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.     

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.     

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     

Influence of two Daphnia species on summer phytoplankton assemblages from eutrophic lakes

We conducted grazing experiments to test whether larger-bodiedDaphnia pulicaria have a different effect from smaller-bodiedDaphnia galeata mendotae on the composition of summer algalassemblages in eutrophic lakes. Three separate cubitainer experimentswere run for 5 days in a replicated factorial design utilizingtwo algal community types and the two Daphnia species. Inorganicphosphorus and nitrogen were added to prevent nutrient limitationof the algae. Both edible and inedible size fractions of chlorophylla increased in cubitainers without Daphnia spp. Grazer additionusually resulted in a reduction in edible chlorophyll; reductionswere greater in D.pulicaria cubitainers. Grazing by Daphniaspp. on presumed inedible chlorophyll was variable. Algal sizewas not always a good predictor of grazeability. The resultsof this study indicate that D.pulicaria, because of its greaterfiltration potential and ability to ingest larger particles,provides a stronger control on inedible-sized algae when comparedto equal numerical densities of D.g.mendotae. However, Aphanizomenonincreased as a response to heavy grazing pressure by D.pulicariaon other algal species. This suggests that biomanipulation effortsthat promote large-bodied Daphnia may not produce desirableresults if nutrient inputs remain high.     

Influence of different sized Daphnia species on chlorophyll concentration and summer phytoplankton community structure in eutrophic Wisconsin lakes

We conducted field observations on 13 eutrophic Wisconsin lakesdominated by either the larger bodied Daphnia pulicaria or thesmaller bodied Daphnia galeata mendotae. While Daphnia numericaldensities were not significantly different between groups oflakes, pulicaria lakes had much higher Daphnia biomasses andfiltration potentials than galeata lakes. Although we foundsignificant differences in chlorophyll (Chl) a between bothgroups of lakes during June, on a seasonal basis populationsof different sized Daphnia were not associated with significantdifferences in Chl a. Filtration potential per se was the majordeterminant of Chl a, regardless of which Daphnia species dominated.However, in pulicaria lakes, the clear-water phase started earlier,lasted longer, and was usually characterized by greater Secchidisc readings than in galeata lakes. For large blue-green algaesuch as Aphanizomenon, D.pulicaria appeared to delay bloom conditions,but ultimately did not prevent the alga from growing. Our resultssuggest that high densities of large-sized Daphnia are a desirablegoal of biomanipulation because they can attain filtration potentialshigh enough to increase summer water clarity in eutrophic lakes.     

Formation of morphological defences in response to YOY perch and invertebrate predation in two <Emphasis Type="Italic">Daphnia</Emphasis> species coexisting in a mesotrophic lake

This study examined the formation of morphological defences by two coexisting Daphnia species, the large-sized D. pulicaria (2 mm) and the small-sized D. mendotae (1.4 mm), in response to the presence of young-of-the-year (YOY) yellow perch (Perca flavescens) and invertebrate predators (Chaoborus, Leptodora) during summer in a mesotrophic lake. We hypothesized that due to differential size-selective predation risk by YOY fish and invertebrates, the large-sized and the small-sized Daphnia species would show different morphological responses to predation threats. We followed changes in two morphological traits (relative length of the tail spine in D. pulicaria and of the helmet in D. mendotae) among different periods during summer according to YOY fish and invertebrate predation. We defined four YOY fish predation periods based on the presence of YOY perch in the pelagic zone of the lake and the relative abundance of Daphnia preys in their gut contents, and two invertebrate predation periods based on exclusive or mutual occurrence of the invertebrate predators. The large-sized (D. pulicaria) and the small-sized (D. mendotae) species showed different morphological responses to YOY fish and invertebrate predators, respectively. The tail spine ratio of the juveniles and adults of D. pulicaria did not change in response to YOY fish predation or to invertebrate predation. A gradual increase in the helmet ratio was observed in the small-sized D. mendotae over the summer period. This change was related to the co-occurrence of the invertebrate predators (Chaoborus and Leptodora) and to YOY fish predation. The warmer temperature cannot be accounted for helmet elongation since it was constant across depths, and not related with the co-occurrence of D. mendotae and YOY perch. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera     

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.     

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.     

The effect of suspended sediments on Lake Texoma Daphnia: field distributions and in situ incubations

1. We measured the abundance and eggs per female of four Daphnia species in turbid and relatively clear regions of Lake Texoma (Oklahoma‐Texas, U.S.A.) on 12 dates over the course of 5 years. 2. Two species, Daphnia lumholtzi and Daphnia parvula, occurred and reproduced in turbid locations, but two other species, Daphnia mendotae and Daphnia pulicaria, occurred almost exclusively in relatively clear conditions. 3. To test the hypothesis that interference with foraging excludes clear‐water Daphnia species from turbid locations, we incubated adult D. mendotae at both a clear and a turbid site. In three successive experiments D. mendotae individuals incubated at the turbid site carried as many or more eggs than individuals incubated at the clear site.     

