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Physical and biological controls on fine sediment transport and storage in rivers

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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
Controls on the delivery, transport and storage of fine sediment, and the resulting ecological responses in river networks. Natural and anthropogenic factors control the quantity and quality of sediment potentially available for delivery to the river channel. Whether or not this potential is realized, and the quantity and composition of sediment conserved during the pluvial phase, depends on delivery pathways that connect the source to the river channel. Once delivered to the river channel, particles may be transported through suspension or saltation, temporarily stored as surface‐deposited material, or become infiltrated in the river bed and stored on a long‐term basis. Which compartment particles are transported or stored in depends on a number of controls, including the inherent properties of the particles, hydraulics, and the activities of biota. The latter is in turn influenced by physicochemical factors (e.g., oxygen, nutrients) that work to limit or enhance biological processes. The hydraulic and biological controls determine the compartments within which particles are transported or stored, structuring the physical habitat, modifying available resources and potentially filtering out unsuitable species from the species pool on the basis of their traits. Such environmental filtering effects may be interfered with by biogeographical processes related to dispersal. The fate of fine sediment, and the extent and nature of the impact it causes is subject to potential lags as sediment is temporarily stored within the landscape. Ecological impacts may also be delayed as time is taken for local populations to reach critical life‐stages where fine sediment limits survival
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Metacommunity theory predicts patterns in dominant community assembly processes in river networks (a) due to variation in spatial, environmental, and demographic influences with stream order (b), with implications for ecosystem responses to stressors. Headwater communities are likely to follow the patch dynamics paradigm, whereby local species diversity is limited by dispersal and community structure becomes a function of stochastic demographic processes involving local extinction and colonization (Leibold et al., ). The structure of lower mainstem communities is predicted to more closely resemble the regional species pool since high dispersal rates drown out the influence of the local environment through “mass effects.” (Leibold et al., ) In mid‐basin locations, species sorting or “environmental filtering” (Poff, ) is made possible as the influence of dispersal is at an optimal level to allow for efficient niche‐based processes (Winegardner, Jones, Ng, Siqueira, & Cottenie, )
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Controls on fine sediment deposition and resuspension exerted through bioflocculation by extracellular polymeric substances (EPS) in biofilms
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Fine sediment transport (black arrows) and deposition (gray arrows) processes in gravel beds. These processes occur in three key load regions (suspended and dissolved load, Brinkman load, and interstitial load) within the water and sediment columns in which such processes occur (left). The model proposes distinct modes of sediment transport in these three regions identified by transitions in the velocity profile V(y) (right). (Reprinted with permission from Casas‐Mulet, Alfredsen, McCluskey, & Stewardson (). Copyright 2017 Elsevier)
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Science of Water > Water Quality
Water and Life > Stresses and Pressures on Ecosystems
Water and Life > Nature of Freshwater Ecosystems

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