Stream baseline conditions shape functional responses to wastewater: evidence from insect-dominated sites

Wastewater treatment plants (WWTP) are a crucial part of modern day infrastructure, cleaning about half of our global wastewater. However, the emergence of micropollutants and higher frequencies of extreme weather events pose unprecedented challenges for society and biodiversity. Conventionally treated wastewater and altered flow regimes create environmental boundaries in rivers, impacting aquatic communities. Previous studies revealed pronounced taxonomic changes in freshwater invertebrate communities in response to WWTP effluents. To explore whether these shifts extend to functional diversity, we studied 338 communities upstream and downstream of 169 WWTPs using commonly applied functional diversity metrics. Surprisingly, we found no clear changes in functional alpha and beta diversity metrics, or community weighted means (CWM), suggesting that trait redundancy offsets the functional impact of the previously observed species turnover. However, in streams dominated by Ephemeroptera, Plecoptera and Trichoptera (EPT), we found more pronounced shifts in CWMs, indicating that the extent of functional changes depends on the baseline condition of the streams. EPT-dominated site-pairs showed significant shifts in traits related to reproduction, dispersal, and feeding, including increased occurrences of ovoviviparity and interstitial locomotion potentially as an avoidance mechanism of high flow and low oxygen saturation. Further, shifts in shredding and absorbing feeding types, aquatic passive dispersal, and hololimnic life cycles might be forms of adaptation to increased nutrient concentrations and reduced intermittency induced by WWTPs. These findings demonstrate that functional responses to wastewater inputs can remain undetected due to the noise inherent in large datasets and are often absent as a result of functional redundancy. In contrast, significant changes emerge in communities dominated by sensitive species, underscoring the value of trait-based approaches for detecting context-dependent ecological impacts.