Critical water quality limits for aquatic freshwater biodiversity

Freshwater ecosystems host a disproportionate share of global biodiversity yet are increasingly exposed to declining water quality driven by nutrient enrichment, chemical contamination, thermal stress, salinisation, and emerging pollutants. While water quality standards and regulatory limits exist for many constituents, their relevance for safeguarding freshwater biodiversity remains fragmented, taxon-specific, and unevenly documented across regions and ecosystem types. This review synthesises current knowledge on critical water quality limits associated with adverse responses of freshwater biodiversity, with a focus on identifying lethal and sublethal thresholds across major aquatic taxa and key water quality constituents.

We systematically assess reported biodiversity responses covering fish, amphibians, macroinvertebrates, reptiles, freshwater-adapted mammals, and where evidence exists, groundwater-associated biota to changes in nutrients (nitrogen and phosphorus), temperature, dissolved oxygen, biochemical oxygen demand, salinity, suspended sediments, metals, plastics, pharmaceuticals, and contaminants of emerging concern, including PFAS and microplastics. Reported thresholds are evaluated across lentic and lotic systems to account for ecosystem-specific sensitivities. Where possible, we distinguish between acute (lethal) and chronic (sublethal) response levels and document observed exceedance events.

Beyond synthesising established thresholds, the review explicitly highlights constituents for which biodiversity-relevant limits are poorly defined or absent, as well as geographic regions where water quality degradation is likely occurring but biodiversity impacts remain under-reported. By consolidating dispersed evidence and identifying critical gaps, this review aims to support biodiversity-relevant water quality assessments, inform monitoring and modelling efforts, and provide a foundation for integrating ecological thresholds into freshwater management and policy frameworks under accelerating global change.