Do nutrients mask the effects of micropollutants on consumer-resource interactions? - a bachelor thesis in the context of the project "EcoImpact" at Eawag
Environmental stress is a fundamental force influencing species distributions and abundances in many habitats globally. The role of environmental stress may be increasing in a wide range of locations due to anthropogenic factors including chemical pollution, habitat degradation, and climate change. These abiotic stressors often combine to cause complex effects and challenge researchers to accurately assess and predict impacts at different levels of biological organisation. Thus, understanding how multiple stressors damage ecological systems is an important research goal for ecotoxicologists and ecologists.
Over recent decades, concern has mounted regarding the threat posed by chemicals discharged to waterways from municipal wastewater, as opposed to industry or agriculture, on aquatic ecosystems. Municipal wastewater treatment plants (WWTPs) are designed to remove contaminants prior to discharge, but total elimination using traditional technologies is often impossible. This means treated wastewater effluent often contributes a wide range of contaminants to the environment, including both micropollutants (e.g., low concentrations of pharmaceuticals and pesticides in mixtures) and nutrients. However, it is poorly understood how the effect of these compounds act together on the receiving ecosystems. My thesis is part of a mesocosm experiment at Eawag to investigate how nutrients modify the toxic effects of micropollutants, and focuses on the responses of the freshwater amphipod Gammarus fossarum as a model study organism to these abiotic stressors.
A recent meta-analysis of multiple stressor effects suggests the most common interaction is antagonistic (i.e., one stressor cancels the other one out), and previous work has shown that nutrients can dampen the effects of pesticides on freshwater invertebrates. Thus, the main hypothesis I tested was that there is an interaction of effects of nutrients and toxicants and that nutrients reduce (‘mask’) the negative effects of micropollutants found in wastewater. Here I assumed that nutrients improve food resources for amphipods (for example, promoting the microbial conditioning of leaf discs). As a result, amphipods might be better able to cope with the stress caused by micropollutants, which should increase metabolic activity and thus represent an energy demand. Therefore, we predicted that a micropollutant treatment would have strong negative effects on gammarids, while the combined presence of nutrients and micropollutants would show effects more similar to the controls, which would constitute the best conditions.
The experiment was conducted using the Maiandros experimental flumes system at ARA Bachwis in Fällanden. The design involved four water quality treatments that were each replicated in four channels (16 channels in total). Each channel contained 14 amphipods in individual cages and four control cages (amphipods absent). River water only from the Glatt was used as the control treatment, and river water dosed with a solvent (methanol) was used as technical control. The third treatment consisted of a mixture of micropollutants representative for wastewater in Switzerland in composition and concentration. For the last treatment, nutrients (nitrogen and phosphorus) were added to this mixture. These two mixtures were dosed to Glatt river water with the methanol solvent. The influence of nutrients on the effects of the micropollutants treatment is therefore the main research interest of this thesis.
In my thesis, I tested the hypothesis that nutrients mask the effects of micropollutants using three Gammarus fossarum endpoints (survival, growth, and consumption) during a five-week experiment.
The results were equivocal and there was large variation observed in the treatments. Survival was reduced in the nutrient treatments compared to micropollutants, while no significant effects of treatments were found for growth and consumption. The initial size of gammarids was evenly distributed over treatments and significantly explained growth but not survival at the end of the experiment.
My results do not strongly support the hypothesis tested that nutrients reduce a negative effect of micropollutants, although they do not necessarily reject it. This shows the complexity of the topic investigated and points to the need for further research regarding the ecosystem effects of multiple stressors in wastewater.