ARC Processes

ARC Processes

Streams are affected by multiple stressors. According to the Asymmetric Response Concept, different processes are relevant during degradation and recovery.

Interactions of stressors affecting environmental variables

Very often, stressors modify the intensity of secondary stressors, which eventually affect biota. Based on an unprecedented dataset, we calculated the share of events in nine hysteresis classes (combination of ‘enrichment’, ‘constancy’, ‘dilution’ with ‘fast response’, ‘intermediate response’, ‘slow response’) for eight water quality parameters. Particularly water temperature, oxygen, pH, ammonium, turbidity and chlorophyll are subject to increasing concentrations (enrichment) as discharge events unfold. These variables also exhibit a slow response, indicated by the occurrence of the concentration peaks usually after the discharge peak. The environmental variables nitrate and conductivity, on the other hand, tend to be governed more by dilution effects.

Interactions of stressors or environmental variables affecting organisms at the individual and population level

According to the Asymmetric Response Concept, tolerances are key to understanding degradation and (to a lesser degree) recovery trajectories. Several RESIST projects addressed species’ tolerances through experiments and meta-analyses. We could mostly confirm that biotic degradation is driven by species’ tolerances, while recovery is mainly driven by dispersal and biotic interactions.

Recovery of benthic invertebrate assemblages with time since restoration in the Boye catchment. While the dispersal capabilities of the establishing community decreases with time since restoration, competition increases, and tolerances against stressors develop differently.

Biotic interactions translating multiple stressor effects on organisms to the community level

Competition, but also other biotic interactions such as parasitism and predation, shape communities and are supposed to be of particular importance in recovery phases. We could show that salinity affects food selection of bacterivorous flagellates and therefore changes the relative grazing pressure on different bacterial phyla. Further, we highlighted that parasites modulate the response of their host to stressors, but at the same time stressor effects lead to a global decline of parasites with complex life cycles.

Dispersal processes

Dispersal is a key factor in recovery, but the drivers of dispersal are poorly understood. We collected data on dispersal distances and the role of dispersing benthic invertebrates on community recovery, showing that macroinvertebrate dispersal follows a leptokurtic distribution. Further we showed that parasites influence drifting behaviour and therefore dispersal processes.