Are zooplankton food resources poor in the vegetated littoral zone of shallow lakes?

1. The distribution of zooplankton in shallow lakes is negatively related to macrophyte density. However, the abundance of their food along density gradients of macrophytes is unknown. A common but untested assumption is that food quantity and quality for pelagic zooplankton is poor in the littoral zone owing to the deleterious influence of macrophytes on phytoplankton. 2. We tested this assumption with a combination of a field survey and laboratory experiments. We collected seston samples from the littoral and pelagic zones of four shallow temperate lakes and related food quantity (phytoplankton biovolume) and quality to macrophyte abundance (per cent volume infested). Seston food quality was assessed in three ways: N/C and P/C ratios, polyunsaturated fatty acid content and phytoplankton community composition. In the laboratory, we measured the growth and reproduction of Daphnia pulicaria on diets consisting of seston from the littoral and pelagic zones in one lake. 3. In our four study lakes, food quantity was not significantly influenced by macrophyte abundance, and food quality was generally high. Laboratory experiments showed increased juvenile growth, but no significant change in D. pulicaria reproduction, when feeding on littoral resources compared to pelagic resources. 4. Our results suggest that there is no nutritional cost to pelagic zooplankton inhabiting the littoral zone. Therefore, it is likely that other factors (e.g. predation, abiotic factors) are involved in determining zooplankton habitat use.     

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.     

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.     

The quick and the dead: might differences in escape rates explain the changes in the zooplankton community composition of Lake Michigan after invasion by <Emphasis Type="Italic">Bythotrephes</Emphasis>?

We demonstrate that zooplankton escape abilities are consistent with the composition of the zooplankton community in the Great Lakes following the invasion of the visually preying invertebrate predator Bythotrephes longimanus. Escape abilities were analyzed by videotaping responses of free-swimming zooplankton to encounters with tethered Bythotrephes. Both maximum speed and maximum acceleration of the escape response were appreciably greater in Daphnia mendotae and diaptomids, whose populations remained relatively unchanged, than those of Daphnia retrocurva and Daphnia pulicaria, whose populations greatly decreased after the Bythotrephes invasion. Maximum speed of all species was higher in the light than in complete darkness, likely due to a different level of activity of Bythotrephes. Contrary to treatments with Bythotrephes, mean and maximum swimming speeds of all species were similar to each other and the same in light and dark in treatments without Bythotrephes. This implies that the prey were responding to infochemicals produced by Bythotrephes.     

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.     

Bacterioplankton communities of Crater Lake,OR: dynamic changes with euphotic zone food web structure and stable deep water populations

The distribution of bacterial and archaeal species in Crater Lake plankton varies dramatically over depth and with time, as assessed by hybridization of group-specific oligonucleotides to RNA extracted from lakewater. Nonmetric, multidimensional scaling (MDS) analysis of relative bacterial phylotype densities revealed complex relationships among assemblages sampled from depth profiles in July, August and September of 1997 through 1999. CL500-11 green nonsulfur bacteria (Phylum Chloroflexi) and marine Group I crenarchaeota are consistently dominant groups in the oxygenated deep waters at 300 and 500 m. Other phylotypes found in the deep waters are similar to surface and mid-depth populations and vary with time. Euphotic zone assemblages are dominated either by β-proteobacteria or CL120-10 verrucomicrobia, and ACK4 actinomycetes. MDS analyses of euphotic zone populations in relation to environmental variables and phytoplankton and zooplankton population structures reveal apparent links between Daphnia pulicaria zooplankton population densities and microbial community structure. These patterns may reflect food web interactions that link kokanee salmon population densities to community structure of the bacterioplankton, via fish predation on Daphnia with cascading consequences to Daphnia bacterivory and predation on bacterivorous protists. These results demonstrate a stable bottom-water microbial community. They also extend previous observations of food web-driven changes in euphotic zone bacterioplankton community structure to an oligotrophic setting.     

