Daphnia Populations Research Articles

Daphnia populations are similar but not identical in tolerance to different protease inhibitors

Cyanobacterial blooms often produce different classes and chemical variants of toxins such as dietary protease inhibitors (PIs) that affect the keystone grazer Daphnia. However, it has been shown that Daphnia populations are able to locally adapt to frequently occurring dietary PIs by modulating their digestive proteases. Up until now, local adaptation has exclusively been tested by making use of single cyanobacterial strains and by measuring average population tolerance. In contrast, we measured juvenile somatic growth rates and egg numbers of several individual clones per each of three different D. magna populations that have previously been found to be either tolerant or sensitive to the Microcystis strain BM25. Clones from the three D. magna populations were either treated with BM25 that produces three different protease inhibitor variants of the class of Ahp-cyclodepsipeptides or another Microcystis strain that produces two other Ahp-cyclodepsipeptide variants. Subsequently, the population growth was calculated as mean of the single-clone growth rates. Both tolerant populations (which originate from ponds with a cyanobacterial history) proved to be similarly tolerant to both Microcystis strains. However, single genotypes of the populations differed in their response to the different strains. Both the tolerant and the sensitive populations contained both sensitive and tolerant genotypes but in different proportions. Furthermore, the genotypes from the sensitive population showed a higher variance in response to one or both strains. Trade-offs between somatic growth rate and clutch size were found in one of the tolerant populations that originated from a pond where cyanobacteria were frequent in the past but completely absent since the pond's restoration. Because of those intra-population difference, we conclude that the tolerant populations were putatively selected by different Ahp-cyclodepsipeptide variants in the past and that all populations still possess the potential to adapt to other environmental conditions by genotype frequency shifts.

Production of Daphnia zooplankton on wastewater-grown algae for sustainable conversion of waste nutrients to fish feed

This study investigates the upcycling of nutrients in anaerobic digestate via algal biomass to zooplankton which is a natural fish feed. There are no published studies, to the authors’ knowledge, on the viability of growing zooplankton on digestate-grown algae. Here, the viability of digestate-grown Chlorella sorokiniana as a feed for the large-bodied generalist zooplankter, Daphnia, was tested. It was found that Daphnia fed with digestate-grown C. sorokiniana led to 1.5- to 14-fold greater Daphnia population growth than Daphnia fed with Ankistrodesmus sp., an established feed. A sterol analysis of C. sorokiniana found 4–6 mg/g of the sterol, ergosterol, and nearly double the α-linolenic acid content of Ankistrodesmus. Sterols and α-linolenic acid are often-limiting nutrients in Daphnia diets. Other factors hypothesized to influence nutrient transfer from algae to Daphnia were also tested, including algal feed concentration, sterol supplementation, and the presence of digestate bacteria in the algal feed. The presence of bacteria and exogenous cholesterol had no significant impacts on Daphnia growth. The higher feed concentration (5 mg C/L) led to 3 times higher Daphnia growth than the low feed concentration (1.5 mg C/L) even though the latter concentration has frequently been used by other researchers. Finally, it was determined that the feed conversion ratio of algae to Daphnia fell in the range of 0.19–0.31 and that trophic transfer of carbon was 25–28% while that of nitrogen was 29–34% in this un-optimized system. These values compare favorably to livestock feed conversion efficiency but additional losses will occur when Daphnia are fed to fish. These results show that cultivation of Daphnia on digestate-grown algae is technically feasible.

The rapid, mass invasion of New Zealand by North American Daphnia "pulex".

