Large Daphnia Research Articles

Negative effects of undesirable fish on common carp production and overall structure and functioning of fishpond ecosystems

Planktivorous undesirable fish may threaten both fishpond ecosystem functioning and common carp ( Cyprinus carpio ) farming. The present study revealed a mechanistic understanding of the adverse ecosystem and production impacts that topmouth gudgeon ( Pseudorasbora parva ), an invasive omnivorous fish species from Asia, could represent for eutrophic fishponds in South Bohemia (Czechia). In an outdoor experiment conducted under natural conditions, six similar ponds of small size (ca. 1 ha) were similarly stocked with the same size-class common carps (1+ year-old age cohort). Three ponds were additionally stocked with heterogenous size-class of topmouth gudgeon, other three were control ponds (stocked solely with common carp). Our sampling covered one vegetation season from March to September 2020. Environmental parameters, fish and zooplankton assemblage were sampled monthly, while zoobenthos bimonthly (in April, June and August). To assess the potential effect of overall fish predation and its change during the season, we calculated a fish stock index combining fish density and biomass at the time, referred as FSI C for stocked fish (common carp) and FSI U for undesirable fish (topmouth gudgeon). Some tenfold increase in FSI U reflected an overall effect of topmouth gudgeon predation. Significantly ( p < 0.05) lower FSI C indeed indicated the adverse effect of topmouth gudgeon on the carp populations compared to the control ponds since August. Our results clearly showed a significant top-down effect of topmouth gudgeon on planktonic crustaceans, especially large Daphnia spp. and other cladoceran species. The complete elimination of cladocerans in the second half of the growing season probably contributed to impaired growth of carp and increased consumption of zoobenthos observed in the undesirable fishponds. Our study suggested indeed that water transparency, often used as a simple indicator of water quality in fishponds, did not correspond well with the ecological status, not even biodiversity, of the fishponds under study. The voracious undesirable fish overexploited zooplankton and significantly out-competed commercially reared common carp, which then switched more to foraging on zoobenthos. Thus, efficient eradicating undesirable fish seems to be a superior tool for overall restoring the eutrophic fishpond ecosystems and improving their ecological status. • Voracious undesirable fish cause a threat to carp production and fishpond ecosystem. • Planktivorous fish out-compete carp, which then switch to foraging on zoobenthos. • Eradicating undesirable fish is a superior tool for restoring eutrophic fishponds.

Openness of Fish Habitat Matters: Lake Pelagic Fish Community Starts Very Close to the Shore

Fish communities differ significantly between the littoral and the pelagic habitats. This paper attempts to define the shift in communities between the two habitats based on the European standard gillnet catch. We sampled the benthic and pelagic habitats from shore to shore in Lake Most and Římov Reservoir (Czech Republic). The 3 m deep pelagic nets were spanned across the water body at equal distances from two boundary points, where the depth was 3.5 m. The benthic community contained more fish, more species, and smaller individuals. The mild sloped littoral with a soft bottom attracted more fish than the sloping bank with a hard bottom and less benthos and large Daphnia. The catch of the pelagic nets was dominated by eurytopic fish—rudd (Scardinius erythrophthalmus) and roach (Rutilus rutilus) in Most and bleak (Alburnus alburnus) in Římov. With the exception of one case where overgrown macrophytes extended the structured habitat, the largest shift from the benthic to the pelagic community was observed only in the first pelagic gillnet above the bottom depth of 3.5 m. Open water catches were relatively constant with small signs of decline towards the middle of the lake. The results indicate that the benthic gillnet catch is representative of a very limited area and volume, while most of the volume is dominated by the pelagic community. This has important consequences for the assessment of the community parameters of the whole lake following the European standards for gillnet sampling design.

