Large Zooplankton Research Articles

Phytoplankton, periphyton, and zooplankton patterns in the pelagic and littoral regions of a large subtropical shallow lake

This study aimed at understanding the patterns among size structure of the phytoplankton, periphyton, and zooplankton considering the spatial variability (pelagic and littoral zones) in a subtropical large shallow polymictic lake (Lake Mangueira, southern Brazil). Water samples were gathered at intervals of 3, 5, and 15 days, for 60 days during summer 2012, for physical, chemical, and biological analyses (abundance and maximum linear dimension of subsurface zooplankton, phytoplankton, and glass slide-colonized periphyton) in both lake zones. A summer storm on the 20th day disturbed the periphyton succession in the littoral and pelagic zones. Phytoplankton size classes varied similarly to the smaller size of zooplankton in the pelagic zone, whereas most of periphyton size classes coincided with the larger zooplankton in the littoral zone, evidencing that zooplankton in Lake Mangueira may exploit both communities and both compartments (littoral and pelagic zones). The study demonstrated that the patterns of those communities are dynamic and closely related to the environmental variability in Lake Mangueira.

The Impact of Anthropogenic Disturbance on Bacterioplankton Communities During the Construction of Donghu Tunnel (Wuhan, China).

Bacterioplankton are both primary producers and primary consumers in aquatic ecosystems, which were commonly investigated to reflect environmental changes, evaluate primary productivity, and assess biogeochemical cycles. However, there is relatively less understanding of their responses to anthropogenic disturbances such as constructions of dams/tunnels/roads that may significantly affect the aquatic ecosystem. To fill such gap, this study focused on the bacterioplankton communities' diversity and turnover during a tunnel construction across an urban lake (Lake Donghu, Wuhan, China), and five batches of samples were collected within 2months according to the tunnel construction progress. Results indicated that both resources and predator factors contributed significant to the variations of bacterioplankton communities, but the closed area and open areas showed different diversity patterns due to the impacts of tunnel construction. Briefly, the phytoplankton, TN, and TP in water were still significantly correlated with the bacterioplankton composition and diversity like that in normal conditions. Additionally, the organic matter, TN, and NH4-N in sediments also showed clear effects on the bacterioplankton. However, the predator effects on the bacterioplankton in the closed-off construction area mainly derived from large zooplankton (i.e., cladocerans), while small zooplankton such as protozoa and rotifers are only responsible for weak predator effects on the bacterioplankton in the open areas. Further analysis about the ecological driving forces indicated that the bacterioplankton communities' turnover during the tunnel construction was mainly governed by the homogeneous selection due to similar environments within the closed area or the open areas at two different stages. This finding suggests that bacterioplankton communities can quickly adapt to the environmental modifications resulting from tunnel construction activities. This study can also give references to enhance our understanding on bacterioplankton communities' response to ecological and environmental changes due to intensification of construction and urbanization in and around lake ecosystems.

Diet source of Euphausia pacifica revealed using carbon- and nitrogen-stable isotopes in the Yellow Sea Cold Water Mass in summer

Euphausia pacifica (E. pacifica) is an important food source for fish and other organisms in marine ecosystems. We analyzed the carbon- and nitrogen-stable isotope ratios in potential diet sources to determine the feeding characteristics of E. pacifica in the Yellow Sea Cold Water Mass (YSCWM), a habitat for oversummering. E. pacifica was collected in the spring and summer from the central and coastal parts of the Yellow Sea from 2012 to 2014, and potential prey were collected in summer 2014. Although the δ13C values of E. pacifica differed significantly in spring each year, they narrowed to a specific range in summer. The δ15N values were always 2‰ higher in summer than in spring, suggesting that the diet of E. pacifica inhabiting the YSCWM was limited. In coastal stations, the contribution of any major food source calculated using an isotope mixing model was not more than 20%, suggesting that E. pacifica is omnivorous. However, at two stations in the YSCWM, 2.0–5.0-mm and > 5-mm plankton contributed 42 and 22% of their diet on average, respectively. These results indicate that E. pacifica inhabiting waters affected by cold water feeds on relatively large zooplankton such as the copepods Calanus sinicus. Therefore, the YSCWM likely plays a role in the trophic shift of E. pacifica during the summer.