Nonindigenous members of the Daphnia pulex complex have been found in many lakes in New Zealand (NZ) in the past 20 years, suggesting a recent invasion. However, very little is known about the precise phylogenetic origin of invasive Daphnia, whether each lake is invaded by a single clone or multiple clones, the lineage of the invasive clones, and whether they are obligately asexual clones. Furthermore, the source and time of arrival of the invasive genotype(s) are unclear. We address these questions by genomic sequencing of Daphnia populations from 13 lakes in the South Island and 1 lake in the North Island, NZ. All biallelic sites in these NZ populations have similar numbers of reads for the two parental alleles, suggesting each NZ population originates from a single asexual clone. Based on 25,643 monomorphic lineage-specific markers, the invasive Daphnia in the South Island were found to be Daphnia pulicaria Forbes, while those in the North Island are hybrids of D. pulicaria Forbes and D. cf. pulex sensu Hebert. Both the South and North Island Daphnia are phylogenetically clustered with North American Daphnia, thereby suggesting their North American origins. We found also that all South Island clones contain identical mitochondrial genomes, suggesting the origin and proliferation from a single founder clone, which we experimentally verified to be an obligate asexual. Estimates from molecular data imply a colonization time for the South Island clones of ~60 years ago, with a likely invasion route associated with the introduction of salmonids from North America.

The thermal regime modifies the response of aquatic keystone species Daphnia to microplastics: Evidence from population fitness, accumulation, histopathological analysis and candidate gene expression

The water bodies are greatly influenced by chemical contamination and global increasing temperature. As an emerging pollutant, microplastics are widely distributed in the freshwater environment, raising concerns regarding their potential toxicity to organisms. Especially for zooplankton filter feeders, many of microplastics are in similar size as their food. Individually, both microplastics and temperature have profound effects on zooplankton populations and their function in ecosystems. However, the strength and direction of their interactive effects are still not clear. Here, we performed a comprehensive biotoxicity assessment providing empirical evidence that the temperature played a key role in shaping the sensitivity of the zooplankter, Daphnia magna, against microplastic toxicity. We found that exposure to microplastics generally caused negative effects on Daphnia individual fitness, such as increased lethality, declined fecundity and reduced population growth rate. This microplastic toxicity was more prominent at 30 °C than at 20 °C, and was rather minor at 15 °C. Moreover, the warming accelerated the ingestion of microplastics, and triggered abnormal ultrastructure of intestinal epithelial cells. In addition, the expression profiling of candidate genes revealed oxidative damage, fecundity impairment and energy retardation by microplastics were amplified with increasing temperature, which may contribute to the enhancement of microplastic toxicity under warming. Given that high temperature fluctuations are becoming more common and difficult to predict, the interactive effects of microplastics and climate warming on Daphnia population dynamics and biomass production may become increasingly aggravated in nature. Collectively, extrapolation for environmental risk assessment studies conducted under different temperature contexts may broaden our knowledge microplastic toxicity on aquatic organism fitness.

Long-term adverse effects of microplastics on Daphnia magna reproduction and population growth rate at increased water temperature and light intensity: Combined effects of stressors and interactions

In many ecosystems, the zooplankton community has been pressured simultaneously by microplastic pollution and alterations resulting from global climate changes. The potential influence of light intensity rise (from 10,830 lx to 26,000 lx) and water temperature rise (from 20 °C to 25 °C) on the long term-toxicity of microplastics (MPs) to Daphnia magna were investigated. Three 21-day laboratory bioassays with model MPs (1–5 μm diameter) were carried out at (i) 20 °C/10830 lx, (ii) 20 °C/26000 lx, and (iii) 25 °C/10830 lx. In each bioassay, one control (no MPs) and three MP concentrations (0.04, 0.09, 0.19 mg/L) were tested. In all the bioassays, MPs caused parental and juvenile mortality, and reduced the somatic growth, reproduction and population growth rate. The MP EC50s on living offspring (95% confidence interval within brackets) were 0.146 mg/L (0.142–0.151 mg/L) at 20 °C/10830 lx, 0.102 mg/L (0.099–0.105 mg/L) at 20 °C/26000 lx, and 0.101 mg/L (0.098–0.104 mg/L) at 25 °C/10830 lx. Relatively to the respective control group, 0.19 mg/L of MPs decreased the mean of the population growth rate by 27% at 20 °C/10830 lx, 38% at 20 °C/26000 lx and 59% at 25 °C/10830 lx. Based on the population growth rate and in relation to 20 °C/10830 lx (control, no MPs), the interaction between increased light intensity (26,000 lx) and MPs was synergism (at all the MP concentrations tested). The interaction between water temperature rise (25 °C) and MPs was antagonism at 0.04 mg/L of MPs and synergism at 0.09 and 0.19 mg/L of MPs. In the present scenario of climate changes and global MP pollution such findings raise high concern because zooplankton communities are crucial for aquatic biodiversity conservation, ecosystem functioning and services provided to humans. Further studies on the combined effects of MPs, other common pollutants, and alterations due to climate changes are needed.

