Disconnected: integrating spatial complexity in multiple stressor research

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Human activities have strongly increased the diversity and spread of stressors that exert pressure on ecosystems. Many studies in different ecosystems have shown that stressors often co-occur and interact with each other regarding their impact on organisms. However, most studies were conducted at local scale, decoupling local patches from their spatial context, which can influence the ecological outcomes. Indeed, ecosystems are connected through the exchange of resources and individuals in the landscape matrix, which holds in particular for the riverine network. In river ecosystems, stressors often occur locally (e.g. wastewater input, field or urban runoff) and affect the biotic community in this reach including the biomass distribution in the local food web. This ultimately changes the quantity and quality of the inorganic or organic material that will flow and subsidize downstream sections in the river network. Hence, local stressors can, directly (i.e. stressor is transported downstream) and indirectly (i.e. through changes of quantity and quality of resources and organisms transported downstream), trigger cascading effects in downstream locations. So far, presumably for practical reasons, most of the multiple stressor research has focused on the effects of stressors at the place of their introduction. To our knowledge only a few stressor-related studies have considered the river network structure and these have mainly been based on simulations or small scale microcosms. When it comes to untangling the effect of multiple stressors, manipulative mesocosm studies have a clear advantage to isolate stressor effects through controlled and replicated experimental units while providing high environmental realism. In this project, we will follow a mesocosm approach but, instead of employing isolated experimental units, we will create simplified spatial networks of connected flumes. We will then manipulate the spatial spread of two different stressors, one chemical stressor that is transported directly to downstream reaches and one non-chemical stressor, i.e. light pollution at night, that is not transported directly.We will combine traditional techniques with molecular methods in a full factorial design to examine effects of stressors on the invertebrate and algal communities and food web structure. We will then measure the exchange of individuals between upstream and downstream reaches by recording the drift and infer the spatial effect of the stressors on the downstream patches via a factorial experiment. Finally, we will evaluate how stressors and river network architecture influence the resource assimilation across trophic levels through an analysis of the energy fluxes and stoichiometric niches. We anticipate that the spatial spread of stressors in the upstream reaches will increase the effect size in downstream communities, preventing any rescue effect.