Zooplankton Research Articles

Early Summer Zooplankton in the Sea of Azov during the Period of Its Salinization

Data on the taxonomic composition of zooplankton in the Sea of Azov and Taganrog Bay are presented for the period of 2005–2015. The main plankton communities have been identified and the changes in the taxonomic diversity of plankton animals and their community structure caused by sea salinization have been analyzed. The salinization of the sea observed these years affected the early summer zooplankton, and the species composition of its communities is shown to become much poorer; their taxonomic diversity decreased 1.4–1.5 times. The increase in salinity caused the replacement of freshwater and brackish water species by the euryhaline copepod Acartia tonsa that spread all over the sea.

Large Vertical Migrations ofPyrosoma atlanticumPlay an Important Role in Active Carbon Transport

AbstractPyrosomes are efficient grazers that can form dense aggregations. Their clearance rates are among the highest of any zooplankton grazer, and they can rapidly repackage what they consume into thousands of fecal pellets per hour. In recent years, pyrosome swarms have been found outside of their natural geographical range; however, environmental drivers that promote these swarms are still unknown. During the austral spring of 2017 aPyrosoma atlanticumswarm was sampled in the Tasman Sea. Depth‐stratified sampling during the day and night was used to examine the spatial and vertical distribution ofP. atlanticumacross three eddies. Respiration rate experiments were performed onboard to determine minimum feeding requirements for the pyrosome population.P. atlanticumwas 2 orders of magnitude more abundant in the cold core eddy (CCE) compared to both warm core eddies, with maximum biomass of 360 mg WW·m−3, most likely driven by high chlorophyllaconcentrations.P. atlanticumexhibited diel vertical migration and migrated to a maximum depth strata of 800–1,000 m. Active carbon transport in the CCE was 4 orders of magnitude higher than the warm core eddies. Fecal pellet production contributed to the majority (91%) of transport, and total downward carbon flux below the mixed layer was estimated at 11 mg C·m−2·d−1. When abundant,P. atlanticumswarms have the potential to play a major role in active carbon transport, comparable to fluxes for zooplankton and micronekton communities.

Structural and Functional Characteristics of Zooplankton in the Ob Estuary and Adjacent Shelf Areas of the Kara Sea in Summer

The study was carried out in the Ob estuary and the adjacent shelf of the Kara Sea in July 2016. For the first time data on the species composition, abundance and distribution as well as on feeding of zooplankton were obtained during the period of intensive river runoff. The biomass of zooplankton in terms of wet weight (without jelly animals and chaetognaths) varied over a wide range from 40 to 1880 mg/m3. The biomass was dominaned by the Limnocalanus macrurus population, represented by all age stages, and Pseudocalanus spp. (an average 60% and 17% of biomass, respectively). Maxima of zooplankton biomass were associated with the boundary of the distribution of freshwater and with the southern periphery of the estuary frontal zone (EFZ). The short-term temporal variation of the abundance of zooplankton was observed when the transect was repeated after 2 days: the biomass at the southern periphery of the EFZ decreased from 1900 to 250 mg/m3. The observed changes is supposed to be caused by the shift of the southern boundary of the EFZ under changing in the wind regime. At high phytoplankton biomass in the estuary (>7 μg Chl-a/L), the daily ingestion of Limnocalanus macrurus was 5.77, 4.46, 2.59 and 1.4 μg C/day in CVI, CV, CIV and CIII, respectively. Energy intake by younger copepodite stages not only covered the metabolic requirements, but also enabled the copepods to growth. At lower Chl-a concentration on the shelf (<2 μg/L), the mean values were lower (1.56 μg C/day and 1.17 μg C/day in CVI and CV, respectively) and hardly met the metabolic demands. The zooplankton grazing impact estimated with the gut fluorescence method varied from 0.2 to 7.3% of phytoplankton standing stock and from 2.2 to 30% of primary production. The results are discussed in relation to the specific hydrophysical conditions in the summer period and in comparison with the data obtained for the autumn season.