Long-term ecosystem change in two highly degraded Lake Ontario (Canada) coastal wetlands

Coastal wetlands are essential to the ecosystem health of the Laurentian Great Lakes (North America) watershed. Multiple anthropogenic stressors have been impacting coastal wetlands since European settlement ca. 1850, and remain a concern for wetland health as watershed development intensifies. We used paleolimnological techniques to explore temporal ecosystem dynamics over the last ~ 100 years in two highly degraded Lake Ontario coastal wetlands located in southern Ontario, Canada, using Cladocera (Branchiopoda, Crustacea) subfossil remains as paleoecological indicators. In Cootes Paradise Marsh (Hamilton, Ontario), cladoceran assemblage changes exhibited a shift in dominance from Chydorus to Bosmina, at the turn of the twentieth century. That shift likely reflected the loss of aquatic macrophytes, and corresponds to the postulated timing of the arrival of invasive carp. Despite recent remediation efforts, including attempts to exclude carp from the wetland, little ecological recovery is evident from the subfossil Cladocera assemblage. No Daphnia remains were observed in our sediment core from Cootes Paradise, in contrast to previous studies on extant zooplankton communities, which reported a large Daphnia population in the west end of the marsh in the 1940s. This could indicate that our sediment core recorded ecological changes solely in the east end of Cootes Paradise Marsh. In Jordan Harbour (Lincoln, Ontario), Bosmina were dominant throughout the sediment record, and increases in littoral cladocerans were observed in the most recent sediments, in particular the appearance of periphytic Pleuroxus taxa after ~ 2008. This suggests that some recovery of aquatic macrophyte communities occurred in response to shoreline remediation efforts. Bosmina size structure exhibited only minimal changes in both wetlands, despite known large changes in historic fish community structure. Overall, our study provides perspectives on the benefits and limitations of paleolimnology for documenting ecological change in the Laurentian Great Lakes coastal wetlands.

Next-generation sequencing of DNA from resting eggs: signatures of eutrophication in a lake’s sediment

Hatching resting stages of ecologically important organisms such as Daphnia from lake sediments, referred to as resurrection ecology, is a powerful approach to assess changes in alleles and traits over time. However, the utility of the approach is constrained by a few obstacles, including low and/or biased hatching among genotypes. Here, we eliminated such bottlenecks by investigating DNA sequences isolated directly (i.e. without hatching) from resting eggs found in the sediments of Lake Constance spanning pre-, peri-, and post-eutrophication. While we expected genome-wide changes, we specifically expected changes in alleles related to pathways involved in mitigating effects of cyanobacterial toxins. We used pairwise FST-analyses to identify transcripts that showed strongest divergence among the four different populations and a clustering analysis to identify correlations between allele frequency shifts and changes in abiotic and biotic lake parameters. In a cluster that correlated with the increased abundance of cyanobacteria in Lake Constance we find genes that have been reported earlier to be differentially expressed in response to the cyanobacterial toxin microcystin and to microcystin-free cyanobacteria. We further reveal the enrichment of gene ontology terms that have been shown to be involved in microcystin-related responses in other organisms but not yet in Daphnia and as such are candidate loci for adaptation of natural Daphnia populations to increased cyanobacterial abundances. In conclusion this approach of investigating DNA extracted from Daphnia resting stages allowed to determine frequency changes of loci in a natural population over time.