Zooplankton, zooplanktivorous fish and their interactions in Southeast Asian waterbodies with special reference to Sri Lanka: a review

Many countries in Southeast Asia have no natural lakes but reservoirs, flood-plains and rivers. The reservoirs are mainly inhabited by riverine fish species and introduced pelagic exotics. Benthivores, herbivores and detritivores dominate, whereas zooplanktivores are relatively rare (generally 4.6-9.5% of the fish community). Therefore, zooplanktivorours fish are not dominant in most Sri Lankan reservoirs and play only a minor role in the food web. In the Southeast Asian region, only small, often pelagic, zooplanktivorous fish species occur. They are either of riverine or marine origin. Observations were made that only six predominantly zooplanktivorous species viz a cyprinid from riverine origin Rasbora daniconius, a half beak from marine origin Hemiramphus limbatus, the glass perchlet Ambassis urotaenia and three freshwater clupeids from marine origin (Clupeichthys aesarnensis, Ehirava fluviatilis, Sardinella tawilis) inhabit the reservoirs in Southeast Asian countries such as Sri Lanka and Thailand and a lake in the Philippines. The highest percentages of zooplanktivory in the tropical region are observed in waterbodies which contain zooplanktivores of marine origin. These small pelagics are very productive, i.e., have relative high Annual Production/Biomass (P/B) ratios. P/B ratios of the small Southeast Asian pelagics ranged generally from 3-6, small clupeids showed the highest P/B ratios. Of the riverine species, R. daniconius is the most successful; it is a common species in the Sri Lankan reservoirs. This species is, however, not able to catch the small cladocerans and the cyclopoid copepods efficiently and fails completely to catch the fast moving calanoid copepods. Instead of feeding on larger zooplankton, they often feed on adult winged midges which are floating on the water surface. H. limbatus is not abundant in Sri Lankan reservoirs and its distribution is limited where only occasionally its densities are high enough to permit a profitable fisheries. Although the freshwater clupeids seem to be superior in their efficiency of collecting food organisms, their distribution is often limited. Despite their ability to build up stable and large populations in a handful of water bodies in the Philippines and Sri Lanka, they have a problem in dispersing into other freshwater lakes and reservoirs. In Thailand, the situation is different because C. aesarnensis is well adapted to riverine conditions and, therefore, present in all reservoirs with inflowing riverine habitats, but in most cases these stocks are still unexploited.

Effects of Extreme Hydrologic Regimes on Juvenile Chinook Salmon Prey Resources and Diet Composition in a Large River Floodplain

AbstractClimate and hydrologic variability are defining characteristics of California rivers. Recently, the region experienced an unprecedented drought, and the probability of similarly warm, dry conditions is predicted to increase. In addition to warming air and water temperatures, climate change projections predict increased flooding and sea level rise, likely aggravating the water resource issues that already challenge the western United States. Water managers balance many public interests, including the conservation of native fishes, such as the Chinook Salmon Oncorhynchus tshawytscha. Given projected changes in climate and hydrology, there is an urgent need to understand how salmon respond to these conditions. In this study, we examined how young salmon responded to extreme drought (2012–2015) versus flood (1998–1999) conditions in the Yolo Bypass, a floodplain–tidal slough complex of the Sacramento River, California. We found that the diets of juvenile Chinook Salmon were dominated by aquatic–riparian insects during flooding and were dominated by zooplankton during the drought. Although juvenile salmon that were caught during the drought seemed to have eaten a higher number of prey items on average, they also had higher metabolic costs. Therefore, it is likely that juvenile salmon must augment their foraging behavior to offset higher temperatures and prey shifts. Finally, preferentially consumed, calorically valuable prey (i.e., larger zooplankton and aquatic–riparian insects) have become rare due to habitat degradation and biological invasions, and resource managers must consider re‐establishing productive off‐channel habitats, such as riparian corridors, floodplains, and wetlands.

