Aquatic ecosystems cover approximately 70% of the Earth’s surface and support a wide range of ecosystem services. Despite their importance, aquatic ecosystems are increasingly exposed to anthropogenic stressors, such as contaminants and climate change impacts. Ecosystems comprise a complex web of interactions both between organisms and between organisms and the abiotic environment. While there is extensive evidence for the importance of ecological processes in determining net ecosystem effects of contaminants, most often their effects are studied in isolation and in a single species setting.

The aim of this thesis is to investigate the ecological effects of contaminants in aquatic ecosystems, ranging from cellular to ecosystem endpoints, by using model ecosystems of increasing complexity. This thesis studies the effects of ionising radiation on the biochemical composition of microalgae and how these may affect consumers (Paper I), as well its effects on an artificial freshwater ecosystem (microcosms) in terms of ecological processes (Paper II) and carbon flows (Paper III). Finally, the thesis investigates the combined effects of a flame retardant and increased temperature on a model ecosystem comprised of a semi-natural Baltic Sea community (Paper IV).

Ionising radiation caused biochemical changes in primary producers that affected the next trophic level, where the consumer responded with an increased feeding rate, suggesting a change in the food quality of the primary producer (Paper I). The microcosms exposed to ionising radiation showed significant dose related effects on photosynthetic parameters for all macrophyte species. Dose dependent trends were seen in snail grazing rates and reproduction indicating a potential for long-term effects (Paper II). Similarly, the carbon flow networks (Paper III) also indicated that the main effect of radiation was a decline in primary production of the macrophytes, while pelagic bacterial production increased. However, the relative distribution of flows from dissolved carbon changed only slightly with increasing dose rates, which mainly triggered an increase in the amount of carbon dissipated through respiration. Finally, in Paper IV, higher temperatures induced the release of PO₄ from the sediment, which stimulated the growth of the cyanobacteria, in turn leading to an increase in copepod abundance.

These results demonstrate that the effects of contaminants on ecosystems depend on ecological processes, which may influence species-specific responses and lead to indirect effects. This thesis builds on a body of literature calling for a more holistic approach of ecotoxicology and radioecology, where ecosystem level responses to contaminants are taken into consideration.