. Krill were collected and incubated shipboard in July 2007 and March 2010 in the Gulf of California. Cleavage patterns were similar to what is observed in broadcast-spawning species, suggesting a close phylogenetic consistency between krill species with distinct spawning strategies. Nyctiphanes simplex , like broadcast-spawning krill species, had eight Kranzzellen (K 1 –K 8 ) cells during the blastula stage. The gonad of ovigerous females with embryos in the cell division stage is in the multiplication stage (Stage I). From the nauplius stage, most females are in previtellogenesis (Stage II). Only females with metanauplii occasionally have gonads in vitellogenesis (Stage III). Gonad maturity (Stage IV) occurs only after the release of the embryos from the ovigerous sac. Females with an ovigerous sac were invariably at the intermolt stage, suggesting a precise synchronization among processes of molting, gonad development and embryo release to produce consecutive broods. Lipid and carbohydrate storage decreased exponentially throughout embryonic and early larval development, depending on their endogenous reserves. When metanauplii leave the ovigerous sac, they have low storage lipid (〈2%) and carbohydrate (〈5%). These proportions suggest a theoretically short period-of-no-return (〈2 days), when they transform into the first feeding stage (calyptopis 1).

Description: One of the problems concerning studies of fish egg distribution is the weak spatial and temporal resolution due to the workload that examination of a large number of samples would demand. Recently, the development of a new laboratory imaging system, the ZooScan, capable of obtaining relatively good resolution images enables automated zooplankton identification using supervised learning algorithms. This new approach was applied to formalin-fixed fish egg samples collected during French winter IBTS (International Bottom Trawl Surveys) in the Eastern English Channel and the Southern North Sea. Fish egg spatial distributions of seven species based on the microscope and ZooScan identifications were compared. Abundance and distribution maps of winter-spawning areas of plaice, long rough dab, cod and whiting were similar for both methods. Low identification accuracy for small size eggs was due to microscope misidentification of standards used for the ZooScan learning (dab and flounder). The potential input of such a tool to quickly acquire valuable data on identification, enumeration, size frequency distribution of fish eggs and map spawning areas is of great interest for understanding and forecasting fisheries recruitment and will support ecosystem-based management.

Description: A new perspective on the role of lipids in zooplankton is proposed, with solid–liquid phase transitions of lipids being a factor regulating their buoyancy. These phase transitions are controlled by zooplankton in relation to their physical environment, through the selective accumulation of specific lipids with optimum levels of unsaturation. The necessity to control buoyancy and maintain an optimum depth is a fundamental evolutionary force, driving anatomical, biochemical and behavioural adaptations of all organisms within the aquatic realm. It is hypothesized that each species adjusts the amount, composition and anatomical location of lipids, to maximize fitness according to their preferred habitat and life history traits. Recent discoveries regarding the role of phase transitions of lipids in marine zooplankton and their role in regulating buoyancy will require re-interpretation of existing data and stimulate future scientific endeavours in zooplankton research.

Description: The accurate identification of individuals in zooplankton samples is a crucial step in many plankton studies. Up to now, this has been done primarily by microscopic analysis of morphological characters, and new molecular methodologies are still relatively rarely applied. Another promising technology is matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), which has had a major impact in applied and systematic microbiology, where it is used for routine high throughput identification of bacteria and fungi. For the present study, we developed a protocol for the rapid acquisition of mass spectra from whole individual copepods. The final protocol enabled us to obtain mass spectra with more than 100 distinct peaks in the mass range of 2000–20 000 Da. A comparison of the mass spectra of three species of Eudiaptomus showed that they could all be clearly discriminated, whereas the mass spectra of different developmental stages and sexes of each particular species were highly similar. Further, a discrimination of con-specific individuals from different habitats was achieved, at least partly, even without extensive optimization of the analytical and statistical procedures. These results indicate the feasibility of identifying copepods by a rapid and simple MALDI-TOF MS analysis, e.g. for population ecology studies.

Description: The rapid and efficient analysis of plankton samples (e.g. enumeration, identification, biomass determination) has been an important driver for recent technological developments in (semi-) automated analysis and imaging instruments. Most focus has been on identification and abundance estimates, while less attention has been given to viability, i.e. the assessment of whether the organisms are dead or alive. However, a wide spectrum of scientific applications requires accurate viability determinations, e.g. the monitoring of invasive species in ship ballast water. The transfer of species through ballast water forms a major threat to marine ecosystems, resulting in significant environmental and economic losses. A variety of viability stains and viability assessment methods are available, but there has been no systematic investigation how these methods perform for larger organisms (≥50 µm). We review the current procedures for viability determination for large plankton and present a cross-comparison of three methods: cell digestion assay (CDA), SYTOX ® Green nucleic acid staining and Neutral Red vital staining. The CDA and SYTOX ® Green methods did not perform well and gave various problems linked to the multicellular nature of zooplankton, autofluorescence and/or constraints set by the definition of cell death. Although some issues remain and there is no universal method, the Neutral Red vital stain proved the most robust viability method in this study and is broadly applicable to both phytoplankton and zooplankton larger than 50 µm.

Description: Changes in the tintinnid community were studied in the neritic area of the middle Adriatic Sea from January 1998 until November 1999. Variability in tintinnid abundance showed a clear seasonal pattern with winter–spring and autumn maxima. The vertical distribution of tintinnid populations was affected by their taxonomic structure. A total of 51 taxa belonging to 11 families and 20 genera were identified. Helicostomella subulata , Codonellopsis schabi , Stenosemella nivalis and Salpingella decurtata were the dominant species. Species richness was positively related to overall abundance temporally at each station, and was also affected by environmental trophic state. The total tintinnid abundance, the dominant species abundance and the dominance grade increased significantly with increasing trophic state, while the diversity indices decreased. Hence we conclude that abundance and dominance may be straight indicators of ecosystem trophic state. In addition to environmental trophic state and food availability, tintinnid species distribution was strongly affected by temperature, which was the most important environmental factor determining their seasonality.

Description: Transects of CTD (to 1000 m) and zooplankton stations (to 200 m in 50 m strata) were made across an anticyclonic eddy in the southern Gulf of California during October 2007 to determine its influence upon the three-dimensional distribution of larval fish assemblages. The eddy was ~90 km in diameter and ~70 m deep. A larval fish assemblage, representing a mix of oceanic and coastal species, was defined mainly in the eddy from 200 m depth to the surface. Mesopelagic species, such as Vinciguerria lucetia , were dominant. Coastal reef ( Diplectrum sp.) and pelagic ( Auxis spp.) species were found mainly in the surface layer. This suggests that, because of the Gulf's relative narrowness, the eddy trapped coastal fish larvae during its formation and trajectory southward, retaining larvae of different adult habits. Another larval fish assemblage was defined off the eastern coast; its high larval abundance and specific richness was probably associated with coastal upwelling. Mesopelagic species (e.g. Triphoturus mexicanus ) dominated this assemblage, and coastal demersal species that were absent from the eddy (e.g. Symphurus williamsi ) were recorded in the surface layer, suggesting that the thermocline was a vertical boundary in this assemblage. The 3D differentiation of planktonic habitats was the result of the mesoscale hydrodynamics in the area sampled, in particular that associated to the eddy life history and characteristics (radius, depth and velocity), and to coastal upwelling, promoting larval retention of a mix of species of different adult habits.

