Invasive species, e.g. the freshwater crayfish Procambarus clarkii, reportedly negatively influence the abundance of various aquatic species. Moreover, these invaders are increasingly linked to ecological degradation of aquatic ecosystems, since invaded habitats show increased levels of turbidity, nitrogen and organic matter concentration. P. clarkii has, among other impacts, been associated with eutrophication in invaded habitats. However, observations indicate that the presence of P. clarkii is often not accompanied by ecosystem degradation, begging the question whether they are drivers of degradation, or function as passive passengers along for the ride driven by other stressors. To test these contrasting hypotheses, we conducted a full factorial experiment in 24 mesocosms with P. clarkii and nutrient pollution (specifically N, P, K), a ubiquitous stressor on aquatic ecosystems. Here we assessed the effects on community compositions of morphologically identified macrophytes and chironomids, and of compositions of bacteria, phytoplankton and diatoms identified using environmental DNA (eDNA) metabarcoding. Nutrient pollution induced significant shifts in macrophyte biomass, and in the composition of the bacterial, diatom and phytoplankton communities. All microbial communities treated with nutrient pollution diverged from the control initially, after which the bacterial and phytoplankton communities converged back to the control in the final weeks. In contrast, we found only marginal effects of P. clarkii, rendering it unlikely as a significant short- to medium-term driver of the tested biodiversity. Since microbial communities respond quickly to changes in the environmental conditions, these results signify that the utilized mesocosms were relatively stable in spite of P. clarkii's presence. The studied crayfish density and timeframe may be leveraged as threshold values in the design and execution of freshwater management strategies that aim to avert potential negative impacts of P. clarkii on ecosystem structure. Ultimately, the importance of nutrient pollution is reinforced as a driver of environmental change in aquatic ecosystems.

Raw output of NGS runs. Methods of sample collection and processes were performed as described in the corresponding manuscript.