Feeding Ecology of Invasive Three-Spined Stickleback (Gasterosteus aculeatus) in Relation to Native Juvenile Eurasian Perch (Perca fluviatilis) in the Pelagic Zone of Upper Lake Constance

The fish community of Lake Constance, a large, deep, oligotrophic lake has undergone drastic changes in recent years, with the sudden rise to dominance of invasive three-spined stickleback (Gasterosteus aculeatus) in the pelagic zone, a rather atypical habitat for this species in Central Europe. The core objective of this study was to compare the feeding ecology of stickleback and young Eurasian perch (Perca fluviatilis) in this unique situation to identify reasons for this unexpected dominance, a possible food niche and feeding time overlap, and to discuss consequences for the reshaped pelagic fish community. The diel feeding patterns and prey compositions of pelagic sticklebacks and juvenile (0+) perch sampled in October 2018 and March 2019 were studied analyzing stomach contents. The diets of both species mostly comprised zooplankton, with copepods appearing in the greatest numbers. Benthic and airborne insects were consumed occasionally, mostly by sticklebacks. Both species exhibited peaks of feeding activity early in the morning, afternoon and dusk, and in both species, stomachs were fullest at dusk. Stickleback stomachs contained about 20% more prey at night than perch, and mean estimated nocturnal stomach fullness values were almost 50% greater. Night feeding in sticklebacks was confirmed by digestive states, pointing to a possible competitive advantage over perch. Dietary composition varied over a 24-h cycle and the pattern of consumption of different prey varied between the species. Perch consumed more comparatively small cladocerans (Bosmina spp.), while larger Daphnia appeared more often in stickleback stomachs. In both species, seasonal variation in diet mirrored food availability, indicating some degree of opportunism. A Morisita-Horn index value of 0.95 confirmed dietary niche overlap between species, suggesting the large population of sticklebacks may exert a competitive effect on juvenile perch when resources are limited. Both the longer feeding periods and greater intake of nutritive high quality prey like daphnids can contribute to the rapid success of stickleback in dominating the pelagic zone of Lake Constance.

The Effect of Salinity and Nutrient Load on the Ecosystems of Selected Lakes in the South of Siberia

Salinity largely determines the species diversity and structure of the food web in lake ecosystems. Many studies have been focused on the salinity tolerance of various plankton species. There are fewer studies investigating the modifications in the food web interactions under the effect of salinity. An appropriate system for such studies is closely located lakes of different salinity. We took samples during the summer stratification and analyzed the physicochemical and biological characteristics of 20 lakes (salinity 0.1-35 g/l) in southern Siberia in order to provide general description of this system and to test a number of hypotheses about the effect of salinity on the structure of the food web. The analysis revealed two key factors structuring ecosystems of the studied lakes. The species composition of zooplankton was determined by salinity, with a decrease in the number of species and the transformation of the community from a diverse community of zooplankton (salinity up to 3 g/l) to the dominance of large daphnia (3-8 g/l), copepods (&gt; 8 g/l), small cladocerans and rotifers (20-30 g/l), and Artemia (&gt; 30 g/l). The top predator (fish) was eliminated at salinity above 10 g/l. The biomass of zooplankton did not depend on salinity, but significantly increased in lakes with an increased nutrient load. The nutrient load also had a significant effect on the concentration of total phosphorus, turbidity and transparency of water, and the concentration of chlorophyll “a”. Thus, we recorded the structuring effect of the nutrient load (bottom-up control) on the biomass of the trophic levels and the parameters of water transparency, while salinity transformed the trophic chain from the top, leading to the disappearance of fish and a change in the dominant species of zooplankton. At the same time, salinity did not affect the biomass of the trophic levels. In order to detect top-down effects in the ecosystems of saline lakes, it is necessary to perform a comparative analysis of the seasonal dynamics of ecosystems of lakes with different salinity

Long-term ecosystem change in two highly degraded Lake Ontario (Canada) coastal wetlands

