Global aquatic environments are under increasing pressure from chemicals and other anthropogenic stressors. In an effort to protect the environment, chemicals must be tested for any potential adverse effects. However, the test methods used to date are highly standardised single-substance protocols and cannot adequately represent natural situations. This thesis therefore explores ways to improve existing ecotoxicity guidelines by including mixtures of contaminants in combination with other prevalent stressors.

A major objective of the thesis is to apply the chemical activity concept when investigating the potential toxicity of non-polar hydrophobic organic contaminants (HOCs). This approach provides a measure that is comparable over different matrices, like water (Paper I, II, IV) and sediment (Paper III). It is further useful, as it allows mixtures of contaminants to be expressed as a single dose metric. Paper I explored the need for more concise neonatal handling before the start of the exposure in the existing guideline for the Daphnia magna acute immobilisation test. The feeding status within the first 24 h of neonatal life affected the ecotoxicity outcome significantly. Thus, it would be beneficial if the guidelines included a suggested feeding regime prior to the start of exposure. The comparability of chemical activity in a mixture toxicity study combining HOCs and metals is tested in Paper II. The addition of zinc, as a model metal, in a Daphnia acute test showed an additive effect of zinc and HOCs on protein content. The feasibility of chemical activity in combination with passive samplers was shown in Paper III, where it was applied in sediment-water toxicity test systems with Lumbriculus variegatus and Chironomus riparius and compared with traditional total concentration approaches. The study illustrated differences between the two approaches and how traditional measurements can lead to uncertainty in tolerance of L. variegatus to HOCs. The increasing thermal pressure aquatic organisms face under global warming was incorporated in ecotoxicity testing with D. magna by using the degree-day approach (Paper IV). Through the use of cumulative thermal experiences, the effects of temperature on physiology could be disentangled from toxic effects, showing that D. magna sensitivity was temperature-independent.

Overall, this thesis provides important insights into feasible improvements of existing standardised ecotoxicity guidelines as well as exploring the usefulness of sublethal endpoints for a better understanding of potential risks in the environment.