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Increasing watershed sediment yields are an important problem in Africa but the sources of these sediment yields have only very rarely been investigated. This study therefor aims to discriminate subsurface and surface sources of fine grain sediments in a representative mesoscale (580 km2) West African savanna watershed. We used a sediment source fingerprinting approach for source apportionment including geochemical and radionuclide (137Cs, 210Pbex, 7Be) composite signatures where 7Be was used as a tracer for the first time in the African environment. Two field campaigns were conducted collecting a total of 258 geochemical and 66 isotope samples. We found that subsurface source categories, dominantly river bank, contributed an unexpected high share of 43% (geochemistry) and 45% (radionuclides) to the sampled fine‐grained sediments. Pairwise comparison of the averaged frequency distributions for predicted source proportions using five geochemical signatures with the frequency distribution generated using the single radionuclide signature suggested that the two distributions are not statistically different. Extrapolating our measured contribution of subsurface erosion to areas with similar yields in comparable environmental settings, we can assume that subsurface sources are an important component of sediment loss across large areas of West Africa. Subsurface erosion, primarily associated with bank rather than gully erosion, is likely to increase in the future with projected rises in runoff due to land use and climate change. Source tracing studies need to be undertaken more widely across Africa to help mitigation planning for sediment‐related and land degradation problems.

Globally, salmon populations are under pressure and in those where numbers are severely depleted, density independent factors such as the accumulation of fine sediment and subsequent mortality of incubating embryos are critical factors in their conservation. Although progress has been made in identifying the processes by which fine sediment can lead to embryo mortality, this has focussed on the physical blockage of gravel pores. This paper presents new knowledge on the role of sediment-associated organic matter in controlling the supply of dissolved oxygen to incubating salmon embryos within gravel river beds. We report a new approach to the measurement of sediment oxygen demand (SOD) from interstitial sediments accumulated within salmon redds and demonstrate this across a range of salmon river types. The effects of varying SOD on dissolved oxygen supply to incubating salmon embryos are quantified for the first time, using the physically based Sediment Intrusion and Dissolved Oxygen (SIDO-UK) model. The results reveal the importance of the mass of accumulating sediment and sediment consumption rates (sediment oxygen consumption [SOC]), which constitute the overall SOD over time for a particular sediment sample. Higher SOC result in reductions in dissolved oxygen that are deleterious to salmonids; however, these are subordinate to the effects of sediment mass. Larger quantities of sediment accumulating within the redd not only create a higher SOD but also physically block the pores within the gravels, leading to a more drastic decline in oxygen supply through the combined effects of reduced seepage velocity and dissolved oxygen concentration. We seek to generalize the results by exploring the influence of catchment characteristics on field measures of SOD and SOC. This work makes a significant and novel contribution to improving our fundamental understanding of hyporheic processes within salmonid spawning gravels whilst providing resource managers with evidence of th

Excessive inputs of fine-grained sediment can damage aquatic ecosystems both by degrading habitat condition and by directly impairing biota. Recent research has improved our understanding of how benthic macroinvertebrates respond to fine-grained sediment stress, leading to the development of a variety of bioassessment indices based on changes in taxonomic composition and biological trait composition. Use of biological traits as indicators of stress has been advocated on the basis of a better mechanistic understanding of the biotic and abiotic factors acting on benthic communities. We quantified changes in the macroinvertebrate biological trait assemblage from a large number of river reaches spanning a national-scale gradient of increasing agricultural fine sediment delivery and retention, having first factored out variation associated with the natural environmental gradient, with the aim of robustly testing predictions of trait response. We found strong support for 2 of 18 predictions of how macroinvertebrate traits would respond to fine sediment stress. Furthermore, using an independent dataset, we were able to confirm the response of 5 of 6 trait classes that partial RLQ-fourth corner analysis found to be significantly associated with the fine sediment gradient. Prevalence of eggs as a resistant form, in combination with either an adult aquatic life stage or crawling, provided the best indication of fine sediment conditions in streams, approaching the performance of taxonomic composition-based sediment indices, CoFSIsp and EPSImtl. This study has robustly confirmed the potential of macroinvertebrate biological traits as indicators of fine sediment impacts.

Excessive mobilisation and delivery of fine sediments to water bodies has detrimental impacts on those biotic elements used for waterbody status classification, including macroinvertebrates, fish and macrophytes. The relationship between fine sediment and diatoms is a reciprocal one, with diatoms influencing the production and retention of fine sediments, as well as being impacted by fine sediment derived from the catchment. Diatoms can increase the retention of fine sediments in benthic environments as a result of various mechanisms, including shear stress modification, surface adhesion, and bed clogging. Enhanced retention of fines can have important implications for the transfer and fate of sediment-associated nutrients and contaminants. Excessive fine sediment loadings impact diatom assemblages via shading, burial and scouring. Indirect impacts of increased fine sediment stress can result from changes in habitat availability, herbivory or predator changes which cascade down the food chain. Indices based on the relative abundance of motile species have been proposed for using diatoms to assess waterbody status. However, disentangling the potential confounding impacts of alternative environmental stressors on these simplistic indices remains a significant challenge. Coupling sediment pressure models, capable of predicting the potential impact of mitigation, with meaningful diatom based indices, remains a challenge for catchment planning for sediment abatement and the attainment of improved, or protection of, ecological status. Existing targets for sediment management in river catchments are largely based on relationships between sediment stress and impacts on fish, but these thresholds have been widely criticised. There remains a need to develop generic modelling toolkits coupling sediment stress and impacts on a range of biological quality elements to support a weight of evidence approach.

Dissolved oxygen (DO) plays a critical role in the development of the juvenile stages of benthic spawning fish and salmonids in particular. Factors influencing the DO regime within spawning gravels include the accumulation of fine sediment, penetration of groundwater or surface water into the gravels, thermal regime, and the consumption of oxygen by sediment and its associated organic fractions. In this field study, we quantify the DO regime within an artificial salmon redd at high temporal resolution. The environment within the redd is shown to be complex, with large variations in DO. Application of a numerical model (SIDO-UK) enables for the first time, the quantification of the relative contributions to DO consumption from thermal regime, sediment accumulation and sediment oxygen demand. Sediment accumulation is shown to have a minor impact on DO in the redd whereas upwelling groundwater is identified as the most likely cause of the major changes in DO. Bed mobility has a minor impact on DO regime of the redd. The effects of fine sediment and oxygen supply on salmon embryo survival are estimated. Implications for river catchment management and prospects for future research are discussed.