On the cost of vertical migration: are feeding conditions really worse at greater depths?

SUMMARY 1. The ultimate explanation for diel vertical migration (DVM) of zooplankton is the avoidance of visual predation in surface waters. Studies on migrating zooplankton have shown that remaining in the cold and food-poor hypolimnion during the day, however, has demographic costs. Higher temperatures and greater food concentrations in the surface waters are thought to be the main reasons why Daphnia species move upwards at night.
2. In this study, we investigated the growth condition of daphniids raised on seston taken from different depths from a lake with and without a deep-water chlorophyll maximum.
3. Juvenile growth rates of Daphnia galeata x hyalina from the lake without a deep-water chlorophyll maximum were similar for all treatments. After temperature correction, however, growth rates were significantly higher on seston taken from the surface layers.
4. In contrast, in the lake with the deep-water chlorophyll maximum, D. galeata growth rates were higher in deeper strata, even after temperature correction. Although this lake had a weak temperature gradient, D. galeata left the food-rich strata at night and migrated into the surface food-poor environment. Invertebrate predation and oxygen depletion are probably not the reasons for the nocturnal upward migration into the surface strata. Therefore, we assume that D. galeata migrates upwards to take advantage of higher temperatures. Using several temperature–egg-development models, we could not, however, fully explain this behaviour.     

Effects of a filter-feeding fish [silver carp, Hypophthalmichthys molitrix (Val.)] on phyto- and zooplankton in a mesotrophic reservoir: results from an enclosure experiment

SUMMARY 1. Silver carp, Hypophthalmichthys molitrix (Val.), feeds on both phyto- and zooplankton and has been used in lake biomanipulation studies to suppress algal biomass. Because reports on the effects of silver carp on lake food webs have been contradictory, we conducted an enclosure experiment to test how a moderate biomass of the fish (10 g wet weight m⁻³) affects phytoplankton and crustacean zooplankton in a mesotrophic temperate reservoir.
2. Phytoplankton biomass <30 μm and particulate organic carbon (POC) <30 μm were significantly higher in enclosures with silver carp than in enclosures without fish, whereas Secchi depth was lower. Total copepod biomass declined strongly in both treatments during the experiment, but it was significantly higher in fish-free enclosures. Daphnid biomass was also consistently higher in enclosures without fish, although this effect was not significant. However, the presence of fish led to a fast and significant decrease in the size at maturity of Daphnia galeata Sars. Thus, the moderate biomass of silver carp had a stronger negative effect on cladoceran zooplankton than on phytoplankton.
3. Based on these results and those of previous studies, we conclude that silver carp should be used for biomanipulation only if the primary aim is to reduce nuisance blooms of large phytoplankton species (e.g. cyanobacteria) that cannot be effectively controlled by large herbivorous zooplankton. Therefore, stocking of silver carp appears to be most appropriate in tropical lakes that are highly productive and naturally lack large cladoceran zooplankton.     

Long-term responses of zooplankton to invasion by a planktivorous fish in a subarctic watercourse

1. Introduced or invading predators may have strong impacts on prey populations of the recipient community mediated by direct and indirect interactions. The long-term progression of predation effects, covering the invasion and establishment phase of alien predators, however, has rarely been documented.
2. This paper documents the impact of an invasive, specialized planktivorous fish on its prey in a subarctic watercourse. Potential predation effects on the crustacean plankton, at the community, population and individual levels, were explored in a long-term study following the invasion by vendace ( Coregonus albula ).
3. Over the 12-year period, the density and species richness of zooplankton decreased, smaller species became more abundant and Daphnia longispina , one of the largest cladocerans, was eliminated from the zooplankton community.
4. Within the dominant cladocerans, including Daphnia spp., Bosmina longispina and Bosmina longirostris , the body size of ovigerous females and the size at first reproduction decreased after the arrival of the new predator. The clutch sizes of Daphnia spp. and B. longirostris also increased.
5. Increased predation pressure following the vendace invasion induced many effects on the crustacean zooplankton, and we document comprehensive and strong direct and indirect long-term impacts of an introduced non-native predator on the native prey community.     

