Daphnia Populations Research Articles

Degree of high phenotypic plasticity in wild populations of Daphnia in early spring

Inducible defenses have been empirically known as defensive phenotypes that are triggered by predator kairomones. We hypothesized that morphological defense of wild Daphnia varies not only with predator density, but also with the predator regime in the field. We observed how the morphological defenses of two Daphnia species (D. ambigua and D. pulex) changed according to the population density in the water column at daytime of predatory insect larvae Chaoborus flavicans in Lake Fukami-ike, Japan, from February to July. In both Daphnia species, the inducible defense morphology was highly expressed in March and April. Its degree of expression decreased, and did not change with increases in predator density from May to July. These seasons are generally considered as when the number of the larvae and fish increases, and predation becomes more active due to their growth and breeding. We suggest that the degree of inducible defense of Daphnia is higher at the end of the overwintering season, when Chaoborus larvae began to inhabit the water column during the daytime and is constant in other seasons regardless of predators regime changes. Field observation of wild populations is important for understanding the seasonal changes in the morphology, and to provide more realistic explanations of phenomena in inducible defense.

Suppression of Alga Blooming by Zooplankton Filter Feeders in Small Water Bodies

The study is focused on the ability of filterers–phytophages Daphnia longispina O.F. Mueller to suppress blooming in shallow-water bodies in Moscow oblast. The effect of phytoplankton development on the composition of daphnia populations is studied. With the predominance of small-cell (<40 μm in length) green, yellow-green, and euglena algae in the phytoplankton and, accordingly, in the nutrient budget of daphnia, the biomass of D. longispina (12 mg/L) is 1.5–6.0 times greater than that in the water bodies where the phytoplankton shows the predominance of Aphanizomenon flexuosum Komarek et Kovacik (filamentous cyanobacteria) and Volvox polychlamys Korshikov (cenobial green algae). The mean fertility of the reproducing organisms of D. longispina (the number of eggs per female reached 11) was also higher in the former case.

Daphnia invest in sexual reproduction when its relative costs are reduced.

The timing of sex in facultatively sexual organisms is critical to fitness, due to the differing demographic consequences of sexual versus asexual reproduction. In addition to the costs of sex itself, an association of sex with the production of dormant life stages also influences the optimal use of sex, especially in environments where resting eggs are essential to survive unfavourable conditions. Here we document population dynamics and the occurrence of sexual reproduction in natural populations of Daphnia magna across their growing season. The frequency of sexually reproducing females and males increased with population density and with decreasing asexual clutch sizes. The frequency of sexually reproducing females additionally increased as population growth rates decreased. Consistent with population dynamic models showing that the opportunity cost of sexual reproduction (foregoing contribution to current population growth) diminishes as populations approach carrying capacity, we found that investment in sexual reproduction was highest when asexual population growth was low or negative. Our results support the idea that the timing of sex is linked with periods when the relative cost of sex is reduced due to low potential asexual growth at high population densities. Thus, a combination of ecological and demographic factors affect the optimal timing of sexual reproduction, allowing D. magna to balance the necessity of sex against its costs.

Parasites driving host diversity: Incidence of disease correlated with Daphnia clonal turnover.

According to the Red Queen hypothesis, clonal diversity in asexual populations could be maintained by negative frequency-dependant selection by coevolving parasites. If common clones are selected against and rare clones gain a concomitant advantage, we expect that clonal turnover should be faster during parasite epidemics than between them. We tested this hypothesis exploring field data of the Daphnia-Caullerya host-parasite system. The clonal make-up and turnover of the Daphnia host population was tracked with high temporal resolution from 1998 until 2013, using first allozyme and later microsatellite markers. Significant differences in the clonal composition between random and infected subsamples of Daphnia populations were detected on six of seven tested occasions, confirming genetic specificity of the host-parasite interaction in this system. We used time series analysis to compare the rates of host clonal turnover to the incidence of parasitism, and found that Caullerya prevalence was significantly associated with microsatellite-based clonal turnover. As alternate hypotheses, we further tested whether turnover was related to a variety of biotic, abiotic, and host demographic parameters. Other significant correlates of turnover were cyanobacterial biomass and (weakly) temperature. Overall, parasitism seems to be a strong driver of host clonal turnover, in support of the Red Queen hypothesis.