Ancestral genetic variation in phenotypic plasticity underlies rapid evolutionary changes in resurrected populations of waterfleas

The role of phenotypic plasticity in adaptive evolution has been debated for decades. This is because the strength of natural selection is dependent on the direction and magnitude of phenotypic responses to environmental signals. Therefore, the connection between plasticity and adaptation will depend on the patterns of plasticity harbored by ancestral populations before a change in the environment. Yet few studies have directly assessed ancestral variation in plasticity and tracked phenotypic changes over time. Here we resurrected historic propagules of Daphnia spanning multiple species and lakes in Wisconsin following the invasion and proliferation of a novel predator (spiny waterflea, Bythotrephes longimanus). This approach revealed extensive genetic variation in predator-induced plasticity in ancestral populations of Daphnia It is unlikely that the standing patterns of plasticity shielded Daphnia from selection to permit long-term coexistence with a novel predator. Instead, this variation in plasticity provided the raw materials for Bythotrephes-mediated selection to drive rapid shifts in Daphnia behavior and life history. Surprisingly, there was little evidence for the evolution of trait plasticity as genetic variation in plasticity was maintained in the face of a novel predator. Such results provide insight into the link between plasticity and adaptation and highlight the importance of quantifying genetic variation in plasticity when evaluating the drivers of evolutionary change in the wild.

Centennial clonal stability of asexual Daphnia in Greenland lakes despite climate variability.

Climate and environmental condition drive biodiversity at many levels of biological organization, from populations to ecosystems. Combined with paleoecological reconstructions, palaeogenetic information on resident populations provides novel insights into evolutionary trajectories and genetic diversity driven by environmental variability. While temporal observations of changing genetic structure are often made of sexual populations, little is known about how environmental change affects the long‐term fate of asexual lineages. Here, we provide information on obligately asexual, triploid Daphnia populations from three Arctic lakes in West Greenland through the past 200–300 years to test the impact of environmental change on the temporal and spatial population genetic structure. The contrasting ecological state of the lakes, specifically regarding salinity and habitat structure may explain the observed lake‐specific clonal composition over time. Palaeolimnological reconstructions show considerable regional environmental fluctuations since 1,700 (the end of the Little Ice Age), but the population genetic structure in two lakes was almost unchanged with at most two clones per time period. Their local populations were strongly dominated by a single clone that has persisted for 250–300 years. We discuss possible explanations for the apparent population genetic stability: (a) persistent clones are general‐purpose genotypes that thrive under broad environmental conditions, (b) clonal lineages evolved subtle genotypic differences unresolved by microsatellite markers, or (c) epigenetic modifications allow for clonal adaptation to changing environmental conditions. Our results motivate research into the mechanisms of adaptation in these populations, as well as their evolutionary fate in the light of accelerating climate change in the polar regions.

Population dynamics and resting egg production in Daphnia: Interactive effects of mercury, population density and temperature

Toxicity studies on freshwater organisms are commonly conducted by quantifying effects on asexual (clonal) reproductive rates in Daphnia, whereas studies of effects on sexual reproductive rates remain relatively rare. Sexual reproduction in Daphnia and the associated production of resting eggs allows them to survive unfavorable environmental conditions and is thus a crucial component of their long-term fitness. It also maintains genetic diversity within Daphnia populations and hence their potential for adaptation to new environmental conditions. This aspect of their biology may therefore be important to consider in toxicity studies. The aim of this study was to investigate for the first time how mercury (Hg) affects sexual versus asexual reproduction in Daphnia under varying environmental conditions. Specifically, we experimentally tested the interactive effects of Hg and temperature on the population dynamics of Daphnia magna. For this purpose, we exposed D. magna to environmentally relevant concentrations (0 μg/L, 0.5 μg/L and 2 μg/L) of Hg (in the form of mercury (II) chloride) found in stream water and measured biomass growth rate resulting from asexual reproduction, and resting egg production resulting from sexual reproduction. This was done at both 17 °C and 24 °C. Biomass growth rate did not vary across Hg treatments and depended mainly on temperature and population density. Density dependence of biomass growth rate was indeed more pronounced at 24 °C than at 17 °C, as resource limitation from intraspecific competition was further exacerbated by the rise in feeding rates with temperature. Density dependence of resting egg production was unaffected by Hg and temperature, but resting egg production was higher under Hg exposure at low temperature. These findings show that depending on environmental conditions, rates of sexual reproduction in D. magna may respond to metal exposure at lower concentrations than those impacting population growth during the asexual phase.

