Effects of Nitrogen Emissions on Fish Species Richness across the World's Freshwater Ecoregions

Nitrogen emissions threaten freshwater ecosystems througheutrophication and toxicity, but such effects have rarely been studied.This study developed spatially explicit effect factors to assess theeffects of nitrogen on freshwater fish biodiversity. The increasing applicationof synthetic fertilizer hastriplednitrogen (N) inputs over the 20th century. N enrichment decreaseswater quality and threatens aquatic species such as fish through eutrophicationand toxicity. However, the impacts of N on freshwater ecosystems aretypically neglected in life cycle assessment (LCA). Due to the varietyof environmental conditions and species compositions, the responseof species to N emissions differs among ecoregions, requiring a regionalizedeffect assessment. Our study tackled this issue by establishing regionalizedspecies sensitivity distributions (SSDs) of freshwater fish againstN concentrations for 367 ecoregions and 48 combinations of realmsand major habitat types globally. Subsequently, effect factors (EFs)were derived for LCA to assess the effects of N on fish species richnessat a 0.5 degree x 0.5 degree resolution. Results show good SSDfits for all of the ecoregions that contain sufficient data and similarpatterns for average and marginal EFs. The SSDs highlight strong effectson species richness due to high N concentrations in the tropical zoneand the vulnerability of cold regions. Our study revealed the regionaldifferences in sensitivities of freshwater ecosystems against N contentin great spatial detail and can be used to assess more precisely andcomprehensively nutrient-induced impacts in LCA.