The release and regulation of rotifers: examining the predatory effects of invasive juvenile common and bighead carp

Ecosystem level effects of common (Cyprinus carpio) and bighead carp (Hypophthalmichthys nobilis) have generally focused on adult life stages. The objective of our mesocosm study was to investigate and contrast the roles of juvenile common and bighead carp in structuring planktonic invertebrate assemblages, with focus on rotifers. We examined whether predation by juvenile carp was indiscriminate or size-selective with respect to prey size. Furthermore, we examined how changes to large and small prey influenced the potential for compensatory increases of some taxa within prey assemblages. Both species of juvenile carp reduced large zooplankton taxa. However, rotifer responses were variable depending on the taxon and predator combination. Juvenile common carp enhanced abundance for Polyartha and Squatinella, but most taxa were unaffected. Juvenile bighead carp had a more varied effect on rotifer abundance, having no effect on most, reducing Keratella and enhancing Anuraeopsis. We also estimated net filtration volume of the zooplankton community for each of the treatments and found partial compensation in net filtration because of the increased abundance of a few rotifer taxa, but this reduction did not match the depletion of macrozooplankton. Rotifers that benefitted from the presence of fish predators likely responded positively because of reduced predation by mesopredators, because of their short generation times, and/or from reduced competition.

Coral Food, Feeding, Nutrition, and Secretion: A Review.

Tropical scleractinian corals are dependent to varying degrees on their photosymbiotic partners. Under normal levels of temperature and irradiance, they can provide most, but not all, of the host's nutritional requirements. Heterotrophy is required to adequately supply critical nutrients, especially nitrogen and phosphorus. Scleractinian corals are known as mesozooplankton predators, and most employ tentacle capture. The ability to trap nano- and picoplankton has been demonstrated by several coral species and appears to fulfill a substantial proportion of their daily metabolic requirements. The mechanism of capture likely involves mucociliary activity or extracoelenteric digestion, but the relative contribution of these avenues have not been evaluated. Many corals employ mesenterial filaments to procure food in various forms, but the functional morphology and chemical activities of these structures have been poorly documented. Corals are capable of acquiring nutrition from particulate and dissolved organic matter, although the degree of reliance on these sources generally has not been established. Corals, including tropical, deep- and cold-water species, are known as a major source of carbon and other nutrients for benthic communities through the secretion of mucus, despite wide variation in chemical composition. Mucus is cycled through the planktonic microbial loop, the benthos, and the microbial community within the sediments. The consensus indicates that the dissolved organic fraction of mucus usually exceeds the insoluble portion, and bothserve as sources for the growth of nano- and picoplankton. As many corals employ mucus to trap food, a portion is taken back during feeding. The net gain or loss has not been evaluated, although production is generally thought to exceed consumption. The same is true for the net uptake and loss of dissolved organic matter by mucus secretion. Octocorals are thought not to employ mucus capture or mesenterial filaments during feeding and generally rely on tentacular filtration of weakly swimming mesozooplankton, particulates, dissolved organic matter, and picoplankton. Nonsymbiotic species in the tropics favor phytoplankton and weakly swimming zooplankton. Azooxanthellate soft corals are opportunistic feeders and shift their diet according to the season from phyto- and nanoplankton in summer to primarily particulate organic matter (POM) in winter. Cold-water species favor POM, phytodetritus, microplankton, and larger zooplankton when available. Antipatharians apparently feed on mesozooplankton but also use mucus nets, possibly for capture of POM. Feeding modes in this group are poorly known.

Influence of environmental factors on vertical distribution of zooplankton communities in humic lakes

The influence of vertical environmental gradients on zooplankton communities was studied in five humic lakes with the high availability of food resources (phytoplankton and bacterioplankton) and low fish pressure. The factors that inhibit the development of large zooplankton in humic lakes are currently widely debated. We have found that relatively productive humic lakes do not offer many niches for zooplankton because of the sharp thermal gradient which results in a shallow layer of oxygenated waters. The results of this study indicated that different taxonomic groups of zooplankton are determined by a different set of environmental variables. This phenomenon explains very low species richness of zooplankton and a possibility of their coexistence in the narrow oxygenated layer. We concluded that due to sharp thermal gradient in humic lakes biomass of herbivores may be reduced which could promote development of phytoplankton.