Transgenerational effects of methyl farnesoate on Daphnia pulex clones: Male and ephippia production and expression of genes involved in sex determination

Abstract Environmental sex determination implies that sex is defined by environmental factors influencing the maternal organism and/or oocytes. It provides population sex ratios that will maximise the sustainability of the population under incipient environmental conditions. Environmental factors responsible for sex determination include temperature, photoperiod, food availability, and population density. These cues stimulate the release of chemical signalling molecules (i.e. hormones) that determine the sex of newborns. We have analysed the responses of three Daphnia pulex clones (I, K, and S), isolated from different natural populations, to juvenile hormone methyl farnesoate (MF) in terms of survivorship, fecundity, production of males and ephippia, and expression of genes involved in sex determination (dsx1) and hormonal regulation (JHAMT, Met, RXR). Clonal individuals were exposed to: (1) high MF concentration (0.8 µM) combined with stressful conditions; (2) low MF concentration (0.15 µM) combined with optimal conditions of population density and food availability in two subsequent generations. The genetically different clones of D. pulex differed in the propensity for male and ephippia production and in the sensitivity to the stimuli that lead to the switch from parthenogenetic to sexual reproduction. A significant induction of male production was observed in all three clones when exposed to MF. In conditions mimicking crowding, only clone S produced male offspring even in absence of MF and showed higher expression levels of JHAMT and dsx1 genes. This response suggests that clone S has a high propensity to produce males probably due to a high sensitivity of receptors to environmental stimuli activating the endogenous biosynthesis of MF. However, clone S exposed to high MF concentration produced fewer males than clones I and K, which generally have a low natural propensity to produce males. High MF concentration coupled with conditions mimicking crowding, probably activated a feedback mechanism in clone S via the differential modulation of genes involved in the synthesis as well in the degradation of MF. Exposure to low MF concentration for two subsequent generations resulted in a reduction of male progeny production from the first (G1) to the second (G2) generation. All analysed genes were up‐regulated in both generations treated with MF, but with lower expression in G2. The reduced up‐regulation of the dsx1 gene matched with the lower male production. Based on our results and literature data, we hypothesised that the maternal dsx1 mRNA probably plays some role early in oocyte development, and causes the eggs that receive the dsx1 signal to develop into males. Exposure to MF for two generations significantly increased ephippia production. These results suggest that the induction of male and ephippia might have a common root in the MF pathway. The effect of MF on life‐history traits (delay in the age at maturity and fecundity reduction), sex ratio, and ephippia production suggests a direct impact of the juvenile hormone on the level of population growth. Thus, as a consequence of the switch from parthenogenetic to sexual reproduction, MF influences Daphnia population metrics and in this way probably maximises the sustainability of the population under variable environmental conditions.

Interactive effects of water temperature and stoichiometric food quality on Daphnia pulicaria