Alteration in microcrustacean secondary production and herbivory effects between the River Danube and its floodplain lake

The aim of this study was to determine microcrustacean secondary production (P/B) and a herbivory ratio (ZB:PB) in the River Danube and a floodplain lake to reveal models in plankton food web functioning. P/B was used to assess population variation under different environmental conditions, while ZB:PB provided information on the zooplankton grazing intensity on algae. Seasonal changes, displayed by the hydrological regime, i.e., related water level fluctuation, were expected to explain different plankton patterns between the study sites as a result of unlike community composition and food quality. The structural equation modelling we used identified specific pathways of measured indexes at the individual localities. Although the direct impact of water level fluctuation on the studied communities was low, the following difference in abiotic components might have caused shifts in zooplankton assemblages that changed P/B and ZB:PB ratios. In the floodplain lake, P/B was primarily influenced by biotic components, and herbivory alteration was expressed through top-down control. On the contrary, P/B in the riverine system was controlled by an environmental element and herbivory was driven by the bottom-up resources. The results show the usefulness of the studied indexes in determining variations of zooplankton functional processes in the hydrologically dynamic environments.

Microplastic-mediated transport of PCBs? A depuration study with Daphnia magna.

The role of microplastic (MP) as a carrier of persistent organic pollutants (POPs) to aquatic organisms has been a topic of debate. However, the reverse POP transport can occur if relative contaminant concentrations are higher in the organism than in the microplastic. We evaluated the effect of microplastic on the PCB removal in planktonic animals by exposing the cladoceran Daphnia magna with a high body burden of polychlorinated biphenyls (PCB 18, 40, 128 and 209) to a mixture of microplastic and algae; daphnids exposed to only algae served as the control. As the endpoints, we used PCB body burden, growth, fecundity and elemental composition (%C and %N) of the daphnids. In the daphnids fed with microplastic, PCB 209 was removed more efficiently, while there was no difference for any other congeners and ΣPCBs between the microplastic-exposed and control animals. Also, higher size-specific egg production in the animals carrying PCB and receiving food mixed with microplastics was observed. However, the effects of the microplastic exposure on fecundity were of low biological significance, because the PCB body burden and the microplastic exposure concentrations were greatly exceeding environmentally relevant concentrations.

Short-term fish predation destroys resilience of zooplankton communities and prevents recovery of phytoplankton control by zooplankton grazing.

Planktivorous fish predation directly affects zooplankton biomass, community and size structure, and may indirectly induce a trophic cascade to phytoplankton. However, it is not clear how quickly the zooplankton community structure and the cascading effects on phytoplankton recover to the unaffected state (i.e. resilience) once short-term predation by fish stops. The resilience has implications for the ecological quality and restoration measures in aquatic ecosystems. To assess the short-term zooplankton resilience against fish predation, we conducted a mesocosm experiment consisting of 10 enclosures, 6 with fish and 4 without fish. Plankton communities from a natural lake were used to establish phytoplankton and zooplankton in the mesocosms. High biomasses (about 20 g wet mass m-3) of juvenile planktivorous fish (perch, Perca fluviatilis) were allowed to feed on zooplankton in fish enclosures for four days. Thereafter, we removed fish and observed the recovery of the zooplankton community and its cascading effect on trophic interactions in comparison with no fish enclosures for four weeks. Short-term fish predation impaired resilience in zooplankton community by modifying community composition, as large zooplankton, such as calanoids, decreased just after fish predation and did not re-appear afterwards, whereas small cladocerans and rotifers proliferated. Total zooplankton biomass increased quickly within two weeks after fish removal, and at the end even exceeded the biomass measured before fish addition. Despite high biomass, the dominance of small zooplankton released phytoplankton from grazer control in fish enclosures. Accordingly, the zooplankton community did not recover from the effect of fish predation, indicating low short-term resilience. In contrast, in no fish enclosures without predation disturbance, a high zooplankton:phytoplankton biomass ratio accompanied by low phytoplankton yield (Chlorophyll-a:Total phosphorus ratio) reflected phytoplankton control by zooplankton over the experimental period. Comprehensive views on short and long-term resilience of zooplankton communities are essential for restoration and management strategies of aquatic ecosystems to better predict responses to global warming, such as higher densities of planktivorous fish.

