Excess fine sediment, comprising particles <2 mm in diameter, is a major cause of ecological degradation in rivers. The erosion of fine sediment from terrestrial or aquatic sources, its delivery to the river, and its storage and transport in the fluvial environment are controlled by a complex interplay of physical, biological, and anthropogenic factors. While the physical controls exerted on fine sediment dynamics are relatively well‐documented, the role of biological processes and their interactions with hydraulic and physicochemical phenomena has been largely overlooked. The activities of biota, from primary producers to predators, exert strong controls on fine sediment deposition, infiltration, and resuspension. For example, extracellular polymeric substances associated with biofilms increase deposition and decrease resuspension. In lower energy rivers, aquatic macrophyte growth and senescence are intimately linked to sediment retention and loss, whereas riparian trees are dominant ecosystem engineers in high energy systems. Fish and invertebrates also have profound effects on fine sediment dynamics through activities that drive both particle deposition and erosion depending on species composition and abiotic conditions. The functional traits of species present will determine not only these biotic effects but also the responses of river ecosystems to excess fine sediment. We discuss which traits are involved and put them into context with spatial processes that occur throughout the river network. While strides towards better understanding of the impacts of excess fine sediment have been made, further progress to identify the most effective management approaches is urgently required through close communication between authorities and scientists. This article is categorized under: Water and Life > Nature of Freshwater Ecosystems Water and Life > Stresses and Pressures on Ecosystems Science of Water > Water Quality

Fine sediment storage within gravel beds is a key component of catchment sediment budgets and affects the health of benthic and hyporheic habitats. Here, we assess the performance of two substrate infiltration traps for the characterization of fine sediment (<2 mm) accumulation. One design, the vertically extending sediment trap, permits both lateral and vertical exchange in the sediment column, whereas the second type, a more traditional fixed-area sediment trap with impermeable side walls, permits only vertical exchange. Traps were deployed at three sites on the River Tame, Birmingham (UK), over varying installation periods (14-401 days). Results indicate that the facilitation of multiple pathways of exchange within the vertically extending sediment traps (vertical and lateral) resulted in a significantly greater amount of fine sediment being accumulated than in adjacent fixed-area sediment traps. This suggests that lateral transport is an important component contributing to fine sediment accumulation. However, there are notable and inherent problems associated with the use of different types of sediment trap and in the way the data should be presented and interpreted. This paper discusses the practical implications of the study findings and reflects on the complexities of undertaking accurate sediment deposition measurements in the field. Excessive quantities of fine sediment stored within river networks is an important driver of aquatic habitat degradation (Descloux, Datry, & Marmonier, 2013;Packman & MacKay, 2003;Phillips & Walling, 1999), which poses a serious long-term threat to in-stream ecosystems (Négrel et al., 2014;Prosser et al., 2001). Fine-grained sediment affects the entire aquatic ecosystem from reducing primary production (Jones, Duerdoth, Collins, Naden, & Sear, 2014;Wagenhoff, Lange, Townsend, & Matthaei, 2013) and altering macroinvertebrate diversity via enhanced drift and direct burial (Larsen & Ormerod, 2010;Wood, Toone, Greenwood, & Armitage, 2005), through to reducing habitat heterogeneity and limiting oxygen exchange within interstitial pore spaces (Huston & Fox, 2015;Owens et al., 2005). Understanding fine --------------------------------------------------------------------------------------------------------------------------------This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Classification of species sensitivity for biomonitoring has been approached under two different frameworks, using either empirical data or expert opinion. Two tools for fine sediment (i.e. clogging and colmation) biomonitoring in the UK tend towards these contrasting approaches. The Proportion of Sediment-sensitive Invertebrates (PSI) index was developed using expert judgement. Empirical weightings were subsequently added at genus or species (EPSI) and mixed (EPSI mixed ) taxonomic levels, but scores remain constrained by the original categories. In contrast, the Combined Fine Sediment Index, composed of separate taxon scores along organic matter and total fine sediment gradients, was developed using a purely empirical approach. We tested the mechanistic bases for these indices by relating taxon scores to species traits. We compared the results with those for the well-established Whalley Hawkes Paisley Trigg index of organic pollution. After controlling for varying sample sizes, Whalley Hawkes Paisley Trigg could be better predicted by a linear combination of all available traits (mean R 2 = 0.92) than any of the fine sediment indices (0.68 < mean R 2 < 0.76). When only traits expected to respond to fine sediment were offered as independent variables, the goodness of fit was substantially reduced for all fine sediment indices (0.27 < mean R 2 < 0.46). Our findings demonstrate the lack of integration between the literature on macroinvertebrate responses to fine sediment, the available trait data and taxon scores. Refinement of the trait database is recommended to build on the valuable work performed to date. As the UK has taken the lead in embedding fine sediment into routine biomonitoring programmes, these findings have important international implications.The test evaluates the null hypothesis that the fit of the global 'all traits' model is no better than the fit of the more parsimonious model ('pruned', 'literature').

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