Characterisation of large zooplankton sampled with two different gears during midwinter in Rijpfjorden, Svalbard

Abstract During a midwinter cruise north of 80°N to Rijpfjorden, Svalbard, the composition and vertical distribution of the zooplankton community were studied using two different samplers 1) a vertically hauled multiple plankton sampler (MPS; mouth area 0.25 m2, mesh size 200 μm) and 2) a horizontally towed Methot Isaacs Kidd trawl (MIK; mouth area 3.14 m2, mesh size 1500 μm). Our results revealed substantially higher species diversity (49 taxa) than if a single sampler (MPS: 38 taxa, MIK: 28) had been used. The youngest stage present (CIII) of Calanus spp. (including C. finmarchicus and C. glacialis) was sampled exclusively by the MPS, and the frequency of CIV copepodites in MPS was double that than in MIK samples. In contrast, catches of the CV-CVI copepodites of Calanus spp. were substantially higher in the MIK samples (3-fold and 5-fold higher for adult males and females, respectively). The MIK sampling clearly showed that the highest abundances of all three Thysanoessa spp. were in the upper layers, although there was a tendency for the larger-sized euphausiids to occur deeper. Consistent patterns for the vertical distributions of the large zooplankters (e.g. ctenophores, euphausiids) collected by the MPS and MIK samplers provided more complete data on their abundances and sizes than obtained by the single net. Possible mechanisms contributing to the observed patterns of distribution, e.g. high abundances of both Calanus spp. and their predators (ctenophores and chaetognaths) in the upper water layers during midwinter are discussed.

Dynamics of cyanobacteria blooms are linked to the hydrology of shallow Florida lakes and provide insight into possible impacts of climate change

Analysis of an 18-year dataset from seven shallow lakes in Florida provides evidence of hydrologic factors controlling dynamics of cyanobacteria blooms. Depth was the most important variable, and there was a synergistic effect with flushing. When the lakes were deep, rainfall led to water discharges and disruption of stagnant conditions that maintained blooms. Factors commonly associated with bloom formation (temperature) or suppression (high organic color, which can be a light-limiting factor) did not play a significant role. A conceptual model illustrates how blooms are regulated. First, the presence of bloom-forming species, high concentrations of nutrients, and lack of large zooplankton create a potential for blooms. Second, climate cycles influence yearly rainfall, resulting in wet years and droughts that affect lake water level and volume. This either stimulates blooms (drought, shallow) or suppresses them (wet, deep). Finally, proximal drivers such as a rain events and a flush of water suppress blooms. The results illustrate how climate variability and weather can control cyanobacteria blooms by affecting several processes long known to influence bloom formation, and provide insight into how climate change might affect occurrence of blooms in nutrient-enriched shallow lakes.

Evidence of Diel Vertical Migration of Mesopelagic Sound-Scattering Organisms in the Arctic

While sound scattering layers have been described previously from ice-covered waters in the Arctic, the existence of a viable mesopelagic community that also includes mesopelagic fishes in the Arctic has been questioned. In addition, it has been hypothesized that vertical migration would hardly exist in these areas. We wanted to check if deep scattering layers (DSLs) was found to the west and north of Svalbard (79°30’N - 82°10’N) during autumn 2015, and if present; whether organisms in such DSLs undertook vertical migrations. Our null hypothesis was that there would be no evidence of diel vertical migration. Multi-frequency acoustic observations by hull mounted echosounders (18, 38 and 120 kHz) revealed a DSL at depths ~210-510 m in areas with bottom depths exceeding ~600 m. Investigating eight geographical locations that differed with respect to time periods, light cycle and sea ice conditions, we show that the deeper layer of DSL displayed a clear ascending movement during night time and a descending movement during daytime. The high-light weighted mean depth (343-514 m) with respect to backscattered energy was statistically deeper than the low-light weighted mean depth (179-437 m) for the locations studied. This behavior of the DSL was found to be consistent both when the sun was continuously above the horizon and after it started to set on 1 September, and both in open water and sea ice covered waters. The weighted mean depth showed an increasing trend, while the nautical area backscattering strength from the DSL showed a decreasing trend from south to north among the studied locations. Hydrographic observations revealed that the diel migration was found in the lower part of the north-flowing Atlantic Water, and was disconnected from the surface water masses above the Atlantic Water during day and night. The organisms conducting vertical migrations were studied by vertical and oblique hauls with zooplankton nets and pelagic trawls. These data suggest that these organisms were mainly various mesopelagic fishes, some few larger fishes, large zooplankton like krill and amphipods, and various gelatinous forms.