Convergent estimates of marine nitrogen fixation.

Uncertainty in the global patterns of marine nitrogen fixation limits our understanding of the response of the ocean's nitrogen and carbon cycles to environmental change. The geographical distribution of and ecological controls on nitrogen fixation are difficult to constrain with limited in situ measurements. Here we present convergent estimates of nitrogen fixation from an inverse biogeochemical and a prognostic ocean model. Our results demonstrate strong spatial variability in the nitrogen-to-phosphorus ratio of exported organic matter that greatly increases the global nitrogen-fixation rate (because phytoplankton manage with less phosphorus when it is in short supply). We find that the inputofnewly fixednitrogen from microbial fixation and external inputs (atmospheric deposition and river fluxes) accounts for up to 50 per cent of carbon export in subtropical gyres. We also find that nitrogen fixation and denitrification are spatially decoupled but that nevertheless nitrogen sources and sinks appear to be balanced over the past few decades. Moreover, we propose a role for top-down zooplankton grazing control in shaping the global patterns of nitrogen fixation. Our findings suggest that biologicalcarbon export in the oceanis higher than expected and that stabilizing nitrogen-cycle feedbacks are weaker than previously thought.

СТРУКТУРНО-ФУНКЦИОНАЛЬНАЯ ХАРАКТЕРИСТИКА ЗООПЛАНКТОННЫХ СООБЩЕСТВ ТЕРМОКАРСТОВЫХ ОЗЕР О. САМОЙЛОВСКИЙ (ДЕЛЬТА РЕКИ ЛЕНЫ, РЕСПУБЛИКА САХА (ЯКУТИЯ))

СТРУКТУРНО-ФУНКЦИОНАЛЬНАЯ ХАРАКТЕРИСТИКА ЗООПЛАНКТОННЫХ СООБЩЕСТВ ТЕРМОКАРСТОВЫХ ОЗЕР О. САМОЙЛОВСКИЙ (ДЕЛЬТА РЕКИ ЛЕНЫ, РЕСПУБЛИКА САХА (ЯКУТИЯ))

Experimental evidence for crustacean zooplankton grazing activity on mixotrophic ciliates

Abstract With their ultraoligotrophic status, the Chilean North Patagonian lakes have mixotrophic ciliates in their pelagic environments as producers, whereas the primary consumers are crustaceans that are low in abundance and species numbers. The aim of the present study was to determine the potential grazer role of mixotrophic ciliates on crustacean zooplankton collected in a lake without mixotrophic ciliates. Three experiments were conducted; one had a control with mixotrophic ciliates and an experimental treatment including the copepod Boeckella gracilipes obtained from Caburgua lake, whereas the second and third experiments had a control without zooplankton, and three treatments with the addition of Daphnia pulex and Mesocyclops araucanus, and a third treatment with equal amounts of both species. The results revealed grazing effects on the mixotrophic ciliates in the experimental treatments. This finding supports the evidence from the field and experiments suggesting that, in the transition from oligotrophy to mesotrophy with consequent changes in zooplankton, species number and abundance of mixotrophic ciliates decrease in their abundance.

An Estimation of the Possible Migration Path of the Pacific Oyster (Crassostrea gigas) Along the Northern Coast of Patagonia

Fil: Worner, Stefania. Ministerio de Defensa. Armada Argentina. Servicio de Hidrografia Naval; Argentina. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina. Universidad de Buenos Aires; Argentina

Zooplankton grazing selectivity regulates herbivory and dominance of toxic phytoplankton over multiple prey generations