Cladoceran assemblages, seasonal succession and the importance of a hypolimnetic refuge

SUMMARY. 1. We investigate the importance of a refuge from fish predation to the abundance, species composition and seasonal succession of zooplankton. Thirty lakes representing a range of depths were sampled twice in summer for physical/chemical parameters and zooplankton community structure.
2. We define the refuge from centrarchid predators to be that space between the thermocline and the zone of anoxia. As lakes vary in rate of oxygen depletion from the hypolimnion. the refuge size and lake depth are independent: refuge size decreases during the summer period.
3. Lake depth and refuge size independently explain variation among lakes in zooplankton species composition, but seasonal community change within lakes is best predicted by loss of refuge size.
4. Refuge size also explains the substantial variation in the relative dominance of the two major daphnid species. Lakes possessing a large refuge are dominated by D. pulicaria ; those with a small refuge are dominated by the smaller, D. galeata mendotae . We suggest that lakes of intermediate refuge size, which are characterized by high species diversity, represent a more equitable balance of predation and competition.     

Hypolimnetic anoxia hampers top–down food-web manipulation in a eutrophic lake

SUMMARY 1. A biomanipulation experiment was carried out in a small (10 ha), but relatively deep (17 m) and highly eutrophic lake in northern Poland. The lake had been stocked in 1996, 1997 and 1998 with a variety of piscivorous fish (pike, catfish, trout and pikeperch), in order to reduce numbers of cyprinid planktivores.
2. Piscivore stocking was associated with a threefold decrease in the offshore fish density (night echosounding). Despite this reduction, the large planktonic cladoceran, Daphnia hyalina , remained scarce, whereas the density of small-sized zooplankton increased greatly.
3. The lack of demographic response in D. hyalina was probably due to the anoxia in the hypolimnetic refuge of this vertically migrating species. The anoxic hypolimnion, below 3–4 m depth, was inhabited day and night by numerous Chaoborus flavicans larvae.
4. Changes in zooplankton were associated with shifts in the taxonomic composition (from single-cell green algae to filamentous cyanobacteria), size structure (from nano- to net phytoplankton) and seasonal dynamics of phytoplankton, but not in the average biomass of planktonic algae. A clear-water phase, which was absent in the prestocking years, developed in spring, with Secchi depth reaching 2.5 m, a value which had never been recorded in the 20 years preceding the biomanipulation. In general, the lake's status was switched from hypertrophic to eutrophic.
5. Deteriorating food conditions, resulting from qualitative changes in the phytoplankton community, combined with predation pressure by the remaining fish and Chaoborus larvae were associated with the ultimate elimination of D. hyalina from the lake.     

Trophic interactions in rockpool food webs: regulation of zooplankton and phytoplankton by Notonecta and Daphnia

We studied trophic interactions in experimental rockpools with three different food web structures: phytoplankton and small-bodied zooplankton; phytoplankton, small-bodied zooplankton and Daphnia ; and phytoplankton, small-bodied zooplankton, Daphnia and Notonecta . Nutrients, primary productivity, chlorophyll a and zooplankton species composition and biomass were measured over eight weeks.
2. Daphnia had a negative impact on other zooplankton and reduced the phytoplankton biomass and primary productivity. In the absence of Daphnia , small-bodied zooplankton species were abundant, in particular cyclopoid copepods. Concentrations of dissolved nutrients were lower and the standing crop of primary producers was higher when Daphnia was absent.
3. The presence of the invertebrate predator Notonecta produced a top-down effect which was similar to that reported for planktivorous fish, i.e. a selective reduction of daphnids followed by an increase of small-bodied zooplankton species and phytoplankton biomass.
4. The study showed that consumer regulation of Daphnia by Notonecta and of algae by Daphnia are important, but also demonstrated that trophic level biomasses were controlled by a combination of predation and resource limitation.     