Description: The most commonly used biomass estimate for microalgae is obtained from cell biovolume, usually calculated from microscopically measured linear dimensions. Although reliable, this is a highly time-consuming and specialized technique. Automated sampling devices that acquire images of cells and use pattern recognition techniques to identify the images have been developed as an alternative to microscopy-based methods. There are some aspects of automatic sampling and classification methods, however, which can be improved for the analysis of field samples including living and non-living particles. In this work, we demonstrate how the accuracy of a state-of the-art technique for plankton classification (Support Vector Machine) can be improved up to 86% if a previous automated step designed to remove non-living images is included. There is a tendency with the currently applied automatic methods to misestimate cell biovolume due to the two-dimensionality of the images. Here, we present a data set of more than 500 samples to show that the greatest effect is caused by the incorrect estimation of biovolume of the chain-forming diatoms. This results in an overestimate of biomass of between 20 and 100% where chain-forming diatoms represent more than the 20% of the biomass of the sample. We show how the classification method can be adapted to provide not only taxonomic but also the morphological classification of cells in order to obtain a more reliable estimate of biovolume according to the predicted cell shape, in a way comparable with microscopy-based estimates.

Description: We evaluate the feasibility of estimating zooplankton biomass using acoustic Doppler profilers (ADP) as deployed on autonomous ocean gliders. A Sontek Spray ADP mounted on a Mocness net was used for simultaneous collection of zooplankton and measurement of acoustic backscatter (ABS). Zooplankton biomass was estimated from optically scanned plankton samples. We found a direct proportionality between ABS and the summed cross-sectional area of the zooplankton as well as with the estimated zooplankton carbon biomass. ABS at 750kHz was most closely related to the summed cross-sectional area of zooplankton and micronekton greater than 1.6 mm equivalent circular diameter.

Description: Recent observations have confirmed the presence of dense accumulations of the diatom Aulacoseira islandica within surface lake ice on Lake Erie. MERIS (Medium Resolution Imaging Spectrometer) full-resolution satellite imagery is shown to clearly distinguish the optical signature of these phytoplankton blooms. The MERIS Level 1 maximum chlorophyll index is applied to Lake Erie under winter, partially (〉50%) ice-covered conditions, demonstrating for the first time the potential for satellite detection of blooms within, or surrounding, surface lake ice.

Description: To analyze the potential trophic interactions among tuna larvae and other fish species, we studied the larval fish assemblages inhabiting the surface mixed layer around the Balearic Islands using data from two surveys carried out in summer 2006 and 2008. Despite the high diversity, Thunnus alalunga , Auxis rochei , Cyclothone pygmaea and Ceratoscopelus maderensis clearly dominated the catch in both years. The spatial distribution of several larval fish assemblages, in which tuna larvae were among the dominant species, was related to the presence of fronts and mixed Atlantic waters (AW). Different developmental stages of tuna were found in similar hydrographical scenarios. Smaller tuna larvae and plausible piscivorous tuna were found in the areas with mixed AW. Later in the season, potential piscivorous tuna were found together with small larvae from other non-tuna species that had been spawned later and transported by the fresh AW flow to offshore stations. That the different life stages of tuna co-occur with mesopelagic species in the mixed layer reveals the interesting possibility of ecological interactions between the different larvae.

Description: The feeding ecology of larvae of the lightfish Maurolicus parvipinnis (Pisces: Sternoptychidae, 2.8–13.4 mm) in fjords and channels of southern Chile (41°–53°S), was analysed during austral spring 2005, 2008 and 2009. Larvae of M. parvipinnis begin external feeding at ~3-mm body length (BL). They are carnivorous predators with similar feeding incidence during day and nighttime, but with higher prey numbers in their guts during day than night. Feeding incidence increased after 8-mm BL, coinciding with an increase in the growth rate of the upper jaw length. The diet of M. parvipinnis larvae was composed of 22 different prey items, mainly invertebrate eggs, nauplii, calanoid, cyclopoid and poecilostomatoid copepodites, ostracods, cladocerans and amphipods. According to the relative importance index [%index of relative importance (IRI)], the most important prey items were Paracalanus parvus copepodites (%IRI = 55.6%), Oithona spp. copepodites (%IRI = 16.3%) and copepod eggs (%IRI = 9.2%), indicating that most of the feeding is on copepod early life stages. Trophic niche breadth was ~0.17 and it did not increase with larval length, suggesting that larval M. parvipinnis have a high degree of selective foraging. Also, we detected a significant relation between feeding success and vertical stratification of the water column, with higher prey volume per gut found in larvae collected in areas with a Brunt-Väissälä frequency of 0.07–0.2 cycles s –1 . We discuss potential effects of the fjord environment on the feeding behavior of this important species along Chilean Patagonia.

Description: The Continuous Plankton Recorder has been deployed in the NE Pacific on two intersecting transects since 2000. Many deployments included a temperature sensor providing in situ temperature data to supplement the species abundance data for ~1300 samples. Twenty-nine copepod taxa were sufficiently abundant to examine their temperature-related distributions. Groups of warm- and cold-water species were identified, with overlapping distributions between 48 and 58°N. Recent fluctuations in ocean climate, from the warmest year on record in 2005 to one of the coldest in decades in 2008, provided ideal conditions to observe temperature-related interannual variability. The abundance and northwards extension of warm water species were significantly positively correlated with mean annual temperature and the Pacific Decadal Oscillation. The cold water species showed no correlations with temperature/Pacific Decadal Oscillation (PDO) within the study region; however, if the 4 years of sampling that extended south to 39°N were examined separately, there was a strong relationship between temperature/PDO and the southern extent of subarctic copepods. Under warm ocean conditions, the range overlap of the two groups will increase as warm water species extend northwards, causing an increase in copepod diversity. Since warm water species are generally smaller and nutritionally poorer, this has implications for higher trophic levels.

Description: The frequent upwelling events characteristic of Oregon's summer may make nearshore retention challenging for early, non-migrating euphausiid life stages inhabiting the surface layer. In contrast, later, vertically migrating life stages spend more time below the strongest offshore flow and are moved offshore less rapidly. We hypothesize that population stage structure will vary in predictable ways as a result of differential transport, a function of occupied depth (and therefore life stage). MOCNESS (Multiple Opening/Closing Net and Environmental Sampling System) samples have indicated that early Euphausia pacifica calyptopes (stages 1 and 2) are non-migrators, whereas late furcilia (stages 4–7) are migrators. We develop a metric (migrator fraction, F M ) of relative densities of these larval stages within a sample to examine potential differential transport with alongshore winds on short timescales (0.5–7 days), for data collected during upwelling seasons from 2000 to 2002 at two stations off Oregon. F M and northward winds had a significant negative relationship in 2002 at the inshore station, indicating differential transport. Upwelling condition-classified sampled densities at nearshore and offshore stations also suggested differential cross-shelf transport, but trends were not always consistent with the hypothesis, suggesting the cross-shelf distribution of non-migrators was not consistent throughout the study period. Several other factors, such as spawning, might also contribute to significant trends observed in F M . The results of this analysis suggest that relationships exist between E. pacifica larval distributions and wind events in the Oregon upwelling region, but that the distributions may be too complex to predict at the event scale without more sophisticated tools, such as biological-physical coupled models.

Description: In experiments designed primarily to investigate viral lysis, we found that the presence of viruses had a positive effect on the growth of Synechococcus . A Landry-Hassett-type stepwise dilution experiment conducted during a Synechococcus bloom in the Gulf of Mexico used both (i) 0.2-µm filtered seawater in which the abundance of bacteria and grazers were reduced but the majority of viruses were retained, and (ii) ultrafiltered (30 000 MW cutoff) virus-free seawater in which the abundance of viruses, bacteria and grazers were reduced. High growth rates and frequency of dividing cells (FDCs) were recorded in 0.2-µm filtered treatments while growth was inhibited in incubations with a high proportion of virus-free ultrafiltered water. In two subsequent experiments using Mediterranean Sea populations, a two-point dilution approach in which viral abundance was reduced by 80–90% yielded similar results, and showed that Synechococcus only grew well in the presence of viruses, bacteria and grazers. In four further Mediterranean experiments viruses removed via ultrafiltration were added back, either untreated, or inactivated by a heat treatment. Growth rates and FDCs were higher in the presence of untreated viruses than with viruses inactivated by heat, suggesting that it was not organic matter in the virus-size fraction but rather the presence of infectious viruses which sustained growth. While Synechococcus was also infected by viruses during these experiments, our data imply that growth of Synechococcus may depend upon viral lysis of heterotrophic bacteria. This finding is consistent with the view that nutrient cycling by viral lysis of heterotrophic bacteria may control phytoplankton growth and ecosystem scale carbon production.

