Trait sorting in Daphnia colonising man‐made lakes

1. We used a zooplankton metacommunity to ask how dispersal, genetic drift and selection act to determine the local and regional distributions of trait variation. Since the formation of the lakes 80 years ago, cladoceran species have sorted into local assemblages that cluster by lake depth. Given this species sorting, we ask whether intraspecific variation in an ecologically important phenotypic trait – body size – has sorted as well. 2. We quantified changes in body size through time by measuring ephippia from D. pulicaria, D. dentifera and D. ambigua recovered from sediment cores from two lakes. We then estimated mean body size of contemporary populations of two competing species, Daphnia pulicaria and D. dentifera, in a laboratory common garden experiment. Finally, we used microsatellite loci to characterise genetic diversity and gene flow among local sites in the metacommunity. 3. Body size was variable both within and among years for the three species of Daphnia examined using sediment cores. For two lakes where we examined body size distributions through time, we observed a significant shift in body size of the first species to arrive after colonisation by other Daphnia species, which suggests selection has occurred historically. 4. Despite heritable variation in body size in the laboratory, evidence for trait sorting was only found for D. pulicaria, which was larger in deeper lakes. Mean body size varied among lakes, but did not sort relative to depth for D. dentifera. 5. Microsatellite data indicated that neutral genetic diversity was low in the region; only 27% of the individuals assayed were unique multi‐locus genotypes. We also found significant patterns of isolation by distance for both species. However, population structure was stronger in D. dentifera than in D. pulicaria. Hence, we conclude that a limited number of colonists have successfully invaded this metacommunity, and those genotypes arriving in this new region have experienced significant dispersal limitation among local sites. 6. Overall, while dispersal and selection have clearly led to the development of predictable community assemblages related to depth in this metacommunity, the distribution of phenotypic traits within species can differ substantially even between two trophically similar species. Our results highlight the complex roles of colonisation history, dispersal, selection and stochasticity in determining inter‐ and intra‐specific patterns in metacommunities.     

Performance trade-off across a natural resource gradient

An important environmental factor determining both phytoplankton and zooplankton community composition is lake depth and thermal stratification. However, there is little information on how the interaction between zooplankton grazers and their phytoplankton food changes along an environmental gradient of lake depth. We contrasted resource availability for daphniid zooplankton populations living in two shallow, unstratified lakes and in two deep, stratified lakes using a novel growth bioassay. Stratified lakes had consistently lower resource richness than shallow unstratified lakes. To test whether resources were important in explaining differences in daphniid composition of shallow and deep lakes, we performed reciprocal transplant experiments. We raised daphniids typical of shallow (Ceriodaphnia reticulata) and deep (Daphnia dentifera) lakes in the resources from replicate shallow and deep lakes and monitored survival and reproduction. The two species exhibited a performance trade-off, measured by life table r and R ₀, across a gradient in natural resource richness. D. dentifera had higher relative fitness than C. reticulata when raised in the poorest resource environment from a deep lake. However, under richer resource conditions typical of shallow lakes, C. reticulata outperformed D. dentifera. We further created a gradient in natural resource quantity (by dilution) to test whether this trade-off in species relative fitness involved aspects of resource quality. No trade-off in species performance was evident across the dilution gradient, indicating that resource quality was important to the trade-off. We conclude that shifts in daphniid species composition along a gradient of lake depth involve an adaptive trade-off in ability to exploit rich versus poor resource quality. Received: 11 May 1998 / Accepted: 15 January 1999     

Contrasting patterns of body size for Daphnia species that segregate by habitat

Summary We investigate how body size of two coexisting Daphnia species varies among 7 lakes that represent a gradient of predation risk. The two species segregate vertically in stratified lakes; D. galeata mendotae is typically smaller and more eplimnetic than D. pulicaria. The extent of vertical habitat partitioning, however, varies seasonally within and among lakes in apparent response to predation intensity by epilimnetic planktivorous fishes. Daphnia pulicaria uses the epilimnion at low levels of fish predation but is restricted to the hypolimnion under high fish predation, whereas D. galaeta mendotae always utilizes the epilimnion. The species display contrasting patterns of genetic variation in neonate size and size at maturity. D. pulicaria is larger in lakes with higher fish and Chaoborus densities whereas D. galeata mendotae is smaller. This contrast in body size in lakes with high predation is associated with greater habitat segregation in those lakes. In lakes with low predation risk, the two species are similar in body size at birth and maturity.Authorship order alphabetical     

