Daphnia community analysis in shallow Kenyan lakes and ponds using dormant eggs in surface sediments

1. Water fleas of the genus Daphnia are considered rare in tropical regions, and information on species distribution and community ecology is scarce and anecdotal. This study presents the results of a survey of Daphnia species distribution and community composition in 40 standing waterbodies in southern Kenya. The study sites cover a wide range of tropical standing aquatic habitats, from small ephemeral pools to large permanent lakes between approximately 700 and 2800 m a.s.l. Our analysis combines data on Daphnia distribution and abundance from zooplankton samples and dormant eggs in surface sediments. 2. Nearly 70% (27 of 40) of the sampled waterbodies were inhabited by Daphnia. Although their abundance in the active community was often very low, this high incidence shows that Daphnia can be equally widespread in tropical regions as in temperate regions. 3. Analysis of local species assemblages from dormant eggs in surface sediments was more productive than snapshot sampling of zooplankton communities. Surface‐sediment samples yielded eight Daphnia species in total, and allowed the detection of Daphnia in 25 waterbodies; zooplankton samples revealed the presence of only four Daphnia species in 16 waterbodies. 4. Daphnia barbata, D. laevis, and D. pulex were the most frequently recorded and most abundant Daphnia species. Canonical correspondence analysis of species–environment relationships indicates that variation in the Daphnia community composition of Kenyan waters was best explained by fish presence, temperature, macrophyte cover and altitude. Daphnia barbata and D. pulex tended to co‐occur with each other and with fish. Both species tended to occur in relatively large (>10 ha) and deeper (>2 m) alkaline waters (pH 8.5). Daphnia laevis mainly occurred in cool and clear, macrophyte‐dominated lakes at high altitudes.     

Crowding‐induced changes in growth,reproduction and morphology of Daphnia

• 1 Daphnia may reach high population densities seasonally, or in patches, in lakes. To test the effects of chemicals released by high daphniid densities on their life‐history traits, nine species of Daphnia, D. magna, D. pulicaria, D. pulex, D. hyalina, D. galeata, D. laevis, D. lumholtzi, D. ambigua and D. cucullata, were grown in water from crowded Daphnia cultures in a flow‐through system in the presence of abundant food.
• 2 Water from Daphnia at 85 L^‐1 depressed growth rate, and lowered body size and clutch at first reproduction of six species of small‐bodied Daphnia (adult body length < 1.8 mm), but had no significant effects on larger species. Two clones of D. pulex differed in their growth rate in response to crowding, indicating that response patterns may vary within species.
• 3 Chemicals released by crowded D. magna reduced tail spine length in D. lumholtzi and D. cucullata by 37% and 11%, respectively, and induced changes in carapace morphology in D. lumholtzi and D. ambigua.
• 4 Chemicals released by crowded conspecifics may provide an additional, density‐dependent mechanism of population regulation; when large species of Daphnia coexist at a high population density with small species, these chemicals may reinforce the competitive advantage of large species.

     

The prevalence of diapause response to risk of size-selective predation in small- and large-bodied prey species

The prevalence of diapause response to the simulated threat of fish predation was compared in three species of planktonic crustaceans of the genus Daphnia (D. magna, D. pulicaria and D. longispina), which due to their different body size vary in vulnerability to fish predation pressure in natural conditions. Higher incidence of diapause response was presumed in the larger-bodied species, which due to their higher conspicuousness and higher energetic content experience the greatest size-selective pressure from visually foraging fish. Small-bodied species were expected to utilize less costly yet less effective active defences, e.g. lower conspicuousness achieved due to slower body growth when facing risk of size-selective predation. Proportions of females forming diapausing structures as well as females body size at the maturation period were compared in experimental beakers containing or not containing fish kairomones (chemical cues of fish predation) in a few clonal lineages of 3 species of Daphnia derived from a single lake inhabited by fish. The highest incidence of diapause response to fish kairomones was observed in D. magna (when measured both as proportion of sensitive individuals and as proportion of inductive clones), the largest of the tested Daphnia species. The lowest proportion of individuals and clones that employed diapause was reported in the smallest tested species, D. longispina. In addition, the large-bodied Daphnia (D. magna) showed a greater reduction in body size in response to fish kairomones than the small-bodied species (D. longispina). The results of the present study support the assumption of higher prevalence of diapause response to the threat of selective predation in larger and more vulnerable prey species.     

