Abstract
Freshwater ecosystems are among the most pollution-impacted habitats on Earth, yet multi-stressor assessments that
simultaneously characterise the effects of nutrient enrichment, organic loading, micropollutants, and heavy metals on
animal community structure remain rare across the Scandinavian and Northern European context. This study quantifies
the individual and combined effects of four pollution categories on macroinvertebrate, fish, and amphibian communities
across 36 freshwater sites spanning a pollution gradient in Sweden, Finland, and Denmark (n = 12,418 individual
records, 2021-2023). Nutrient enrichment (total phosphorus > 0.10 mg/L) was the most prevalent stressor (present at
72.2% of sites), significantly reducing EPT (Ephemeroptera, Plecoptera, Trichoptera) richness by 48.3 +- 6.1% relative to
reference sites (GLMM: z = 6.84, p < 0.001). Organic pollution (BOD5 > 4 mg/L) reduced fish species richness by 34.7 +-
5.8% and shifted assemblages towards pollution-tolerant cyprinids. Micropollutant mixtures (pharmaceutical compounds,
pesticide residues) at peri-urban sites were associated with intersex prevalence of 31.4 +- 7.2% in roach (Rutilus rutilus),
consistent with endocrine disruption by estrogenic compounds. Heavy metal gradients (Cu, Zn, Pb) in mine-affected
streams correlated negatively with macroinvertebrate ASPT scores (r = -0.81, p < 0.001). Multi-stressor interaction
analysis revealed predominantly additive effects of nutrient and organic stressors on EPT richness, but synergistic effects
of micropollutants combined with nutrient enrichment on fish endocrine endpoints. These findings provide
stressor-specific management targets for achieving EU Water Framework Directive good ecological status in
Scandinavian and Northern European rivers.