AbstractHow grazer selectivity regulates the primary producer community is a core topic in ecology. Yet, the role of zooplankton grazing selection on phytoplankton dynamics is poorly understood. Few studies have compared the effect of grazers with contrasting selectivity on mixed phytoplankton prey, and none over multiple phytoplankton generations. We tested the hypothesis that a selectively grazing copepod (Eudiaptomus gracilis) would facilitate the dominance of a toxic cyanobacterium (Microcystis aeruginosa) by grazing on a competing eukaryotic microalga (Cryptomonas pyrenoidifera), while a generalist cladoceran (Daphnia magna) would have no effect on the dominance of cyanobacteria in 4‐d laboratory cocultures. Experiments started with a ninefold initial dominance of Cryptomonas over Microcystis by biomass. Each grazer type was added to cocultured phytoplankton and the abundance of phytoplankton was compared to no‐grazer controls. As predicted, Daphnia had no effect on the relative abundance of its prey and the copepod facilitated Microcystis dominance, although the strength of facilitation slightly declined with time. As the copepod reduced mostly the biomass of the edible algae, it pushed the system toward the dominance of toxic prey, which likely reduced the efficiency of selective grazing on the last day. Hence, while the selective grazer promoted cyanobacterial dominance, the effect may be weaker than predicted from extrapolating grazing rates obtained from short‐term (i.e., hourly) assays. Overall, predicting the role of zooplankton selectivity on phytoplankton dynamics—especially harmful algal blooms—would benefit from accounting for fluctuations in grazer effects due to shifting abundance and growth of each prey over time.

Linking Marine Plankton Ecosystems and Climate: A New Modeling Approach to the Warm Early Eocene Climate

AbstractThe fossil record reveals large changes in marine plankton ecosystems linked with both environmental and ecological change across the Cenozoic. An understanding of the drivers of these changes is key to understanding the marine carbon cycle. The response of plankton ecosystems in past warm climates also provides a key analogue for current climate change. While models are employed to quantify interactions between the environment and the biota, current Earth system models strongly encode our understanding of modern marine ecosystems. By contrast, trait‐based models aim to describe the marine plankton ecosystem in terms of fundamental ecological and physiological rules that are less likely to change through time. This provides a unique opportunity to assess the interactions between marine ecosystem and paleoclimate. For the first time, we apply a size‐structured trait‐based plankton ecosystem model embedded in the Earth system model of intermediate complexity, cGENIE, to model plankton communities for the warm climate of the early Eocene. Compared to modern, we find the warm climate is associated with an increase in the mean cell size of plankton communities and export production, particularly in the southern high latitudes, along with lower total phytoplankton biomass. Paleogeography has an important role in regulating the effect of ecosystem structure via changes in ocean circulation and nutrient cycling. Warmer temperatures also drive changes due to enhanced zooplankton grazing. An integration of the fossil record with plankton ecosystem models will provide a powerful tool to assess the impacts of warm climates on marine systems.

A perturbed biogeochemistry model ensemble evaluated against in situ and satellite observations

Abstract. The dynamics of biogeochemical models are determined by the mathematical equations used to describe the main biological processes. Earlier studies have shown that small changes in the model formulation may lead to major changes in system dynamics, a property known as structural sensitivity. We assessed the impact of structural sensitivity in a biogeochemical model of intermediate complexity by modelling the chlorophyll and dissolved inorganic nitrogen (DIN) concentrations. The model is run at five different oceanographic stations spanning three different regimes: oligotrophic, coastal, and the abyssal plain, over a 10-year timescale to observe the effect in different regions. A 1-D Model of Ecosystem Dynamics, nutrient Utilisation, Sequestration, and Acidification (MEDUSA) ensemble was used with each ensemble member having a combination of tuned function parameterizations that describe some of the key biogeochemical processes, namely nutrient uptake, zooplankton grazing, and plankton mortalities. The impact is quantified using phytoplankton phenology (initiation, bloom time, peak height, duration, and termination of phytoplankton blooms) and statistical measures such as RMSE (root-mean-squared error), mean, and range for chlorophyll and nutrients. The spread of the ensemble as a measure of uncertainty is assessed against observations using the normalized RMSE ratio (NRR). We found that even small perturbations in model structure can produce large ensemble spreads. The range of 10-year mean surface chlorophyll concentration in the ensemble is between 0.14 and 3.69 mg m−3 at coastal stations, 0.43 and 1.11 mg m−3 on the abyssal plain, and 0.004 and 0.16 mg m−3 at the oligotrophic stations. Changing both phytoplankton and zooplankton mortalities and the grazing functions has the largest impact on chlorophyll concentrations. The in situ measurements of bloom timings, duration, and terminations lie mostly within the ensemble range. The RMSEs between in situ observations and the ensemble mean and median are mostly reduced compared to the default model output. The NRRs for monthly variability suggest that the ensemble spread is generally narrow (NRR 1.21–1.39 for DIN and 1.19–1.39 for chlorophyll profiles, 1.07–1.40 for surface chlorophyll, and 1.01–1.40 for depth-integrated chlorophyll). Among the five stations, the most reliable ensembles are obtained for the oligotrophic station ALOHA (for the surface and integrated chlorophyll and bloom peak height), for coastal station L4 (for inter-annual mean), and for the abyssal plain station PAP (for bloom peak height). Overall our study provides a novel way to generate a realistic ensemble of a biogeochemical model by perturbing the model equations and parameterizations, which will be helpful for the probabilistic predictions.

