Freshwater ecosystems provide essential ecological and socio-economic benefits but are increasingly subjected to the combined pressures of climate change and chemical pollution. Although multiple stressor interactions are recognised as a major challenge in ecotoxicology, the extent to which climate-related warming modifies contaminant toxicity in freshwater primary producers remains poorly resolved. This thesis investigates how moderate increases in temperature influence the toxicity of established and emerging contaminants to freshwater primary producers and whether responses differ among taxonomic groups. The study represents a feasibility assessment of a standardised experiment method for studying multi-stressor bioassays.
Representative species of green algae (Pseudokirchneriella subcapitata), cyanobacteria (Anabaena flos-aquae), and diatoms (Nitzschia palea) were exposed to toxicologically relevant concentrations of five contaminants, including the metals copper and zinc, the pharmaceuticals erythromycin and sulfamethoxazole, and the pesticide metazachlor. Experiments were conducted under three temperature regimes (15, 20, and 25 °C) using a standardised 96-hour growth inhibition assay in accordance with OECD Test Guideline 201. Concentration–response relationships were modelled using four-parameter log-logistic functions, from which EC₅₀ values were derived to quantify changes in sensitivity across temperatures. Targeted chemical analysis was performed alongside biological testing to confirm exposure concentrations and assess contaminant stability across temperature treatments.
Across taxa and contaminants, moderate warming did not result in increased toxicity. Instead, EC₅₀ values frequently increased with temperature, indicating a modest reduction in sensitivity at higher temperatures. These patterns suggest that organisms were operating closer to their thermal optima within the tested range, consistent with the inverted V-shaped temperature–toxicity relationships described in the ecotoxicological literature. While clear differences in toxicity were observed among contaminant classes, chemical identity remained the primary driver of toxic potency, with temperature exerting a comparatively minor and context-dependent influence.
Overall, this study demonstrates that moderate warming within ecologically realistic limits does not necessarily amplify chemical toxicity in freshwater primary producers. By applying a controlled and standardised experimental framework, the research indicates that temperature effects on toxicity are non-linear and species-specific rather than uniformly synergistic. These findings have important implications for ecological risk assessment, biomonitoring, and regulatory frameworks seeking to incorporate climate-sensitive toxicity data in a warming freshwater environment.