Ecological consequences of intraspecific variation in lake Daphnia

1. Although populations harbour considerable diversity, most ecological studies still assume they are homogeneous. However, mounting evidence suggests that intraspecific diversity is not only common, but also important for interactions with community members. Here, intraspecific variation in Daphnia dentifera in haemoglobin content is shown to be a marker of hypolimnion use. 2. Hypolimnion use differed substantially within and among D. dentifera populations. Daphnia dentifera with haemoglobin resided primarily in the hypolimnion, while D. dentifera lacking haemoglobin migrated vertically. These ‘deep’ and ‘migratory’D. dentifera had different seasonal phenologies and dynamics. 3. Deep and migratory D. dentifera had qualitatively different relationships with an important competitor, Daphnia pulicaria. Deep D. dentifera density was negatively correlated with D. pulicaria density, whereas migratory density was not correlated with D. pulicaria density. 4. Given that D. pulicaria tends to reside in the hypolimnion, this negative correlation probably reflects competition between D. pulicaria and the deep D. dentifera. This pattern would have been missed if only the relationship between the overall lake populations of D. dentifera and D. pulicaria had been studied. 5. Abundances of deep D. dentifera and D. pulicaria were both correlated with the size of the hypolimnetic refuge from fish predation, but in opposite directions. Lakes with large refuges generally had high D. pulicaria and low deep D. dentifera densities.     

Habitat partitioning of native and exotic Daphnia in gradients of temperature and food: mesocosm experiments

1. Invasion of tropical zooplankton into temperate lakes provides an interesting opportunity to explore habitat segregation in a thermal gradient. 2. We explored differing vertical positioning of native and exotic Daphnia (Daphnia mendotae and Daphnia lumholtzi) in a large indoor mesocosm system (Plön plankton towers) during 2 month‐long experiments. The two towers were manipulated to provide gradients of both temperature (15–29 °C) and algal food (0.05–0.58 mg C L^?1) and a day–night cycle. 3. Both juvenile and adult D. lumholtzi showed a ‘typical’ vertical migration pattern, with higher densities in the epilimnion at night than during the day. They avoided the food‐poor middle layer. In contrast, D. mendotae adults showed little tendency to migrate into the epilimnion at night, remaining in the cooler hypolimnion while juveniles migrated. The vertical distribution of D. mendotae appeared unaffected by the presence of D. lumholtzi. 4. The strong migration behaviour of D. lumholtzi in the absence of fish cues suggests that this behaviour may be a constitutive trait. Habitat partitioning of the two species is probably the result of different thermal tolerances, with D. mendotae constrained to remaining in deeper water by high temperatures in the epilimnion and the tropical D. lumholtzi able to use the warm epilimnion at night.     

Patterns and sources of variation in Daphnia phosphorus content in nature

It has recently been shown that Daphnia can vary in the phosphorus (P)-content of their body tissues, but the relative importance of genetic versus environmental causes for this variation is unexplored. We measured variation in P-content (as % body mass) of Daphnia from eight lakes and conducted experiments to contrast three sources of variation: interspecific variation, clonal variation and phenotypic plasticity. Daphnia P-content decreased with increasing seston C:P ratio across lakes. This relationship reflected both inter- and intraspecific variation. Daphnia parvula and D. dubia exhibited high P-content and were found in shallow lakes with low C:P seston, whereas D. pulicaria had low P-content and was found in deep, stratified lakes having high C:P seston. Populations of D. dentifera spanned this lake gradient and exhibited P-content that was negatively related to seston C:P. Evidence for phenotypic plasticity came from experiments with D. pulicaria and D. dentifera collected from a lake with P-deficient seston and fed a P-sufficient diet in the laboratory. In addition, populations of D. dentifera differed in P-content even after 7 d of feeding on P-sufficient resources, suggesting within-species clonal variation. However, mesocosm experiments revealed broad and surprisingly continuous variation in the P-content of individual clones of D. pulex (range 1.54–1.05%) and D. mendotae (1.51–1.07%) over a gradient in dietary C:P. The broad range in P-content exhibited by individual clones, acclimated for generations, suggests that variation in Daphnia P-content from laboratory experiments needs to be interpreted with caution. These results also show that phenotypic variation in response to environment can be a larger source of variation in P-content than genetic differences within or among species.     