Satellite‐Based Estimation of Particulate Organic Carbon Export in the Northern South China Sea

AbstractKnowledge of particulate organic carbon export flux (EP) is the key to understanding the marine biological carbon pump. In this study, a satellite remote sensing algorithm based on the food‐web model established by Siegel et al. (2014; https://doi.org/10.1002/2013GB004743) was used to estimate EP in the northern South China Sea (NSCS), which consists of direct sinking flux of large phytoplankton and associated aggregates by gravity and flux of zooplankton feces by grazing. This is the first time that fine spatiotemporal variation of satellite‐derived EP in the NSCS is unveiled. Compared with the results on 1° spatial scale products, the model results based on the satellite products with 1/3° and 1/12° resolutions showed better consistency with the observations. Validation with in situ EP showed that the model exhibited a good performance in the NSCS basin, but the predicted EP in the shelf regions were smaller compared with measurements. The satellite‐derived annual‐mean EP in the shelf areas was 8.47 and 5.56 mmol C·m−2·d−1 in the NSCS basin, on the 1/3° spatial scale, with relative differences from observations were about −50% and −15% for the shelf and basin, respectively. The flux of feces from zooplankton grazing, especially that from microzooplankton grazing, might account for the major fraction of the EP in the NSCS. Accurate products of ocean net primary production and empirical coefficients in the food web (e.g., export efficiency of zooplankton grazing and specific mortality rate of nongrazing phytoplankton) suitable for the NSCS, are essential for performance improvement of the satellite‐based EP model.

Pseudo-nitzschia, Nitzschia, and domoic acid: New research since 2011

Some diatoms of the genera Pseudo-nitzschia and Nitzschia produce the neurotoxin domoic acid (DA), a compound that caused amnesic shellfish poisoning (ASP) in humans just over 30 years ago (December 1987) in eastern Canada. This review covers new information since two previous reviews in 2012. Nitzschia bizertensis was subsequently discovered to be toxigenic in Tunisian waters. The known distribution of N. navis-varingica has expanded from Vietnam to Malaysia, Indonesia, the Philippines and Australia. Furthermore, 15 new species (and one new variety) of Pseudo-nitzschia have been discovered, bringing the total to 52. Seven new species were found to produce DA, bringing the total of toxigenic species to 26.We list all Pseudo-nitzschia species, their ability to produce DA, and show their global distribution. A consequence of the extended distribution and increased number of toxigenic species worldwide is that DA is now found more pervasively in the food web, contaminating new marine organisms (especially marine mammals), affecting their physiology and disrupting ecosystems.Recent findings highlight how zooplankton grazers can induce DA production in Pseudo-nitzschia and how bacteria interact with Pseudo-nitzschia. Since 2012, new discoveries have been reported on physiological controls of Pseudo-nitzschia growth and DA production, its sexual reproduction, and infection by an oomycete parasitoid. Many advances are the result of applying molecular approaches to discovering new species, and to understanding the population genetic structure of Pseudo-nitzschia and mechanisms used to cope with iron limitation. The availability of genomes from three Pseudo-nitzschia species, coupled with a comparative transcriptomic approach, has allowed advances in our understanding of the sexual reproduction of Pseudo-nitzschia, its signaling pathways, its interactions with bacteria, and genes involved in iron and vitamin B12 and B7 metabolism.Although there have been no new confirmed cases of ASP since 1987 because of monitoring efforts, new blooms have occurred. A massive toxic Pseudo-nitzschia bloom affected the entire west coast of North America during 2015–2016, and was linked to a ‘warm blob’ of ocean water. Other smaller toxic blooms occurred in the Gulf of Mexico and east coast of North America. Knowledge gaps remain, including how and why DA and its isomers are produced, the world distribution of potentially toxigenic Nitzschia species, the prevalence of DA isomers, and molecular markers to discriminate between toxigenic and non-toxigenic species and to discover sexually reproducing populations in the field.

