Multiple stressors in periphyton - comparison of observed and predicted tolerance responses to high ionic loads and herbicide exposure

As a result of the increasing human impact on aquatic ecosystems, freshwater organisms are often exposed to multiple stressors simultaneously. The joint actions between stressors can result in combined effects and unexpected ecological effects. Therefore, a better understanding of the interactive effects on ecosystems is required. This study aimed to identify potential interactions between high ionic loads and herbicides. A microcosm study, using periphyton as model community, was conducted with a factorial design. Two levels of ionic loads were used as single stressor and in combination with prometryn. Structural (biomass, algal class and diatom composition) and functional parameters (tolerance development) were determined over a growth period of 6weeks. The concept of pollution-induced community tolerance (PICT) was applied to quantify integrated community responses. Long-term community responses to the combined exposure were predicted using the model of independent action. No co-tolerance of high ionic loads and prometryn or vice versa was found. Stress-induced succession resulted in a distinct community structure for each stressor and combination of stressors. Multiple stressors led to the selection of opportunistic species and higher tolerances to prometryn than predicted by the model of independent action. However, joint effects for high ionic loads and prometryn were concentration and time-dependent. The PICT concept enabled the quantification of community-level effects in systems receiving multiple stresses.Synthesis and applications. Multiple stressors might explain the failure to achieve good ecological status for many European water bodies within the context of the EU-Water Framework Directive (WFD). We propose PICT as a diagnostic tool for investigative monitoring to clarify stressor conditions by testing the tolerances of local communities to preselected site-specific compounds.