Alan J. Tessier

Body Size, Energy Reserves, and Competitive Ability in Three Species of Cladocera

The purpose of this study was to identify mechanisms of competitive ability among planktonic Clodocera. We used three species (Daphnia magna, Daphnia galeata, and Bosmina longirostris) in life—table and birth—schudle experiments to analyze the effect of low food levels on each population, and in competition experiments. Demographic traits of all species were affected by low food levels, but the smaller species suffered higher death rates. D. magna matured and reproduced well in the lowest food level studied; growth rate of the population was 0.143 d—1. Survivorship for D. galeata cohorts was poor at the lowest food level but many animals did mature and reproduce; population growth rate was 0.082 d—1. Few Bosmina individuals matured and reproduced and all were dead by the third adult instar; the population growth rate was —0.136 d—1. In the competition experiments, D. magna was the numerical dominant in high—food cultures with D. galeata, and D. galeata was the numerical dominant in cultures with Bosmina. ...

CRYPTIC TROPHIC CASCADE ALONG A GRADIENT OF LAKE SIZE

Lake size and depth mediate the strength of interaction between fish and zooplankton. We test whether this variation in zooplanktivory indirectly affects the phy- toplankton by comparing 19 lakes that represent two food webs resulting from the absence of piscivores in small, partial-winterkill lakes. Lakes with piscivorous fish are further distinguished by thermal stratification, which provides a refuge for zooplankton to avoid fish predation. We contrasted phytoplankton abundance in these three categories of lakes over six years, using both direct measures of concentration and a growth bioassay that measures phytoplankton from the perspective of a standard grazer (Daphnia). Contrary to expectations, phytoplankton abundance was largely unaffected by trophic structure or the presence of a deep-water refuge. However, grazer growth differed dramatically among the three categories of lakes. Consistent with trophic cascade, increased fish planktivory resulted in more phytoplankton food as measured from the grazers perspective. This effect was independent of lake productivity or total abundance and size of phytoplankton. Instead, variation in food quality for grazers was associated with compositional differences in phytoplankton. These results indicate that persistent trophic cascades are more dramatic in the plankton than previously realized but primarily influence composition, rather than bio- mass. Although cryptic, such top-down effects create functional variation in grazer-resource coupling.

Resource quantity and offspring quality in Daphnia

Many species of zooplankton display temporal changes in the size of the offspring they produce. We address the question, is offspring size variation by Daphnia a predictable response to changes in resource abundance? For each of two species of Daphnia (D. pulicaria and D. pulex), we raised several distinct genotypes at each of three different food levels. Differences in neonate body length, dry mass, and C:N ratio were measured at each food level. The two species responded differently to food level; neonate mass decreased in D. pulex with decreasing food, but the largest (mass) D. pulicaria neonates were produced at the intermediate food level. For both species, the lightest (mass) neonates were produced at the lowest food, but these neonates were often equal (D. pulex), or longer (D. pulicaria) in body length than those produced at high food. The production of small-mass, but relatively large-length neonates in the lowest food level was associated with a lower C:N ratio of both adults and neonates compared with the higher food levels. We conclude that qualitative changes in the production of offspring can result from simple quantitative changes in resource abundance. We suggest that these responses may represent adaptive plasticity in reproduction.