Description: The goal of this study was to determine the morphology and ultrastructure of doliolid pellets using light, epifluorescence and transmission electron microscopy and compare the results to observations of calanoid copepod pellets. For (ultra)structural analyses, pellets of gonozooids of Dolioletta gegenbauri and females of the copepod Eucalanus pileatus were produced in feeding experiments at close to environmental food concentrations. Thin sections of a representative doliolid pellet revealed that these pellets were mainly composed of intact diatom valves, a few fragmented valves and intact flagellates. While the larger diatoms, Rhizosolenia alata , were completely digested (empty valves), the smaller diatoms, Thalassiosira weissflogii , were partly, or not digested at all. The phytoflagellate, Isochrysis galbana , appeared to be hardly digested. Aggregations of bacteria occurred mostly inside pellets associated closely with intact I. galbana flagellates and partly digested T. weissflogii cells; some scattered bacteria were found among fragmented valves. No, or little, bacterial colonization was associated with empty R. alata valves, and hardly digested T. weissflogii cells. Whereas doliolid fecal pellets were loosely packed and composed of fully, incompletely and/or hardly digested food particles, pellets of the copepod E. pileatus were densely packed and consisted mainly of fragmented diatom valves. Pellets of doliolids and calanoid copepods can represent a high percentage of the particulate organic carbon in the water-column on subtropical continental shelves.

Description: Changes in lake thermal structure, which are predicted with future climate warming, may alter predator–prey interactions if foraging rates or the spatial overlap of predators and prey depend on thermal conditions. Small Boreal Shield lakes are particularly responsive to weather-induced changes in thermal structure. They are often fishless, with macroinvertebrate predators regulating crustacean zooplankton communities. We performed a mesocosm experiment to examine how thermal structure (stratified and isothermal) influences the predation impact of surface-orienting Buenoa macrotibialis and vertically migrating Chaoborus punctipennis on crustacean zooplankton. We expected predation from surface-orienting predators to be greatest in stratified conditions when food resources are concentrated near the surface in proximity with predators. Surprisingly, surface predators had no effect on zooplankton abundance, and zooplankton avoided surface predators regardless of thermal habitat structure. In contrast, Chaoborus had a strong predation impact and reduced total zooplankton abundance, but only in isothermal conditions. We hypothesize that this predation effect was due to increased predator metabolism, foraging and ingestion rates when migrating through a thermally homogenous warm water column without access to cool bottom waters. These results demonstrate that changes in lake thermal structure may result in strong, unexpected consequences for predator–prey dynamics.

Description: The majority of studies of mechanisms regulating bacterioplankton processes have focused on assessing the isolated effects of nutrients and predation. However, in natural ecosystems, microorganisms may interact strongly with larger organisms in an array of complex, direct and indirect interdependencies. In this work, we report the results of a field mesocosm experiment in which, over 7 weeks, we evaluated the individual and interactive effects of resource availability (N and P addition) and indirect effects originating from the presence of an omnivorous fish species ( Hyphessobrycon bifasciatus ) on bacterioplankton production (BP). Nutrient addition and fish presence both had individual positive effects on BP, but bottom-up control effects were stronger than indirect top-down control effects. The positive effect of nutrients on BP was mainly direct, through increasing the availability of inorganic N and P. There was no significant interaction between fish and nutrients. The positive indirect top-down effects of omnivorous fish on BP were probably related to both fish-mediated changes in the zooplankton community structure and fish-mediated cross-habitat nutrient regeneration. Our results show that changes in food web structure, due to the presence or absence of vertebrate macroconsumers, can also affect heterotrophic microbial processes.

Description: Food limitation is one of the most important extrinsic factors influencing population dynamics in aquatic ecosystems, but the spatial patterns of resource limitation and their influence on zooplankton species have not been investigated. We tested the hypothesis that the occurrence and intensity of food limitation may vary both spatially and temporally, and that the irregular population dynamics of the rotifers Brachionus group plicatilis and Hexarthra jenkinae in a saline, warm-monomictic, oligotrophic lake (Alchichica, Central Mexico) can be attributed to resource limitation. We estimated plankton abundance and composition and performed 19 laboratory experiments to assess the intensity of food limitation ( r ) at three selected depths in the water column. Most of the phytoplankton biomass consisted of algae inedible to rotifers, and good-quality food (phytoplankton ≤20 µm + small heterotrophic flagellates) was frequently below the threshold food concentration for Brachionus species. The vertical distribution and abundance of rotifers were correlated with the availability of food resources. Food limitation was observed during mixing and late stratification; however, the intensity of limitation was rather similar throughout the water column owing to the overall scarcity of resources in the lake. Hexarthra seldom appeared. We conclude that the irregular pattern of rotifer abundance we observed is a consequence of the interplay between the harsh conditions of scarce food availability, resulting in intense resource competition and opportunities for rapid exploitation of opportunities when food outbursts occur.

Description: Cyanobacterial blooms of Aphanizomenon spp. occur worldwide, with deleterious effects on aquatic communities. The present work aimed to investigate the effects of different levels of phosphorus and nitrogen on the growth of Aphanizomenon bloom-forming strains, belonging to the species A. issatschenkoi , A. aphanizomenoides and A. gracile . Algal inhibition tests were also performed to assess the potential allelopathic effects of the strains' filtrates on the growth of four green algae ( Pseudokirchneriella subcapitata , Chlorella vulgaris , Pandorina morum and Coelastrum astroideum ). Phosphorus had a significant effect on the growth of all the strains tested; orthophosphate concentrations ≤0.3 mg PO 4 3– L –1 led to a decrease of more than 50% in the cyanobacterial growth of most strains. The growth of A. gracile strains was unaffected by variation in nitrate levels, but strains of A. issatschenkoi (UADFA1) and A. aphanizomenoides (UADFA6, UADFA7 and UADFA13) were moderately to extremely sensitive to nitrate depletion in the medium. These results strengthen the idea that intra-specific variation may play an important role in bloom dynamics; however, culture conditions and mutations may explain the morphological and physiological changes observed. Allelopathic assays revealed significant inhibition of microalgal growth by filtrates of strains from the three species, but also indicated intra-specific differences in allelopathic activity.

Description: The rhythmic migration pattern of the dinoflagellate Ceratium furca is a result of ecological adaptation to avoid high irradiance. The high proportions of dividing cells at deeper depths are likely to be an ecological response to maintain their population away from the turbulence in the near-surface layer.

Description: The MERIS (MEdium Resolution Imaging Spectrometer) maximum chlorophyll index was applied to Lake of the Woods, an inland water body under significant water quality pressure from recurring cyanobacteria blooms, in order to address the commonly raised concern that blooms on the lake have increased recently. Recent trends in bloom characteristics (intensity, timing and aerial extent) were analysed in relation to local climate variables, offering important new insights into the mechanisms driving algal bloom occurrences on the lake.