Trophic interactions across lake–stream boundaries in mountain lakes

Fish can affect the mass flux of invertebrates across aquatic ecosystem boundaries, as shown by the effects of fish on export of zooplankton from lakes to outlet streams in 12 oligotrophic lakes in Rocky Mountain National Park (RMNP), Colorado (6 lakes and streams with fish; 6 without fish). Lakes without fish exported to streams 5 times as much zooplankton biomass as lakes with fish because of strong suppression of large zooplankton in lakes with fish. Of the zooplankton biomass exported to streams from both types of lakes, 95% was lost within 500 m of the lakes; the greatest rate of loss occurred within 10 m. Higher rates of loss for large zooplankton in streams below lakes without fish suggest that fish predation is not the primary cause for loss of zooplankton below lakes. In streams with and without fish, small zooplankton were lost at much lower rates downstream than large zooplankton. Daylight and nocturnal sampling showed much higher exports of zooplankton and, especially for large taxa, from lakes to streams at night, presumably because of diel vertical migration. Stream locations near a lake outlet had high total macroinvertebrate biomass, especially for filtering and gathering collectors as well as detritivores, indicating that macroinvertebrate predation is likely related to zooplankton mass flux. Large zooplankton also could have high vulnerability to interception by the channel substrate, which could stimulate detritivory. Biomass of all macroinvertebrates, and specifically of the filtering and gathering collectors, was lower in streams with fish because fish suppressed the export of zooplankton food for macroinvertebrates from the lakes and directly consumed macroinvertebrates. Therefore, the RMNP lakes and their outlet streams demonstrate trophic compression of stream macroinvertebrates by fish, during which macroinvertebrates are simultaneously pressured by increasing top-down forces from direct predation and reduced bottom-up forces because zooplankton are suppressed by fish in lakes.

Nutrient supply, surface currents, and plankton dynamics predict zooplankton hotspots in coastal upwelling systems

AbstractA simple combination of wind‐driven nutrient upwelling, surface currents, and plankton growth/grazing equations generates zooplankton patchiness and hotspots in coastal upwelling regions. Starting with an initial input of nitrate from coastal upwelling, growth and grazing equations evolve phytoplankton and zooplankton over time and space following surface currents. The model simulates the transition from coastal (large phytoplankton, e.g., diatoms) to offshore (picophytoplankton and microzooplankton) communities, and in between generates a large zooplankton maximum. The method was applied to four major upwelling systems (California, Peru, Northwest Africa, and Benguela) using latitudinal estimates of wind‐driven nitrate supply and satellite‐based surface currents. The resulting zooplankton simulations are patchy in nature; areas of high concentrations coincide with previously documented copepod and krill hotspots. The exercise highlights the importance of the upwelling process and surface currents in shaping plankton communities.

Responses of trophic structure and zooplankton community to salinity and temperature in Tibetan lakes: Implication for the effect of climate warming

Warming has pronounced effects on lake ecosystems, either directly by increased temperatures or indirectly by a change in salinity. We investigated the current status of zooplankton communities and trophic structure in 45 Tibetan lakes along a 2300 m altitude and a 76 g/l salinity gradient. Freshwater to hyposaline lakes mainly had three trophic levels: phytoplankton, small zooplankton and fish/Gammarus, while mesosaline to hypersaline lakes only had two: phytoplankton and large zooplankton. Zooplankton species richness declined significantly with salinity, but did not relate with temperature. Furthermore, the decline in species richness with salinity in lakes with two trophic levels was much less abrupt than in lakes with three trophic levels. The structural variation of the zooplankton community depended on the length of the food chain, and was significantly explained by salinity as the critical environmental variable. The zooplankton community shifted from dominance of copepods and small cladoceran species in the lakes with low salinity and three trophic levels to large saline filter-feeding phyllopod species in those lakes with high salinity and two trophic levels. The zooplankton to phytoplankton biomass ratio was positively related with temperature in two-trophic-level systems and vice versa in three-trophic-level systems. As the Tibetan Plateau is warming about three times faster than the global average, our results imply that warming could have a considerable impact on the structure and function of Tibetan lake ecosystems, either via indirect effects of salinization/desalinization on species richness, composition and trophic structure or through direct effects of water temperature on trophic interactions.