Relationships among climate variability, Cladocera phenology and the pelagic food web in deep lakes in different trophic states

Achieving a better understanding of the role of climate change in altering population phenology, seasonal cycles in freshwater organisms, and ecosystem structure and function is of high scientific and economic value. The present paper has demonstrated the different food-web responses to teleconnection indices, which are proxy of climate fluctuations, in lakes characterised by different trophic levels. We analysed an 18-year long-term dataset (1998–2015) recorded in the deep eutrophic Lake Iseo and we compared our results to those concerning deep southern-Alpine oligotrophic lakes. Our results confirmed that winter large-scale circulation patterns (for Mediterranean area: East Atlantic Pattern and Eastern Mediterranean Pattern) control a chain of linked causal factors, affecting the winter air temperature, spring water temperature, the resulting water vertical-mixing depth and epilimnetic concentration of total phosphorus. We highlighted that in a lake, characterised by high phosphorus concentration, the spring enrichment in nutrients did not result in either a considerable increase of phytoplankton growth, nor, consequently, in a zooplankton density peak. Whereas in oligotrophic lakes, cascading effects influenced the algal carrying capacity and Daphnia population density. We observed that climatic fluctuations, mediated by, for example, colder water temperature in spring, postponed the timing of population recovery after diapause of both primary consumers (Daphnia and Eubosmina) and secondary consumers (Bythotrephes and Leptodora). The latter being verified for the first time in a eutrophic lake.

Population transcriptomics in Daphnia: The role of thermal selection.

The complex interplay of forces influencing genetic divergence among populations complicates the discovery of the genetic basis underlying local adaptation. Here, we utilized for the first time a combined reverse ecology and population transcriptomic approach to assess the contribution of thermal selection to population differentiation, thereby considering transcriptome-wide variation in both gene expression profiles and DNA sequences. We compared transcriptomes among four Daphnia galeata populations and identified transcripts potentially responding to local thermal selection based on an extensive literature search for candidate genes possibly under thermal selection in arthropods. Over-representation of temperature-relevant candidate genes among transcripts strongly contributing to sequence divergence among two populations indicates that local thermal selection acted on the coding sequence level. We identified a large number of transcripts, which may contribute to local thermal adaptation based on outlier tests and distinctive expression profiles. However, among these, temperature-relevant candidate genes were not over-represented compared to the global gene set, suggesting that thermal selection played a minor role in divergence among Daphnia populations. Interestingly, although the majority of genes contributing strongly to sequence divergence did not contribute strongly to divergence at the expression level and vice versa, the affected gene functions were largely consistent between the two data sets. This suggests that genetic and regulatory variation constitutes alternative routes for responses to natural selection. Our combined utilization of a population transcriptomics approach and literature-based identification of ecologically informative candidate genes represents a useful and powerful methodology with a wide range of applications in evolutionary biology.

Evolutionary history of Daphnia drives divergence in grazing selectivity and alters temporal community dynamics of producers.

Consumers with different seasonal life histories encounter different communities of producers during specific seasonal phases. If consumers evolve to prefer the producers that they encounter, then consumers may reciprocally influence the temporal composition of producer communities. Here, we study the keystone consumer Daphnia ambigua, whose seasonal life history has diverged due to intraspecific predator divergence across lakes of New England. We ask whether grazing preferences of Daphnia have diverged also and test whether any grazing differences influence temporal composition patterns of producers. We reared clonal populations of Daphnia from natural populations representing the two diverged life history types for multiple generations. We conducted short‐term (24 hr) and long‐term (27 days) grazing experiments in equal polycultures consisting of three diatom and two green algae species, treated with no consumer, Daphnia from lakes with anadromous alewife, or from lakes with landlocked alewife. After 24 hr, life history and grazing preference divergence in Daphnia ambigua drove significant differences in producer composition. However, those differences disappeared at the end of the 27‐day experiment. Our results illustrate that, despite potentially more complex long‐term dynamics, a multitrophic cascade of evolutionary divergence from a predator can influence temporal community dynamics at the producer level.

Whole genome amplification and sequencing of a Daphnia resting egg.