Brachionus falcatus and Platyias patulus indicating organic pollution in Ouémé River’s basin, Republic of Benin

Zooplankton is a biological compartment widely used in the bio-monitoring of aquatic ecosystems. It allows early detection of environmental disturbances even before reaching the upper compartments of interest to humans. The present study evaluated the ability of two rotifer species Brachionus falcatus and Platyias patulus to indicate organic pollution in the Oueme River basin. Sampling was done between October 2014 and September 2015. Plankton net of 20 μm mesh size was used. Parameters such as NH 4 + , NO 2 - and PO 4 3- were measured in water and used for the calculation of the Organic Pollution Index (IPO). The results showed that the occurrence of B. falcatus and P. patulus was 96.66% and 81.11%, respectively. Both species were particularly abundant in sites such as Kaboua and Vossa as well as downstream of the basin (Agonlin-lowe and Bonou). These strong abundances are linked to a very strong organic pollution in Kaboua and Vossa then to a high organic pollution at downstream. The least organically polluted station (Kpassa) had the lowest abundance of both species. B. falcatus and P. patulus are therefore two indicator species of organic pollution in the Oueme River’s basin in Benin.

Latitudinal variations of zooplankton community structure and productivity in the Barents sea (summer 2013)

The Barents Sea is regarded as one of the world’s most unique regions with its high summer productivity. However, there were a few studies dealing with estimation of daily secondary production in the Barents Sea. The aim of the present study was to conduct an investigation of the pelagic mesozooplankton assemblage in the Barents Sea, to calculate daily production levels of mesozooplankton and to analyze the relationship between the community structure and environmental drivers.Structure of zooplankton communities was investigated within the main water masses of the Barents Sea in the summer 2013 (June–July). Calanus finmarchicus presented as copepodites IV–V dominated in terms of the total zooplankton abundance and biomass in Murmansk Coastal Water and Atlantic Water. Spring phase of zooplankton succession cycle was found in Arctic Water where Copepoda nauplii were the most abundant and Calanus glacialis dominated by zooplankton biomass. Zooplankton of the Barents Sea Water had features of spring and summer phases of zooplankton succession cycle. Spatial variations in the zooplankton distribution were associated with hydrological conditions and phytoplankton density. Maximum of the mean zooplankton biomass was registered in Atlantic Water (30,7 mg dry mass/m3) while the minimum was found in Barents Sea Water (5,4 mg dry mass/m3). Calculated daily production of zooplankton ranged from 0,01 to 1,57 mg dry mass/m3per day. Total zooplankton stock for the study area of 281 000 km2was estimated to be &gt; 740 000 t of dry mass. The obtained values of zooplankton biomass and stock were considerably lower than in previous years, which are probably connected with higher predator pressure — grazing of zooplankton by capelin and other predators as well as with possible climatic influence.

The Effects of an Herbicide and Antibiotic Mixture on Aquatic Primary Producers and Grazers.