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.     

New microsatellite primers for Daphnia galeata mendotae

In order to study genetic changes in populations of Daphnia galeata mendotae, I characterized seven polymorphic microsatellite loci. Primers to amplify these loci were tested on individual eggs from the resting egg bank of Onondaga Lake, NY. Levels of polymorphism and cross‐amplification in D. g. galeata indicate that they will be useful markers for ecological genetic studies on both adults and diapausing eggs of these species.     

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.     

Temperature and kairomone induced life history plasticity in coexisting Daphnia

We investigated the life history alterations of coexisting Daphnia species responding to environmental temperature and predator cues. In a laboratory experiment, we measured Daphnia life history plasticity under different predation risk and temperature treatments that simulate changing environmental conditions. Daphnia pulicaria abundance and size at first reproduction (SFR) declined, while ephippia (resting egg) formation increased at high temperatures. Daphnia mendotae abundance and clutch size increased with predation risk at high temperatures, but produced few ephippia. Thus, each species exhibited phenotypic plasticity, but responded in sharply different ways to the same environmental cues. In Glen Elder reservoir, Kansas USA, D. pulicaria dominance shifted to D. mendotae dominance as temperature and predation risk increased from March to June in both 1999 and 2000. Field estimates of life history shifts mirrored the laboratory experiment results, suggesting that similar phenotypic responses to seasonal cues contribute to seasonal Daphnia population trends. These results illustrate species-specific differences in life history plasticity among coexisting zooplankton taxa.     

Using morphological characters of subfossil daphniid postabdominal claws to improve taxonomic resolution within species complexes

Daphnia subfossils from lake sediments are useful for exploring the impacts of environmental stressors on aquatic ecosystems. Unfortunately, taxonomic resolution of Daphnia remains is coarse, as only a small portion of the animal is preserved, and so the identification of daphniid subfossils typically relies upon postabdominal claws. Daphniid claws can be assigned to one of two species complexes: D. longispina or D. pulex. Both complexes contain species with differing environmental optima, and therefore improved taxonomic resolution of subfossil daphniid claws would aid paleolimnological analyses. To identify morphological features that may be used to help differentiate between species within complexes, we used species presence/absence data from net tows to select lakes in central Ontario (Canada) containing only a single species from a particular complex, then used remains preserved in surface sediments of these lakes to isolate four Daphnia species: D. ambigua and D. mendotae from the D. longispina complex, and D. pulicaria and D. catawba from the D. pulex complex. Our analyses demonstrate that, within the D. longispina complex, postabdominal claw length (PCL) and spinule length can be used to distinguish D. mendotae from D. ambigua. In addition, within the D. pulex complex, there are differences between D. pulicaria and D. catawba in the relative lengths of the proximal and middle combs on the postabdominal claw. However, the number of stout spines on the middle comb is an unreliable character for differentiating species. Overall, our data demonstrate that greater resolution within Daphnia species complexes is possible using postabdominal claws; however, the process is arduous, and applicability will likely decrease with the number of taxa present.     

At the edge and on the top: molecular identification and ecology of Daphnia dentifera and D. longispina in high-altitude Asian lakes

The occurrence of members of the highly diverse Daphnia longispina complex in Southern and Central Asian high-mountain lakes has been recognized for more than a century. Until now, however, no molecular data have been available for these populations inhabiting the “Roof of the World.” Here, we present the first identification for D. gr. longispina from that region based on a molecular phylogeny. Our findings show that alpine lakes in the Pamir and Himalaya mountains host populations of widespread species of the complex, for which these are the highest known localities. A spineless morph from the Himalaya region, previously labeled as D. longispina var. aspina, was clustering tightly with D. dentifera, while a population from the Pamir mountain range was grouped with D. longispina. In addition, we analyzed ecological data available for lakes in the Khumbu region (Himalaya) to investigate ecological preferences of non-pigmented D. gr. longispina. The identified factors can at least partly be related to avoidance of high UV conditions by this species. We conclude that the widespread species D. dentifera and D. longispina also colonized the Asian high-mountain lakes, and identify the need for further research to trace the possible effect of rapid environmental changes in this region on the diversity and ecology of high-altitude Daphnia populations.     