An Atlantis model of the southern Benguela upwelling system: Validation, sensitivity analysis and insights into ecosystem functioning

Ecosystem models are valuable tools for evaluating the effects of different environmental stressors on an ecosystem and to increase system understanding. The Atlantis modelling framework is an end-to-end model that includes information and processes from the abiotic environment to the human component of a system. An application of Atlantis for the Benguela and Agulhas Currents was updated and validated against time series of biomass and catch (1990–2013). Five univariate skill metrics were used to evaluate model performance of biomass data of selected species. A sensitivity analysis, focused on the growth rate of small and large phytoplankton and large zooplankton, was conducted to evaluate the influence of these parameters on model outputs. A total of 26 alternative model parameterisations based on a variation of ±20% around the growth parameter of the selected groups were compared to the baseline model. Skill metrics suggest that the model captures the main emergent properties of the southern Benguela, the magnitude and trends in biomass and catch of most modelled groups. Large variations in biomass were observed among the sensitivity runs compared to the baseline. The most marked differences in relative biomass among runs seem to be attributed to changes in the growth rate of large zooplankton. The Atlantis model presented here has the capability to be used as a tool to provide strategic information for fisheries management purposes in the southern Benguela system.

Light climate and dissolved organic carbon concentration influence species-specific changes in fish zooplanktivory

Dissolved organic carbon (DOC) in lakes reduces light penetration and limits fish production in low nutrient lakes, reportedly via reduced primary and secondary production. Alternatively, DOC and light reductions could influence fish by altering their visual feeding. Previous studies report mixed effects of DOC on feeding rates of zooplanktivorous fish, but most investigators tested effects of a single concentration of DOC against clear-water, turbid, or algal treatments. We used a controlled laboratory study to quantify the effects of a DOC gradient (3–19 mg L−1) on average light climate and the zooplankton feeding rate of 3 common, north temperate fishes. Light availability, which was inversely related to DOC concentration, had a positive and linear effect on zooplankton consumption by juvenile largemouth bass (Micropterus salmoides) and bluegill (Lepomis macrochirus), explaining 22% and 28% of the variation in consumption, respectively. By contrast, zooplankton feeding rates by fathead minnow (Pimephales promelas) were best predicted by a nonlinear, negative influence of light (R2 = 0.13). In bluegill feeding trials we found a general trend for positive selection of larger zooplankton (Cladocera and Chaoboridae); however, the light climate did not influence the selection of prey type. Largemouth bass selected for larger-bodied zooplankton, with weak evidence that selectivity for large Cladocera changed from negative to neutral selection based on electivity values across the light gradient. Our results suggest that the effect of DOC on the light climate of lakes may directly influence fish zooplanktivory and that this influence may vary among fish species.

Effects of common carp (Cyprinus carpio) and bream (Abramis brama) on the structure of the littoral community in a mesotrophic lake (Mesocosm Experiments)

The effect of common carp and bream on hydrochemical parameters, abundance, and structure of phyto-, zooplankton, and macrozoobenthos has been studied in semienclosed mesocosms installed in the littoral zone of a mesotrophic lake. Significant differences in the biomass of different algal groups in mesocosms with fish in respect to the control were established only for diatoms and were not found for other phytoplankton groups. Common carp had a greater effect on the abundance of large zooplankton species (Diaphanosoma brachyurum) compared to bream. The abundance of the small Bosmina longirostris increased in mesocosms both with bream and common carp. The macrozoobenthos biomass reduced at higher rates in mesocosms with common carp than in those with bream, with the strongest effect of common carp on mayfly larvae. The differences between the consumption of chironomid and oligochaete larvae were not established in mesocosms with common carp. Bream mainly affected the larvae of mayflies and oligochaetes and, to a lesser degree, the chironomid larvae.

