b'Session V: Impact of multiple stressors on aquatic ecologyMultiple stressorsa challenge for freshwater assessment and impact mitigationKelly-Quinn M1 1School of Biology & Environmental Science, University College DublinMost freshwater ecosystems are subject to more than one stressor acting simultaneously. These include a range of chemical pollutants but also physical stressors such as fine sediment that originate from drivers such as agricultural, forestry and urban land-uses, industry andtransport. The various stressors can interact in complex ways, have different exposure pathways and impact different receptors as well as varying in intensity across spatial and temporal scales. The resulting impacts are complex, they are seldom simply the sum of the effects of theindividual stressors, but can be greater (synergism) or lesser (antagonism), and can even lead to so called ecological surprises. Furthermore, climate change is likely to superimpose other stress factors such as increased temperature and altered flow regimes. This is in addition to the growing number of contaminants of emerging concern. The resulting stressor cocktails challenge our ability to detect the impacting stressors, prioritise which stressors should be targeted and apply effective mitigation measures to protectecological health and biodiversity but also the supply of essential ecosystem services. In the context of the Water Framework Directive this has implications for developing effective River Basin Management Plans and associated policy. This paper will introduce the issue of multiple stressors in freshwaters and highlight what research to date has shown in terms of stressor interactions. The results of multi-stressor mesocosm experiments carried out in Ireland will be included. The presentation will conclude with a summary of knowledge gaps, key questionsrelevant for multi-stressor management and options for using the available information toassess and manage the ecological health of freshwaters in a multi-stressor environment. 24'