Predation by underyearling perch (Perca fluviatilis) on a Daphnia galeata population in a short-term enclosure experiment

1. The predation impact of underyearling perch (20 mm total length) on the dynamics of Daphnia galeata was studied in three 6.5-m³ enclosures during a 17-day experiment in June 1995. These data were compared with zooplankton succession in three fish-free control enclosures and in the pelagic zone of Bautzen reservoir, Germany. 2. Due to individual growth during the experiment, fish biomass in the enclosures increased from 210 mg wet body mass (w.b.m.) m^–3 to 830 mg w.b.m. m^–3, equivalent to an increase from 20 kg ha^–1 to 75 kg ha^–1. 3. In the enclosures with fish, biomass of daphnids decreased steadily to values below 1 mg wet weight (w.w.) l^–1 within 17 days, whereas in the fish-free controls and in Bautzen reservoir the Daphnia biomass fluctuated around 8 mg w.w. l^–1. Other zooplankton species exhibited little or no change. Approximately 60 kg ha^–1 was calculated as the critical underyearling perch biomass which may induce a drastic decline of the Daphnia galeata population in Bautzen reservoir. Comparison with values from other lakes is difficult due to differences in water depths and Daphnia biomasses. 4. Mean individual biomass of daphnids, egg ratio and proportion of adult daphnids were significantly lower in the enclosures with fish compared with the control enclosures at the end of the experiment. This may be explained partly by preferential predation of the large, egg-carrying daphnids by fish. However, no difference was found in clutch size and size at first reproduction, possibly due to the short duration of the experiment. 5. It must be assumed that the ‘summer depression’ of daphnids observed in many waters is not the exclusive effect of direct reduction of daphnids by fish predation. Even the high biomass of underyearling perch stocked in the enclosures did not completely account for Daphnia mortality. Instead, the selective loss of large size classes, combined with low food resources and reduced reproduction rates, may induce the marked declines in daphnids.     

Temperature impact on the midsummer decline of Daphnia galeata: an analysis of long-term data from the biomanipulated Bautzen Reservoir (Germany)

1. The influence of water temperature on occurrence and duration of a midsummer decline (MSD) of Daphnia galeata was studied in the biomanipulated Bautzen Reservoir in Germany. The proportion of piscivores in the fish community of the reservoir has been enhanced experimentally since 1981. As a consequence, Daphnia galeata has dominated the zooplankton. Over 18 years of study (1981–1998), a long‐lasting MSD (longer than 30 days) occurred in 7 years, whereas a short MSD (shorter than 30 days) was observed in 6 years. During the remaining 5 years, an MSD was not observed.
2. Two hypotheses were examined to explain the observed patterns. First, we postulated that high water temperature during winter and early spring (January–April) leads to an MSD after an early and high spring peak of daphnids. On the other hand, low temperature during winter and early spring should not cause an MSD owing to a slower increase of the population, resulting in a later peak of daphnids. Second, we hypothesized that the mean water temperature during early summer (May and June) influences the occurrence of an MSD (by controlling young‐of‐the‐year (YOY) fish predation on daphnids).
3. The water temperature during winter and early spring explains 83%, and the early summer water temperature 55%, of interannual variation in the occurrence of an MSD.
4. The interannual variation in duration of an MSD was neither explained by temperature during winter and early spring nor by early summer temperature alone, but in 14 of the 18 years (78%) by a combination of both.
5. We conclude that water temperature during winter and early spring had a strong impact on Daphnia mortality by influencing height and timing of the spring peak which, in turn, influenced the extent of overexploitation of their food resources. By contrast, the water temperature during early summer probably influenced the mortality of daphnids caused by predation of YOY fish. The relative timing of both sources of mortality, which depends on the temperature regime during the first 6 months of the year, is the key process in controlling the occurrence and duration of an MSD. A long‐lasting MSD, therefore, is likely in Bautzen Reservoir only if temperatures are high during winter and early spring, as well as during early summer.
6. As a consequence of climate warming, recent climate records reveal warming during winter, spring and early summer in middle Europe, rather than an increase in mean annual temperatures. If our findings and conclusions are related to this regional and temporal pattern of climate warming, an increasing frequency of years with a long‐lasting MSD and, consequently, a decreasing efficiency of biomanipulation can be predicted.     

