Sediment transport is regarded as an abiotic process driven by geophysical energy, but zoogeomorphological activity indicates that biological energy can also fuel sediment movements. It is therefore prudent to measure the contribution that biota make to sediment transport, but comparisons of abiotic and biotic sediment fluxes are rare. For a stream in the UK, the contribution of crayfish bioturbation to suspended sediment flux was compared with the amount of sediment moved by hydraulic forcing. During base flow periods, biotic fluxes can be isolated because nocturnal crayfish activity drives diel turbidity cycles, such that nighttime increases above daytime lows are attributable to sediment suspension by crayfish. On average, crayfish bioturbation contributed at least 32% (474 kg) to monthly base flow suspended sediment loads; this biotic surcharge added between 5.1 and 16.1 t (0.21 to 0.66 t km−2 yr−1) to the annual sediment yield. As anticipated, most sediment was moved by hydraulic forcing during floods and the biotic contribution from baseflow periods represented between 0.46 and 1.46% of the annual load. Crayfish activity is nonetheless an important impact during baseflow periods and the measured annual contribution may be a conservative estimate because of unusually prolonged flooding during the measurement period. In addition to direct sediment entrainment by bioturbation, crayfish burrowing supplies sediment to the channel for mobilization during floods so that the total biotic effect of crayfish is potentially greater than documented in this study. These results suggest that in rivers, during base flow periods, bioturbation can entrain significant quantities of fine sediment into suspension with implications for the aquatic ecosystem and base flow sediment fluxes. Energy from life rather than from elevation can make significant contributions to sediment fluxes.

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The distribution of macroinvertebrates in the heads and tails of riffles were examined in an in situ field experiment under stable baseflow conditions. Paired colonisation cylinders were used to examine the influence of vertical hydraulic exchange (upwelling and downwelling) and horizontal interstitial flow on the patterns of sedimentation and invertebrate colonisation. Sedimentation rates were greatest in cylinders permitting vertical and horizontal flow (VHE cylinders), and were significantly lower (29%) in cylinders where only vertical flow and ingress of fine sediment were possible (VE cylinders). The results demonstrate that horizontal interstitial flows represent an important pathway for fine sediment transport. Differences in fine sediment accumulation were also observed between riffle heads and tails. Significantly higher sedimentation rates were recorded in riffle tails, with the macroinvertebrate communities characterised by larger proportions of fine sediment tolerant taxa. In contrast, riffle head communities were characterised by greater proportions of sediment sensitive taxa, and in the case of VHE cylinders, shredders and EPT taxa. The results demonstrate that spatial differences in fine sediment deposition are evident at the riffle scale as a function of vertical and horizontal subsurface flows and that these factors play a key role in the distribution of macroinvertebrate fauna.

Summary Sedimentation and clogging of benthic and hyporheic zone substrata is increasingly being recognised as one of the greatest threats to the ecological integrity of riverine ecosystems globally. This ex situ study examined the influence of sedimentation (surface and subsurface) and pattern of hydrological exchange on the vertical distribution of the freshwater shrimp Gammarus pulex within the experimental substrata of running water mesocosms. Six sediment treatments representing a continuum from a clean gravel substratum to heavy sediment loading of both surface (benthic) and subsurface (hyporheic) substrata were used to examine the distribution of G. pulex in relation to the direction of hydrological exchange (downwelling, upwelling and no exchange). The distribution of G. pulex between the sediment layers was dependent on the pattern of hydrological exchange, sediment treatment and the interaction between these two factors. Sedimentation of the surface layer under no‐exchange conditions resulted in a lower proportion of G. pulex being recorded in the benthic sediments, whilst there were no significant differences under downwelling and upwelling flow conditions. Sedimentation of multiple layers of the column (benthic and subsurface) reduced the ability of individuals to utilise the subsurface layers of the substratum (i.e. the hyporheic zone) under no‐exchange and upwelling conditions. However, with downwelling conditions, the abundance of G. pulex declined with depth regardless of the fine sediment distribution or volume. This study demonstrates that faunal movement, and use of benthic and hyporheic substrata, may be influenced by sedimentation and modified by the pattern of vertical hydrological exchange. Severe sedimentation (colmation) has the potential to prevent benthic fauna from accessing the hyporheic zone and its resources which may ultimately lead to a reduction in stream diversity and metabolism, thereby limiting overall productivity and lotic ecosystem resilience.

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Ponds are among the most biodiverse and ecologically important freshwater habitats globally and may provide a significant opportunity to mitigate anthropogenic pressures and reverse the decline of aquatic biodiversity. Ponds also provide important contributions to society through the provision of ecosystem services. Despite the ecological and societal importance of ponds, freshwater research, policy, and conservation have historically focused on larger water bodies, with significant gaps remaining in our understanding and conservation of pond ecosystems. In May 2019, pond researchers and practitioners participated in a workshop to tackle several pond ecology, conservation, and management issues. Nine research themes and 30 research questions were identified during and following the workshop to address knowledge gaps around: (1) pond habitat definition; (2) global and long-term data availability; (3) anthropogenic stressors; (4) aquatic-terrestrial interactions; (5) succession and disturbance; (6) freshwater connectivity; (7) pond monitoring and technological advances; (8) socio-economic factors; and (9) conservation, management, and policy. Key areas for the future inclusion of ponds in environmental and conservation policy were also discussed. Addressing gaps in our fundamental understanding of pond ecosystems will facilitate more effective research-led conservation and management of pondscapes, their inclusion in environmental policy, support the sustainability of ecosystem services, and help address many of the global threats driving the decline in freshwater biodiversity.

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