Description: To examine the relationship between carbon and nitrogen stable isotope (SI) ratios ( 13 C and 15 N) of zooplankton, we analyzed samples collected bimonthly from March to October 2009, from the euphotic layers of the Oyashio current along the A-line in the western North Pacific. Isotopic ratios of higher trophic levels such as predatory zooplankton and/or long-lived zooplankton varied little with season, while those of short-lived zooplankton were variable on the 15 N– 13 C map. We also analyzed preserved samples taken from the warm-core ring 86-B derived from the Kuroshio extension region. Although the zooplankton groups in the two regions exhibited different values in 15 N, the 15 N versus 13 C slopes for each ecosystem do not show significant differences. Statistical analysis conducted together with previously published data from the Antarctic Ocean and the Gulf of Alaska suggested a similar 15 N versus 13 C slope throughout the four regions. We attributed this common slope to physiological aspects of feeding processes (e.g. the kinetic isotope effects inherent in the processes of amino acid synthesis). The common pattern for all four oceanic regions suggests that SIs may be used to elucidate general patterns in ecosystems and biogeochemical cycles.

Description: In the North Atlantic, Calanus finmarchicus , C. glacialis and C. hyperboreus are identified based on subtle morphological traits, which is fastidious, or based on their allegedly non-overlapping prosome length ranges. We reappraised the prosome length-based diagnosis for the copepodite stage V stage by coupling prosome length and molecular identifications (mtDNA, 16S gene) for 1159 individuals collected over 2 years from 15 stations off the Canadian coast from the Arctic to the Atlantic. We observed spatial but no intra-annual variation in species' average prosome length. At sympatric coastal sites, prosome length overlap was frequent between C. finmarchicus and C. glacialis and restricted to the Estuary/Gulf of St. Lawrence between C. glacialis and C. hyperboreus . We used discriminant analyses to redefine prosome length criteria to minimize errors in species identification. Species abundances were corrected and this affected mostly C. glacialis . In the St. Lawrence Estuary and on the Labrador shelf, abundance of C. glacialis was underestimated by 19 and 35%, respectively, with important interannual variations since 2000. This increase in abundance could enhance estimation of the role of C. glacialis in the food web and potentially alter our view of the long-term changes along the eastern Canadian coast.

Description: Zooplankton in the ocean respond to visual and hydro-mechanical cues such as small-scale shear in turbulent flow. In addition, they form strong aggregations where currents intersect sloping bottoms. Strong and predictable tidal currents over a sill in Knight Inlet, Canada, make it an ideal location to investigate biological behaviour in turbulent cross-isobath flow. We examine acoustic data (38, 120 and 200 kHz) collected there during the daylight hours, when the dominant zooplankters, Euphausia pacifica have descended into low light levels at ~90 m. As expected, these data reveal strong aggregations at the sill. However, they occur consistently 10–20 m below the preferred light depth of the animals. We have constructed a simple model of the flow to investigate this phenomenon. Tracks of individual animals are traced in the flow and a variety of zooplankton behaviours tested. Our results indicate that the euphausiids must actively swim downward when they encounter the bottom boundary layer (bbl) to reproduce the observed downward shift in aggregation patterns. We suggest that this behaviour is cued by the small-scale shear in the bbl. Furthermore, this behaviour is likely to enhance aggregations found in strong flows at sills and on continental shelves.

Description: This study focused on how phosphorus and carbon control pelagic bacteria in lakes over a gradient of dissolved organic carbon (DOC from 6.7 to 29.5 mg C L –1 ) and phosphorus (P-tot from 5 to 19 µg L –1 ). Five oligotrophic lakes in southern Sweden were sampled in late autumn. Phosphate-P and glucose-C alone or in combination (0.01 and 0.3 mg L –1 , respectively) was added to 1.0 µm filtered lake water and incubated in darkness at 20°C. Additions of glucose (C) and phosphorus (P) alone did not lead to changes in the rates of bacterial metabolic processes, whereas bacterial respiration and bacterial production responded positively to C + P enrichment for most of the lakes sampled. Bacterial growth efficiency showed a wide range (2.5–28.7%) and low mean value (12%). These variations were not correlated with the DOC concentration. Our results show that heterotrophic bacterial carbon mineralization in this kind of system during autumn is conditioned by the combined availability of labile carbon and phosphorus, with the assimilated carbon mainly transformed to inorganic carbon in respiration, contributing to CO 2 supersaturation in these systems.

Description: We exposed the small-bodied cladoceran Moina micrura either to fish (vertebrate) or to Chaoborus (invertebrate) kairomones for three consecutive generations. We hypothesized that reactions to Chaoborus should be stronger than reactions to fish kairomones. Significant effects of predators and number of generations exposed to a predator were observed in size at primipara, clutch sizes and neonate body lengths. However, stronger responses were detected in females exposed to fish. For intrinsically small-bodied species such as Moina , resource allocation shifts favoring somatic growth might not be effective against invertebrate predators. Thus, our study highlights the relevance of evolutionary and developmental constraints imposed by a small body size that prevents the formation of certain types of defences.

Description: The genus Pseudocalanus (Copepoda, Calanoida) consists of seven species, all of which are known to co-occur with two or more sibling species in some areas of their geographic ranges. Despite the ecological importance of this abundant genus, there is no available method that can reliably and accurately identify Pseudocalanus species without knowledge of origin. We present evidence of several observations of Pseudocalanus moultoni [ Frost (1989) Can. J. Zool., 67, 525–551] in fjords of Svalbard and northern Norway; this species has previously been known to occur only on the east and west coasts of North America. Patterns of DNA sequence variation of the mitochondrial cytochrome oxidase subunit I (COI) gene allow us to confidently identify the species, discriminate it from co-occurring sibling species and infer relationships among the newly discovered and previously sampled P. moultoni populations. Our observations suggest that NE Atlantic populations of P. moultoni are self-sustaining and we discuss potential source populations and pathways of transport. In light of recent reports of climate-driven shifts in distributional ranges of marine zooplankton, accurate species identification is essential for monitoring and understanding marine ecosystems.

Description: Morphological characters for distinguishing among Calanus sinicus , C. jashnovi and C. pacificus are described for all copepodite stages using specimens collected from the western North Pacific. We applied species-specific polymerase chain reaction (PCR) to identify the three species. The three species differ morphologically either in (i) prosome length (PL), (ii) length and width of caudal ramus, or (iii) shape and width of cephalosome. At copepodite stages III–VI female (CIII–CVI female), PLs of C. jashnovi were greater than those of both in C. sinicus and C. pacificus . At CIII, PL of C. jashnovi slightly overlapped with that of C. sinicus , but C. jashnovi also differed from the other species in caudal ramus length (CRL) and width (CRW). CIII–CVI females of C. sinicus and C. pacificus differed in CRL. At CI–CII, the C. sinicus cephalosome was oval-shaped, with a minor axis longer than that of C. pacificus , whereas C. jashnovi had a rhomboid rather than oval cephalosome. Species composition of Calanus copepodites was investigated at a location in the Oyashio–Kuroshio mixed region using the aforementioned morphological characters. Copepodites of C. sinicus were the most abundant of the three species in the region.

Description: The timing, sources and pathways for incorporation of larval fishes into a developing anti-cyclonic eddy off south-western Australia were investigated. Larval fish assemblages within the study region were structured by location (shelf, eddy and oceanic) and water mass. The larval fish assemblage within the eddy was significantly distinct from that characterizing the surrounding oceanic water. The eddy assemblage, which was comprised largely of larvae of oceanic meso-pelagic fishes (especially Diaphus spp. and Vinciguerria spp.) and less abundant neritic taxa, reflected its Leeuwin Current (LC), shelf and oceanic source waters. The occurrence of neritic taxa in the eddy confirmed the hypothesis that these larvae were incorporated as it developed in proximity to the shelf break. The significantly larger larval size of temperate neritic taxa (e.g. Sardinops sagax , Engraulis australis ) in the eddy compared with the shelf suggests that these larvae were transported from the shelf adjacent to the developing eddy. The occurrence of tropical neritic taxa (e.g. Acanthuridae, Lutjanidae, Pomacentridae) highlighted the LC as an important transport route to higher latitudes. Coupling the sampling of larval fishes with the trajectories of Lagrangian drifters provided insight into how larval fish assemblages changed during development of the eddy.