Biotic relations affecting species structure in zooplankton communities

Mathematical modelling and laboratory experiments were used to study how exploitative competition and predation influence the species structure in cladoceran community. For five species of Cladocera (Sida crystallina, Daphnia magna, Simocephalus vetulus, Daphnia longispina, and Diaphanosoma brachyurum), representing a gradient of body size, population characteristics were described as functions of food concentration. Abundance dynamics were simulated in mixed species cultures and invasion experiments under different levels of food supply corresponding to oligo-, meso-, and eutrophic conditions. Separate simulations were also run including and excluding (fish) predation. The competitive ability of each species was estimated as the values of the population equilibrium food concentration. Simulation results showed that for the no-predator scenario, increases in the level of food supply promoted species coexistence while under lower food concentrations only one species remained at the end of the simulation runs. When predation was allowed, the number of species that coexisted at the end of the simulations increased up to four species, indicating that predator pressure facilitated species coexistence because it shortened periods of food depletion. Simulation results were verified in laboratory experiments which suggested that population equilibrium food concentration can be used as an estimate of competitive ability. Finally, species structure and relative abundance in Lake Naroch (Belarus) during the summer of 2004 was found to be consistent with our results from computer simulation and laboratory experiments with regard to competition and predation impacts on zooplankton community. Handling editor: S. I. Dodson     

Relative vulnerabilities of natural rotifer and ciliate communities to cladocerans: laboratory and field experiments

SUMMARY. 1. Two experiments with plankton communities from Storrs Pond (NH), one conducted in the laboratory and one in field enclosures, assessed the impact of different cladocerans on rotifers and ciliated protozoa.
2. The smallest cladoceran, Bosmina longirostris , did not depress rotifer or ciliate growth rates while the intermediate sized dadoceran, Daphnia galeata mendotae , reduced ciliate growth rates in the enclosure experiment but had only a marginal effect in the jar experiment. D. galeata mendotae had no effect on any of the rotifers in either experiment.
3. In both experiments the largest cladoceran, Daphniapulex , depressed the growth rates of ciliates and those rotifers known to be vulnerable to interference competition. Polyarthra vulgaris , previously shown to be resistant to cladoceran interference, was the only rotifer unaffected by D. pulex in the field experiment but was depressed by the much higher densities of this cladoceran in the laboratory experiment.
4. Cladocerans did not affect phytoplankton or bacterioplankton abundance in either experiment. Therefore the mechanism most likely to be responsible for the suppressive effect of cladocerans on rotifers and ciliates in these experiments is direct mechanical interference or predation, rather than exploitative competition.     

Trophic relationships in the pelagic zone of Mondsee,Austria

Data are presented on nutrient concentrations, phytoplankton biovolume development, zooplankton composition and population dynamics, and fish from a deep, stratifying, alpine lake (Mondsee, Austria) during a three-year period between 1982 and 1984. Development of the phytoplankton is closely related to structuring events of the physico-chemical environment. Dissolved silicate and phosphorus concentrations are critical for the summer situation. During summer algal abundance is largely affected by grazing of zooplankton, but no clear-water phase was observed at the end of the spring peak of phytoplankton.Temperature and food are factors responsible for the timing and growth of the zooplankton populations. Because of close overlap in the epilimnion, exploitative and mechanical interference competition and predation by invertebrate and vertebrate predators are the main structuring forces acting on the zooplankton community, and hence influence phytoplankton indirectly.     

Direct and indirect effects of predation on the genetic structure of a Daphnia population

The effects of fish predation on zooplankton communities arewell documented, but relatively little is known about how predationstructures the genetic composition of individual populations.This study illustrates how a perturbation in the timing andstrength of predation by rainbow trout directly and indirectlyaltered the genetic composition of a Daphnia pulicaria populationin a Minnesota (USA) lake. Trout were stocked in autumn in thefirst 2 years of the study and in spring in the second 2 years.In autumn stocking years, predation was highest over winterand in spring but relatively low during summer stratification.In contrast, in spring stocking years, predation was low overwinter and high during summer stratification. In all years,the Daphnia population became genetically differentiated withrespect to depth, as summer stratification became more pronounced.In addition, allozyme analyses of trout stomach contents revealedselective predation on Daphnia in the metalimnion. In springstocking years, when trout were abundant during the summer,this directly caused a shift in the dominant clone type froma metalimnetic to a hypolimnetic specialist. The fisheries manipulationindirectly affected the genetic composition of the Daphnia populationby altering the importance of recruitment from diapausing embryosin the sediment (the egg bank). In autumn stocking years, whenthe over-wintering population was small, genotype frequenciesin early summer indicated the recent emergence of sexually derivedindividuals from the egg bank. Conversely, in spring stockingyears when over-wintering populations of Daphnia were large,no emergence events were detected.     