Zooplankton community response to the winter 2013 deep convection process in the NWMediterranean Sea

AbstractThe Gulf of Lion is an important area of deep convection, where intense winter vertical mixing brings nutrients up from deeper layers, promoting the largest bloom in the Mediterranean at the end of winter/early spring. The DEWEX program conducted cruises in February and April 2013 to investigate the ecosystem level impacts of deep water convection. Zooplankton data were collected through net sampling and imaging with an Underwater Vision Profiler. In winter, low zooplankton abundance and biomass were observed in the Deep Convection Zone (DCZ) and higher values on its periphery. In spring, this pattern reversed with high biomass in the DCZ and lower values on the periphery. On average for the whole area, the potential grazing impact was estimated to increase by one order of magnitude from winter to spring. In April, all areas except the DCZ incurred top‐down control by zooplankton on the phytoplankton stock. In the DCZ, the chlorophyll‐a values remained high despite the high zooplankton biomass and carbon demand, indicating a sustained bottom‐up control. The zooplankton community composition was comparable for both periods, typified by high copepod dominance, but with some differences between the DCZ and peripheral regions. In spring the DCZ was characterized by a strong increase in herbivorous species such as Centropages typicus and Calanus helgolandicus, and an increase in the number of large zooplankton individuals. Our study indicates that the DCZ is likely an area of both enhanced energy transfer to higher trophic levels and organic matter export in the North Western Mediterranean Sea.

Cascading effect of exotic fish fry on plankton community in a tropical Andean high mountain lake: a mesocosm experiment

<p>Fishless Andean high mountain lakes may be vulnerable to fish invasion because they tend to be small, oligotrophic and contain low zooplankton diversity. During the first decades of the twentieth century, rainbow trout, <em>Oncorhynchus mikiss</em>, was introduced in South America, and stocking of juvenile stages (fry) in lakes continues today. However, their effect on plankton in these lakes has been little studied. We performed a mesocosms experiment to assess effects of trout of different ontogenetic stages on zooplankton and phytoplankton in a tropical-Andean high mountain lake. The presence of trout fry resulted in declines in several large zooplankton taxa, increases in chlorophyll <em>a</em> and dissolved inorganic nitrogen (DIN). Our results showed that small fry consume large copepods (<em>Colombodiaptomus brandorffi</em>) at a faster rate than larger fry, and also consumed medium sized copepods (<em>Tropocyclops prasinus</em>) that are not affected by the larger trout fry. Fish of both sizes consumed <em>Ceriodaphnia quadrangula</em>, a midsize cladoceran. Fish predation had weak effect on the phytoplankton biomass, but we found a correlation between zooplankton biomass and phytoplankton richness, and significantly larger cell of the alga <em>Peridinium</em> in the presence of fish. Our results indicate that trout introduction produces cascading ecological and phenotypic effects on the plankton communities of tropical mountain lakes, similar to those observed in temperate latitudes.</p>

Relationships among cyanobacteria, zooplankton and fish in sub-bloom conditions in the Sulejow Reservoir

<p>The occurrence of cyanobacteria is particularly characteristic of shallow eutrophic waters, and they often form massive ‘blooms’ that can affect aquatic invertebrates and fish. However, even a low abundance of cyanobacteria can be hazardous to aquatic organisms, due to the production of toxic metabolites. The aim of this study was to investigate the relationship between cyanobacteria and their toxicity (biological activity) towards zooplankton and fish communities, when only low concentrations of cyanobacterial chlorophyll <em>a</em> (less than 20 μg L<sup>-1</sup>) are detected, <em>i.e.</em> in sub-bloom conditions. Measurements were performed in Sulejow Reservoir (Central Poland), a shallow, lowland, eutrophic reservoir, in which cyanobacterial blooms occur regularly. Fish were assessed using echo-sounding (distribution) and by gillnetting (species composition). Simultaneously, zooplankton, cyanobacteria and physico-chemical characteristics were studied at 14 stations situated along hydroacoustic transects. Parameters that characterized the cyanobacteria (cyanobacterial chlorophyll <em>a</em> concentration, the number of 16S rRNA and the <em>mcy</em>A gene copies and microcystin (MC) concentration) were consistently correlated (based on a principal component analysis), and the highest values were found in the downstream region of the study area. This ‘cyano-complex’ was also positively correlated with oxygen concentration, pH and phosphate levels, but was negatively correlated with temperature and the concentrations of nitrates and nitrites. In Sulejow Reservoir in 2013 the biomass of large zooplankton filter feeders decreased along with increasing MC concentration and fish densities, while small filter feeders did not present such relationships with regards to fish densities. Fish abundance tended to decrease at stations with a lower abundance of cyanobacteria and with growing toxic genotype copies and MC concentration.</p>