Zooplankton seasonal dynamics in a recently filled mine pit lake: the effect of non-indigenous <Emphasis Type="Italic">Daphnia</Emphasis> establishment

We examined the temporal and vertical dynamics of zooplankton in Weavers Lake, New Zealand, between October 2004 and October 2005, at a time when it was colonised by a non-indigenous Daphnia species. Zooplankton community composition changed during the study from one of rotifer dominance (e.g. Asplanchna, Polyarthra, Brachionus and Keratella species) to cladoceran (Daphnia dentifera) dominance. Temporal changes in zooplankton community composition were strongly associated with a gradual increase in lake water clarity, and were attributable to the highly efficient filter feeding of D. dentifera. The corresponding reduction in rotifer densities may have resulted from the superior competitive abilities of the newly established Daphnia. As Daphnia were rare inhabitants of New Zealand lakes before 1990, the arrival and rapid spread of the non-indigenous D. dentifera has lead to widespread changes in both water clarity and zooplankton community composition. An apparent lack of mixing in the lake was facilitated by the lake’s extremely small surface area:depth ratio. However, we conclude that physical features of the lake had minimal influence on water clarity relative to the invasion of D. dentifera.     

The outcome of the invasion of Florida lakes by Daphnia lumholtzi

1. We examined 7–12 years of monthly to quarterly historical data from 15 lakes in FL, U.S.A. to determine the extent and outcome of invasion by the alien cladoceran Daphnia lumholtzi Sars. 2. The alien species was found in 10 of the 15 lakes, including Florida’s three largest lakes: Okeechobee, George and Apopka. All the surveyed lakes had resident populations of the smaller native species Daphnia ambigua Scourfield. 3. In most of the lakes, D. ambigua occurred seven to ninefold more often in plankton samples than D. lumholtzi, and at 10‐ to 100‐fold higher maximal densities. One exception was a small lake in central Florida (Lake Jesup), where D. lumholtzi attained high densities on several occasions in the 10 years of sampling. 4. In Lake Okeechobee, where data were of sufficient quality and quantity to perform statistical analyses, the results of canonical correlation analysis indicated that high densities of D. lumholtzi were correlated with lower concentrations of suspended solids, high algal biomass and higher temperature, whereas the opposite conditions were correlated with high densities of D. ambigua. 5. Based on the majority of data, D. lumholtzi has successfully invaded many lakes in Florida, yet it has not become a substantive component of the zooplankton. Additional research is needed to determine whether resources, fish predation or some other factor is responsible for this outcome of invasion.     

Evolutionary history of alpine and subalpine Daphnia in western North America

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The genusDaphnia in Cameroon,West Africa

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Selective predation on an introduced zooplankter ( Bythotrephes cederstroemi ) by lake herring ( Coregonus artedii ) in Harp Lake,Ontario

1. Bythotrephes cederstroemi (Crustacea: Onychopoda: Cercopagidae) invaded Harp Lake, Ontario in 1993, since when the zooplankton community has shifted from dominance of small-bodied to large-bodied species. During 1995 diets of adult lake herring (Coregonus artedii), Harp Lake’s primary planktivorous fish, were examined to determine the extent to which this conspicuous zooplankter has become integrated into the lake’s foodweb and whether fish condition has been affected in consequence. 2. Bythotrephes and Daphnia galeata mendotae were strongly preferred prey, whilst Holopedium gibberum and calanoid and cyclopoid copepods were negatively selected by lake herring. Predation on Bythotrephes and Holopedium was not size-selective, although D. galeata mendotae and calanoid copepods (Leptodiaptomus minutus and L. sicilis) consumed by herring were significantly larger than co-occurring conspecifics in the lake. 3. Caudal spines of Bythotrephes may form boluses in lake herring stomachs. However, the number of caudal spines in fish digestive tracts did not differ significantly from the number of Bythotrephes’ mandible pairs, indicating that the former were not differentially retained. 4. Lake herring weight-at-length relationships in lakes in Muskoka, Ontario, invaded by Bythotrephes did not differ from those in adjacent non-invaded lakes, indicating that Bythotrephes invasion of lakes apparently has not affected condition of lake herring.