Current attempts to synthesise the effects of multiple stressors on freshwater ecosystems include review papers, meta-analyses and the comparative analysis of original data on multiple stressor experiments and field studies. While all these studies give insight into the frequency of stressors and their combinations, a mechanistic understanding of how multiple stressors affect freshwater ecosystems has not yet emerged. Conceptual models on the mechanisms of multiple stressor effects have not yet been tested with dedicated experimental or field exercises.
Project Z03 will serve as an overall synthesis project of the results obtained by RESIST’s individual projects and aims at deriving generic principles on the mechanisms of multiple stressors affecting freshwater ecosystem degradation and recovery. A synthesis committee composed of the PIs (Principal investigators) of Z03, selected PIs of other RESIST projects and external advisors will guide the project and will contribute to the synthesis work, together with personnel employed by Z03.
The analytical units for the synthesis are named ‘cases’, i.e. a biological response to two or more stressors. There are cases for degradation (e.g. the stressor application phases of AquaFlow and ExStream systems) and cases of recovery (e.g. the consecutive experimental phases when stressors have been removed). Depending on the number of biological variables investigated in the individual experiments and field studies and considered in the synthesis, there will be between 500 and 1000 cases resulting from the CRC. Optionally, data from other sources (e.g. Birk et al., 2020; results of the MARS project) can be included into selected analyses.
The results of all individual cases will be collated by Z-INF into a common database. As a first analytical step, the effect type (i.e. stressor dominance, additive effect, antagonistic interaction, synergistic interaction, reversal interaction, opposing interaction) will be derived for each individual case using a harmonised methodology. In a second step, proxies will be assigned to each case that describe how the four ARC components potentially influence the way multiple stressors affect the communities or functions (e.g., species sensitivity to stressors; distance to recolonisation sources). This will result in a database of individual cases (stressor 1, stressor 2, response), the associated effect type and proxies for the four ARC components. This data source will be the raw material for testing the CRC’s Main Hypotheses.
For each of RESIST’s Main Hypotheses, including their sub-hypotheses, a strategy for testing has been defined and will be executed by Z03. This involves comparing the explanatory power of proxies of the four ARC processes during degradation and of recovery (MH1), relation of the explanatory power of proxies for the four ARC processes to individual organism groups, to body size and to population size (MH2), and comparing the rate of change for functions and communities and unravelling the differences in degree of change in functions and communities for micro- and macroorganisms (MH3).
Additional research questions may emerge based on preliminary results, discussions within the synthesis committee and within the consortium. The synthesis committee will regularly review these and initiate work on additional research questions, if necessary.