Interclonal variation in diel horizontal migration behaviour of the water flea <Emphasis Type="Italic">Daphnia magna</Emphasis>—searching for a signature of adaptive evolution

In shallow temperate lakes, zooplankton populations may exhibit diel horizontal migration (DHM) and move towards macrophytes during the day to avoid fish. Using a natural Daphnia magna population, we undertook an experimental investigation aimed to describe the genetic variation for DHM and to study whether an adaptive micro-evolutionary response occurred to changes in macrophyte coverage and fish predation pressure through time. Twenty-seven D. magna clones were hatched from ephippia in the sediment of shallow Lake Ring, Denmark. This lake was eutrophied during the 20th century and was subject to restoration measures in the 1970s. The DHM behaviour of the clones was observed both in the presence and absence of fish kairomone. Significant interclonal variation in DHM behaviour occurred in both treatments. To study the micro-evolutionary response of the Lake Ring D. magna population, two approaches were used. First, we compared the DHM behaviour of clones derived from ephippia collected at different depths. A comparison was conducted between clones resurrected from the period of eutrophication (1960–1980) and from the period of recovery (1986–2000). A significant treatment (presence and absence of fish kairomone) × period interaction effect was identified, suggesting a significant micro-evolutionary response for DHM behaviour. The D. magna clones exhibited a significantly stronger horizontal migration response during the period of eutrophication than in the recovery phase. Second, clonal means, representing the influence of the genotype on the trait, were correlated with environmental conditions (macrophyte cover, fish predation pressure and Secchi depth). The results of this analysis also suggest that a micro-evolutionary response by Daphnia has occurred in reaction to changes in fish predation pressure. In periods with high fish predation pressure, Daphnia migrated more strongly towards the plants. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera     

Sediments,not plants,offer the preferred refuge for Daphnia against fish predation in Mediterranean shallow lakes: an experimental demonstration

1. Different behavioural responses of planktonic animals to their main predators, fish, have been reported from shallow lakes. In north temperate lakes, large‐bodied zooplankton may seek refuge from predation among macrophytes, whereas in subtropical lakes, avoidance of macrophytes has been observed. The prevalent behaviour probably depends on the characteristics of the fish community, which in Mediterranean lakes is typically dispersed in both the open water zone and in the littoral, as in temperate lakes, and is dominated by small size classes, as in subtropical lakes. 2. We performed ‘habitat choice’ experiments to test the response of Daphnia magna to predation cues at both the horizontal and vertical level by mimicking a ‘shallow littoral’ zone with plants and a ‘deeper pelagic’ zone with sediments. 3. Initial separate response experiments showed that natural plants, artificial plants and predation cues all repelled D. magna in the absence of other stimuli, while sediments alone did not trigger any significant response by D. magna. 4. The habitat choice experiments showed that, in the presence of predation cues and absence of plants, Daphnia moved towards areas with sediment. In the presence of both plants and sediments, Daphnia moved away from the plants towards the sediments under both shallow and deep water treatment conditions. 5. Based on these results, we suggest that Daphnia in Mediterranean shallow lakes avoid submerged macrophytes and instead prefer to hide near the sediment when exposed to predation risk, as also observed in subtropical shallow lakes. This pattern is not likely to change with water level alterations, a common feature of lakes in the region, even if the effectiveness of the refuge may be reduced.     