Description: Viruses are the most abundant biological entities in aquatic systems, infecting from bacteria to mammals. However, there has been little study so far on their impact on marine heterotrophic flagellates (HFs). For this reason, four experiments were carried out between April 2006 and February 2007 in the Blanes Bay Microbial Observatory (NW Mediterranean coast). We investigated whether viruses could affect HF communities, and specifically two uncultured marine stramenopile groups (MAST-4 and MAST-1C). For each experiment, four microcosms containing 12 L of 5-µm filtered seawater were prepared, two received active viruses and the other two received heat-inactivated viruses. Microcosms were then incubated for 48 h in order to measure changes in the abundance of the target groups. In three of the four experiments, both the growth rates of HF and MAST-4 cells and the percentage of MAST-4 cells with respect to HF after 48 h were higher in the heat-inactivated treatment compared with the active viruses treatment. These results indicate that viruses can negatively affect the HF community either directly via lysis of protists or indirectly via lysis of bacteria, and highlight the interactions between, virus, bacteria and protists.

Description: The cylindrospermopsin (CYN)-producing cyanobacterium Aphanizomenon ovalisporum is currently known from the Mediterranean area of Europe and the Middle East and from North America and Australia. Aphanizomenon ovalisporum was observed for the first time in Lake Kinneret (Sea of Galilee, Israel) in 1994. It appeared with an exceptional bloom and has persisted since then. To gain further insight into biodiversity and chemotype composition of this invasive cyanobacterium, we isolated, cultured and analyzed six putative A. ovalisporum strains from Lake Kinneret in 2010. In a polyphasic approach, the strains were investigated for their morphology and phylogeny (based on 16S rRNA gene and PC-IGS) as well as for CYN production [by enzyme-linked immunosorbent assay (ELISA) and LC-MS] and the presence of four fragments of the CYN-encoding gene cluster (aoaA/ cyrA , aoaB/ cyrB , aoaC/ cyrC and cyrJ ). Two isolated strains were assigned to A. ovalisporum , whereas four strains were identified as Anabaena bergii . Unlike previous reports of toxic A. ovalisporum from Lake Kinneret, all strains isolated in this study tested negative for CYN by ELISA and LC-MS. All strains contained aoaA/ cyrA , aoaB/ cyrB and aoaC/ cyrC fragments of the CYN gene cluster. The cyrJ fragment was not detected in any of the isolated strains. This is the first report describing non-CYN-producing A. ovalisporum strains and the presence of the cyanobacterium A. bergii in L. Kinneret.

Description: New baseline information is presented on biogeography, abundance and distribution of euphausiids from discrete depth samples collected throughout the water column to 3000 m at 42 locations along the Mid-Atlantic Ridge (MAR) between Iceland and the Azores. Eighteen species were recorded, with Euphausia krohni and Thysanoessa longicaudata being most abundant. Eight species had not been recorded in the area previously. The Subpolar Front is a northern boundary to some southern species, but not a southern boundary to northern ubiquitous species that show submergence. Four major species assemblages were identified and characterized in terms of spatial distribution and species composition. Numerical abundance was highly variable but decreased logarithmically with depth. The greatest average abundance and the greatest spatial variation in abundance occurred in northern waters, over the Reykjanes Ridge. Abundance declined less with depth in the sub-Arctic Intermediate Water/North Atlantic Current Water than in other areas and water masses. The near bottom layer (0–200 m from the seabed) had typically low density of euphausiids. The MAR is generally deeper than the usual depth distribution of euphausiids (0–300 m), but many species conduct vertical migrations to ridge crest depths. Euphausiid distribution and abundance patterns appeared only weakly affected by the MAR.

Description: This study defined the status quo of biogeographic domains and examined spatial patterns of stable isotopes (SIs) of carbon and nitrogen in relation to biophysical groupings to gain greater insight into how mesozooplankton may respond to continuous environmental change in the Canadian Arctic and sub-Arctic regions. Mesozooplankton communities were sampled during the summer of 2007 along a transect from Belle-Isle Strait, NFL, to Kugluktuk, NU (Canada), and during the early autumn of 2009 along a transect extending from Pelly Bay to Hall Beach, NU. Five broad water mass types corresponded to geographical regions. In general, we found relationships between water mass and species composition; however, this relationship was not always straightforward. Mesozooplankton community composition varied along the transect, revealing eight species assemblages. Calanus finmarchicus was abundant in the warmer and saltier Atlantic waters of the Labrador Sea, whereas Calanus hyperboreus , Calanus glacialis and Metridia longa were most abundant in the cold Arctic waters of Central Baffin Bay and in the eastern portion of the Canadian Arctic Archipelago. Nitrogen and carbon SI analysis revealed that 15 N (but not 13 C) varied spatially for C. glacialis , C. hyperboreus , Paraeuchaeta spp. and Themisto libellula . 15 N values were less enriched in Davis Strait and more enriched in the Gulf of Boothia. Seasonality, oceanic fronts and changes in the trophic structure at the base of each regional food web may explain some of the observed variability. This study represents the first broad-scale characterization of the composition and isotopic signatures for mesozooplankton communities ranging from the sub-Arctic Atlantic to the western Central Arctic Archipelago. Our study provides a baseline of the zooplankton community for monitoring species biogeographical range.

Description: To explore the influence of dispersal on community composition, we conducted an experiment to quantify the migration necessary to homogenize diverse, taxonomically distinct zooplankton communities. While the communities in unmixed controls showed divergent or null trajectories over time, dispersal magnitudes in excess of ~1% total mesocosm volume resulted in significant community convergence. These results provide the first experimentally derived threshold for dispersal, beyond which zooplankton communities begin to homogenize in composition.

Description: In a laboratory study, we determined the influence of temperature, salinity and irradiance on the growth of the paralytic shellfish poisoning (PSP) toxin producer Alexandrium catenella , which can form toxic blooms in the Thau lagoon (western Mediterranean Sea). The strain studied, ACT03, was grown in an artificial seawater medium. The influence of temperature and that of salinity were analysed using 48 different combinations of 6 salinities (10–40 psu) and 8 temperatures (9–30°C) under saturating irradiance (100 µmol photons m –2 s –1 ). ACT03 appeared to be an euryhaline strain that can survive at salinities as low as 10 psu and can grow at salinities up to 40 psu. This strain can grow between 15 and 30°C. The highest growth rates (〉0.4 day –1 ) were observed between 35 and 40 psu and 15 and 27°C. The influence of irradiance on growth and cell pigment content was tested between 10 and 260 µmol photons m –2 s –1 at 20°C and 38 psu. The results revealed both a low compensation irradiance and that light saturation was reached at 90 µmol photons m –2 s –1 . Temperature had the greatest influence on growth. The ecophysiological characteristics reported here are consistent with the environmental conditions encountered in the Thau lagoon. A. catenella exhibited important adaptive capacities over the large range of tested physical factors. This flexibility helps us to explain its ability to bloom seasonally on the Mediterranean coast, where the physico-chemical environmental conditions are characterized by high seasonal variations.

Description: Escape success of copepod nauplii (N5–6) of the copepod Acartia tonsa was significantly lower than that of copepodites (C1–2) when exposed to a coral reef fish Acanthemblemaria paula , but no difference was observed when exposed to the seahorse Hippocampus zosterae . The addition of low level turbulence inhibited feeding by the seahorse but did not affect overall capture success in A. paula , although a shift in predator behavior occurred. This study highlights the importance of considering predatory mode and behavior in the context of planktonic predator–prey dynamics.

