The development of a novel macroinvertebrate indexing tool for the determination of salinity effects in freshwater habitats

Salinisation is a global threat to freshwater habitats and is predicted to worsen with climate change. Increases in salinity can result in substantial modification of freshwater biotic communities through both direct toxic effects and indirect effects such as altering prey resources, competitive interactions, predator abundances, and facilitating the spread of invasive species. Traditional techniques to determine salinity, such as point sampling chemical assessment, are typically periodic and may not reveal intermittent changes in salinity concentration. Halo-stratification and a lack of standardised depth to collect chemical data further complicates using these methods. More importantly, such methods do not show the ecological impacts of salinity increases in freshwater habitats. Complementing traditional techniques with biological assessments may resolve these issues. Pressure-specific biotic indices using aquatic macroinvertebrate community data have long been used to assess aspects of aquatic habitats, although relatively few have focussed on salinity. This paper presents the Salinity Association Group Index (SAGI), a novel aquatic macroinvertebrate index to assess salinity in freshwater habitats. SAGI is compatible with data derived from established survey techniques employed by regulatory bodies and for Water Framework Directive assessments, amongst others. The method integrates taxonomic resolution beyond the family level with taxon abundance weighting in a scoring matrix to increase the efficacy of the tool and make the best use of publicly available data. Application of SAGI in case studies demonstrates a positive, moderate to strong correlation with conductivity used as a measure of salinity. The range of correlations (R2 = 0.57–0.91) compares favourably with pressure-specific, macroinvertebrate-based monitoring tools used in Europe for WFD monitoring. Furthermore, SAGI is shown to be highly effective in comparison to alternative salinity-specific biotic indices