An evolutionary perspective on the resistance of Daphnia to the epizoic rotifer Brachionus rubens

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Evidence that fish structure the zooplankton communities of turbid lakes and reservoirs

1. Visually foraging fish typically exclude large zooplankton from clear‐water lakes and reservoirs. Do fish have the same effect in turbid waters, or does turbidity provide a refuge from visual predation? 2. To test the hypothesis that fish exclude large zooplankton species from turbid sites, I searched for populations of medium or large Daphnia species in turbid, fish‐containing reservoirs of south‐central Oklahoma and north‐central Texas, U.S.A., and surveyed the literature for accounts of Daphnia species in turbid habitats worldwide. 3. Only small Daphnia species and the exuberantly spined Daphnia lumholtzi were detected in the turbid reservoirs. The Daphnia species in the reservoirs are smaller than other Daphnia species that occur in the area but were not detected. An extensive survey of the literature suggests that large Daphnia may be found in the lakes of extreme turbidity [Secchi disk depth (SD) < 0.2 m] but that only small and spiny Daphnia are likely to occur in more typical turbid locations (1.0 m > SD > 0.2 m) unless some additional factor reduces the influence of fish predation in such sites. 4. The field samples from Texas and Oklahoma together with the literature review suggest that the effect of visually foraging planktivorous fish on the size structure of turbid‐water zooplankton communities may often be as strong or even stronger than the effect of fish on clear‐water zooplankton communities.     

Competition, predation and the relative abundances of two species of Daphnia

We investigated the factors controlling the relative abundancesof two Daphnia species, D.pulex and D.laevis, in a small Wisconsinpond. D.pulex was the dominant Daphnia species in fall 1977and summer-fall 1978; D.laevis was the only Daphnia speciespresent in summer 1979. The abundance of D.laevis was positivelycorrelated with the abundance of the notonectid, Buenoa confusa.In predation trials, notonectides exhibited a distinct preferencefor D.pulex over similarly-sized D.laevis, but Chaoborus larvaefed at similar rates on both Daphnia species. Behavioral observationsrevealed that Buenoa adults were much less efficient at capturingD.laevis than D.pulex. Quantitative results of these predationtrials were combined with estimates of predator and prey densityand distribution to evaluate the effect of predation on thedaphnid populations. The effect of predation varied throughtime and microhabitat, and only infrequently could predationaccount for total prey mortality. D.laevis was most abundantat times and in places where Buenoa predation was most intense.Competition experiments illustrated the competitive superiorityof D.pulex over D.laevis. D.pulex was able to competitivelyexclude D.laevis in long term experiments, and D.pulex's fecunditywas higher than that of D.laevis in shorter experiments. Inlong-term experiments, Chaoborus larvae at natural densitieswere able to keep both Daphnia species at low, constant levelsand neither species clearly dominated when Chaoborus was present.The relative abundances of D.pulex and D.laevis were controlledby a complex of biotic and abiotic factors. Pond depth and predatordensity determined the intensity of predation on daphnid populations.When notonectid predation was intense, D.laevis dominated; whenthe intensity of predation by notonectids was low, D.pulex dominateddue to its superior competitive abilities. At different timesselective predation or high resource levels promoted the co-existenceof these two species. ¹Current address of both authors: Department of Biological Sciences,University of California, Santa Barbara, CA 93106, USA     

The relationship between temperature,competition and the potential for colonization of a subtropical pond by Daphnia magna

The role that temperature plays in excluding large daphnid species from subtropical and tropical ponds, and competitive relationships between Daphnia laevis and Daphnia magna, are explored. D. magna, a large temperate species is probably not excluded from subtropical or tropical systems by elevated temperatures. However, D. magna was a poorer competitor, under a restricted set of laboratory conditions, than D. laevis, the only pond dwelling daphnid in subtropical Florida. Competition is proposed as one mechanism that may limit the number of daphnid species in subtropical and tropical ponds and lakes. Reduced environmental fluctuations in subtropical and tropical systems (compared with temperate systems) may allow zooplankton populations to reach an environmental carrying capacity where competition limits the number of similar, coexisting species in a habitat.     