Coastal wetlands are essential to the ecosystem health of the Laurentian Great Lakes (North America) watershed. Multiple anthropogenic stressors have been impacting coastal wetlands since European settlement ca. 1850, and remain a concern for wetland health as watershed development intensifies. We used paleolimnological techniques to explore temporal ecosystem dynamics over the last ~ 100 years in two highly degraded Lake Ontario coastal wetlands located in southern Ontario, Canada, using Cladocera (Branchiopoda, Crustacea) subfossil remains as paleoecological indicators. In Cootes Paradise Marsh (Hamilton, Ontario), cladoceran assemblage changes exhibited a shift in dominance from Chydorus to Bosmina, at the turn of the twentieth century. That shift likely reflected the loss of aquatic macrophytes, and corresponds to the postulated timing of the arrival of invasive carp. Despite recent remediation efforts, including attempts to exclude carp from the wetland, little ecological recovery is evident from the subfossil Cladocera assemblage. No Daphnia remains were observed in our sediment core from Cootes Paradise, in contrast to previous studies on extant zooplankton communities, which reported a large Daphnia population in the west end of the marsh in the 1940s. This could indicate that our sediment core recorded ecological changes solely in the east end of Cootes Paradise Marsh. In Jordan Harbour (Lincoln, Ontario), Bosmina were dominant throughout the sediment record, and increases in littoral cladocerans were observed in the most recent sediments, in particular the appearance of periphytic Pleuroxus taxa after ~ 2008. This suggests that some recovery of aquatic macrophyte communities occurred in response to shoreline remediation efforts. Bosmina size structure exhibited only minimal changes in both wetlands, despite known large changes in historic fish community structure. Overall, our study provides perspectives on the benefits and limitations of paleolimnology for documenting ecological change in the Laurentian Great Lakes coastal wetlands.

Long-term dynamics of a cladoceran community from an early stage of lake formation in Lake Fukami-ike, Japan.

An increase in nutrient levels due to eutrophication has considerable effects on lake ecosystems. Cladocerans are intermediate consumers in lake ecosystems; thus, they are influenced by both the bottom‐up and top‐down effects that occur as eutrophication progresses. The long‐term community succession of cladocerans and the effects cladocerans experience through the various eutrophication stages have rarely been investigated from the perspective of the early‐stage cladoceran community assemblage during lake formation. In our research, long‐term cladoceran community succession was examined via paleolimnological analysis in the currently eutrophic Lake Fukami‐ike, Japan. We measured the concentration of total phosphorus and phytoplankton pigments and counted cladoceran and other invertebrate subfossils in all layers of collected sediment cores, and then assessed changes in the factors controlling the cladoceran community over a 354‐year period from lake formation to the present. The cladoceran community consisted only of benthic taxa at the time of lake formation. When rapid eutrophication occurred and phytoplankton increased, the benthic community was replaced by a pelagic community. After further eutrophication, large Daphnia and high‐order consumers became established. The statistical analysis suggested that bottom‐up effects mainly controlled the cladoceran community in the lake's early stages, and the importance of top‐down effects increased after eutrophication occurred. Total phosphorus and phytoplankton pigments had positive effects on pelagic Bosmina, leading to the replacement of the benthic cladoceran community by the pelagic one. In contrast, the taxa established posteutrophication were affected more by predators than by nutrient levels. A decrease in planktivorous fish possibly allowed large Daphnia to establish, and the subsequent increase in planktivorous fish reduced the body size of the cladoceran community.

Diel changes in vertical and horizontal distribution of cladocerans in two deep lakes during early and late summer

