Assessing the impact of fine sediment on high status river sites

The European Union (EU) Water Framework Directive (WFD) designates as "high status" rivers, lakes, transitional and coastal waters that are close to natural status and relatively un-impacted by anthropogenic activities. These high status water-bodies (HSWs) are sensitive areas that require special attention. Ireland had a globally important distribution of HSWs (10.5% of rivers and 16.2% of lakes classified as high ecological status in Europe occurred in Ireland), but there have been declines of almost 50% between 1987 and 2018, with excessive sediment implicated as a pressure. In this study, an extensive assessment of macro-invertebrate sediment metrics were used to assess sediment as a pressure in sixty-five high or formerly high status river sites in Ireland that were determined to have either: "Lost" their high status (e.g. gone from high to good, moderate, poor or bad; 20 sites); consistently "Maintained" high status (24 sites); or "Gained" in status (e.g. from good to high; 21 sites). Macro-invertebrate taxa occurring in the sixty-five sites were pre-dominantly sediment sensitive taxa. However, for two specific sediment metrics, the Proportion of Sediment-sensitive Index (PSI) and the Empirically-weighted PSI (E-PSI), significant differences were observed between sites that Lost status and those that Maintained status, implying that at some sites, sediment is impacting on macro-invertebrates. However, no significant difference between Lost and Gained sites was observed, leaving an important caveat. While weak to moderate relationships were observed between the macro-invertebrate sediment metrics and the physical sediment variables, no difference between status categories for any of the physical sediment variables was observed. Further research priorities should consider the sampling resolution of these physical variables (e.g. patch vs reach scale), the properties of sediment (e.g. chemical composition) in addition to concentration, the potential interaction of multiple-stressors, and the life cycle characteristics of invertebrate taxa.