Role of melatonin in the control of depth distribution of <Emphasis Type="Italic">Daphnia magna</Emphasis>

Previous studies confirmed the presence of melatonin in Daphnia magna and demonstrated diurnal fluctuations in its concentration. It is also known that in several invertebrate species, melatonin affects locomotor activity. We tested the hypothesis that this hormone is involved in the regulation of Daphnia diel vertical migration (DVM) behaviour that is well recognized as the adaptive response to predation threat. Using ‘plankton organs’, we studied the effect of three concentrations of exogenous melatonin (10⁻⁵, 10⁻⁷, 10⁻⁹ M) on DVM of both female and male D. magna in the presence or absence of chemical cue (kairomone) of planktivorous fish. Depth distribution was measured six times a day, using infrared-sensitive closed circuit television cameras. Our results showed a significant effect of melatonin on the mean depth of experimental populations, both males and females, but only when melatonin was combined with fish kairomone. Females stayed, on average, closer to the surface than males, both responding to the presence of kairomone by descending to deeper strata. In the presence of exogenous melatonin and with the threat of predation, Daphnia stayed closer to the surface and their distribution was more variable than that of individuals, which were exposed to the kairomone alone. Approaching the surface in the presence of predation threat seems to be maladaptive. We postulate the role of melatonin as a stress signal inhibitor in molecular pathways of response to predation threat in Cladocera.     

A quantitative test of the size efficiency hypothesis by means of a physiologically structured model

According to the size‐efficiency hypothesis (SEH) larger bodied cladocerans are better competitors for food than small bodied species. In environments with fish, however, the higher losses of the large bodied species due to size‐selective predation may shift the balance in favor of the small bodied species. Here we present a theoretical framework for the analysis of the competitive abilities of zooplankton species that takes both competition and predation into account in one coherent analysis. By applying the conceptually well‐understood framework of physiologically structured population models we were able to predict the relative difference in predation rates necessary to cause a shift in dominance of the large‐bodied species (Daphnia pulicaria) to the small‐bodied species (D. galeata). These predictions depend only on seven easily interpretable parameters per species: size at birth, size at maturity and maximum size, age at maturity, maximal clutch size, egg development time and finally the half‐saturation constant for food. The critical equilibrium mortality of D. pulicaria was 0.16 d^?1 at food concentrations close to the critical food concentration of D. galeata, i.e. D. pulicaria will win the competition as long as its mortality rate is below 0.16 d^?1. At higher food concentrations the differential mortality curve (plotting equilibrium mortalities of both species against each other) approached a linear function with a slope of one and an intercept equal to the difference in maximal population birth rates. The prediction of critical predation rates was independent of the ingestion rate of the cladocerans and the algal carrying capacity and food regeneration rate of the environment although the mechanism works through competition for a shared algal food resource. We interpret these findings in terms of the relative predation risk large and small‐bodied cladocerans will face in various freshwater ecosystems.     

Increasing dominance of small zooplankton with toxic cyanobacteria

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Predator-released compounds, ambient temperature and competitive exclusion among differently sized Daphnia species

1. We studied the effects of fish water and temperature on mechanisms of competitive exclusion among two Daphnia species in flow‐through microcosms. The large‐bodied D. pulicaria outcompeted the medium sized D. galeata × hyalina in fish water, but not in the control treatment. Daphnia galeata × hyalina was competitively displaced 36 days earlier at 18 °C than at 12 °C. 2. It is likely that the high phosphorus content of fish water increased the nutritional value of detrital seston particles by stimulating bacterial growth. Daphnia pulicaria was presumably better able to use these as food and hence showed a more rapid somatic growth than its competitor. This led to very high density of D. pulicaria in fish water, but not in the controls. The elevated D. pulicaria density coincided with high mortality and reduced fecundity in D. galeata × hyalina, resulting in competitive displacement of the hybrid. 3. It is clear that the daphnids competed for a limiting resource, as grazing caused a strong decrease in their seston food concentration. However, interference may also have played a role, as earlier studies have shown larger Daphnia species to be dominant in this respect. The high density of large‐bodied D. pulicaria in fish water may have had an allelopathic effect on the hybrid. Our data are inconclusive with respect to whether the reached seston concentration was below the threshold resource level (R*) of the hybrid, where population growth rate and mortality exactly balance, as it would be set in the absence of interference, or whether interference actually raised the hybrid's R* to a value above this equilibrium particle concentration. 4. Our results do clearly show that fish‐released compounds mediated competitive exclusion among zooplankton species and that such displacement occurred at a greatly enhanced rate at an elevated temperature. Fish may thus not only structure zooplankton communities directly through size‐selective predation, but also indirectly through the compounds they release.     