Temporal and spatial heterogeneity in the distribution of cladocerans in lakes could be caused by abiotic (wind, water currents) and biotic factors (reproduction, food resources, predation). Diel horizontal and vertical distribution of cladoceran assemblages was studied in two deep lakes (Milada and Most Lakes, Czech Republic) in early (June) and late (September) summer. The objective was to study diel vertical migration (DVM) and diel horizontal migration (DHM) of cladocerans under conditions of different macrophyte cover in littoral areas (rich in Milada Lake, poor in Most Lake) and fish assemblages (non-specialised planktivorous species in Milada Lake, and abundant planktivorous fish - maraena whitefish, Coregonus maraena - in open water habitats in Most Lake). Temporal variations in cladoceran assemblages were reported in both lakes in the two sampling periods. DVM was observed in the two lakes, performed by the most vulnerable species to fish predation (the larger Daphnia spp.), but with different patterns (direct and reverse) probably linked with the local fish community and other biotic and abiotic factors in each lake. Horizontal movements were only observed in Most Lake: D. longispina increase its abundance in open waters at night compared to the littoral points; while Ceriodaphnia spp. showed the inverse pattern. In both lakes, higher densities were often found at night in surface layers, producing a great “diurnal deficit”: cladocerans remain undetected in some zones during the day (especially in the littoral areas) moving to surface layers at night.

The Effects of Sodium Percarbonate Generated Free Oxygen on Daphnia—Implications for the Management of Harmful Algal Blooms

Increasing frequencies and durations of harmful algal blooms are a nuisance in many aquatic ecosystems. This has led to the use of a variety of control methods to prevent their appearance or to disperse them following their establishment. Most of these methods are not selective; consequently, research into alternative selective methods has been ongoing. Reactive oxygen species generated following the addition of hydrogen peroxide have been shown to selectively target the cyanobacterial component of harmful algal blooms in experimental and field settings. This study assesses the effects of increasing concentrations of reactive oxygen species from the addition of sodium percarbonate on zooplankton in a small experimental setting using a natural plankton sample. It was found that the genus Daphnia showed moderate sensitivity to sodium percarbonate. Preliminary evidence suggests that the size of an individual may affect the probability of survival, with larger individuals having a lower likelihood of survival. Lower survival rates of large Daphnia were hypothesized to have been caused by higher relative filtration rates of larger individuals. From the zooplankton data obtained, we suggest that a safe concentration of sodium percarbonate for Daphnia individuals would be below 10.0 mg·L−1 sodium percarbonate (2.8 mg·L−1 hydrogen peroxide).

Responses of aquatic organisms to cyanobacteria and elodea in microcosms

The aim of the work was to investigate the effect of cyanobacteria and elodea on population characteristics (abundance, biomass, fecundity) and “health” indicators (embryo status, heart rate and thermal resistance) of aquatic invertebrates in microcosms with controlled conditions. In July-August 2018, four treatments with environmental conditions of the coastal zone of the Rybinsk Reservoir during the period of cyanobacterial bloom were modeled. Mass species of plankton (with 80% dominance of large Daphnia longispina) and benthic animals (Unio pictorum, Gmelinoides fasciatus, Asellus aquaticus) and Elodea canadensis were placed in microcosms. In treatments with cyanobacteria, the toxic microcystins-LR were detected (24-53% of the sum of toxins, or 6,6-66,5 µg/l). The cyanobacteria Aphanizomenon flos-aquae, Microcystis aeruginosa and Gloeoitrichia spp. affected the structure of plankton reflecting in a decrease in the amount of green algae and an increase in the abundance of small crustaceans. Under the influence of cyanobacteria, variables of benthic animals differed significantly from control treatment, including a reduced level of thermal resistance of mollusks and amphipods, an increased recovery time of the heart rate after loading (salinity test) in mollusks, a decrease in individual fecundity, and an increase in the frequency of malformations and embryo mortality in amphipods. The presence of elodea enhanced these effects. Thus, the mass development of cyanobacteria and elodea affects the physiological status of sensitive species of aquatic animals, reducing their adaptability to natural stresses and worsening the quality of offspring.