Influence of diet shifts in underyearling fish on phosphorus recycling in a hypertrophic biomanipulated reservoir

1. In addition to effects of direct predation by planktivorous fish, nutrient recycling by fish may also contribute to structuring foodwebs in lakes. There is little evidence, however, about whether underyearling fish undergoing several ontogenetic diet shifts may have a comparable bottom-up impact. 2. This study examined seasonal patterns of phosphorus (P) concentration and external load, phytoplankton, zooplankton and benthos, and diet shifts in three underyearling fish [perch (Perca fluviatilis), roach (Rutilus rutilus) and ruffe (Gymnocephalus cernuus)] in the shallow, hypertrophic biomanipulated Bautzen reservoir, Germany. Phosphorus metabolism of fish was calculated by a balanced bioenergetics model on the basis of fish diet, growth and water temperature. 3. The fish showed several shifts from planktivory to other food sources during the sampling period from May to September. These shifts were probably caused by the seasonal succession of the zooplankton community, mainly the midsummer decline of Daphnia galeata. 4. The diet shifts in fish also had consequences for the amount of P consumed and released. During periods of dominant zooplanktivory, the excretion of P did not exceed the removal of P stored in pelagic prey. By contrast, if benthivory dominated, fish subsidized the pelagic P pool by excreting more P from benthic prey than had been removed from the pelagic area. This occurred predominantly in perch and ruffe during periods of low zooplankton biomass, whereas the roach ate more algae and therefore excreted less P of benthic origin. 5. Phosphorus release by underyearling fish was estimated at a maximum of 0.1 mg m^–3 JY day^–1. This value was negligible compared with both the external load of P to Bautzen reservoir and the concentration of P in the pelagic area during summer. It is therefore concluded that both the predominance of underyearling zooplanktivorous fish and the high Daphnia biomass during certain periods of the year in the Bautzen reservoir may be the reason that nutrient release by the fish structured the foodweb only marginally. 6. This study suggests that biomanipulation has altered both top-down and bottom-up impacts of fish in Bautzen reservoir. The highest efficiency of foodweb manipulations may be obtained after reduction of the external P loading below a certain threshold. In turn, if external restoration of eutrophied lakes is not accompanied by changes in fish community, then the combined forces of strong zooplanktivory and high P recycling of dense stocks of zooplanktivorous and benthivorous fish may hold the water in a eutrophic-like stage, even if external load has been significantly reduced.     

Is the midsummer decline of Daphnia really induced by age-0 fish predation? Comparison of fish consumption and MDaphnia mortality and life history parameters in a biomanipulated reservoir

We analysed the temporal pattern of a Daphnia galeata populationand the development of the age-0 Ash community in a long-termbiomanipulated lake (Bautzen reservoir, Germany) during springand early summer of two successive years. In Bautzen reservoir,the age-0 Rsh-Daphnia interaction is a key process within thefood web due to the low abundance of adult zooplanktivorousfish. Daily consumption of daphnids by age-0 fish, as estimatedby a bioenergetics model, was compared to daily mortality ratesof daphnids. In addition, we estimated life history parametersof Daphnia that may indicate predation impact by fish. A midsummerdecline of daphnids occurred only in July of 1996, whereas in1995 the Daphnia biomass remained >4 mg wet weight 1^–1for the entire summer. The percentage of total Daphnia mortalitydue to fish predation before the onset of the midsummer declinewas –2% day^–1. Temporal patterns of individual size,clutch size and size at maturity also indicated that the 1996midsummer decline of daphnids was not the exclusive consequenceof age-0 fish predation. Instead, low reproductive capacityof daphnids also contributed significantly to the decline. Consequently,year-to-year variation of the Daphnia dynamics may be determinedby a fine-tuned ‘timing’ between the period of reducedfecundity and the time of the strongest predation impact byage-0 fish.     

