Spatially divergent impacts of global nutrient emission scenarios on freshwater community intactness

Eutrophication presents a persistent challenge to biodiversity conservation in freshwater systems worldwide. Yet, a global assessment of potential future nutrient-induced losses of freshwater biodiversity integrity is currently lacking. Here, we evaluated the effects of freshwater nitrogen (N) and phosphorus (P) concentrations on freshwater community intactness for three potential future development scenarios from the shared socio-economic pathway (SSP) framework: the sustainability scenario (SSP1), the regional rivalry scenario (SSP3), and the fossil-fuelled development scenario (SSP5). We first derived novel relationships between freshwater invertebrate and macrophyte community intactness (quantified with the mean species abundance (MSA) indicator) and nutrient concentrations, based on a recently published database of N and P addition experiments. We then used these relationships to estimate changes in MSA values from 2010 to 2070 based on global nutrient concentration maps for the different SSP scenarios. All three scenarios resulted in further declines in global mean MSA, with the smallest declines for the sustainability scenario and the largest for the regional rivalry scenario. Future declines in community intactness were most prominent in African river basins (Nile, Congo), whereas we observed future increases in community intactness in river basins in Europe (Rhine, Dnieper), North America (Mississippi), and Eastern Asia (Ganges, Yangtze), particularly for the sustainability scenario. Our results indicate that halting further freshwater biodiversity decline requires increased and concerted scientific and societal efforts to reduce eutrophication, in particular in river basins where biodiversity intactness is currently high yet large future decline are projected.