Resting eggs banks are unique windows that allow us to directly observe shifts in population genetics, and phenotypes over time as natural populations evolve. Though a variety of planktonic organisms also produce resting stages, the keystone freshwater consumer, Daphnia, is a well-known model for paleogenetics and resurrection ecology. Nevertheless, paleogenomic investigations are limited largely because resting eggs do not contain enough DNA for genomic sequencing. In fact, genomic studies even on extant populations include a laborious preparatory phase of batch culturing dozens of individuals to generate sufficient genomic DNA. Here, we furnish a protocol to generate whole genomes of single ephippial (resting) eggs and single daphniids. Whole genomes of single ephippial eggs and single adults were amplified using Qiagen REPLI-g Single Cell kit reaction, followed by NEBNext Ultra DNA Library Prep Kit for library construction and Illumina sequencing. We compared the quality of the single-egg and single-individual amplified genomes to the standard batch genomic DNA extraction in the absence of genome amplification. At mean 20× depth, coverage was essentially identical for the amplified single individual relative to the unamplified batch extracted genome (>90% of the genome was covered and callable). Finally, while amplification resulted in the slight loss of heterozygosity for the amplified genomes, estimates were largely comparable and illustrate the utility and limitations of this approach in estimating population genetic parameters over long periods of time in natural populations of Daphnia and also other small species known to produce resting stages.

QTL mapping of a natural genetic polymorphism for long-term parasite persistence in Daphnia populations.

Knowing the determinants of the geographic ranges of parasites is important for understanding their evolutionary ecology, epidemiology and their potential to expand their range. Here we explore the determinants of geographic range in the peculiar case of a parasite species - the microsporidian Hamiltosporidium tvaerminnensis - that has a limited geographic distribution in a wide-spread host - Daphnia magna. We conducted a quantitative trait loci (QTLs) analysis with monoclonal F2 D. magna populations originating from a cross between a susceptible northern European genotype and a resistant central European genotype. Contrary to our expectations, long-term persistence turned out to be a quantitative trait across the F2 offspring. Evidence for two QTLs, one epistatic interaction and for further minor QTL was found. This finding contrasts markedly with the previously described bimodal pattern for long-term parasite persistence in natural host genotypes across Europe and leaves open the question of how a quantitative genetic trait could determine the disjunct geographic distribution of the parasite across Europe.

The heat is on: Genetic adaptation to urbanization mediated by thermal tolerance and body size.

Worldwide, urbanization leads to tremendous anthropogenic environmental alterations, causing strong selection pressures on populations of animals and plants. Although a key feature of urban areas is their higher temperature ("urban heat islands"), adaptive thermal evolution in organisms inhabiting urban areas has rarely been studied. We tested for evolution of a higher heat tolerance (CTMAX ) in urban populations of the water flea Daphnia magna, a keystone grazer in freshwater ecosystems, by carrying out a common garden experiment at two temperatures (20°C and 24°C) with genotypes of 13 natural populations ordered along a well-defined urbanization gradient. We also assessed body size and haemoglobin concentration to identify underlying physiological drivers of responses in CTMAX . We found a higher CTMAX in animals isolated from urban compared to rural habitats and in animals reared at higher temperatures. We also observed substantial genetic variation in thermal tolerance within populations. Overall, smaller animals were more heat tolerant. While urban animals mature at smaller size, the effect of urbanization on thermal tolerance is only in part caused by reductions in body size. Although urban Daphnia contained higher concentrations of haemoglobin, this did not contribute to their higher CTMAX . Our results provide evidence of adaptive thermal evolution to urbanization in the water flea Daphnia. In addition, our results show both evolutionary potential and adaptive plasticity in rural as well as urban Daphnia populations, facilitating responses to warming. Given the important ecological role of Daphnia in ponds and lakes, these adaptive responses likely impact food web dynamics, top-down control of algae, water quality, and the socio-economic value of urban ponds.

Standardized toxicity testing may underestimate ecotoxicity: Environmentally relevant food rations increase the toxicity of silver nanoparticles to Daphnia.

Daphnia in the natural environment experience fluctuations in algal food supply, with periods when algal populations bloom and seasons when Daphnia have very little algal food. Standardized chronic toxicity tests, used for ecological risk assessment, dictate that Daphnia must be fed up to 400 times more food than they would experience in the natural environment (outside of algal blooms) for a toxicity test to be valid. This disconnect can lead to underestimating the toxicity of a contaminant. We followed the growth, reproduction, and survival of Daphnia exposed to 75 and 200 µg/L silver nanoparticles (AgNPs) at 4 food rations for up to 99 d and found that AgNP exposure at low, environmentally relevant food rations increased the toxicity of AgNPs. Exposure to AgNP at low food rations decreased the survival and/or reproduction of individuals, with potential consequences for Daphnia populations (based on calculated specific population growth rates). We also found tentative evidence that a sublethal concentration of AgNPs (75 µg/L) caused Daphnia to alter energy allocation away from reproduction and toward survival and growth. The present findings emphasize the need to consider resource availability, and not just exposure, in the environment when estimating the effect of a toxicant. Environ Toxicol Chem 2017;36:3008-3018. © 2017 SETAC.