Diet,stable isotopes and morphology of Eurasian perch (Perca fluviatilis) in littoral and pelagic habitats in the northern Baltic Proper

We present morphology, stable isotope signals and stomach contents of Eurasian perch (Perca fluviatilis) from littoral and pelagic habitats in a brackish water embayment in the northern Baltic Proper. Studies conducted in freshwater habitats repeatedly state that littoral perch have deeper bodies than the pelagic ones. In this study we observed the opposite; the perch from the pelagic study site had deeper bodies than the littoral ones, indicating that more factors than habitat structure affect the perch morphology. A possible explanation to this discrepancy is the diet choice; the pelagic perch in this study were more benthivorous than freshwater pelagic perch. Our results on stable isotope signals combined with the stomach contents also shed new light on the dietary preferences of perch. Perch is known to be a generalist predatory fish, but our results indicate that perch have individual diet preferences. Based on our results, it seems that at some point in their lives the perch in brackish water choose between the littoral and pelagic habitats and also specialise in a certain diet. This study shows that the perch morphology and diet in the Baltic Sea coast differ among habitats, but the patterns are not similar to those observed in freshwater studies.     

Horizontal migration of zooplankton in a littoral zone of the lowland Sulejow Reservoir (Central Poland)

Horizontal migrations of zooplankton between macrophyte patches and open areas were investigated in the sparsely vegetated littoral zone of the Sulejow Reservoir in June-July 2000 and 2001, using one-litre plastic traps. Large-bodied zooplankton: daphnids and copepods generally swam towards the open water at dusk and towards submerged macrophytes at dawn. Small-bodied zooplankton (Bosmina sp., Chydorus sp.) did not show any pattern of horizontal movement. At the time of the research the phytoplankton community was dominated by eatable diatoms (Cyclotella sp.), whose biomass reached 14 mg l⁻¹. Thus, bottom-up forces (food scarcity) are not likely to be responsible for the observed zooplankton migrations. Analyses of fish stomach contents showed high contribution of large zooplankters to the food of juvenile roach (Rutilus rutilus) and perch (Perca fluviatilis) which densely inhabited the littoral zone of reservoir. High fish pressure in the littoral zone along with high density of the predatory cladoceran, Leptodora kindtii in the open water, suggest that top-down forces (predatory pressure) were responsible for the migration of large zooplankton. At dusk predatory pressure of fish fry exceeded that of L. kindtii, forcing endangered zooplankton to escape from macrophytes towards open water. The opposite situation occurred at dawn. The consequences of the relationships for both zooplankton and fish fry communities dynamics are discussed.

     

Habitat shift in roach (Rutilus rutilus) induced by pikeperch (Stizostedion lucioperca) introduction: predation risk versus pelagic behaviour

Changes in the fish community structure and habitat use were followed after the introduction of pikeperch (Stizostedion lucioperca) to the roach-dominated Lake Gjersjøen. Quantitative echosounding showed that the density of juvenile roach (Rutilus rutilus) was dramatically reduced in pelagic areas, from 12 000–15 000 fish/ha to 250 fish/ha, while total fish density remained unchanged in littoral areas. At the same time, the habitat segregation between different size groups of roach was altered as larger roach utilized the pelagic zone after pikeperch introduction. The loss of the pelagic refuge for juvenile roach increased the availability of juvenile roach to littoral predators, notably perch. In littoral areas, the fish community changed from one dominated by roach (> 95%) to one dominated by perch (> 50%).     

Climate-driven warming during spring destabilises a Daphnia population: a mechanistic food web approach