Widespread use of agrochemicals increases their likelihood of entering aquatic systems in mixture. Despite different modes of action, atrazine (herbicide) and tetracycline (antibiotic) adversely affect non-target photosynthetic organisms individually, but the effects of simultaneous exposure to both contaminants are untested. We created microcosms containing microalgae (Chlorella sp.), floating macrophytes (Lemna minor), and a zooplankton grazer (Daphnia magna). Microcosms were exposed to environmentally relevant concentrations of atrazine and tetracycline, alone and together, for 10days. Atrazine decreased Chlorella sp. abundance, but not enough to reduce food availability to D. magna whose reproduction and mortality were unaffected. In contrast, tetracycline and atrazine appeared to have additive effects on L. minor abundance and growth inhibition. These additive adverse effects suggest increased potential for L. minor population decline over the long term, and potential for altered species interactions in aquatic systems receiving agricultural runoff.

Transcriptomic analysis dissects the mechanistic insight into the Daphnia clonal variation in tolerance to toxic Microcystis

AbstractCyanobacteria have become more prevalent than other phytoplankton in freshwater assemblages during summer. In such conditions, cyanobacterial traits may reduce zooplankton fitness and the energy flow efficiency from primary producers to aquatic herbivores. Cladocerans, as the dominant zooplankton grazers in freshwater ecosystems, exhibit clonal variation in their tolerance to cyanobacteria with an increasing gradient in eutrophication history. Hitherto, research on the full modes of action (MoAs) of Daphnia clonal differences in tolerance to toxic Microcystis still remains in its infancy. We conducted fitness and transcriptome analyses on two Daphnia clones, clone TH09 and TH14. A significant decline in body growth rate was detected in the sensitive clone TH09 compared with the tolerant clone TH14 at the presence of toxic Microcystis. Furthermore, transcriptional analysis indicated that major MoAs such as glutathione metabolism, protein processing in endoplasmic reticulum, amino sugar/nucleotide sugar metabolism, and arachidonic acid metabolism were linked to the tolerance fitness in Daphnia similoides. These results provided mechanistic insights into the pathways of genetic and biological processes involved in cyanobacteria tolerance in the Daphnia clonal variation, and propose that the genetic architecture of this fitness‐related trait would be helpful to clarify how zooplankton clones adapt to harmful algal blooms.

Successful restoration of a tropical shallow eutrophic lake: Strong bottom-up but weak top-down effects recorded

Fish manipulation has been used to restore lakes in the temperate zone. Often strong short-term cascading effects have been obtained, but the long term-perspectives are less clear. Fish manipulation methods are far less advanced for warm lakes, and it is debatable whether it is, in fact, possible to create a trophic cascade in warm lakes due to the dominance and high densities of fast-reproducing omnivorous fish. However, removal of benthic feeding fish also reduce disturbance of the sediment, which not only affects the nutrient level but also the concentration of suspended organic and inorganic matter with enhanced water clarity and potentially better growth conditions for submerged macrophytes. We conducted a biomanipulation experiment in one of the basins in Chinese Huizhou West Lake that have remained highly turbid after extensive nutrient loading reduction. Another basin was used as control (control-treatment pairing design). Removal of a substantial amount of plankti-benthivorous fish was followed by planting of submerged macrophytes and stocking of piscivorous fish. We found strong and relatively long-lasting effects of the restoration initiative in the form of substantial improvements in water clarity and major reductions in nutrient concentrations, particularly total phosphorus, phytoplankton and turbidity, while only minor effects were detected for crustacean zooplankton grazers occurring in low densities before as well as after the restoration. Our results add importantly to the existing knowledge of restoration of warm lakes and are strongly relevant, not least in Asia where natural lakes frequently are used extensively for fish production, often involving massive stocking of benthivorous fish. With a growing economy and development of more efficient fish production systems, the interest in restoring lakes is increasing world-wide. We found convincing evidence that fish removal and piscivores stocking combined with transplantation of submerged macrophytes may have significant effects on water clarity in warm shallow lakes even if the zooplankton grazing potential remains low, the latter most likely as a result of high predation on the zooplankton.