A FUNDAMENTAL TRADE-OFF IN RESOURCE EXPLOITATION BY DAPHNIA AND CONSEQUENCES TO PLANKTON COMMUNITIES

The genus Daphnia (Crustacea: Branchipoda) includes a diversity of species that are generalist grazers on plankton in freshwater lakes. Despite substantial intraspecific variation in body size, explanations of functional differences among Daphnia species con- tinue to emphasize body size. In this study, we ask whether Daphnia in a relatively narrow range of body sizes differ in ability to exploit the array of resources commonly encountered in lakes. We compared juvenile growth rates of several clones and species of Daphnia fed natural planktonic resources that differed in overall dietary richness. We observed, both among and within species, a trade-off between growth rates in rich and poor resource conditions. We compared the ability of different species and clones to exploit natural resources in a lake mesocosm experiment, testing the prediction that clones displaying the greatest growth rates in rich resource conditions would have the highest minimum resource requirements. Using 8000-L whole water column enclosures, replicate populations of D. rosea and two different morphs of D. pulicaria were established separately in monoculture in a thermally stratified lake. An additional treatment without crustacean zooplankton was also established. We quantified resources in the enclosures using multiple indirect measures (e.g., chlorophyll a) and a growth bioassay. Results supported predictions; rankings of resource levels achieved in the different treatments suggest that Daphnia trade off high maximum growth rates for low minimum resource requirements. This field experiment also documented large differences among the species and clones of Daphnia in their impact on phytoplankton resources, water clarity, and formation of calcite particles (whiting). We suggest that these large ecological effects stem from the trade-off in abilities to use rich vs. poor resources and constitute an important aspect of grazer-resource interactions. We review the literature on growth and reproductive responses of zooplankton species to resource level and find general agreement with the concept of a trade-off between abilities to exploit rich and poor resource conditions. A reanalysis of data from a comprehensive study on Daphniidae growth confirms our results and suggests that the breadth of species in this family conforms to the trade-off. Furthermore, the trade-off is only weakly, and not significantly, associated with body size. Our study presents a new framework for under- standing the functional significance of daphniid diversity to plankton food webs and illus- trates how an ecological trade-off provides a means of linking species diversity to ecosystem function.

Quality matters: resource quality for hosts and the timing of epidemics

Epidemiologists increasingly realize that species interactions (e.g. selective predation) can determine when epidemics start and end. We hypothesize here that resource quality can also strongly influence disease dynamics: epidemics can be inhibited when resource quality for hosts is too poor and too good. In three lakes, resource quality for the zooplankton host (Daphnia dentifera) was poor when fungal epidemics (Metschnikowia bicuspidata) commenced and increased as epidemics waned. Experiments using variation in algal food showed that resource quality had conflicting effects on underlying epidemiology: high-quality food induced large production of infective propagules (spores) and high birth rate but also reduced transmission. A model then illustrated how these underlying correlations can inhibit the start of epidemics (when spore production/birth rate are too low) but also catalyse their end (when transmission becomes too low). This resource quality mechanism is likely to interface with other ones controlling disease dynamics and warrants closer evaluation.

SEASONAL SUCCESSION AND THE STRENGTH OF INTRA- AND INTERSPECIFIC COMPETITION IN A DAPHNIA ASSEMBLAGE'

Species of the genus Daphnia are generalized filter-feeders and potentially strong competitors, yet several species often coexist in the open waters of lakes. We in- vestigated the population dynamics and competitive interactions of two Daphnia species in Gull Lake, Michigan. In each of 3 yr D. pulicaria was always common, while D. galeata mendotae became abundant only in late summer. The summer invasion of D. galeata mendotae was associated with a decline in abundance of D. pulicaria in some, but not all years. Decline of D. pulicaria was strongly predicted by the loss of oxygen from deep water during summer, which resulted in greater spatial overlap between the species, and by changes in the quality of the phytoplankton resources. We employed in situ enclosures to manipulate the density of each species and quantify the intensity of intra- and interspecific competition. Repeated competition experiments demonstrated that the relative exploitative ability of the species reversed from early to late summer. Strong interspecific competition was often observed, but its strength varied with juvenile or adult stage, and with season. Daphnia pulicaria juveniles were always affected strongly by interspecific effects, while D. galeata mendotae juveniles were impacted largely by intraspecific effects. The relative strength of intra- and interspecific competition on adults reversed from early to late summer for each species. The magnitude of intra- and interspecific effects on D. pulicaria adults was similar during early summer, but by late summer interspecific effects were much stronger than intraspecific effects. The reverse was true for D. galeata mendotae adults; interspecific effects greatly decreased in late summer. This seasonal shift in the nature of competitive interactions provides an explanation for seasonal succession in this Daphnia assemblage.

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

SummaryWe 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.

