Beyond discharge concentrations: effects of wastewater nutrient loads on river macroinvertebrate communities

Excessive anthropogenic inputs of nitrogen and phosphorus into surface waters remain a major challenge for river ecosystem management, with wastewater treatment plants (WWTPs) representing key point sources of nutrients, particularly in densely populated regions. In this study, we investigated the effects of nutrient loads from WWTPs of different sizes (i.e. small, medium, and large) on macroinvertebrate communities across the Po River District (Northern Italy). Using basin-scale nutrient load estimates and macroinvertebrate metrics derived from Water Framework Directive, we assessed taxonomic, functional, and trait-based response to nitrogen and phosphorus loads. Results showed that large and small WWTPs contributed the greatest nutrient loads, and that these loads were significantly associated with shifts in community structure and functional diversity. High nutrient loads favoured tolerant taxa, reduced diversity of sensitive groups, and promoted trait modalities linked to enriched and depositional habitats. Responses of functional traits were particularly pronounced for small WWTPs, indicating strong ecological effects despite their limited size. Our findings underscore the need to integrate load-based approaches into regulatory frameworks, moving beyond concentration-based standards to capture cumulative nutrient pressures, especially under increasing hydrological intermittence and water scarcity in river networks.