The role of light for fish–zooplankton–phytoplankton interactions during winter in shallow lakes – a climate change perspective

1. Variations in the light regime can affect the availability and quality of food for zooplankton grazers as well as their exposure to fish predation. In northern lakes light is particularly low in winter and, with increasing warming, the northern limit of some present-day plankton communities may move further north and the plankton will thus receive less winter light.
2. We followed the changes in the biomass and community structure of zooplankton and phytoplankton in a clear and a turbid shallow lake during winter (November–March) in enclosures both with and without fish and with four different light treatments (100%, 55%, 7% and <1% of incoming light).
3. In both lakes total zooplankton biomass and chlorophyll- a were influenced by light availability and the presence of fish. Presence of fish irrespective of the light level led to low crustacean biomass, high rotifer biomass and changes in the life history of copepods. The strength of the fish effect on zooplankton biomass diminished with declining light and the effect of light was strongest in the presence of fish.
4. When fish were present, reduced light led to a shift from rotifers to calanoid copepods in the clear lake and from rotifers to cyclopoid copepods in the turbid lake. Light affected the phytoplankton biomass and, to a lesser extent, the phytoplankton community composition and size. However, the fish effect on phytoplankton was overall weak.
5. Our results from typical Danish shallow eutrophic lakes suggest that major changes in winter light conditions are needed in order to have a significant effect on the plankton community. The change in light occurring when such plankton communities move northwards in response to global warming will mostly be of modest importance for this lake type, at least for the rest of this century in an IPCC A2 scenario, while stronger effects may be observed in deep lakes.     

Temperature effects on chemical signalling in a predator–prey system

SUMMARY 1. We investigated the effect of temperature on chemical signalling in a predator–prey model system (planktivorous fish and Daphnia galeata ). Life-history changes in Daphnia in response to chemical cues (kairomones) derived from fish have become a paradigm for chemically induced anti-predator defences.
2. As temperature can affect both predator and prey, we carried out two experiments to disentangle these effects. In order to test for temperature effects on the predator, we kept prey at a single temperature and exposed them to kairomones from fish exposed to two different temperatures. Daphnia exhibited a higher intrinsic rate of population increase ( r ) when exposed to fish kairomones produced at high rather than low temperature. Assuming a positive correlation between r (because of an earlier maturation and/or increased clutch sizes) and kairomone concentration, our results suggest that kairomone production increases with rising temperature.
3. In the second experiment, to study the influence of temperature on the prey, Daphnia were kept at two different temperatures and exposed to fish kairomones produced at one constant temperature. We found no interaction between the effects of fish kairomone and temperature on Daphnia life history, suggesting that temperature does not directly alter life-history responses to fish kairomones.
4. Our results suggest that temperature influences Daphnia life history through its effects on fish kairomone concentration, but that temperature does not affect the strength of the response of Daphnia to the presence of fish.     

Selective predation on Cladocera by brown trout Salmo trutta L.

The size and species composition of zooplankton ingested by brown trout in Lough Sheelin was studied. Zooplankton and fish samples were taken simultaneously. The zooplankton samples consisted of Daphnia hyalina var. galeata , Copepoda and Bythotrephes longimanus in the proportion of 246: 10: 1. The entire stomach contents of the fish were examined with particular attention paid to the zooplankton component. The results indicated a high degree of selection for B. longimanus .     