Bottom-Up Forces Drive Increases in the Abundance of Large Daphnids in Four Small Lakes Stocked with Rainbow Trout (Oncorhynchus mykiss), Interior British Columbia, Canada

The introduction of salmonids into lakes of western North America for sport fishing is a widespread phenomenon. While numerous investigations have documented cascading trophic interactions upon the introduction of fish into naturally fishless systems, little research has been done to investigate the importance of natural fish status (fishless vs. fish bearing) in modulating historical food web response to dual forcing by bottom-up (resource regulation from nutrients) and top-down (planktivory from stocked fish) processes. We used the paleolimnological record to reconstruct food web changes in four lakes in interior British Columbia that have been stocked with rainbow trout since the early to mid-1900s. Analysis of pigments, diatoms, and Cladocera was undertaken in cores from all lakes. We predicted that if fish were important in structuring cladoceran abundance and composition, we would document a decline in the abundance of large daphnids post-stocking in our two naturally fishless lakes, and little change in the two fish-bearing lakes. Instead, we documented increased abundance of large daphnids after stocking in all lakes in the early to mid-1900s, a finding inconsistent with size-selective predation from planktivorous fish. Further, our data suggest that deep, low-oxygen refugia may be important in sustaining populations of large Daphnia, a process which was enhanced by increased nutrients and lake production according to sub-fossil diatom and pigment analyses. This study shows that fish stocking does not invariably result in a decrease in large-bodied Cladocera and that nutrients and lake type can modulate the response of invertebrate planktivores.

The stoichiometric ratios (C:N:P) in a pelagic food web under experimental conditions

Interactions between phytoplankton and zooplankton affect the overall functioning of lakes. Herbivores are habitually confronted with food of inferior quality, usually a result of low nutrient concentrations in plant material. Large-bodied cladocerans are better competitors for food than small-bodied species but they are more vulnerable to low food quality. Understanding the effects of food quality on zooplankton structure and competition between small - large bodied herbivorous is of considerable interest. We want to find out how differences in C:N:P ratios between phytoplankton and zooplankton communities affect their abundances in a freshwater food web. We want also to assess the role of phytoplankton and zooplankton as sinks of the phosphorus and nitrogen. Therefore, we conducted a 31-day mesocosms experiment with water from a mesotrophic and a eutrophic lake (with natural plankton communities). To simulate changes in the plankton communities large-bodied Daphnia magna and Daphnia pulicaria were added. Samples for zooplankton, phytoplankton and water chemistry were taken every 10 days. Samples for elemental analysis (C:N:P) of seston and zooplankton were collected on the first, and on the final day of the experiment.Our mesocosms experiment showed mismatch in C:P between seston (high) and zooplankton (low), which suggests that most of the phosphorus is incorporated in zooplankton biomass. This evidenced that zooplankton is an effective sink of phosphorus, while nitrogen is accumulated mainly by primary producers. Our results also indicated more stability in stoichiometry with increasing trophic levels of organisms. However, there were significant changes in the zooplankton structure. The increasing dominance of large Daphnia resulted in reduction of C:P ratio in zooplankton. Low food quality (C:P) did not limit the growth of large Daphnia in the experimental conditions, which competed effectively with small planktonic cladocerans and with Rotifera. Over time, inedible algae began to dominate resulting in increase of relative biomass of periphyton grazers, which suggests that plankton community is transformed into littoral system in mesocosms for about 30 days.

Trophic cascade strength is influenced by size frequency distribution of primary consumers and size-selective predation: examined with mesocosms and modeling

Understanding variation in trophic cascades is critical for developing management strategies for invasive and introduced species as well as in designing biomanipulation experiments. We evaluated, in fish driven trophic cascades, the responses of invertebrate biomass, body size, and production both experimentally and through a model. Experimental mesocosm treatments of small (Moina), large (Daphnia pulex), and mixed size zooplankton communities were characterized with and without size-selective fish (bluegill, Lepomis macrochirus). Experimental responses were typical of trophic cascades showing reduced biomass, body size and production of zooplankton and generally enhanced chlorophyll a. Although zooplankton biomass was similarly reduced in all fish treatments, production for the fish D. pulex treatment was lower than the other fish treatments because larger taxa have a lower ratio of production to biomass (P/B). As would be expected based on zooplankton production among fish treatments, response of chlorophyll a in treatments with D. pulex was faster and final concentration higher than in Moina or mixed size treatments. A model of changes in production under a range of reductions in body size and/or biomass, from no reduction to 99% reduction, supported experimental results indicating invertebrate production can be resilient to reduction of biomass when body size is reduced. Strength of trophic cascades may therefore, in part, be related to interactions among reduction in biomass, size-selective predation, and size frequency distribution and plasticity of prey community. Because size-selective predation is a common trait in trophic cascades, some level of compensation is likely in most trophic cascades.

