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

The increasing application
of synthetic fertilizer has
tripled
nitrogen (N) inputs over the 20th century. N enrichment decreases
water quality and threatens aquatic species such as fish through eutrophication
and toxicity. However, the impacts of N on freshwater ecosystems are
typically neglected in life cycle assessment (LCA). Due to the variety
of environmental conditions and species compositions, the response
of species to N emissions differs among ecoregions, requiring a regionalized
effect assessment. Our study tackled this issue by establishing regionalized
species sensitivity distributions (SSDs) of freshwater fish against
N concentrations for 367 ecoregions and 48 combinations of realms
and major habitat types globally. Subsequently, effect factors (EFs)
were derived for LCA to assess the effects of N on fish species richness
at a 0.5 degree × 0.5 degree resolution. Results show good SSD
fits for all of the ecoregions that contain sufficient data and similar
patterns for average and marginal EFs. The SSDs highlight strong effects
on species richness due to high N concentrations in the tropical zone
and the vulnerability of cold regions. Our study revealed the regional
differences in sensitivities of freshwater ecosystems against N content
in great spatial detail and can be used to assess more precisely and
comprehensively nutrient-induced impacts in LCA.