Temperature-driven changes in interactions between populations are crucial to the estimation of the impact of global warming on aquatic food webs. We analysed inter-annual variability in two data sets from Bautzen reservoir, Germany. In a long-term data set (1981–1999) we examined the pelagic phenology of Daphnia galeata, a keystone species, the invertebrate predator Leptodora kindtii, phytoplankton and Secchi depth in relation to water temperature and the North Atlantic Oscillation index. In a short-term data set (1995–1998) we examined food web relations, particularly the consumption of D. galeata by young-of-the-year (YOY) percids and L. kindtii and rates of population change of D. galeata (abundance, recruitment pattern and non-consumptive mortality). The start of the clear-water stage (CWS) was correlated with winter temperatures. It started 5.8 days earlier per degree warming after warm winters (mean January–March temperature ≥2.5°C) compared to cold winters (mean temperature <2.5°C). However, the end of the CWS remained relatively constant. Predation by L. kindtii and YOY percids on D. galeata started distinctly earlier, i.e. by 13.0 and 6.5 days per degree warming, respectively, in years when the average May temperature was high (≥14°C) compared to years when it was low (<14°C). Significant reductions of Daphnia abundance in midsummer occurred only in years in which the mean May temperature exceeded 14°C. This temperature regime resulted in a match of over-exploitation of food resources by Daphnia during the CWS and strong predation by YOY percids and L. kindtii. Consumptive mortality increased at higher rates with a rise in temperature than net recruitment, resulting in lower Daphnia densities at the end of the CWS. Our data suggest that even low warming by 1.7°C during a short, but critical seasonal period, resulting in the coincidence of two or more factors adversely affecting a keystone species, such as Daphnia, may induce changes in whole lake food webs and thus alter entire ecosystems. Priority programme of the German Research Foundation—contribution 9.     

Visual conditions and habitat shape the coloration of the Eurasian perch (Perca fluviatilis L.): a trade‐off between camouflage and communication?

In theory, selection for effective camouflage (i.e. dull coloration) in fish should be strongest when the conditions for visual predation are most favourable, such as in structurally simple pelagic habitats. By contrast, in more sheltered (e.g. littoral) habitats, selection may favour effective intra‐specific communication (i.e. bright coloration) (at the expense of crypsis). Poor transparency, as in highly humic waters, should constrain colour adaptations. We investigated phenotypic variation in body coloration of Eurasian perch (Perca fluviatilis L.) in littoral and pelagic habitats of four humic boreal lakes. Perch from the most transparent lake had the lightest and less coloured belly and perch were more colourful in the littoral habitats than in the pelagic areas, with the pattern being clearest in the most transparent lake. In addition, perch in the most transparent lake exhibited sexual dichromatism, with males having a more colourful belly than the females, whereas no indications of sexual dichromatism were found in more humic lakes. Moreover, in the most transparent lake, the condition of fish correlated with bright belly coloration in the littoral, but with dull belly coloration in the pelagic habitat. The results obtained in the present study suggest that selection on perch coloration may differ between lakes as a result of visual properties of the water, and within lakes as a result of divergent selection for camouflage and communication in pelagic and littoral habitats. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 47–59.     

Multiple defence strategies of <Emphasis Type="Italic">Daphnia</Emphasis><Emphasis Type="Italic">galeata</Emphasis> against predation in a weakly stratified reservoir

In the presence of size-selective fish daphnids were shown to exhibit two alternative inducible defence strategies: They may either escape predation by active migration or adopt a life history strategy, e.g., reproduce earlier and at a smaller size. Depending on the type of habitat, migration may either be vertically (in deep stratified lakes) or horizontally (in shallow lakes with macrophytes) oriented. Concerning behavioural defence strategies, daphnids living in medium-deep, weakly stratified water bodies with a poorly developed littoral face a dilemma, since the littoral provides no shelter and the availability of a deep-water refuge is unpredictable. We studied the population dynamics, life history changes (size at maturity) and daytime vertical distribution of Daphnia galeata in a weakly stratified reservoir in relation to predation by juvenile fish during 6 years. While temperature gradients were usually small, oxygen concentrations suggest that a low-oxygen refuge for daphnids was available in every year to some extent. Our results indicate that, depending on predation intensity and stratification patterns, daphnids exhibit both, behavioural and life history defences. In years with a high biomass of young-of-the-year (YOY) perch Daphnia abundance declined rapidly at the end of the clear water stage while at the same time the vertical distribution at daytime shifted to deep strata providing a low-oxygen refuge and the size at maturity decreased. However, while the life history response in some years lasted throughout most of the summer period, a shift in daytime vertical distribution was exhibited for much shorter periods. Both traits were much less expressed in years with low YOY fish densities and no negative correlation between them could be verified. We suggest that under high predation pressure in this relatively shallow reservoir no strictly alternative (either behavioural or life history) strategies exist, but that daphnids make use of the full range of possible anti-predator defences available, at least during short periods when predation is most intense. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera     

