Nitrogen pollution in rivers as potential driver of invertebrate species turnover

Nitrogen pollution represents one of the most significant threats to European freshwater ecosystems, with nitrite (NO2-N) standing out as a highly toxic compound for aquatic organisms, particularly vertebrates. Despite its recognized toxicity, little is known about its effects on invertebrates, even as riverine ecosystems experience profound species turnover. Here, we investigated the lethal and sublethal effects of nitrite on three representative amphipod species (Gammarus fossarum, G. pulex, and G. roeselii), which occupy distinct river sections and ecological niches. These species serve as models for assessing how nitrogen pollution may shape invertebrate communities across freshwater habitats. A series of laboratory bioassays revealed that G. fossarum, a species associated with upstream sections and pristine conditions, was the most sensitive to nitrite exposure, followed by the midstream species G. pulex and the long-established downstream species G. roeselii. To contextualize these findings, we compared the nitrite vulnerability of these amphipods with that of other freshwater invertebrates, offering a comprehensive perspective on how nitrogen pollution reshapes aquatic communities. While many invertebrate groups exhibit lower vulnerability to nitrite due to their reliance on hemocyanin - an oxygen-transport molecule mostly unaffected by nitrite oxidation - our results underscore significant interspecific differences in tolerance. For sensitive insect species, lethal effects occurred already at environmentally relevant concentrations, highlighting their exceptional vulnerability. In contrast, more tolerant groups such as amphipods survived higher concentrations, yet still displayed sublethal impairments, most notably a reduced leaf litter consumption - a key process in stream nutrient cycling - and altered behavioral responses at comparable exposure levels. Molluscs exhibit the highest tolerance, whereas insects are the most sensitive. Among crustaceans, tolerance varies widely, with a relationship to chloride content of the water mitigating the toxicity of nitrite. Chloride concentrations generally rise along the course of a river, placing upstream regions with naturally low chloride levels and their species at heightened risk. These differences highlight the potential role of nitrogen pollution as a driver of species turnover, particularly in multistressor environments. By linking species-specific sensitivity to broader ecological processes, like leaf litter consumption, this study provides critical insights into cascading effects of nitrogen pollution on freshwater biodiversity and ecosystem stability.