Stream Invertebrate Responses to Fine Sediment Depend on the Organic and Inorganic Components

ABSTRACT 1. Excess fine sediment (<2mm) is a pervasive stressor of aquatic systems at a global scale. To date, most research quantifying ecological response to deposited fine sediment has been conducted without distinguishing between the organic and inorganic components of fine sediment, leaving mechanistic understanding of how fine sediment affects macroinvertebrates lacking. 2. To address this research gap, we examined the ecological responses of macroinvertebrates to various components of fine sediment stress (inorganic, organic and total mass). Using data collected from field surveys, first, we quantified invertebrate responses to fine sediment in two countries: New Zealand and the United Kingdom and second, we quantified whether ecological responses to the fine sediment components differed in spring and autumn in the United Kingdom. 3. Results indicated that the response of invertebrates varied dependent on both the component of fine sediment and the facet of community quantified (i.e., taxonomic or functional). The organic component was highly influential in structuring aquatic communities, particularly in New Zealand, whilst in the United Kingdom, there were less pronounced differences between the relative importance of organic and inorganic sediment. Ephemeroptera, Trichoptera and Plecoptera taxa (and derived indices) were consistently found to be strongly related to all fine sediment gradients. Further, according to threshold analysis, changes in taxonomic measures of the community in the United Kingdom occurred at lower quantities of fine sediment in autumn than in spring, particularly for the organic component. However, individual taxon and trait responses were more evident in spring. 4. Our study highlights several implications for the global monitoring and management of fine sediment. We have demonstrated that macroinvertebrates have distinctive responses to individual components of fine sediment (i.e., inorganic, organic and total mass) which may be missed from qualitative assessments of gross fine sediment coverage on streambeds alone. Further, aggregating multiple seasons into annual assessments may overlook important nuances in invertebrate responses to the different fine sediment components exhibited in individual seasons