Reduction of the filtration rate of Daphnia galeata by dissolved photosynthetic products of edible phytoplankton

The filtration rate of Daphnia galeata was determined in in situ experiments in Bautzen Reservoir and in laboratory experiments, where daphnids were exposed to filtrates that previously contained either natural phytoplankton or cultured eukaryotic algae (Scenedesmus obliquus or Asterionella formosa), respectively. Individual filtration rate (FR) was measured using fluorescent beads, taking into account ingested beads in the gut only. Compared to heated control treatments (100 °C), dissolved compounds released by the nutritious cultured algae during the preconditioning phase or by the natural phytoplankton assemblages from Bautzen Reservoir strongly reduced the filtration rate of D. galeata (down to 60%). Heating deactivated these dissolved compounds. A significant correlation was found between primary production measured in situ and the reduction of FR in the filtrate of reservoir water, indicating that extra-cellular products released during photosynthesis triggered the reduction of the filtration rate. The ratio of ingested to collected beads was used to quantify the proportion of food, which was not only collected but passed the mouth of D. galeata. The ratio of ingestion to collection was compared between filtered and unfiltered reservoir water both media identical with respect to the concentration of dissolved compounds, whereas other factors (e.g. food concentration, temperature, filtration rate) were different. The changes in this ratio between filtered and unfiltered reservoir water suggest that D. galeata is capable of a chemosensory control of the ingestion behaviour by detecting external metabolites.     

Life history of Daphnia galeata in a hypertrophic reservoir and consequences of non-consumptive mortality for the initiation of a midsummer decline

SUMMARY 1. Field and laboratory investigations were combined in a 2‐year study on the initiation of a midsummer decline of Daphnia galeata Sars in a hypertrophic reservoir. Quantitative field samples were taken twice a week, and, adult and juvenile mortality rates were calculated. Patterns of reproduction and survival of daphnids born during spring and early summer under fluctuating food conditions were determined in life‐table experiments. 2. The abundance of Daphnia increased strongly in early May and declined in June 1998 (midsummer decline). In 1999, Daphnia density increased only slowly in spring and remained constantly high throughout the summer. 3. Food conditions (concentrations of POC_{<30 μm}) for daphnids deteriorated in both years in response to increasing Daphnia densities, resulting in a clear‐water phase of about 4 weeks. When Daphnia abundance declined in 1998, POC_{<30 μm} concentrations increased greatly, whereas in 1999 food conditions improved only slightly and Secchi depth remained high. 4. Survival of daphnids in life‐table experiments decreased greatly after food became rare and was strongly reduced in those animals born during the clear‐water phase compared with those born later. In addition, age at first reproduction was retarded during the clear‐water phase, resulting in very low population growth rates. Survivorship patterns in life‐table experiments suggest a strong impact of non‐consumptive mortality on Daphnia population dynamics. 5. Field data of mortality point to differences in mortality patterns between years, probably resulting from different predation impacts of juvenile fish. In both years, however, adult mortality contributed substantially to overall mortality at the end of the clear‐water phase. As bottom‐up effects on D. galeata were very similar in both years, the significance of non‐consumptive mortality on the initiation of midsummer declines appears to depend largely on recruitment patterns before the clear‐water phase. A high impact can be expected when Daphnia populations are dominated by a peak cohort of nearly identical age during the clear‐water phase.     