Sedimentary and egg-bank DNA from 3 European lakes reveal concurrent changes in the composition and diversity of cyanobacterial and Daphnia communities

Eutrophication generally favours the growth of cyanobacteria over eukaryotic green algae in freshwater lakes. Cyanobacteria constitute a poor food source for the waterflea Daphnia, an important primary consumer of phytoplankton in lakes. While it is known that some Daphnia species are adapted to eutrophic conditions and can cope with cyanobacteria in their diet, it is less known whether cyanobacterial community composition can influence Daphnia population structure in lakes. We studied the variation in genetic diversity of Daphnia resting eggs and cyanobacterial DNA preserved in sediment cores from three European lakes impacted by eutrophication. Our retrospective analysis confirms that D. galeata invaded the two pre-alpine lakes around the middle of the twentieth century, hybridized with and became dominant over D. longispina. This coincides with the presence in all lakes and the increase in the proportion of colonial and filamentous cyanobacteria in the pre-alpine lakes. The recent re-oligotrophication of the lakes did not reverse the cyanobacterial and Daphnia assemblages to their pre-eutrophication composition and genetic structure, suggesting that both changed irreversibly due to anthropogenic influence on the ecosystems. Genetic analyses applied to lake sedimentary archives have the potential to unveil how different compartments of the food web covary in a changing environment.

Predator-induced plasticity does not alter the pathway from evolution to ecology among locally adapted populations of Daphnia

In the growing field of eco-evolutionary dynamics, evidence for an influence of rapid shifts in phenotype on ecological processes is accumulating, yet, the contributions of phenotypic plasticity versus genetic change to these observed ecological changes are unclear. In one of the best studied ecosystems in terms of eco-evolutionary dynamics, landlocked versus anadromous alewife (Alosa pseudoharengus) have caused strong evolutionary divergence in their key zooplankton prey (Daphnia ambigua). We previously showed that such evolutionary differences have cascading ecological effects on consumer-resource dynamics and primary production. Yet, these locally adapted populations of Daphnia also differ in trait plasticity, which may, in turn, modify the pathway from evolution to ecology. Here we compared Daphnia from lakes with landlocked versus anadromous alewife for differences in rates of population growth in the presence and absence of predator cues over the course of a 39-day experiment. We predicted that predator-induced shifts in life history traits would facilitate faster rates of population growth. Contrary to our expectations, predator cue exposure did not alter rates of population growth. We instead found that Daphnia from lakes with landlocked alewife ultimately attained higher population densities (and exhibited faster population growth) when compared with Daphnia from lakes with anadromous alewife. Based on our previous work, these population level responses were unexpected, as Daphnia from lakes with landlocked alewife exhibit slower rates of somatic growth and delayed maturation. We discuss our results in lieu of the known differences in plasticity and how the population growth patterns may be influenced by resource limitation.

Maladaptation to Acute Metal Exposure in Resurrected Daphnia ambigua Clones after Decades of Increasing Contamination.

Human environmental impacts have driven some of the strongest and fastest phenotypic changes recorded in wild animal populations. Across populations, this variation is often adaptive, because populations evolve fitness advantages in response to human-modified environments. Yet some populations fail to adapt to changing environments. Evidenced by declines in relative fitness, such seemingly maladaptive outcomes are less common but may be more likely in human-modified contexts. Further, our ability to investigate the dynamics of these adaptive and maladaptive responses over time is typically limited in natural systems. I combined resurrection ecology and paleolimnology approaches to examine evolutionary responses of the freshwater zooplankter Daphnia to exposure to heavy metal contamination over the past 50-75 years, using animals hatched from diapausing egg banks. In contrast to the predicted trend of adaptation to metal exposure over time, I observed an increase in sensitivity to both copper and cadmium exposure associated with increasing historic contamination. This potentially maladaptive trend occurred in Daphnia populations in three lakes. Given that the release of toxicants such as heavy metals is widespread and that other researchers have observed local maladaptation to toxicant exposure, it is important to understand the drivers and implications of this pattern.

How does the cladoceran Daphnia pulex affect the fate of Escherichia coli in water?

