Evaluating the relationship between biotic and sediment metrics using mesocosms and field studies

An ongoing research challenge is the detection of biological responses to elevated sediment and the identification of sediment-specific bioassessment metrics to evaluate these biological responses. Laboratory mesocosms and field observations in rivers in Ireland were used to evaluate the relationship between a range of biological and sediment metrics and to assess which biological metrics were best at discerning the effects of excess sediment on macroinvertebrates. Results from the mesocosm study indicated a marked decrease in the abundance of sensitive taxa with increasing sediment surface cover. % EPT (Ephemeroptera, Plecoptera, Trichoptera) and % E abundances exhibited the strongest negative correlation with sediment surface cover in the mesocosm study. The field study revealed that % EPT abundance was most closely correlated with % sediment surface cover, explaining 13% of the variance in the biological metric. Both studies revealed weaker relationships with a number of other taxonomy-based metrics including total taxon abundance, total taxon richness and moderate relationships with the Proportion of Sediment-sensitive Invertebrates metric (PSI). All trait-based metrics were poorly correlated with sediment surface cover in the field study. In terms of sediment metrics, % surface cover was more closely related to biological metrics than either re-suspendable sediment or turbidity. These results indicate that % sediment surface cover and % EPT abundance may be useful metrics for assessing the effect of excessive sediment on macroinvertebrates. However, EPT metrics may not be specific to sediment impact and therefore when applied to rivers with multiple pressures should be combined with observations on sediment cover.