Search

River channels, riparian and floodplain forms and dynamics are all influenced strongly by biological processes. However, the influence of macroinvertebrates on entrainment and transport of river sediments remains poorly understood. We use an energy‐based approach to explore the capacity of benthic animals to move surficial, gravel‐bed particles in field and laboratory settings and use the results to assess the relative significance of biological and physical benthic processes. Our results showed that in 11 British gravel‐bed rivers, the maximum energy content (i.e., calorific content) of macroinvertebrate communities generally matched the flow energy associated with median discharges and, at multiple sites, exceeded that of the 10‐year return interval flood. A series of laboratory experiments used to estimate the minimum energy expended by signal crayfish (Pacifastacus leniusculus) when performing geomorphic work established that crayfish move gravel particles at energy levels below that expected of the flow, complicating direct comparisons of the capacity for macroinvertebrates and fluvial flows to influence bed mobility. Our findings suggest that the influence of macroinvertebrate communities in either promoting or suppressing, the mobilisation of the bed may be large compared to equivalent values of fluvial energy. Based on these findings, we conclude that in the gravel‐bed rivers studied, the macroinvertebrate community's potential to perform geomorphic work matches or exceeds the stream power during most of the year. Although our study examined biological and fluvial energy systems separately, it is important to recognise that in nature, these systems are highly interactive. It follows that utilising the energy framework presented in this paper could lead to rapid advances in both fluvial biogeomorphology and river management and restoration. [ABSTRACT FROM AUTHOR]

Suitable gravel availability is critical for the spawning success of lithophilous fishes, including redd builders. Redd construction during spawning can alter substrate characteristics, thereby influencing hydraulic conditions and sediment transport, highlighting the importance of spawning as a zoogeomorphic activity. Here, interactions between redd‐building fish and their spawning environment were investigated for European barbel Barbus barbus with a comparative approach across three English rivers: Teme (western), Great Ouse (eastern) and Idle (central). Sediment characteristics of spawning habitats were similar across the rivers, including subsurface fine sediment (<2 mm) content (≈20% dry weight), but elevated subsurface silt content and coarser surface sediments were found in the river Teme. Water velocities were similar at spawning sites despite differences in channel width and depth. Redds were characterized by a pit and tailspill, with no differences in surface grain‐size characteristics between these and the surrounding riverbed, but with topographic alteration (dimensions and tailspill amplitude) in line with those of salmonids. Estimates of the fraction of the bed that spawning barbel were capable of moving exceeded 97% in all rivers. Estimated reproductive potential varied significantly between the rivers Idle and Teme (3,098 to 9,715 eggs/m2), which was largely due to differences in barbel lengths affecting fecundity. Larger barbel, capable of producing and depositing more eggs, but in more spatially extensive redds, meaning fewer redds per given surface area of riverbed. Predictions of barbel egg mortality based on sand content were low across both rivers. The effects of silt on barbel egg and larvae development are unknown, but the levels detected here would significantly impact salmon egg mortality. Similarities in fish length to redd area and the size of moveable grains by spawning barbel and salmon suggest they have similar geomorphic effects on sediments, although fine sediment tolerance is highly divergent. [ABSTRACT FROM AUTHOR]

The spawning success of lithophilic salmonids is strongly influenced by the fine sediment content ("fines") of spawning substrates, yet knowledge on the impacts of fines on the spawning of non‐salmonid lithophiles remains limited, despite their ecological and socio‐economic importance in European rivers. Consequently, the aim here was to use an ex‐situ experiment to investigate the impact of sand content on egg survival and timing of larval emergence of the surface‐spawning cyprinid European barbel Barbus barbus. Thirty incubator boxes within a recirculating system were filled with one of five experimental sediment mixtures (0%–40% sand by mass) that each contained 300 fertilised eggs at a depth of 50 mm. Emerged, free‐swimming larvae were captured and counted daily to assess grain‐size effects on larval survival and emergence. Specifically, total proportion of emerged larvae, cumulative daily proportion of emerged larvae and time required to reach 50% emergence were measured during the study. Whilst the proportion of sand in the sediments did not have a significant impact on egg‐to‐emergence survival (mean survival per treatment 75%–79%), it significantly affected the timing of larval emergence to the water column; early emergence was detected in treatments with elevated sand content (on average, 50% emergence after 12–13 days versus 19 days in the control). Similar to findings from salmonid studies, these results suggest high sand content in spawning gravels can influence timing of larval emergence and potentially cyprinid lithophilic fish survival. [ABSTRACT FROM AUTHOR]