The faecal indicator Escherichia coli plays a central role in water quality assessment and monitoring. It is therefore essential to understand its fate under various environmental constraints such as predation by bacterivorous zooplankton. Whereas most studies have examined how protozooplankton communities (heterotrophic nanoflagellates and ciliates) affect the fate of E. coli in water, the capacity of metazooplankton to control the faecal indicator remains poorly understood. In this study, we investigated how the common filter-feeding cladoceran, Daphnia pulex, affects the fate of E. coli under different experimental conditions. Daphnia ingested E. coli and increased its loss rates in water, but the latter rates decreased from 1.65 d-1 to 0.62 d-1 after a 1,000-fold reduction in E. coli initial concentrations, due to lower probability of encounter between Daphnia and E. coli. The combined use of culture and PMA qPCR (viability-qPCR) demonstrated that exposure to Daphnia did not result into the formation of viable but non-culturable E. coli cells. In lake water, a significant part of E. coli population loss was associated with matrix-related factors, most likely due to predation by other bacterivorous biota and/or bacterial competition. However, when exposing E. coli to a D. pulex gradient (from 0 to 65 ind.L-1), we observed an increasing impact of Daphnia on E. coli loss rates, which reached 0.47 d-1 in presence of 65 ind.L-1. Our results suggest that the filter-feeder can exert a non-negligible predation pressure on E. coli, especially during seasonal Daphnia population peaks. Similar trials using other Daphnia species as well as stressed E. coli cells will increase our knowledge on the capacity of this widespread zooplankter to control E. coli in freshwater resources. Based on our results, we strongly advocate the use of natural matrices to study these biotic interactions in order to avoid overestimation of Daphnia impact.

Copy number variation of a protease gene of Daphnia: Its role in population tolerance.

Copy number variation (CNV) of genes coding for certain enzymes has been shown to be responsible for adaptation of arthropods to anthropogenic toxins. Natural toxins produced by cyanobacteria in freshwater ecosystems, that is, protease inhibitors (PIs), have been demonstrated to increase in frequency over the last decades due to eutrophication and global warming. These PIs inhibit digestive proteases of Daphnia, the major herbivore of phytoplankton and cyanobacteria. The adjustment of isoforms, differences in gene expression, and activity of gut proteases determine tolerance to dietary PIs in single Daphnia genotypes. Here, we tested whether similar mechanisms are also responsible for differences in tolerance among Daphnia population. We developed a droplet digital PCR (ddPCR) method for the analysis of CNV of Daphnia proteases. We report that one Daphnia protease gene showed CNV between populations and that CNV correlates with chymotrypsin gene expression among populations. We showed that populations of Daphnia magna differ in tolerance to cyanobacterial PIs according to the cyanobacterial background of their lake of origin, which hints at local adaptation. The tolerance of the populations correlates with IC50 values of their chymotrypsins, which is probably due to a combined effect of CNV (translating into gene expression differences) and positive selection of tolerant protease isoforms. This is the first study using ddPCR to demonstrate CNV of a gene with ecologically relevant function, and the first report of differences in tolerance to cyanobacterial PIs among Daphnia populations in combination with the assessment of underlying molecular mechanisms.

Rapid evolution of tolerance to road salt in zooplankton

Organisms around the globe are experiencing novel environments created by human activities. One such disturbance of growing concern is the salinization of freshwater habitats from the application of road deicing salts, which creates salinity levels not experienced within the recent evolutionary history of most freshwater organisms. Moreover, salinization can induce trophic cascades and alter the structure of freshwater communities, but knowledge is still scarce about the ability of freshwater organisms to adapt to elevated salinity. We examined if a common zooplankton of freshwater lakes (Daphnia pulex) could evolve a tolerance to the most commonly used road deicing salt (sodium chloride, NaCl). Using a mesocosm experiment, we exposed freshwater communities containing Daphnia to five levels of NaCl (15, 100, 200, 500, and 1000 mg Cl− L−1). After 2.5 months, we collected Daphnia from each mesocosm and raised them in the lab for three generations under low salt conditions (15 mg Cl− L−1). We then conducted a time-to-death experiment with varying concentrations of NaCl (30, 1300, 1500, 1700, 1900 mg Cl− L−1) to test for evolved tolerance. All Daphnia populations exhibited high survival when subsequently exposed to the lowest salt concentration (30 mg Cl− L−1). At the intermediate concentration (1300 mg Cl− L−1), however, populations previously exposed to elevated concentrations (i.e.100–1000 mg Cl− L−1) had higher survival than populations previously exposed to natural background levels (15 mg Cl− L−1). All populations survived poorly when subsequently exposed to the highest concentrations (1500, 1700, and 1900 mg Cl− L−1). Our results show that the evolution of tolerance to moderate levels of salt can occur within 2.5 months, or 5–10 generations, in Daphnia. Given the importance of Daphnia in freshwater food webs, such evolved tolerance might allow Daphnia to buffer food webs from the impacts of freshwater salinization.