Effects of habitat and food resources on morphology and ontogenetic growth trajectories in perch

Studies on resource polymorphism have mainly been considered at the end stage of ontogeny, whereas many species undergo diet changes as they grow. We conducted a field survey to analyze the role of adaptive variation during ontogeny in Eurasian perch (Perca fluviatilis). We caught perch from the littoral and pelagic zones of a lake to investigate whether perch differ in morphology and diet between these habitats. We also investigated whether there were any differences in morphological trajectories during the ontogeny of perch collected from the two habitats. We found that perch caught in the littoral habitat, independently of size, had a deeper body, larger head and mouth and longer fins than perch caught in the pelagic zone. Macroinvertebrates and fish dominated the diet of littoral perch, whereas the diet of the pelagic perch consisted mainly of zooplankton and to some extent fish. Independently of size, the more streamlined individuals had a larger proportion of zooplankton and a smaller proportion of macroinvertebrates in their diet than the deeper-bodied individuals, indicating a relation between diet and morphology. Some morphological characters followed different ontogenetic trajectories in the two habitats; e.g. the changes to a deeper body and a larger head were faster in the littoral than in the pelagic perch. The relationship between the length of perch and the size of the mouth and fins also differed between perch from the two habitats, where the increase in the length of the pelvic fin and the area of the mouth increased faster with size in the littoral perch. Our findings show that variation in morphology between habitats differs during ontogeny in a way that corresponds to functional expectations for fish species that occupy these habitats.     

Chaos and stability of age-0 fish assemblages in a temperate deep reservoir: unpredictable success and stable habitat use

Large year-to-year variability in different fish species recruitment has been confirmed by previous studies while diurnal patterns of occupation in two basic reservoir habitats (pelagic and littoral) by different age-0 fish species in late summer are still unclear. Data collected over an 11-year period regarding late-summer age-0 fish assemblages in pelagic and littoral habitats of a reservoir were used to test the recruitment instability and to investigate diurnal habitat use. Trawling was conducted in the pelagic habitat at night while beach seining was conducted in the littoral habitat during day and night. Fluctuations in age-0 fish abundance and species composition were observed with both sampling methods; however, the following spatio-temporal patterns were relatively stable in most investigated years: (1) pelagic species (pikeperch; Sander lucioperca, small perch; Perca fluviatilis, bream; Abramis brama at night), (2) littoral species (large perch, asp; Leuciscus aspius, dace; Leuciscus leuciscus), (3) migratory species likely performing diel horizontal migrations (bleak; Alburnus alburnus), (4) species abundant in the littoral habitat both during day and night and also in pelagic habitat at night (roach; Rutilus rutilus) and (5) species detected in both habitats exclusively at night (ruffe; Gymnocephalus cernuus).     

Diet-dependent body morphology and ontogenetic reaction norms in Eurasian perch

Individual morphology and performance are directly or indirectly under the influence of variation in resource levels. To study the effects of different resource conditions and their effects on morphology and ontogenetic reaction norms in young‐of‐the‐year (YOY) perch (Perca fluviatilis), we used three different approaches. First, we examined the morphological trajectories over early ontogeny in relation to lake‐specific resources in a field study. Second, one lake that lacked perch recruitment was stocked with perch eggs from a control lake in a whole‐lake experiment to study ontogenetic reaction norms. Third, we compared the development of YOY perch in the three lakes that mainly inhabited the littoral zone with YOY perch experimentally confined to enclosures in the pelagic zone of the lakes.
Overall body morphology of the YOY perch changed both as a function of size and as a function of diet. As perch increased in size they developed a deeper body morphology corresponding to an increased proportion of benthic macroinvertebrates in their diet. In pelagic enclosures where perch were constrained to feed mainly on zooplankton they had a more fusiform body morphology than perch in the lakes that fed on a mixture of zooplankton and macroinvertebrates. Similarly, the ontogenetic reaction norm of perch was related to the diet and lake‐specific zooplankton levels in the whole‐lake experiment.
In the pelagic enclosures, perch with high growth rates had a more fusiform body morphology than slow‐growing perch, whereas the opposite was found in the lakes, where perch included more macroinvertebrates in their diets. Perch in lakes with a higher proportion of macroinvertebrates in the diet also had deeper body morphology. The opposite morphology – growth rate relationship found between perch in the pelagic versus those using the whole lake suggest a morphological trade‐off between foraging on zooplankton and foraging on macroinvertebrates. Our results suggest that YOY perch show different ontogenetic reaction norms as a function of lake‐specific resource levels, which may allow YOY recruitment to later stages. Our results further suggest that diet‐related changes in morphology are a rapid process.     