Daphnia predation on the amphibian chytrid fungus and its impacts on disease risk in tadpoles

Direct predation upon parasites has the potential to reduce infection in host populations. For example, the fungal parasite of amphibians, Batrachochytrium dendrobatidis (Bd), is commonly transmitted through a free‐swimming zoospore stage that may be vulnerable to predation. Potential predators of Bd include freshwater zooplankton that graze on organisms in the water column. We tested the ability of two species of freshwater crustacean (Daphnia magna and D. dentifera) to consume Bd and to reduce Bd density in water and infection in tadpoles. In a series of laboratory experiments, we allowed Daphnia to graze in water containing Bd while manipulating Daphnia densities, Daphnia species identity, grazing periods and concentrations of suspended algae (Ankistrodesmus falcatus). We then exposed tadpoles to the grazed water. We found that high densities of D. magna reduced the amount of Bd detected in water, leading to a reduction in the proportion of tadpoles that became infected. Daphnia dentifera, a smaller species of Daphnia, also reduced Bd in water samples, but did not have an effect on tadpole infection. We also found that algae affected Bd in complex ways. When Daphnia were absent, less Bd was detected in water and tadpole samples when concentrations of algae were higher, indicating a direct negative effect of algae on Bd. When Daphnia were present, however, the amount of Bd detected in water samples showed the opposite trend, with less Bd when densities of algae were lower. Our results indicate that Daphnia can reduce Bd levels in water and infection in tadpoles, but these effects vary with species, algal concentration, and Daphnia density. Therefore, the ability of predators to consume parasites and reduce infection is likely to vary depending on ecological context.     

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.     

The Relative Role of Interference Competition in Regulation of a Rotifer Community during Spring Development in a Eutrophic Reservoir

The relative importance of interference and exploitative competition from Daphnia was tested in contrast to known role of Cyclops vicinus predation, influencing the rotifer community in Římov Reservoir during spring period. The abundances of five dominant rotifer species and their birth and death rates were analysed, together with changes in chl‐a concentration, HNF, bacteria biomass and the abundances of predator and competitor species, from high frequency sampling. Development of loricated species (Keratella cochlearis, K. quadrata and Kellicottia longispina) was driven predominantly by food availability. The dominant soft‐bodied species Polyarthra spp. and Synchaeta lakowitziana were affected predominantly through C. vicinus predation. The role of mechanical interference from D. galeata, although may be important in a limited periods was overclouded by predation and exploitative competition in general. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Trophic interactions and habitat segregation between competing Daphnia species

Summary Two commonly coexisting species of Daphnia segregate by habitat in many stratified lakes. Daphnia pulicaria is mostly found in the hypolimnion whereas D. galeata mendotae undergoes diel vertical migration between the hypolimnion and the epilimnion. I examined how habitat segregation between these two potentially competing species might be affected by trophic interactions with their resources and predators by performing a field experiment in deep enclosures in which I manipulated fish predation, nutrient levels, and the density of epilimnetic Daphnia. The results of the experiment indicate that habitat use by D. pulicaria can be jointly regulated by competition for food from epilimnetic Daphnia and predation by fishes. Patterns of habitat segregation between the two Daphnia species were determined by predation by fish but not by nutrient levels: The removal of epilimnetic fish predators resulted in higher zooplankton and lower epilimnetic phytoplankton densities and allowed D. pulicaria to expand its habitat distribution into the epilimnion. In contrast, increased resource productivity resulted in higher densities of both Daphnia species but did not affect phytoplankton levels or habitat use by Daphnia. The two species exhibit a trade-off in their ability to exploit resources and their susceptibility to predation by fish. D. g. mendotae (the less susceptible species) may thus restrict D. pulicaria (the better resource exploiter) from the epilimnion when fish are common due to lower minimum resource requirements than those needed by D. pulicaria to offset the higher mortality rate imposed by selective epilimnetic fish predators. D. g. mendotae does not appear to have this effect in the absence of fish.     

