The response of Daphnia pulex to multiple environmental stressors

Thesis abstract
Natural and anthropogenic sources produce a wide range of stressors, which influence the water quality and food web’s components including Daphnia populations. Current environmental risk assessment remains dominated by evaluation of single substances and single species. However, typically, aquatic organisms are exposed to various chemicals and stressors, which together cause additive, synergistic or antagonistic effects. Therefore, understanding population and community response to various stressors requires understanding how the stressors interact and how different genotypes/species respond. Both are necessary to evaluate the risks associated with pollutants.
Because Daphnia spp. are keystone species regulating algae biomass, supporting invertebrate and vertebrate consumers and influencing the provision of freshwater, impacts on them may influence the structure of entire communities. Within populations, stressors and other conditions, such as water chemistry, may directly influence phenotype and digestive physiology of daphnids. In this thesis, I examined how copper (Cu), cadmium (Cd) and predation risk influence Daphnia pulex life history, foraging activity and digestive, and oxidative stress responses.
I first examined, via meta-analysis of 32 published papers, the sub-lethal effects of Cu and Cd
concentrations on Cladocera spp. life history (reproduction, maturation age, and somatic growth rate). I found that Cladocera spp. showed different variations in their responses to both metals, and D. magna was found to be less sensitive to sub-lethal changes in reproduction. The effects were largely consistent for aqueous vs. dietary food. Further, water hardness and exposure duration (only Cu) had no detectable effect for D. magna reproductive response.
Literature-based toxicity data on Cladocera is focused on examination of single toxicants and single genotypes/species. Further, in ecotoxicology, a major concern is that genotypes of a certain species can respond differently to stressors in both laboratory and field tests. Therefore, I examined, using response surface design, whether the effects of Cu and Cd are additive or interactive on foraging and life history traits using three genotypes of Daphnia pulex. I further replicated our experiment under standard and high food levels. I found that that the studied traits were affected in non-additive ways by the following factors: mixtures of stressors, genotypes and resource levels.
Next, I examined the response of different biomarkers (digestive such as Amylase, Trypsin and Esterase and antioxidant enzymes such as superoxide dismutase, glutathione peroxidase and glutathione S-transferases) under lethal exposures of Cu and Cd (alone and in a mixture). I found, as with the life history responses, numerous instances of synergistic interactions of Cu and Cd on both digestive and antioxidant enzyme activity.
Finally, I examined the interaction between the anthropogenic stress (Cu) and natural stress (predation risk) by examining morphology, life history and body condition. I also manipulated the microbiome of D. pulex to test whether the response to each and to multiple stressors was influenced by the gut bacterial community. I found multiple instances of non-additive, interactive effects of Cu and predation risk across life history traits. I also found that the community structure of the natural gut microbiota responds to any stress in non-additive way. However, manipulation of the gut microbiota at young ages generated unique communities later in life and many instances where life history responses were reversed from untreated conditions.
This thesis contributes to further understanding of the risk associated with multiple stressors, in particular highlights the metals interaction together and with other stressors in the context of risk assessment and water quality guidelines.