Summer depth selection in crustacean zooplankton in nutrient‐poor boreal lakes is affected by recent residential development

1. Strong vertical gradients in light, water temperature, oxygen, algal concentration and predator encounters during summer in stratified lakes may influence patterns of depth selection in crustacean zooplankton, especially Daphnia species. 2. To test how crustacean depth selection varies among lakes along a gradient of catchment disturbance by recent residential development and land use change, we calculated the weighted mean depth distribution of the biomass of crustaceans by day and night in eight nutrient‐poor boreal lakes. 3. Generally, the greatest biomass of crustaceans was located at the metalimnion or at the lower boundary of the euphotic zone during thermal stratification in July. The crustacean zooplankton avoided warm surface layers and tended to stay in colder deep waters by both day and night. They also remained at greater depths in lakes with a more extensive euphotic zone. 4. There was some evidence of upward nocturnal migrations of large Daphnia and copepods in some lakes, and one case of downward migration in a lake inhabited by chaoborid larvae. 5. Multivariate regression trees (MRT) were used to cluster crustaceans and Daphnia species in homogeneous groups based on lake natural and disturbance factors. For crustaceans, the depth of the euphotic zone, the sampling depth (epilimnion, metalimnion and hypolimnion), time (day or night) of sampling and the biomass of chlorophyll a were the main driving factors. For Daphnia species, the drainage area, the sampling depth, the cleared land surface area within the catchment and the concentration of total dissolved phosphorus were the main factors.     

Timing of predation by rainbow trout controls Daphnia demography and the trophic status of a Minnesota lake

1. Stocking of lakes with rainbow trout is a common practice that presents a potential conflict for lake managers who must balance the interests of anglers with those concerned that zooplanktivory by trout may trigger a trophic cascade and result in decreased water clarity. 2. This study examined how the timing of trout stocking (autumn versus spring) in a Minnesota (U.S.A.) lake affected (i) the population dynamics of their zooplankton food supply (Daphnia pulicaria), (ii) phytoplankton biomass and water clarity and (iii) trout survival. Sizes of both Daphnia and trout populations were estimated acoustically with high‐frequency (192 kHz) sonar. 3. Daphnia were nearly eliminated from the lake during winters after trout were stocked in autumn. In both of these years (1996 and 1997), the Daphnia population was small in the spring, and grew during the summer and into the autumn as the trout population diminished. 4. The lake was then stocked in spring for 2 years (1998 and 1999). This fisheries manipulation alleviated predation over the winter, but increased predation on D. pulicaria during the spring, summer and autumn. However, the high mortality caused by the spring‐stocked trout was offset by even higher rates of reproduction by the relatively large populations of fecund Daphnia that survived the winter in 1998 and 1999. 5. Grazing by these dense populations of Daphnia produced clear‐water phases during May and June that were inhibited in autumn stocking years. In addition, the large Daphnia populations present during the spring and early summer of 1998 and 1999 provided abundant forage for trout. 6. This fisheries manipulation achieved seemingly mutually exclusive management objectives: a robust planktivorous sport fishery, and clear water for other forms of recreation.     

Habitat use and ecological specialization within lake Daphnia populations

Many species of planktonic cladocerans display substantial variation in habitat use (mean depth and diel vertical migration), both among and within populations. We examined whether clonal segregation and specialization contributes to such behavioral variation within several lake populations of the cladoceran, Daphnia pulicaria. Electrophoretic and quantitative genetic analysis of clonal lines isolated from different depths at night revealed that clonal habitat specialization was common. Clones that utilized shallow water at night were genetically smaller at maturity and lower fecundity under standard laboratory conditions than the deep-water clones. The magnitude of this clonal habitat specialization varied among lakes: populations displaying broad use of depth habitats contained greater genetic variance than populations with more constrained habitat use. These results are consistent with known differences in selective factors in different depth habitats and suggest that substantial clonal specialization can occur within single populations. Since previous work has discovered a heritable basis to habitat selection in several Daphnia species, including D. pulicaria in our study lakes, it is likely that clonal/depth specialization is an important factor affecting the trophic ecology of Daphnia. Received: 18 April 1996 / Accepted: 25 September 1996     

Clonal integration facilitates invasiveness of the alien aquatic plant Myriophyllum aquaticum L. under heterogeneous water availability

The avoidance of visually feeding fish has long been considered as the primary driver of diel vertical migration of zooplankton. The diurnal vertical distribution of Cyclops gr. abyssorum, Arctodiaptomus alpinus, and Daphnia gr. longispina from 13 alpine lakes with fish (Salvelinus fontinalis) and without, was compared in order to understand whether fish in transparent lakes reduce the presence of large zooplankton from the irradiated zone. We used the light level at each sampling depth and the size of each specimen as proxies of predation risk, and we tested two predictions: (P1) the relative abundance of zooplankton in the well-lit surface waters vs. the darker waters will be greater in fishless lakes; (P2) the size of zooplankton in the well-lit surface waters vs. the deeper, darker waters will be greater in fishless lakes. We did not find any evidence of the validity of P1, but we confirmed P2 for Arctodiaptomus alpinus. These results support with new field data the Transparency Regulator Hypothesis, which argues that in transparent lakes, fish predation is less important for the vertical distribution of zooplankton than ultraviolet radiation, and further suggest that zooplankton size rather than vertical distribution may be more effective against visual predators in transparent lakes.