Description: This study investigates the turnover of polysaccharides by heterotrophic bacterioplankton in the northern Bay of Biscay, a productive marine system on the continental margin of the temperate Atlantic Ocean. Bacterial biomass production (BBP) near the surface ranged from 0.5 to 25.7 nmol C L –1 h –1 during small phytoplankton blooms in May and June that occurred after the main spring bloom. A direct relationship between BBP and total polysaccharides strongly suggests the dependence of bacterial growth on the availability of semi-labile organic matter. Concentrations of combined glucose as well as rate constants of extracellular glucosidase activity and glucose uptake were determined to estimate the actual carbon fluxes from bacterial polysaccharide turnover. Results reveal that the degradation of polysaccharides in the upper 100 m of the water column sustained a glucose flux of 15.2–32.3 mg C m –2 d –1 that was available for bacterial consumption. The mean turnover time for polysaccharides was 170 and 165 days for α - and β -glycosidic linked polymers, respectively. Incorporation of free glucose supported 0.4–19.6% of BBP. The availability of nitrate plus nitrite (NO x ) was identified as one factor increasing bacterial incorporation of glucose in most samples. Our results demonstrate that the bacterial recycling of polysaccharides generated a significant flux of organic carbon in microbial food-webs and biogeochemical processes.

Description: DNA-based molecular methods such as clone libraries constructed of 18S rRNA amplicons have been increasingly employed in surveys of marine protist communities but are known to have underlying biases, and especially haptophytes are known to be underestimated in eukaryotic clone libraries. Using controlled mixtures of laboratory strains, we show that the recovery of organisms in clone libraries is both influenced by their concentration in the environment as well as bias exerted by certain co-occurring organisms.

Description: We investigated the space–time dynamics of picophytoplankton in South Australian continental shelf waters from February 2008 to January 2009, focusing on localized physical events. We discriminated six picophytoplankton populations by flow cytometry, including Synechococcus (SYN1, SYN2), Prochlorococcus (PROC1, PROC2) and small and large picoeukaryotes (EUKS, EUKL). Local physical events observed included downwelling and dense waters outflowing from a nearby gulf in winter–early spring 2008, upwelling in summer and early spring 2008 and eddy formation in January 2009. Each population responded differently to these events, which resulted in up to four orders of magnitude changes in their abundances. Population-specific hotspots reflected a succession of distinct dominant communities associated with the strength of upwelling events, changes in fluorescence maximum depths and local downwelling and mixing processes. The unexpected high abundances and local dominance of Prochlorococcus in summer reflected the possible influence of eastward and westward current transports and the presence of a High-Light (PROC1)- and Low-Light (PROC2)-adapted ecotypes. This study highlights the role of localized physical events in the dominance of all three picophytoplankton groups that may be critical for the high productivity of the study region, and suggests the importance of hydroclimatic forcing for inter-annual changes in picophytoplankton communities.

Description: Dense localized blooms of the toxic dinoflagellate Alexandrium ostenfeldii are a recent phenomenon in the low salinity waters of the Baltic Sea. This study reports results from laboratory experiments investigating the interaction between PSP toxin producing strains of A. ostenfeldii and two copepod species, Eurytemora affinis and Acartia bifilosa , from the northern Baltic Sea. Copepod grazing rates were studied by incubating the copepods with mixed cultures of A. ostenfeldii isolated from a bloom area. The effect of A. ostenfeldii on copepod condition and behavior was studied by exposing copepods to the mixed cultures and cell-free filtrates. Grazing on A. ostenfeldii cells was negligible for both copepod species. The 24 h incapacitation incubations indicated that the condition of both copepod species was negatively affected by the presence of A. ostenfeldii . The behavioral changes observed took place during the first hours of exposure to A. ostenfeldii cells, and little or no change was observed in the condition of the copepods after 6 h compared to 24 h. Similar observations were made for E. affinis incubated in cell-free filtrates of A. ostenfeldii , while the response of A. bifilosa was less pronounced. Our study shows that the copepods do not graze on A. ostenfeldii due to rapid behavioral disturbance and incapacitation by the algal cells and their exudates. This represents the first observation of a negative effect of the A. ostenfeldii on co-occurring biota in the northern Baltic Sea.

Description: This study uses Laser Optical Plankton Counter (LOPC) data to study the vertical distribution of the copepod Calanus finmarchicus in relation to hydrography in the NE Norwegian Sea, in January 2009 and January 2010. The high sampling efficiency of the LOPC produced data with higher vertical resolution and allowed a larger number of stations to be sampled in a smaller area compared with previous studies. There were no significant differences between 2009 and 2010 in any of the quantities, median depth, C. finmarchicus layer thickness and temperature and salinity at median depth, indicating that the vertical distributions for the two different years were overall very similar. There was, however, considerable spatial variability in median overwintering depth, and this was strongly correlated with the depth of the overlying Atlantic Water layer. We also observed a significant correlation between C. finmarchicus layer thickness and the mean abundance in the layer, indicating that a vertical stretching and squeezing of the copepod distribution took place in the area. These results suggest that copepods were subject to vertical movement caused by local hydrodynamic variability which is in addition to other vertical motion due to e.g. buoyancy forces. Finally, the results are discussed with respect to future sampling strategies in the area.

Description: We investigated the effect of suspended sediments on the vital rates of the copepods Calanus finmarchicus , Pseudocalanus sp. and Metridia longa in a Greenland sub-Arctic fjord. The fjord had a gradient of suspended particulate matter (SPM) with high concentrations (〉50 mg L –1 ) in the inner fjord due to glacial melt water runoff. Laboratory experiments showed that when feeding on the diatom Thalassiosira weissflogii specific ingestion rates were low at high concentrations of suspended sediment for C. finmarchicus (〉20 mg L –1 ) and Pseudocalanus sp. (〉50 mg L –1 ), while no effect was found for M. longa . For C. finmarchicus , a relatively constant fecal pellet production (FPP) and fecal pellet volume suggested ingestion of sediment, which probably led to reduction in egg production rates (EPRs) at high sediment concentrations. For Pseudocalanus sp., FPP decreased with increasing sediment concentrations, while no effect was observed on EPR. No significant difference was observed in FPP for M. longa feeding on the diatom T. weissflogii compared to the ciliate Strombidium sulcatum . The study shows that high sediment concentrations influence the capability of carbon turnover in C. finmarchicus and Pseudocalanus sp., while M. longa appears to be more tolerant to high sediment loads. Therefore, high concentrations of SPM could potentially influence the species composition of glacially influenced fjords.

Description: Comparative phylogenetic analysis of non-marine planktonic picocyanobacterial isolates (based on the 16S ribosomal RNA gene and the phycocyanin operon) revealed that the geographic restriction of several clades is the outcome of under-sampling or database search bias. This result supports the high dispersive potential of planktonic picocyanobacteria and emphasizes that endemic genotypes do not seem as widespread as suggested by the latest publications.

Description: Dormant egg banks provide a means for many species to survive adverse conditions. This study describes a short-term experiment evaluating predation by the notostracan Triops cancriformis on dormant eggs buried in sediment, i.e. Daphnia ephippia and Triops dormant eggs (cannibalism). Significant predation was recorded on both dormant stages, with no apparent selection. These results indicate that Triops could eradicate the dormant eggs in surface sediments and thereby impact the benthic-pelagic coupling in aquatic systems.

Description: Behaviour and life-history strategies of zooplankton have evolved in response to seasonal cycles in food availability, predation risk and abiotic conditions. A key challenge is to understand how different activities over the year are linked. For instance, how does a change in spring activities, such as the timing or amount of egg production, influence autumn activities, for instance energy storage or migration? Trade-offs viewed in relation to individual lifetime fitness consequences couple these events. The framework of optimal annual routines provides theory and methodology for consistent analyses of these temporal trade-offs. Here I describe the key parts of optimal annual routine models and how the models can be used to: (i) study phenology, life-history strategies, and population dynamics; (ii) predict responses to environmental change; and (iii) guide future zooplankton studies. I mainly discuss the adaptations of zooplankton species inhabiting high latitude oceans where the seasonal cycle and its effects are particularly strong. Empirical challenges include issues of seasonal resolution, state-dependent processes and individual variability. Two ecological problems with avenues for future work are discussed in particular detail: the role of sea ice and ice algae in the life cycle of copepods and krill, and the adaptive value and ecological consequences of semelparous versus iteroparous reproductive strategies.

