Salinity thresholds shape global patterns of freshwater biodiversity

Accelerating human-driven environmental change is reorganizing biodiversity across the world’s ecosystems1–3. Freshwater ecosystems, though limited in extent, support a disproportionate share of global biodiversity4,5 and are highly sensitive to even subtle changes in water chemistry6. Salinity is rising in many lakes worldwide due to land use and climate change7,8, with growing evidence that it alters biodiversity and species composition9,10. A key uncertainty is whether salinization causes biodiversity to erode gradually10,11 or collapse suddenly once critical thresholds are exceeded11,12, limiting our ability to predict resilience and recovery13. Here, we compiled data on macroinvertebrate communities across 684 lakes distributed globally across a broad salinity gradient. Biodiversity metrics and total abundance exhibited clear threshold responses to salinization: evenness declined above ~42 mg/L, richness above ~160 mg/L, and total abundance only beyond ~10,000 mg/L. These threshold declines were largely driven by shifts in community composition, as many taxa failed to persist beyond critical salinity levels. Our findings suggest that nonlinear biodiversity responses to anthropogenic stressors may be widespread, posing challenges for anticipating ecological change and designing effective interventions.