An update on the global stressors and constraints affecting Daphnia populations in large Chilean Patagonian lakes (39-51°S)

The zooplankton communities in Chilean Patagonian lakes (39-51°S) are characterized by the low presence of daphniid cladocerans, i.e., under conditions of the usual native oligotrophy and, thus, low mineral concentrations. Nevertheless, from some northern Patagonian lakes (39-41°S) a marked abundance of daphniids has been reported. According to the literature, as well as evidence from field observations, the main causes of such marked changes in the local zooplankton assemblages would be changes in phytoplankton quantity and quality, providing increased energetic resources for the zooplankton communities. Such energetic resources might decrease the stress effects of low calcium concentrations, as calcium constitutes an essential nutrient for caparace formation in Daphniidae; in addition, energetic expenses needed to perform vertical migrations due to fish predation and exposure to natural ultraviolet radiation can better be provided for. This is a markedly different situation compared to North Patagonian lakes, where daphniid abundance and number of species are related only to phytoplankton quantity and quality in term of phosphorus/nitrogen molar ratio.

Environment not dispersal limitation drives clonal composition of Arctic Daphnia in a recently deglaciated area

One of the most prominent manifestations of the ongoing climate warming is the retreat of glaciers and ice sheets around the world. Retreating glaciers result in the formation of new ponds and lakes, which are available for colonization. The gradual appearance of these new habitat patches allows us to determine to what extent the composition of asexual Daphnia (water flea) populations is affected by environmental drivers vs. dispersal limitation. Here, we used a landscape genetics approach to assess the processes structuring the clonal composition of species in the D.pulex species complex that have colonized periglacial habitats created by ice-sheet retreat in western Greenland. We analysed 61 populations from a young (<50years) and an old cluster (>150years) of lakes and ponds. We identified 42 asexual clones that varied widely in spatial distribution. Beta-diversity was higher among older than among younger systems. Lineage sorting by the environment explained 14% of the variation in clonal composition whereas the pure effect of geographical distance was very small and statistically insignificant (Radj2=0.010, P=0.085). Dispersal limitation did not seem important, even among young habitat patches. The observation of several tens of clones colonizing the area combined with environmentally driven clonal composition of populations illustrates that population assembly of asexual species in the Arctic is structured by environmental gradients reflecting differences in the ecology of clones.

Thermal variation and factors influencing vertical migration behavior in Daphnia populations

The antipredator behavior diel vertical migration (DVM), common in aquatic keystone species Daphnia, involves daily migration from warmer surface waters before dawn to cooler deeper waters after dusk. Plasticity in Daphnia DVM behavior optimizes fitness via trade-offs between growth, reproduction, and predator avoidance. Migration behavior is affected by co-varying biotic and abiotic factors, including light, predator cues, and anthropogenic stressors making it difficult to determine each factor's individual contribution to the variation in this behavior. This study aims to better understand this ecologically significant behavior in Daphnia by: (1) determining how Daphnia pulicaria thermal preferences vary within and among natural populations; (2) distinguishing the role of temperature verses depth in Daphnia vertical migration; and (3) defining how two anthropogenic stressors (copper and nickel) impact Daphnia migratory behavior.Simulated natural lake stratification were constructed in 8L (0.5m tall, 14.5cm wide) water columns to monitor under controlled laboratory conditions the individual effects of temperature gradients, depth, and metal stressors on Daphnia vertical migration. Three major findings are reported. First, while no difference in thermal preference was found among the four populations studied, within lake populations variability among isolates was high. Second, decoupling temperature and depth revealed that depth was a better predictor of Daphnia migratory patterns over temperature. Third, exposure to environmentally relevant concentrations of copper or nickel inhibited classic DVM behavior. These findings revealed the high variability in thermal preference found within Daphnia populations, elucidated the individual roles that depth and temperature have on migratory behavior, and showed how copper and nickel can interfere with the natural response of Daphnia to fish predator cues. Thus contributing to the body of knowledge necessary to predict how natural populations of Daphnia will be affected by climate related changes in lake temperatures and increased presence of anthropogenic stressors.