Hydrological variability, zooplankton availability and the shift between planktivore-benthivore feeding behaviour in the visual predator fish, Odontesthes bonariensis

Hydrology is a key factor in the dynamic of zooplankton community in lakes with high water renewal rate. Crustaceans’ zooplankton biomass can sharply decline during flooding periods as consequence of wash-out effect. The hydrologically induced changes in zooplankton community may certainly affect the feeding behaviour of planktivorous fishes, but this has not been studied yet. The inland silverside, Odontesthes bonariensis (Valenciennes, 1835), is the main visual planktivorous fish in temperate shallow lakes of South America. We analyzed the diet composition and feeding strategy of O. bonariensis in a large run-of-the-river shallow lake, along two consecutive spring-summer periods with contrasting hydrological conditions. Water residence time varied one order of magnitude between flood and drought period. As it was expected, silversides showed size selective feeding on large cladocerans during drought period when zooplankton biomass was high. On the other hand, during high flushing period, zooplankton abundance decreased noticeable and silversides drastically changes its feeding behaviour from visual planktivore to benthivore. The analysis of the feeding strategy of O. bonariensis showed that this species largely behaves as a specialist irrespective of the hydrological conditions. However, the main involved prey varied from large Daphnia during low flow conditions to large Ostracoda during floods. Our study reveals that the variation in the availability of suitable prey induced by hydrological variability is associated with a drastic change in feeding grounds and prey selection of O. bonariensis. This flexible predatory behaviour may have consequences on food web dynamics of shallow lakes that are further discussed.

Echyridella menziesii (Bivalvia: Hyriidae) as a predator of zooplankton of different sizes; are large non-indigenous Daphnia a potential food source?

ABSTRACTInteractions between two recent invaders to New Zealand, the cladocerans Daphnia galeata and D. pulex, and native filter-feeding freshwater mussels, are unknown. We examined predation rates of the common native mussel Echyridella menziesii (kākahi, kāeo) on non-native Daphnia (comparatively large zooplankton in New Zealand), relative to two common native species, the smaller cladoceran Bosmina meridionalis and rotifer Brachionus calyciflorus. Controlled laboratory experiments were conducted in which each zooplankton species was exposed to bivalve predation for a two-hour period. Comparing treatments to non-mussel controls, removal rates of non-indigenous D. pulex and native B. calyciflorus were statistically significant. Nevertheless, kākahi removal rates may not be ecologically significant for daphnids (1.7% D. galeata and 7.4% D. pulex). Kākahi removed 8.8% of B. meridionalis and 30.2% of B. calyciflorus, suggesting that small, feeble zooplankton species (particularly rotifers) are most susceptible to predation, and potentially function as an energy source for native freshwater mussels.

Oddity, predation risk and social decisions in aquatic invertebrates

AbstractGroup living is widespread across animal taxa, incurring benefits such as increased foraging efficiency or an enhanced chance of surviving a predator's attack. The chances of escaping a predator are often lower for odd‐looking individuals, as these are detected at a higher rate than uniform looking group members. While this “oddity effect” shall operate in animals differing in any given phenotype, including colour, size or species identity, it has been experimentally tested mainly for odd‐coloured individuals. We examined the oddity effect and swarming preferences in two differently sized species of freshwater planktonic crustaceans (largeDaphnia magnaand smallDaphnia pulex). We experimentally investigated whether odd individuals in a swarm of heterospecificDaphniawere more vulnerable to predation by a common predator, the three‐spined stickleback (Gasterosteus aculeatus). Furthermore,Daphnia'sswarming preference was tested by giving individuals the opportunity to choose between conspecific/heterospecific odour and a neutral control. In contrast to the predictions of the oddity effect, odd individuals were not always preyed on earlier; instead three‐spined stickleback preferentially predated large, more nutritious individuals.Daphniaof both species reacted towards the perception of con‐ and heterospecifics odours. WhileD. pulexgenerally avoided the smell of other daphnids,D. magnaavoided conspecifics but tended to prefer heterospecifics over the neutral control. These findings provide new insights into swarming strategies and social preference of an invertebrate and how this behaviour can influence predation risk.