Predator-induced diapause in Daphnia magna may require two chemical cues

The production of diapausing eggs by Daphnia magna stimulated by fish exudates can be explained as an anti-predator defence ensuring genome protection in periods of high risk from fish predation. The combined effects on the induction of D. magna diapause of an “alarm” chemical originating from injured conspecific prey and fish kairomones were tested. The results of the experiment showed that the cues when present together promote both the production of ephippial eggs and male formation, indicating their role in the synchronization of the entire mode of Daphnia sexual reproduction. Ephippial eggs were only produced in the presence of both fish kairomone and conspecific alarm chemicals, while male offspring occurred in the treatments where both, one or none of the cues were present. However, production of males was the highest when both cues were provided. D. magna responded similarly to the tested cues whether or not the hypothetical alarm substance associated with predator odour came from Daphnia specimens actually eaten by fish or from crushed conspecific individuals. However, chemicals from crushed chironomid larvae combined with fish kairomones did not induce a similar response in D. magna. The relative advantage of utilization of alarm cues or predator kairomones in the induction of defence responses in prey organisms is discussed. Received: 8 June 1998 / Accepted: 11 January 1999     

Larval fish-induced phenotypic plasticity of coexisting Daphnia: an enclosure experiment

1. The indirect effects of predators on lower trophic levels have been studied without much attention to phenotypically plastic traits of key food web components. Phenotypic plasticity among species creates phenotypic diversity over a changing environmental landscape. 2. We measured the indirect effects of planktivorous larval walleye (Stizostedion vitreum) on phytoplankton biomass through their effects on the dominant herbivore species, Daphnia pulicaria and D. mendotae. 3. Fish had no effect on phytoplankton biomass or overall Daphnia density. We observed a compensatory response to predation by functionally comparable species within a trophic level in the form of shifting dominance and coexistence of Daphnia species. We hypothesized that this phenotypically plastic response to predation decoupled a potential trophic cascade in this freshwater pelagic system. Daphnia pulicaria density decreased over time with fish predation, but D. mendotae density increased over time with fish predation. 4. Phenotypically plastic life history trait shifts and reproductive rates differed between species in fishless and fish enclosures, accounting for population trends. Daphnia pulicaria were also proportionally higher in walleye larvae stomachs than in the enclosures, indicating that walleye preferred to feed on D. pulcaria over D. mendotae. The resultant shift in dominance may partially explain the overall benign effect of fish on grazers and supports the hypothesis that trophic level diversity can decouple a trophic cascade.     

Fish Faeces as the Primary Source of Chemical Cues Inducing Fish Avoidance Diapause in Daphnia Magna

In a laboratory batch culture experiment, females of Daphnia magna were exposed to five different experimental media containing either:(1) water from an aquarium with fish, (2) extract of fish faeces, (3) mixture of both media, (4) extract of homogenised conspecific Daphnia, or (5) control water without the addition of extra cues. The experiment was planned to test potential pathways of excretion of the chemical cues that induce resting-egg formation in D. magna and to find an effective way of collecting these chemical cues. The results indicate that fish faeces are the prevailing source of the chemical cues that induce resting-egg production in D. magna. The ease of collection and the possibility of storing it in a frozen state make it a convenient cue for inducing diapause response in Daphnia. The results of the experiment imply that in natural conditions Daphnia may face high concentration of the inductive signals once migrating to the bottom zone where fish faeces commonly accumulate.