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Effects of Suspended Sediment Transport on Invertebrate Drift

Invertebrate drift plays an important role in river ecosystems. Although drift has been studied extensively, the relative importance of the various factors that initiate drift during disturbances remains unclear. Instream gravel mining releases fine sediment and so provides an opportunity to assess the influence of suspended sediment on drift, without the confounding effects of hydraulic changes and bed‐material entrainment associated with floods. This paper examines invertebrate drift responses to increases in suspended sediment during an episode of mining in a Pyrenean river. During short periods of mining activity, suspended sediment concentrations and thus suspended sediment loads (SSLs) increased one order of magnitude at downstream monitoring sections, with maxima similar to those observed during natural floods in the river. Maximum SSLs were recorded at the sections closest to the mining, with downstream transport patterns suggesting that the majority of suspended material was deposited within 1.5 km. Invertebrate drift rates, the number of taxa drifting and the taxonomic structure of the drift changed at sections close to the mining when suspended sediment concentrations and SSLs were high; such changes were not observed at the section 1.5 km downstream. There were significant relationships between SSL and drift, positive for some groups (Ephemeroptera, Plecoptera and Trichoptera) and negative for others (Chironomidae). Our work shows that increases in suspended sediment alone are sufficient to trigger changes in drift, although further studies are needed to elucidate the underlying mechanisms, and especially to explain the varying responses shown by different taxonomic groups. Copyright © 2017 John Wiley & Sons, Ltd.