The Influence of Invertebrate Predators on Daphnia Spatial Distribution and Survival in Laboratory Experiments: Support for Daphnia Horizontal Migration in Shallow Lakes

The behavioural response of Daphnia cucullata to the presence of the pelagic invertebrate predator Leptodora kindtii, and the predation rate of littoral dragonfly nymphs on this species were investigated under laboratory conditions. Results of this study revealed a strong hiding response of Daphnia cucullata in the presence of the predatory cladoceran, L. kindtii, which was similar to the response of Daphnia in the presence of juvenile perch. This suggests that pelagic invertebrate predators may cause Daphnia to hide in the littoral zone which could result in increased exposure to predation by littoral invertebrates. A strong influence of dragonfly nymphs on D. cucullata, both in the presence and absence of macrophytes, was found. The average predation rate of Odonata larvae was about 5 prey ind^–1 h^–1 and did not differ significantly between treatments. Quantification of dragonfly pressure on Daphnia populations will require cross‐verification with field experiments since in the natural conditions Daphnia seeks a shelter in the vegetation stands against predation by Leptodora, despite the occurrence of odonates. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Some like it deep: Intraspecific niche segregation in ruffe (Gymnocephalus cernua)

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Carbon as an indicator of Daphnia condition in an alpine lake

Carbon–length relationships can be used to indicate the condition of daphnids in natural situations. We examined the Daphnia galeata population of an alpine lake (Oberer, Arosasee, Switzerland), where most individuals display diel vertical migration behaviour (DVM). Normally, migrating daphnids face a trade-off between `predator safe areas' in the hypolimnion and `food rich areas' with high predation risk. However, in alpine lakes, with chlorophyll a and POC maxima typically in deeper layers and weak temperature gradients, migrating species are confronted less with this trade-off. We investigated the seasonal changes in carbon content of D. galeata in different depth strata in relation to environmental parameters of the lake. The carbon content of D. galeata was high in spring, but declined considerably in summer and increased slightly in autumn. The low values indicate that D. galeata are food limited for much of the year. The slopes of the regression lines between carbon content and body length varied seasonally, but were not significantly different among depths on a given date. In summer, D. galeata individuals residing in the deep layers during the day had a significantly higher carbon content than individuals in the surface layers. During the rest of the year, the carbon content of individuals was similar among all depth strata. We conclude that migrating D. galeata and individuals remaining in deep layer had better body conditions compared to non-migrating D. galeata in this alpine lake during summer, when migration amplitudes were highest.     

Impact of fish predation on coexisting Daphnia taxa: a partial test of the temporal hybrid superiority hypothesis

Fish predation was tested as a factor mediating the coexistence of Daphnia taxa in the shallow, hypertrophic Lake Blankaart. Naturally co-occurring populations of D. galeata and the hybrid D. galeata x cucullata were subjected to different levels of fish predation in in situ enclosures. In control enclosures without fish, the largest taxon D. galeata rapidly became dominant over the intermediate sized D. galeata x cucullata, mainly as a result of higher birth rates. In enclosures with fish, population densities of D. galeata dropped relative to D. galeata x cucullata, due to higher mortality rates. These results are in concordance with the `temporal hybrid superiority hypothesis', and can be explained by a higher vulnerability of the large and more conspicuous D. galeata to the size selective predation exerted by visually hunting planktivorous fishes. After approximately one month, however, population growth rates of D. galeata and D. galeata x cucullata in the enclosures with fish converged, due to a relative reduction in the mortality rate of D. galeata. This suggests that, in the presence of fish, D. galeata may co-exist with hybrids due to a decrease in its relative vulnerability to visual predation with time. Indeed, both D. galeata and the hybrid showed strong reductions in adult body size in the enclosures with fish, but this size reduction tended to be stronger in D. galeata than in D. galeata x cucullata. In addition, turbidity increased in the enclosures with fish and may additionally have reduced the relative advantage of D. galeata x cucullata with regard to mortality caused by visual predation.