Ecotoxicological risk of sulfate in Baltic Sea brackish water assessed using model-averaged species sensitivity distribution

Sulfate is a major ion commonly found in natural environments, but excess amounts of it can be harmful to aquatic life. Effects of sulfate on freshwater organisms have been explored previously, but its effects on brackish water biota are poorly known. The elevating human-induced sulfate load to the Baltic Sea necessitates understanding how brackish water species tolerate the additional sulfate. In this study, we conducted chronic sulfate toxicity tests on species from coastal areas of the Baltic Sea. Species of both freshwater and marine origin in the Baltic Sea were tested. Species sensitivity distribution (SSD) with a model-averaging approach was used to derive the fifth percentile hazardous concentration (HC5). For comparison, HC5s were also derived from three alternative single-distribution models. Sulfate sensitivity varied notably between the test species. The most sensitive species were found to be common water moss Fontinalis antipyretica and European whitefish Coregonus lavaretus. The chronic HC5 with 95% confidence interval was 1,204 (472-2,429) mg/L for the coastal Baltic Sea areas, with a salinity range from 2 to 6‰. To account for SSD uncertainties, the HC5 is divided by the assessment factor (AF) to derive the predicted no effect concentration (PNEC). Different AF scenarios were evaluated through deriving the percentages of species affected under each PNEC. The PNEC that would protect over 95% of species with a 95% probability was 469 mg/L, corresponding to an AF of 2.57, which accounted for the statistical uncertainties from the SSD modeling. Compared to species inhabiting freshwater ecosystems in general, the Baltic Sea species showed significantly higher tolerance to sulfate. The toxicity tests performed here on a wide range of Baltic Sea species significantly contribute to the understanding of the sensitivity of brackish water species to sulfate and advanced the development of its risk assessment in brackish water.