Is fine sediment deposition a main driver for the composition of benthic macroinvertebrate assemblages?

Intensive agriculture causes increased fine sediment and nutrient runoff into running waters. Despite various approaches to quantify the entry of fine sediment into streams, its biological impact is not well understood. In particular, it is unclear whether the potential effects on the biota can be explained by fine sediment loads alone or in combination with other physicochemical and/or hydrological impacts. In this study, we investigated the impact of fine sediment deposition relative to other impacts on the benthic macroinvertebrates in small headwaters in Luxembourg, a region that is sparsely populated and characterised by agricultural land use on the hills and wooded stream valleys. The surficial-deposited fine sediment, content of inorganic sediment, proportions of organic matter, and carbon to nitrogen (C/N) ratio were recorded 1 year. The stream macroinvertebrate assemblages were recorded in the spring and autumn using multi-habitat sampling. A partial canonical correspondence analysis (pCCA) was applied to quantify and verify the impact of individual sediment components and other environmental variables on the community composition, whereas redundancy analysis (RDA) was used to examine the impact of environmental variables on the macroinvertebrate diversity and functional metrics. Oxygen content, C/N ratio as well as fine gravel explained best both the taxa composition and macroinvertebrate metrics, whereas large-scale variables such as land use were less important. The biological response to oxygen deficits and the C/N ratio of the deposited fine sediment indicate the potential effects of fine sediment deposition through oxygen consumption.Our results show that the chemical composition of the deposited sediment is more important than the amount of sediment, as the C/N ratio alone explained a substantial amount of variance in species composition. Thus, we suggest that future studies on the impact of fine sediment on the macroinvertebrate community should focus on small-scale factors, including the chemical composition of the deposited fine sediment in combination with the physicochemical and hydromorphological stream parameters.