Foraging by benthivorous fish can affect bed material mobility and sediment flux. This paper collates evidence of benthic feeding effects at local scales and evaluates the possibility that large numbers of foraging fish, each of which accomplishes a small amount of geomorphic work when feeding, may have a cumulative effect across river systems. A first synthesis of research from several disciplines provides a deeper understanding of how fish disturb and condition bed materials with implications for sediment mobility. To evaluate the spatial extent of benthic feeding and therefore the potential for it to have a large‐scale effect, the distribution of benthivorous fish is established across a large river network. After quality control, the dataset yields a comprehensive set of fish community information based on over 61 000 individuals and 30 species at 176 sites. The factors that are likely to mediate foraging and its geomorphological effectiveness are considered. A novel scoring system that incorporates three key controls (fish feeding behaviour, fish abundance and fish body size) is then applied across the river network to predict where geomorphologically effective benthic feeding is feasible and its possible relative magnitude. Our results demonstrate, for the first time, that the potential for zoogeomorphic impacts is widespread but variable in space as a function of community composition and the abundance of key benthivores. An initial calibration against measured field impacts suggests that benthic feeding may cause measurable geomorphological disturbance at more than 90% of sites in this large network. Together, previous work and this unique analysis suggest that benthic feeding is sufficiently effective and extensive to warrant additional research. Investigating the role of benthivorous fish in fluvial geomorphology is important because it may yield results that challenge the assumption that biota are irrelevant sources of energy in geomorphological systems. Key research questions and a roadmap to facilitate progress are identified. © 2018 John Wiley & Sons, Ltd. Critical evaluation of the evidence for bed disturbance by feeding fish and novel analysis of the spatial extent and likely intensity of foraging across a large river network are used to: (1) argue that benthic feeding fish are zoogeomorphic agents with the potential to have important cumulative impacts on fluvial processes; and (2) develop key research questions and a roadmap to evaluate this hypothesis. [ABSTRACT FROM AUTHOR]

• Water Injection Dredging altered benthic macroinvertebrate communities. • Burrowing macroinvertebrate taxa were particularly affected by WID. • Fish and marginal macroinvertebrate communities were relatively unaffected by WID. • WID provides an ecologically sensitive alternative to traditional dredging methods. Subaqueous dredging is a management activity undertaken globally to improve navigation, remove contaminants, mitigate flood risk and/or generate aggregate. Water Injection Dredging (WID) is a hydrodynamic technique involving the turbation and downstream displacement of fine sediments using vessel-mounted water jets. Despite the technique being widely applied internationally, the environmental and ecological effects of WID are poorly understood. For the first time, this study used a Before-After-Control-Impact (BACI) experimental design to assess the effects of WID on water physicochemistry, and macroinvertebrate and fish communities within a 5.7 km-long reach of tidal river. WID targeted the central channel (thalweg) to avoid disturbance of the channel margins and banks. Mean but not peak turbidity levels were substantially elevated, and dissolved oxygen levels were reduced during periods of WID, although effects were relatively short-lived (≈3 h on average). Dredging resulted in significant reductions in benthic macroinvertebrate community abundance (particularly taxa that burrow into fine sediments), taxonomic richness and diversity. In contrast, minor changes were detected in marginal macroinvertebrate communities within and downstream of the dredged reach following WID. Reductions in fish taxonomic richness and diversity were recorded downstream of the dredged reach most likely due to behavioural avoidance of the sediment plume. No visibly stressed or dead fish were sampled during dredging. Results suggest that mobile organisms and marginal communities were largely unaffected by thalweg WID and that the technique represents a more ecologically sensitive alternative to traditional channel margin mechanical dredging techniques. [ABSTRACT FROM AUTHOR]

Fine sediments can impact river biota, with egg and larval stages of lithophilic fish particularly sensitive to deposition of sand- to clay-sized particles (‘fines’) in spawning gravels. Mitigation and restoration methods include jetting to cleanse gravels of fines. Despite wide application, impacts of jetting on gravel composition and quality have rarely been quantified. Here, gravel jetting impacts on sediment composition in the River Great Ouse (UK), were tested during an in-situ experiment completed at riffle (55.6 ± 13.4 m 2 ) and patch (0.3 m 2 ) scales to determine its magnitude and persistence on surface and subsurface substrate conditions. Before-after (riffle) and control-impact (patch) designs were used, with bedload sediment traps installed downstream of experimental patches to investigate the sediments mobilised during jetting. At the riffle scale, surface grain size was significantly altered; fines were removed resulting in coarser and better-sorted sediments. Similar patterns were detected at the patch scale, although sediment sorting was not significantly altered. Despite reduced fine sediment content of subsurface gravels at the riffle scale, the overall grain size composition was not significantly altered. At the patch scale, no subsurface improvements were detected. Temporally, at the riffle scale, no changes in surface or subsurface sediments lasted more than 12 months; patch scale changes generally persisted for less than 3 months. Thus, whilst gravel jetting could improve spawning gravel quality for surface spawning fishes, including European barbel Barbus barbus , its effects are short-lived. Because subsurface sediments are not affected by gravel jetting, the benefits are limited for redd-building fishes, such as salmonids. Consequently, reducing fine sediment delivery to rivers, such as by changes in agricultural practices, is more sustainable for managing excessive river sedimentation. [ABSTRACT FROM AUTHOR]