Abstract While water temperatures vary widely within and among lakes, most studies of food quality on lake zooplankton have been conducted across a relatively narrow temperature range. Given the strong links between food, temperature, and metabolism, there is a need to assess the interactive effects of water temperature and food quality on the life history and population growth rates of zooplankton. This study examined Daphnia pulicaria life history and population growth rates exposed to variable food P content (molar C:P ratios c. 100–900) across a range of temperatures (c. 12–30°C). Mass‐specific growth rate experiments were conducted for a period of 5 days, while Daphnia reproduction and survival metrics were measured for 28 days. Mass‐specific growth rates of D. pulicaria varied with food‐quality but the size of this effect depended on temperature. We observed a thermal limit of 28°C above which mortality reached 100%, while survival between 25 and 28°C was substantially reduced, after c. 20 days, in animals consuming food C:P ratios of 100. Poorer quality food (C:P 300–700) altered these temperature effects and increased the probability of survival in animals exposed to 25–28°C temperatures. When we examined the population‐level effects of these treatments, we found a rise in intrinsic rate with temperature with higher quality food, driven by an earlier age of first reproduction and shorter generation time. However, this increased population growth with temperature was not observed in animals consuming food with low P content. Altogether, we found strong interactive effects of temperature and food quality on Daphnia, with especially dynamic changes seen among differentially nourished animals raised in water above 25°C. These results demonstrate the complex non‐additive interactions of food quality and temperature responses in freshwater invertebrates and indicate that the effects of warming temperatures on zooplankton communities must be considered within the context of the animals’ nutrition and metabolism if monitoring and mitigation efforts are to be successful.

Survival of Bada (Rasbora sp.) and Daphnia sp. using biofloc from Lake Maninjau as natural feed

Biofloc is one of the water bioremediation agents in aquaculture. A consortium of microorganisms in biofloc, which include heterotrophic bacteria, phytoplankton and zooplankton, can also function as natural food sources for aquatic biota. This study aims to investigate the potential of biofloc microorganisms consortium as natural food for commercial fish feed-less aquaculture. The study was conducted using Lake Maninjau water taken from two locations at Lake Maninjau, namely Nagari Sungai Batang and Nagari Maninjau, as test samples. Maninjau Lake was chosen because it had a high content of organic compounds and various microorganisms which could be used as an indigenous biofloc starter. Daphnia sp. and Bada (Rasbora sp.) were used as an experimental biota without periodical additional feed during the trial. The results showed that biofloc could grow well using Maninjau Lake water media. Bada and Daphnia could live in the biofloc system for 20 days without supplementary feeding and afterwards entering the death phase. The decline in the population of Bada and Daphnia was thought to be caused by the decrease of biofloc role as natural feed and its ability in maintaining the water quality.

Metal exposure causes rDNA copy number to fluctuate in mutation accumulation lines of Daphnia pulex

Ribosomal (r)DNA is a highly dynamic, conserved, multigene family whose sequence homogeneity is thought to be maintained by intra- and interchromosomal recombination, which are capable of changing rDNA copy number. It is generally not known how environmental stress such as sublethal exposure to environmentally relevant concentrations of metals impacts rDNA copy number. To determine how chronic metal exposure affects rDNA, we measured copy number of the 18S rRNA gene in 355 copper and nickel-exposed samples and 132 metal-free samples derived from 325 mutation accumulation (MA) lines of two genetically distinct Daphnia pulex lineages. The MA lines were sampled at four time points over 100+ generations of clonal propagation. The copy number of rDNA was also measured in 15 individuals sampled from a metal-free non-MA control population established from the same progenitor as one of the MA lineages. We found that mean rDNA copy number fluctuated across lines exposed to metals with a tendency to decrease over time. In contrast, mean rDNA copy number in the metal-free control lines and the non-MA population remained stable over time. It is generally accepted that extreme rDNA loss results in the loss of organism fitness. Thus, fluctuations in rDNA copy number, including losses, could affect the long-term viability of natural populations of Daphnia in metal-contaminated habitats.