Fish-mediated plankton responses to increased temperature in subtropical aquatic mesocosm ecosystems: Implications for lake management

Although it is well established that climate warming can reinforce eutrophication in shallow lakes by altering top-down and bottom-up processes in the food web and biogeochemical cycling, recent studies in temperate zones have also shown that adverse effects of rising temperature are diminished in fishless systems. Whereas the removal of zooplanktivorous fish may be useful in attempts to mitigate eutrophication in temperate shallow lakes, it is uncertain whether similar mitigation might be achieved in warmer climates. We compared the responses of zooplankton and phytoplankton communities to climate warming in the presence and absence of fish (Aristichthys nobilis) in a 4-month mesocosm experiment at subtropical temperatures. We hypothesized that 1) fish and phytoplankton would benefit from warming, while zooplankton would suffer in fish-present mesocosms and 2) warming would favor zooplankton growth but reduce phytoplankton biomass in fish-absent mesocosms. Our results showed significant interacting effects of warming and fish presence on both phytoplankton and zooplankton. In mesocosms with fish, biomasses of fish and phytoplankton increased in heated treatments, while biomasses of Daphnia and total zooplankton declined. Warming reduced the proportion of large Daphnia in total zooplankton biomass, and reduced the zooplankton to phytoplankton biomass ratio, but increased the ratio of chlorophyll a to total phosphorus, indicating a relaxation of zooplankton grazing pressure on phytoplankton. Meanwhile, warming resulted in a 3-fold increase in TP concentrations in the mesocosms with fish present. The results suggest that climate warming has the potential to boost eutrophication in shallow lakes via both top-down (loss of herbivores) and bottom-up (elevated nutrient) effects. However, in the mesocosms without fish, there was no decline in large Daphnia or in total zooplankton biomass, supporting the conclusion that fish predation is the major driver of low large Daphnia abundance in warm lakes. In the fishless mesocosms, phytoplankton biomass and nutrient levels were not affected by temperature. Our study suggests that removing fish to mitigate warming effects on eutrophication may be potentially beneficial in subtropical lakes, though the rapid recruitment of fish in such lakes may present a challenge to success in the long-term.

Effect of the vital activity products of beaver (Castor fiber L.) on the formation of zooplankton structure: Changes in the quantitative parameters of two cladocera species of different sizes in a beaver pound (in situ experiment)

The experiments were performed in microcosms situated on a control site and on a site regulated by beavers. Two cladoceran species of different sizes were placed in microcosms in different proportions. It was found that the vital activity products of Castor fiber L. promote an increase in the concentration of total phosphorus (P), a decrease in the N/P ratio in water, and an increase in the chlorophyll a concentration and the abundance and biomass of bacterioplankton. In such conditions, the abundance and biomass of the large species Daphnia (Ctenodaphnia) magna Straus was increased to the greatest extent when compared with background values when it was placed in combination with Ceriodaphnia dubia Richard or without it. Moreover, in the case of the combination of two crustacean species in microcosms, the abundance of C. dubia was lower when compared with the control site, while the abundance of D. magna was higher. The results of a bioassay showed that the productivity of C. dubia decreases in waters where D. magna is most abundant. We concluded that the formation of zooplankton in beaver ponds with specific features determined by the presence of large cladoceran species is provided by changes in the quantitative and qualitative parameters of food resources. This contributes to the massive distribution of large D. magna and its competitive relations with small C. dubia. According to the results of a bioassay, the vital activity products of D. magna can inhibit the fecundity of C. dubia.