Description: Limnocalanus macrurus from Bothnian Bay in the northern part of the Baltic Sea was studied during the ice-free period (April–December) in order to understand its life history and feeding biology. Our data on the population dynamics indicated that reproduction occurred during the ice-covered period, during which lipid storage was reduced to a minimum. From spring to late summer, the lipid reserve increased by a factor of 3, while the gonads of adult females were immature during this period, continuing to December as indicated by the small size of the eggs. Average stomach fullness was always ca. 50% indicating continuous feeding activity. A newly developed denaturing high-performance liquid chromatography method was used to analyse the gut contents over the study period. More than 30 taxa (at different taxonomic levels) could be identified. However, phytoplankton was only represented by one taxon (Diatomophycea), and was restricted to July. Thus, adult L. macrurus seems to have a strongly carnivorous feeding preference in the northern Baltic Sea.

Description: While the importance of biotic interactions in shaping zooplankton communities has long been established in lentic environments, river plankton is traditionally thought to be primarily abiotically constrained. The role of interactions among zooplankters has thus only occasionally been examined and mainly regarded as secondary or negligible in rivers. This study addresses the question whether and under which conditions biotic interactions within the zooplankton can become the chief drivers of community structure and dynamics in riverine environments. A short-term, high-frequency sampling campaign was performed in the potamal reach of the Po River (Italy). To test for the presence of association patterns among zooplankters, which might be suggestive of potential trophic interactions (predation/competition), taxa were aggregated into functional groups according to their feeding ecology, and time series of their abundances were analysed by means of a variance ratio test, which revealed the occurrence of compensatory dynamics among functional groups under low and stable, although truly advective, discharge conditions. Evidence on the importance of predator–prey interactions and intra-population regulation mechanisms also came from further analyses of the dominant predator's gut contents and of the main prey population dynamics. Our results show that, under certain conditions, zooplankton exhibit internal, self-regulatory mechanisms also in the main current of large rivers. The view of riverine zooplankton as a mere assemblage of taxa exclusively abiotically controlled is therefore oversimplified.

Description: We describe a climate-driven range expansion of the red-tide dinoflagellate Noctiluca scintillans into the Southern Ocean (45°31'S 147°E). Sea surface height data showed that a warm-core eddy moving southwards from Tasmania was the potential vector for the transport of Noctiluca . We provide evidence for active feeding of Noctiluca on Southern Ocean phytoplankton. Possible competition with other grazers may have implications for food web dynamics were Noctiluca to become established in the Southern Ocean.

Description: A prototype shipboard filter skid (p1SFS) was designed and built to facilitate shipboard collection and concentration of ≥50 µm planktonic organisms from large volumes of water. The p1SFS consisted of two stainless steel filter housings, each containing a filter bag, arranged in parallel. Validation of the device examined the particle retention efficiency of filter bags (using inert polystyrene microbeads), the potential toxicity of filter skid materials and the capture efficiency of plankton collected with a filter skid versus a plankton net at two sample volumes (5 and 10 m 3 ). Microbead recovery in filter bags was 〉89 and 100% for microbeads of 50 and 150 µm diameters, respectively. Exposure to the sealant used to close the filter bags' seams or stainless steel did not lead to mortality of two model zooplankton species. Overall, the concentration of ≥50 µm plankton in the p1SFS relative to concentrations in the plankton net (i.e. the capture efficiency, CE) was 108 ± 66% (mean ± 1 SD, n = 6). The p1SFS CE was higher in experiments with 5 m 3 sample volume (147 ± 74%; n = 3) relative to experiments with a 10 m 3 sample volume (69 ± 28%, n = 3), although the difference in CE between the sample volumes was not significant. Consequently, these experiments suggest this or similarly validated filter skids are appropriate for in-line sampling of plankton from relatively large volumes of water.

Description: The community-wide trophic cascade due to collapsed fish stocks and population outburst of invasive predators after the early 1990s resulted in different pelagic and benthic food web structures in the Black Sea. The northeastern region assumed their most simplified forms characterized by increasing mesozooplankton and polychaete biomass. The ecological degradation is more severe in the northwestern part, whereas the southern basin maintains an anchovy fishery under relatively weak gelatinous predation.

Description: We studied diel vertical migration (DVM) of the six copepodite stages of two of the most abundant crustacean zooplankton in the Baltic Sea, the calanoid copepods Eurytemora affinis and Acartia spp. The study was conducted monthly from May through October in a bay in the northwestern Baltic proper. Fish biomass, phytoplankton abundance and temperature were obtained in conjunction with the zooplankton sampling. Both copepod species performed DVM. With the exception of females, all E. affinis copepodite stages performed migrations of over 10 m with only a slight increase with the copepodite stage. Adult female E. affinis remained at depth with only slight upward movement at night. In Acartia spp., DVM amplitude increased with stage and size, suggesting an ontogenetic shift in behaviour; although they had a less pronounced DVM than E. affinis. Although DVM amplitude increased with size, indicative of visual predation, fish biomass did not correlate with the amplitude of DVM. However, fish were present throughout the study period. We surmise that these ontogenetic shifts in behaviour are due to size increase and therefore visibility to predators and that the difference in DVM between the species may well be a result of physiological differences and reproductive strategy.

Description: Denaturing high-performance liquid chromatography (DHPLC) is a relatively new method for separating amplicons in a mixture, and was recently developed for parasite detection in the blue crab Callinectes sapidus . That assay used a peptide nucleic acid (PNA) PCR hybridization blocking probe (PNA–PCR–DHPLC) to decrease the generic PCR bias of dominant templates (the host) in the mixture prior to separation on the DHPLC column, thus enhancing the less abundant parasite DNA. The same assay and rational can be used to investigate predator–prey interactions. However, in ecosystem studies with many predator–prey relationships, development of specific PNA-blocking probes for each predator would be too laborious. Here, we have developed a PCR–DHPLC assay excluding the dominant predator amplicons in a first DHPLC run, followed by re-amplification of the non-predator retention volumes and further separation and characterization in a second DHPLC run. This assay generated data on the specific trophic interactions between the calanoid copepod Limnocalanus macrurus and its prey from a seasonal sampling programme. The assay provides an efficient way for an unbiased screening of predator–prey relationships, and although developed for L. macrurus in this study, the approach has wide applicability for any predator–prey interaction.

Description: Dilution grazing experiments are the most commonly used method for quantifying microzooplankton grazing rates on phytoplankton. However, in some cases, the outcomes of these experiments yield non-significant grazing impacts, even when microbial grazers are numerous, or result in unexpected non-linear functions or positive slopes. Here, we argue that some of these situations and others that may go unnoticed can be explained by trophic cascades occurring in the community confined within the incubation experiments. When trophic cascades are present, simple linear models relating dilution levels to phytoplankton net growth rates can be distorted into artificial saturation curves, V-shaped responses or a non-significant fit and may even invert the sign of the slope. These artefacts have minor consequences for true estimates of phytoplankton mortality rates when the dilution level is sufficiently high (ca. 5–10%), but they may result in significant underestimation of the microzooplankton grazing impact when working at lower, typical maximum dilution levels (e.g. 25%). These underestimations can be particularly important when the microzooplankton community is already under feeding-saturated conditions.