Sex dependent sexual reproduction strategies in a cyclic parthenogen—A case study from intermittent urban pond

The fitness of animals inhabiting highly unpredictable intermittent ponds depends on the effectiveness of the production of their resting stages. Daphnia living in such an environment produce sexual eggs as well as the males needed to fertilize them. We hypothesize that the strategy of permanent male presence should coexist with the synchronization of the production of males with that of sexual eggs. To test this hypothesis, we collected plankton samples from a model urban pond, two times a week, throughout the growing season. We analyzed in detail a number of environmental factors and the population dynamics of two Daphnia populations. The percentage of ephippial females and males periodically reached c.a. 50 % of the population. Depending on the moment of the growing season, this proportion was primarily influenced by population crowding, the richness of invertebrate predators in the habitat, extreme high temperatures and the occurrence of the autumnal photoperiod. Our results confirm the hypothesis that Daphnia produce simultaneously long-living males and males synchronized with receptive females. Additionally, we have shown that the number of resting eggs deposited by temporally-isolated populations varied significantly; thus the fitness of a particular genotype depends on the season and on the particular timing of its activation.

Defective defence in Daphnia daughters: silver nanoparticles inhibit anti-predator defence in offspring but not in maternal Daphnia magna

One major environmental problem of our time are emerging contaminants in the aquatic environment. While nanoparticles exhibit attractive features such as antimicrobial properties in the case of silver nanoparticles (AgNPs), earlier studies suggest that NPs are not completely filtered out at wastewater treatment plants and may therefore be continuously introduced into the aquatic environment. Although adverse effects of AgNPs on aquatic organisms have been extensively studied, there is still a lack of knowledge on how this chemical stressor interacts with natural cues on the maternal and subsequent generation of aquatic organisms. We tested whether AgNPs (NM-300K, 14.9 ± 2.4 nm, concentration range: 2.5 µg/L – 20 µg/L) affect the kairomone-induced adaptive anti-predator defence mechanism in maternal Daphnia and their offspring. While maternal Daphnia developed typical anti-predator defence mechanisms when exposed to kairomones and AgNPs, their offspring could not develop such adaptive defensive traits. The lack of this defence mechanism in offspring could have dramatic negative consequences (e.g. reduced Daphnia population) for the entire complex food web in the aquatic ecosystem. For a realistic risk assessment, it is extremely important to test combinations of chemical stressors because aquatic organisms are exposed to several natural and artificial chemical stressors at the same time.

Modeling diel vertical migration with membrane computing

Diel vertical migration (DVM) is an important ecological phenomenon in which zooplankton migrate vertically to deal with trade-offs associated with greater food availability in shallow waters and lower predator risk in deep waters due to lower light availability. Because of these trade-offs, DVM dynamics are particularly sensitive to changes in light intensity at the water surface. Therefore, changes in the proportion of cloudy and sunny days have the potential to disrupt DVM dynamics. We propose a new membrane computing model that captures the effect of cloud cover on DVM in Daphnia, and we use it to explore the impacts of an increased proportion of cloudy days that are predicted to occur with climate change. Our 2-dimensional, spatially explicit model integrates multiple trophic levels from abiotic nutrients to Daphnia predators. We analyzed the effect that different proportions of cloudy and sunny days throughout the summer have on our model. The model simulations suggest that an increase in sunny days promotes a high phytoplankton concentration near the surface but does not necessarily promote an increased abundance of Daphnia. Our model also suggests that a higher proportion of cloudy days would increase Daphnia abundance due to a shift in the vertical distribution of Daphnia populations towards superficial waters. Our results highlight that climate changes in multiple regions will affect animal migrations leading to altered food web dynamics in freshwater ecosystems, and emphasize the potential of membrane computing as a modeling framework for spatially and temporally explicit ecological processes.

Temperature elevation reduces the sensitivity of invasive cladoceran Daphnia lumholtzi to filamentous cyanobacterium Raphidiopsis raciborskii