Effects of piscivore-mediated habitat use on growth, diet and zooplankton consumption of roach: an individual-based modelling approach

SUMMARY 1. We used an individual based modelling approach for roach to (i) simulate observed diel habitat shifts between the pelagic and littoral zone of a mesotrophic lake; (ii) analyse the relevance of these habitat shifts for the diet, activity costs and growth of roach; and (iii) quantify the effects of a hypothetical piscivore-mediated (presence of pikeperch) confinement of roach to the littoral zone on roach diet, activity costs and growth.
2. The model suggests that in the presence of pikeperch, roach shifts from zooplankton as the primary diet to increased consumption of less nutritious food items such as macrophytes, filamentous algae and detritus.
3. The growth of roach between May and October was predicted to be significantly higher in the absence of pikeperch, although the net activity costs were about 60% higher compared with the scenario where pikeperch were present.
4. These modelling results provide quantitative information for interpreting diel horizontal migrations of roach as a result from a trade-off between food availability and predation risk in different habitats of a lake.
5. Altering the habitat selection mode of planktivorous roach by piscivore stocking has the potential to reduce zooplankton consumption by fish substantially, and could therefore be used as a biomanipulation technique complementing the reduction of zooplanktivorous fish.     

Field and experimental evidence of the effect of Jenynsia multidentata, a small omnivorous–planktivorous fish, on the size distribution of zooplankton in subtropical lakes

1. Small cladocerans, copepod nauplii and rotifers often dominate the zooplankton community in tropical and subtropical lakes. This is probably because of high predation pressure by small omnivorous–planktivorous fish, but experimental evidence is scarce.
2. This study used two approaches to test the effect of the small omnivorous–planktivorous fish species Jenynsia multidentata , which is frequently abundant in (sub)tropical eutrophic lakes in South America, on the size distribution of zooplankton. In Lake Blanca (Uruguay), which lacks any piscivores, we sampled seasonally for both fish and zooplankton. We also conducted an outdoor mesocosm experiment with treatments containing or lacking J. multidentata .
3. Together, the empirical and experimental data suggest that J. multidentata predation plays an important role in modulating the size structure of the zooplankton community in subtropical lakes. In the absence of J. multidentata , stocked large-sized zooplankters like Daphnia obtusa were abundant in the experiments, while small-sized zooplankton dominated in the presence of fish, as they did in the lake itself from spring to the end of the season.     

Daphnia hybridization along ecological gradients in pelagic environments: the potential for the presence of hybrid zones in plankton

The relative homogeneity of pelagic environments has been regarded as the reason for the absence of hybrid zones for hybridizing planktonic Daphnia (Crustacea: Cladocera); occasional dominance of interspecific hybrids over parental species was explained by their temporal superiority in fluctuating environments. However, water bodies with spatially varying environmental conditions might facilitate the formation of hybrid zones in plankton. We studied the distribution of species and hybrids of the Daphnia longispina complex in 11 canyon-shaped reservoirs, localities characterized by horizontal environmental gradients (particularly of food supply and size-selective predation); we also analysed patterns of carapace size and fecundity among coexisting taxa. Spatial distribution of taxa agreed with their ecological characteristics; those showing different affinities along longitudinal reservoir profiles differed in size according to the presumed fish predation gradient. Only hybrids of Daphnia galeata with Daphnia cucullata and D. longispina (=hyalina) were recorded. The latter two species preferred opposite ends of gradients, such spatial segregation probably explaining the absence of their hybrids. Distributional patterns were relatively stable in two consecutive summers, apart from a substantial decline of D. galeata X cucullata in the second year. The observed pattern of a hybrid-dominated zone in intermediate conditions suggests that local Daphnia hybrid zones may indeed form within reservoirs.     

Refuge availability: a key to understanding the summer disappearance of Daphnia

SUMMARY. 1. The mid-summer declines of Daphnia species in three small lakes were investigated to examine the relative roles of reduced natality and increased mortality. Reduced natality (assessed by quantifying clutch size, lipid index, and available food) could not account for the decline in daphnid abundance in any of the populations examined.
2. The role of increased mortality imposed by zooplanktivorous fish was assessed by estimating the sizes of the mid-water refuge areas where daphnids could escape fish predation. The boundaries of the refuge areas were estimated from field measurements and literature values and were based on gradients of temperature, dissolved oxygen and light. Observed decreases in refuge thicknesses correlated well with the mid-summer declines of large-bodied Daphnia species in all three lakes. Intermediatesized daphnids were less affected as the refuges thinned and small-bodied species increased in abundance.
3. The importance of refuge thickness in modifying zooplanktivore-induced mortality was further tested in large enclosures where refuge thickness was experimentally modified. In the presence of zooplanktivorous fish, large-bodied Daphnia , which used the refuge, persisted when the refuge was thick but disappeared when it thinned. Daphnia galeata mendotae , which did not occupy the refuge zone, was rapidly eliminated regardless of refuge thickness. We conclude that refuge availability plays a major role in Daphnia population dynamics.     