Microcystis aeruginosa strengthens the advantage of Daphnia similoides in competition with Moina micrura

Microcystis blooms are generally associated with zooplankton shifts by disturbing interspecific relationships. The influence of Microcystis on competitive dominance by different sized zooplanktons showed species-specific dependence. We evaluated the competitive responses of small Moina micrura and large Daphnia similoides to the presence of Microcystis using mixed diets comprising 0%, 20%, and 35% of toxic M. aeruginosa, and the rest of green alga Chlorella pyrenoidosa. No competitive exclusion occurred for the two species under the tested diet combinations. In the absence of M. aeruginosa, the biomasses of the two cladocerans were decreased by the competition between them. However, the Daphnia was less inhibited with the higher biomass, suggesting the competitive dominance of Daphnia. M. aeruginosa treatment suppressed the population growths of the two cladocerans, with the reduced carrying capacities. Nonetheless, the population inhibition of Daphnia by competition was alleviated by the increased Microcystis proportion in diet. As a result, the competitive advantage of Daphnia became more pronounced, as indicated by the higher Daphnia: Moina biomass ratio with increased Microcystis proportions. These results suggested that M. aeruginosa strengthens the advantage of D. similoides in competition with M. micrura, which contributes to the diversified zooplankton shifts observed in fields during cyanobacteria blooms.

Bigger is better: changes in body size explain a maternal effect of food on offspring disease resistance.

Maternal effects triggered by changes in the environment (e.g., nutrition or crowding) can influence the outcome of offspring–parasite interactions, with fitness consequences for the host and parasite. Outside of the classic example of antibody transfer in vertebrates, proximate mechanisms have been little studied, and thus, the adaptive significance of maternal effects on infection is not well resolved. We sought to determine why food‐stressed mothers give birth to offspring that show a low rate of infection when the crustacean Daphnia magna is exposed to an orally infective bacterial pathogen. These more‐resistant offspring are also larger at birth and feed at a lower rate. Thus, reduced disease resistance could result from slow‐feeding offspring ingesting fewer bacterial spores or because their larger size allows for greater immune investment. To distinguish between these theories, we performed an experiment in which we measured body size, feeding rate, and susceptibility, and were able to show that body size is the primary mechanism causing altered susceptibility: Larger Daphnia were less likely to become infected. Contrary to our predictions, there was also a trend that fast‐feeding Daphnia were less likely to become infected. Thus, our results explain how a maternal environmental effect can alter offspring disease resistance (though body size), and highlight the potential complexity of relationship between feeding rate and susceptibility in a host that encounters a parasite whilst feeding.

Food availability and initial relative abundance determine the outcome of interspecific competition between two different‐sized cladocerans

Competition amongst different‐sized cladocerans has attracted the interest of many researchers because of their similar life strategies and overlap in feeding habits. To test the combined effects of food concentration and initial relative abundance on the competition outcomes between small‐ and large‐bodied cladocerans, we grew large Daphnia magna and small Moina micrura with two initial relative abundances under five algal densities. At the lowest food concentration (0.10 mg L−1 carbon), D. magna could not maintain a population in the presence or absence of M. micrura. Under conditions in which both species grew together, food levels as high as 0.30 mg L−1 favoured superior population growth of D. magna, irrespective of inoculation abundance. Under conditions in which both species had equal initial abundance, population growth of M. micrura was inhibited by the presence of Daphnia at ≥0.30 mg L−1 food levels. M. micrura was superior to D. magna at higher food concentrations (≥0.60 mg L−1) and high initial abundance. These results suggest that population growth of large Daphnia was more successful than that of small Moina when the food availability was low, while the small Moina dominated under conditions of high initial abundance and high food supply. Therefore, interspecific competition amongst cladocerans depends not only on the size of competing species, but also on their initial relative abundance and food availability.