Abstract Exotic cladoceran Daphnia lumholtzi is a highly invasive species in the north and south American continents and can potentially also invade European freshwaters and outcompete native Daphnia populations. However, European waterbodies are frequently dominated by less edible filamentous cyanobacteria including also invaders such as Raphidiopsis raciborskii, which might affect the fitness of D. lumholtzi. Furthermore, temperature may influence the sensitivity of D. lumholtzi to R. raciborskii filaments. In this study, we determined whether the presence of R. raciborskii could obstruct the invasion of Europe by D. lumholtzi, through reducing its fitness, and whether this depends on temperature. We compared the population growth rate (r) and the somatic growth rate of D. lumholtzi maintained at two temperatures (20 or 26°C) and fed with two diets: green microalgae alone or green microalgae mixed with filaments of R. raciborskii. Three clones of D. lumholtzi were used to evaluate potential variation in response to the treatments among different clones. At 20°C, the population growth rate of D. lumholtzi fed with cyanobacterial filaments declined sharply. This reduction was caused by increased egg abortion, egg degeneration, and mortality of newborn daphnids. At 26°C, R. raciborskii lost its harmful effect on the population growth of D. lumholtzi. The presence of cyanobacteria did reduce the somatic growth rate of D. lumholtzi at both temperatures and in all three clones except for one that had a similar somatic growth rate on both diets at 26°C. The presence of filamentous cyanobacteria does affect growth in D. lumholtzi and may thereby substantially reduce its invasive potential, but only at lower temperatures. Therefore, the presence of filamentous cyanobacteria may not present an obstacle to the invasion of Europe by D. lumholtzi in a warmer future climate.

Bottom-Up Forces Drive Increases in the Abundance of Large Daphnids in Four Small Lakes Stocked with Rainbow Trout (Oncorhynchus mykiss), Interior British Columbia, Canada

The introduction of salmonids into lakes of western North America for sport fishing is a widespread phenomenon. While numerous investigations have documented cascading trophic interactions upon the introduction of fish into naturally fishless systems, little research has been done to investigate the importance of natural fish status (fishless vs. fish bearing) in modulating historical food web response to dual forcing by bottom-up (resource regulation from nutrients) and top-down (planktivory from stocked fish) processes. We used the paleolimnological record to reconstruct food web changes in four lakes in interior British Columbia that have been stocked with rainbow trout since the early to mid-1900s. Analysis of pigments, diatoms, and Cladocera was undertaken in cores from all lakes. We predicted that if fish were important in structuring cladoceran abundance and composition, we would document a decline in the abundance of large daphnids post-stocking in our two naturally fishless lakes, and little change in the two fish-bearing lakes. Instead, we documented increased abundance of large daphnids after stocking in all lakes in the early to mid-1900s, a finding inconsistent with size-selective predation from planktivorous fish. Further, our data suggest that deep, low-oxygen refugia may be important in sustaining populations of large Daphnia, a process which was enhanced by increased nutrients and lake production according to sub-fossil diatom and pigment analyses. This study shows that fish stocking does not invariably result in a decrease in large-bodied Cladocera and that nutrients and lake type can modulate the response of invertebrate planktivores.

Free-living parasite infectious stages promote zooplankton abundance under the risk of predation.

Free-living parasite infectious stages, such as the cercariae of trematodes (flatworms), can represent substantial biomass in aquatic ecosystems, yet their interactions with other planktonic fauna are poorly understood. Given that cercariae are consumed by various aquatic predators, sometimes even preferentially over zooplankton, their presence may decrease predation pressure on free-living organisms within similar trophic niches by serving as alternate prey. Here, we experimentally examined how the presence of cercariae (Plagiorchis sp.) affected the population dynamics of common freshwater zooplankton (Daphnia sp.) in the presence of a predator (the larval dragonfly, Leucorrhinia intacta) known to consume both. After seeding 48 mesocosms with starting populations of Daphnia, we used four treatments (12 replicates each) representing a factorial combination of the absence/presence of both cercariae and dragonfly larvae and tracked Daphnia populations over 4 weeks. We found a significant interaction between the presence of cercariae and predators on Daphnia population size. When faced with predation pressure, Daphnia reached ~ 50% higher numbers when accompanied by cercariae than without, suggesting a "protective" effect of the latter by acting as substitute prey. Within aquatic ecosystems, an abundance of trematodes may prove advantageous for zooplankton communities that share common predators, but further studies will be needed to determine how this varies depending on the predator, trematode, and zooplankton taxa involved.