Combined effects of environmental factors and predator–prey interactions on zooplankton assemblages in five high alpine lakes

The examination of a yearly cycle of plankton density (rotifer and microcrustacean) and of 419 stomach contents of four species of salmonid fishes, arctic charr, Salvelinus alpinus, brown trout, Salmo trutta, rainbow trout, Onchorynchus mykiss and lake trout, Salvelinus namaycush, living in five high altitude lakes in the French Alps, shows an impact of abiotic variates and the effects of predation on the composition of zooplankton assemblages. The lakes studied may be divided in two groups. The colder, Muzelle and Puy Vachier, are characterised by a low level of food resources, an important impact of predation, and the near absence of planktonic crustaceans. The second group, consisting of Lakes Les Pisses, Petarel and Palluel, is characterised by a low density rotifer assemblage and a more abundant crustacean population controlled by a low level of fish predation.     

Cost of predation avoidance in young-of-year lake trout (Salvelinus namaycush): growth differential in sub-optimal habitats

Selection of habitat to avoid predation may affect the diet of young-of-year (YOY) lake trout (Salvelinus namaycush). YOY lake trout may use inshore habitat to avoid predation; this habitat may be sub-optimal for growth. To test this, YOY lake trout were penned in nearshore and offshore pelagic areas of two arctic lakes. Toolik Lake had a lake trout population, the other lake, S6, did not. YOY lake trout in Toolik Lake lost weight, but those offshore lost less weight. The YOY lake trout in Lake S6 gained weight and those offshore gained more weight. The primary diet item of the YOY lake trout in both lakes during this experiment was the zooplankter Diaptomis probilofensis; it was also one of the most abundant species. However, its density inshore in Lake S6 was similar to inshore and offshore densities in Toolik Lake. The increased availability of alternative zooplankton prey in Lake S6 may account for the growth differential of YOY lake trout in Lake S6 relative to Toolik Lake. Bioenergetic modeling of YOY lake trout suggests that growth similar to that in the offshore of Lake S6 would be necessary for successful recruitment. If the reduced zooplankton availability in Toolik Lake leads to the reduced growth of YOY in the inshore and offshore pelagic areas, then these fish will be more susceptable to winter predation/starvation. For YOY lake trout to survive in Toolik Lake they most likely shift to feeding on benthic prey before the end of their first summer. Dept. of Chemical Engineering     

Growth and survival of Daphnia in epilimnetic and metalimnetic water from oligotrophic lakes: the effects of food and temperature

SUMMARY 1. In oligotrophic lakes, phytoplankton and bacteria growing in the deep chlorophyll maximum in the cool metalimnion of lakes often dominate biomass and production, but the importance of this source of food for zooplankton is unknown.
2. During much of the day, Daphnia rosea in two mountain lakes inhabited deep chlorophyll layers where food availability was at least equal to that in the epilimnion.
3. To determine the importance of the two strata (epilimnion and metalimnion) for Daphnia , we used a cross-classified factorial experiment to measure how epilimnetic and metalimnetic food and temperature (10 and 16 °C) influenced survival, growth and reproduction.
4. Daphnia survived and grew better when fed seston from the epilimnion of one lake, although chlorophyll, particulate nitrogen and particulate carbon were 2–2.5 times greater in the metalimnion.
5. Temperature had no significant influence on Daphnia survival or growth. Similar results were obtained with food from the second lake, with Daphnia surviving and reproducing better when provided with epilimnetic, rather than metalimnetic food, although the quantities of chlorophyll and carbon in the two strata were similar.
6. Food quality, rather than quantity or temperature, appeared to be the most important determinant influencing survival, growth and reproduction, and the greater food quantity in the metalimnia was not used effectively by the Daphnia .