Comparative Population Regulation of Two Planktonic Cladocera (Holopedium Gibberum and Daphnia Catawba)

The abundance of two planktonic grazers (Holopedium gibberum and Daphnia catawba) fluctuated seasonally in a nonparallel manner in Lake Lacawac, Pennsylvania. Holopedium gibberum was abundant in early and midsummer, while D. catawba was common only in spring and late summer. I sought to explain this seasonal succession by identifying seasonal factors which regulate each pop- ulation. I monitored population abundance, individual energy reserves (lipid content), egg production, and body size distributions in plankton samples from April to October in three years. Food resources and vertebrate predator abundance were also examined seasonally. Results of experimental manip- ulations of food availability and predators, conducted seasonally in both small (6.4-L) and large (6000- L) enclosures, supported interpretations drawn from descriptive data. Contrasting seasonal abundance patterns of the species are explained by differences in the intensity of food stress and predation affecting each species. Food supply restricted population growth of H. gibberum at most times. In the spring of each year, starvation was a major cause of mortality; during summer, individual energy reserves and fecundity were near zero for H. gibberum. Predation losses were negligible for adult H. gibberum, but neonate (smallest age class) mortality by fish was occasionally large. Daphnia catawba experienced a seasonal change in the intensity of regulating factors; food supply restricted population growth only in spring and late summer, but predation controlled growth during summer. During late summer, predation on D. catawba was reduced and its population abundance increased sharply. Experimental removal of fish caused a comparable increase of D. catawba during midsummer. At peak animal densities in late summer, both species were food stressed. Life table experiments conducted at this time suggest that competition for food occurs between D. catawba and H. gibberum, and results in a reduction of H. gibberum density.

HOW LONG TO REST: THE ECOLOGY OF OPTIMAL DORMANCY AND ENVIRONMENTAL CONSTRAINT

Dormancy is a common mechanism employed by short-lived organisms for persistence in a variable environment. Theory suggests that the fraction of propagules that terminate dormancy each year should be <100% when recruitment success varies temporally. Moreover, the fraction of propagules that resumes development should vary across habitats that differ in the probability of successful recruitment or the probability of survival during dormancy. We tested these predictions by using dormant eggs from five populations of the freshwater cladoceran Daphnia pulicaria that differ in their ability to recruit to and persist in the water column. In two separate experiments, newly produced dormant eggs were incubated in situ for one year at various sites on the bottom of the lakes. A series of reciprocal transplants among four of these populations separated the effects of lake-specific environmental cues from the genetic and maternal effects of the different populations. Additional eggs were incubated in the laboratory under photoperiod–temperature combinations representative of those in the field. We found that the annual hatching fraction ranged from 6% to 50% among lakes, and that hatching fraction was primarily driven by environmental cues rather than being a result of the source of the eggs. However, laboratory incubations demonstrated significant differences among populations in the trajectories of the hatching curves, and a much higher rate of hatching than the field incubations. Our results suggest that variation in dormancy strategies within these systems is likely influenced both by the seasonal risk experienced by the active individuals and by risks associated with entering the dormant egg bank.

NATURAL GENETIC VARIATION OF LIFE SPAN, REPRODUCTION, AND JUVENILE GROWTH IN DAPHNIA

The evolutionary theory of senescence predicts that high extrinsic mortality in natural populations should select for accelerated reproductive investment and shortened life span. Here, we test the theory with natural populations of the Daphnia pulex‐pulicaria species complex, a group of freshwater zooplankton that spans an environmental gradient of habitat permanence. We document substantial genetic variation in demographic life‐history traits among parent and hybrid populations of this complex. Populations from temporary ponds have shorter life spans, earlier and faster increases of intrinsic mortality risk, and earlier and steeper declines in fecundity than populations from permanent lakes. We also examine the age‐specific contribution to fitness, measured by reproductive value, and to expected lifetime reproduction; these traits decline faster in populations from temporary ponds. Despite having more rapid senescence, pond Daphnia exhibit faster juvenile growth and higher early fitness, measured as population growth rate (r). Among populations within this species complex we observed negative genetic correlations between r and indices of life‐history timing, suggesting trade‐offs between early‐ and late‐life performance. Our results cannot be explained by a trade‐off between survival and fecundity or by nonevolutionary theories of senescence. Instead, our data are consistent with the evolutionary theory of senescence because the genetic variation in life histories we observed is roughly congruent with the temporal scale of environmental change in the field.