Multiple stressors may result in cumulative and un-anticipated effects that modulate the toxicity of chemicals to organisms. Our activities focus on a mechanistic understanding of effects of chemical mixtures in combination with non-chemical stressors. We follow combined stress responses in complex communities like environmental biofilms, field-derived populations (endemic Gammarus species from Lake Baikal, project LABEGLO) or model organisms. We use molecular methods as well as ecological approaches like the concept of stress-induced community tolerance (SICT) to understand multiple stress effects. Experimental systems scale from controlled laboratory experiments to mesocosms (MOBICOS) and field studies.