Description: Phytoplankton nutrient limitation patterns were investigated in an anthropogenically influenced coastal ecosystem with a high nutrient load. Weekly nutrient limitation bioassays, and water chemistry and stoichiometry measurements were performed in the Bay of Brest, France, from February to July 2011. Each limitation bioassay included phosphorus, nitrogen, silicate and all possible combined nutrient additions, and lasted over 72 h. Results showed that the phytoplankton community experienced a general P limitation from March to July. N limitation alone was observed only during 1 week in early March. Subsequently, all limitation bioassays revealed primary P limitation, indicated by significantly increased growth rates in all samples containing P additions. Besides P, Si was the second limiting nutrient for phytoplankton during 2 weeks at end of May to mid-June. Seston C:P ratios ranged between 198:1 and 749:1, and N:P ratios between 28:1 and 104:1. The highest effects on phytoplankton growth were reached with NPSi addition, suggesting synergistic macronutrient effects under single nutrient limitation. Our results demonstrate that dissolved nutrient ratios are not reliable in predicting actual phytoplankton nutrient limitation, since other processes and organisms strongly influence nutrient availability and resupply. The dominant P limitation emphasizes its importance for consideration in coastal water management.

Description: Because we found previously that Chlamydomonas reinhardtii produces allelochemicals to which the rotifer Brachionus calyciflorus is sensitive, we explored its effects on other freshwater plankton. We used Chlamydomona s under light-, nitrogen- and phosphorus-limitation to test its allelopathic effect on Microcystis aeruginosa , Cryptomonas ozolinii , Ochromonas danica , Tetrahymena thermophila and Paramecium aurelia . Allelopathy depended strongly on the target organism. Only Cryptomonas suffered a marked negative effect. Among the resource limiting regimes, light limitation exerted the greatest effect.

Description: The effect of ambient medium viscosity on the motility and flagellar beating motion of the dinoflagellate Prorocentrum minimum was experimentally investigated. Prorocentrum minimum is a thecate desmokont dinoflagellate with two flagella that emerge from the flagellar pore at the anterior end of the cell. A digital holographic particle tracking velocimetry technique was employed to capture the swimming trajectories of hundreds of P. minimum cells and to extract the kinematic motion parameters. The viscosity of the surrounding medium was changed from 1.12 cP (natural sea water at 22°C) to 11.31 cP by adding methylcellulose to increase fluid viscosity. In natural sea water, the swimming speeds of P. minimum cells ( n = 77) in a helical motion ranged from 5.4 to 138.4 µm/s with a mean speed of 51.3 ± 27.9 µm/s. The helix radius and pitch of the swimming trajectories are 3.08 ± 0.64 and 25.34 ± 7.96 µm/s, respectively. The longitudinal flagellum beats with a planar wave at a beating frequency of 87.10 ± 10.96 Hz. On the other hand, the transverse flagellum beats with a helical wave at a beating frequency of 45.38 ± 13.61 Hz. As the ambient medium viscosity increases, the beating frequency of flagella decreased and consequently, the swimming speed of P. minimum was reduced.

Description: This research focused on the specific diversity of diatoms carried in ballast sediments of ships reaching Canadian ports on the Pacific, Atlantic and the Great Lakes during 2007 and 2009. The inventory of 180 taxa included Thalassiosira , Chaetoceros spores, Cyclotella , Actinocyclus , Aulacoseira , Melosira and Pseudo - nitzschia as the most species-rich genera found in 142 samples of the matter settled at the bottom of ship tanks. We also tested and showed evidence that diatom species composition identified in any given tank was the product of: (i) species intrinsic survival strategies, (ii) the most recent ballast water exchange (BWE), and (iii) the cumulative end result of past ballast operations. Multivariate analysis reduced our data set to three assemblages. Propagule pressure of these potentially colonizing assemblages may be enhanced due to repeated introduction attempts by ships belonging to different ballast management categories (transoceanic, intra-coastal with our without BWE) but that actually carried the same assemblage type. A non-linear relationship between colonization and propagule pressure confirms that the early stages of invasion (ballast uptake, survival in the tank) structure the later stages. Coastal floras in tanks that underwent offshore BWE (particularly persisting freshwater species) corroborate concern with the low efficiency of this management option.

Description: Egg development times (DTs) of copepods are used to scale DTs of life stages to temperature and to calculate egg production rate in sac spawners. Here, we report egg DTs at various temperatures for two copepod species from the upper San Francisco Estuary measured using video observations of individual egg sacs. The egg DTs (days) of Eurytemora affinis and Pseudodiaptomus forbesi were related to temperature ( T , °C) by DT = 11.5e –0.106T and DT = 22.7e –0.121T , respectively. Video observations decreased manpower requirements, increased the frequency of observations and provided more detailed observations of individual hatching patterns compared with historical techniques including direct observations and regression analysis.

Description: Euphausia pacifica occupy a remarkably wide range of ecosystems. We examined the differences in brood sizes from eight regions of the North Pacific to explore how variation in the brood size might contribute to their success over such a range. We show a compilation and analysis of short-term E. pacifica brood size incubations from around the North Pacific rim, ranging from the Yellow Sea (YS) (Korea) to southern California (SC), USA. Brood sizes for E. pacifica are highly variable in all regions and range from 5 to 697 eggs overall. Median brood sizes were largest in the YS and off Newport, Oregon and the maximum brood sizes were greatest in the regions off the coasts of Oregon and Washington, USA followed by the YS. Quantile regression analysis revealed that there was a significant relationship between the female length and the maximum brood size (95% quantile) for broods from SC, Oregon, and Washington, USA as well as Toyama Bay, Japan, indicating that the maximum brood size is limited by the female length in these regions. This suggests that other factors, hydrographic conditions, food quantity/quality or physiological condition of females, may limit maximum brood sizes in the other regions examined here.

Description: This is the first characterization of the structure and temporal variation of the plankton communities comprising the complete food web in five peat bog pools related to environmental factors over two consecutive ice-free periods in Tierra del Fuego (54°S). Remarkably, picophytoplankton was composed solely of eukaryotic cells, surpassing the dominance expectations for these acidic water bodies, whereas testaceans were virtually absent, even as tychoplankters. Abundances of the different planktonic communities were slightly higher than those reported for Northern Hemisphere peat bogs and humic lakes. Mixotrophic nutrition prevailed among nano- and microphytoplankters, a strategy also common in humic lakes. The structures in spring of the planktonic communities were similar. In contrast, in late summer there were differences in the abundance and biomass of the different trophic compartments among small, shallow water bodies and large ones. These seem to be dictated by distinct pool size-driven patterns of water temperature variation. A general shift in the control of heterotrophic flagellates abundance in the pools occurred, changing from bottom-up regulation in spring to top-down control in late summer related to temperature-dependant variations in zooplankton abundance and composition. We hypothesize that changes in the trophic interactions affecting the entire food web occur over the open water period in these aquatic ecosystems, and that these are dictated by pool morphometry and related abiotic features.

Description: We analysed the structure of the microbial plankton communities of different types of freshwater environments from the southernmost region of South America (Tierra del Fuego). Water bodies were grouped in four categories: humic lakes, clear oligotrophic lakes, beaver ponds and steppe shallow lakes, which differed in their nutrient and dissolved organic carbon (DOC) contents. We tested if microbial planktonic communities were different among lakes with dissimilar nutrient and DOC concentrations, analysing to what extent the known large-scale patterns of lake trophic structure applies to a diverse but localized set of lakes. We found that mixotrophs dominated over strict autotrophs in both humic and clear oligotrophic systems, whereas in eutrophic lakes autotrophy was a successful strategy. The functional phytoplankton approach also allowed the separation between oligotrophic (clear and humic) and eutrophic systems, with different functional groups. The lowest abundances of picoplankton were found in oligotrophic lakes, picoeukaryotes being more abundant than picocyanobacteria in beaver ponds and humic lakes. Our results show that in low nutrient environments, mixotrophic strategies thrive over strict autotrophs suggesting the paramount importance of the microbial loop when compared with high trophic status systems where the prevalence of autotrophy indicates that the energy flux depends on phytoplankton.