Reduced bed material stability and increased bedload transport caused by foraging fish: a flume study with juvenile Barbel (Barbus barbus)

The plants and animals that inhabit river channels may act as zoogeomorphic agents affecting the nature and rates of sediment recruitment, transport and deposition. The impact of benthic-feeding fish, which disturb bed material sediments during their search for food, has received very little attention, even though benthic feeding species are widespread in rivers and may collectively expend significant amounts of energy foraging across the bed. An ex situ experiment was conducted to investigate the impact of a benthic feeding fish (Barbel Barbus barbus) on particle displacements, bed sediment structures, gravel entrainment and transport fluxes. In a laboratory flume changes in bed surface topography were measured and grain displacements examined when an imbricated, water-worked bed of 5.6 to 16 mm gravels was exposed to feeding juvenile Barbel (on average, 0.195 m in length). Grain entrainment rates and bedload fluxes were measured under a moderate transport regime for substrates that had been exposed to feeding fish and control substrates which had not. On average, approximately 37% of the substrate, by area, was modified by foraging fish during a four-hour treatment period, resulting in increased microtopographic roughness and reduced particle imbrication. Structural changes by fish corresponded with an average increase in bedload flux of 60% under entrainment flows, whilst on average the total number of grains transported during the entrainment phase was 82% higher from substrates that had been disturbed by Barbel. Together, these results indicate that by increasing surface microtopography and undoing the naturally stable structures produced by water working, foraging can increase the mobility of gravel-bed materials. An interesting implication of this result is that by increasing the quantity of available, transportable sediment and lowering entrainment thresholds, benthic feeding might affect bedload fluxes in gravel-bed rivers. The evidence presented here is sufficient to suggest that further investigation of this possibility is warrante

Foraging fish as zoogeomorphic agents: An assessment of fish impacts at patch, barform, and reach scales

Flume studies have demonstrated that foraging by fish can modify the structure and topography of gravel substrates, thereby increasing particle entrainment probabilities and the amount of sediment mobilized during subsequent experimental high flows. However, the zoogeomorphic impact of benthic foraging has not previously been investigated in the field. This paper reports field experiments that examined the nature and extent of disturbance of riverbed gravels by foraging fish, predominately Cyprinids, at patch, riffle, and reach scales and complementary ex situ experiments of the impacts on bed stability. At patch scale, benthic feeding fish displaced particle sizes ≤90 mm in diameter, increased bed surface microtopography and grain protrusion, and loosened surface structures. Although enhanced mobility was expected from these structural changes, foraging also caused localized coarsening of sediments, and the ex situ experiments recorded significantly reduced grain entrainment, bedload flux, and total transported mass from foraged patches. Foraging disturbed bed materials at all 12 riffles in the study reach and, on average, disturbed 26.1% of riffle area per 24 h feeding period. These findings demonstrate for the first time that foraging fish, which are widespread and feed perennially, can act as zoogeomorphic agents in rivers, affecting grain-size distributions and bed material structure, with potential implications for bed stability and bedload transport at reach and river scales. Whether fish increase or reduce bed mobility is probably dependent on a host of factors, including the net effects of both structural disturbance and biogenic particle sorting, as these affect entrainment stresses under subsequent competent flows

Bed disturbance via foraging fish increases bedload transport during subsequent high flows and is controlled by fish size and species

Benthic foraging by fish can modify the nature and rates of fine sediment accrual and the structure and topography of coarse-grained fluvial substrates, with the potential to alter bed material characteristics, particle entrainment thresholds, and bedload transport fluxes. However, knowledge of what controls the nature, extent, and intensity of benthic foraging and the consequent influence of these controls on geomorphic impact remain rudimentary. An ex-situ experiment utilising Barbel Barbus barbus and Chub Leuciscus cephalus extended previous work by considering the role of fish size and species as controls of sediment disturbance by foraging and the implications for bed material characteristics and bedload transport. In a laboratory flume, changes in bed microtopography and structure were measured when a water-worked bed of 5.6-22.6. mm gravels was exposed to four size classes of Barbel (4-5″, 5-6″, 6-8″, 8-10″ in length) and a single size class of Chub (8-10″). In line with other studies that have investigated animal size as a control of zoogeomorphic agency, increasing the size of Barbel had a significant effect on measured disturbance and transport metrics. Specifically, the area of disturbed substrate, foraging depth, and the fish's impact on microtopographic roughness and imbrication all increased as a function of fish size. In a comparison of the foraging effects of like-sized Barbel and Chub, 8-10″ in length, Barbel foraged a larger area of the test bed and had a greater impact on microtopographic roughness and sediment structure. Relative to water-worked beds that were not foraged, bed conditioning by both species was associated with increased bedload transport during the subsequent application of high flows. However, the bedload flux after foraging by Barbel, which is a specialist benthivore, was 150% higher than that following foraging by Chub, which feed opportunistically from the bed, and the total transported mass of sediment was 98% greater. An interesting implication of these results, given the abundance and widespread distribution of foraging fish, is that numerous fish species belonging to a variety of functional groups may be acting as zoogeomorphic agents in rivers, directly affecting bed material conditions and sediment transport fluxes in proportion to their body size and feeding traits

Does sand content in spawning substrate result in early larval emergence? Evidence from a lithophilic cyprinid fish

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

Zoogeomorphological behaviours in fish and the potential impact of benthic feeding on bed material mobility in fluvial landscapes

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 provide the first prediction of where geomorphologically effective benthic feeding is feasible and its possible relative magnitude. Our results demonstrate that the potential for zoogeomorphic impacts is widespread but variable in space as a function of community composition and the abundance of key benthivores. A preliminary calibration against measured field impacts suggests that benthic feeding may cause measurable geomorphological disturbance at more than 90% of sites. 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

Quantifying the habitat and zoogeomorphic capabilities of spawning European barbel Barbus barbus, a lithophilous cyprinid

© 2019 The Authors. River Research and Applications published by Wiley Periodicals, Inc. 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

The effects of water injection dredging on low-salinity estuarine ecosystems: implications for fish and macroinvertebrate communities

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

Hydromorphological and ecological impacts of water injection dredging in the River Parrett, Somerset Levels, Somerset, UK [Poster]

The Somerset Levels (‘The Levels’ hereafter) are a low-lying and flood-prone agricultural landscape in South West England, UK. The Levels drainage network is heavily managed for navigation and flood relief purposes and there is a legacy of dredging, to help mitigate flood risk. Despite widespread application of the technique internationally, knowledge of the short to medium-term effects of dredging in the wider ecosystem remains limited. Furthermore, the potential impacts of new dredging technologies, including water injection dredging, have rarely been considered or quantified, nor have the effects of these operations on instream biota. A programme of work therefore investigated the hydromorphological (bathymetry, bed material grain size, water physicochemistry) and ecological (macroinvertebrate, diatom, fish) effects of water injection dredging temporally and spatially within the River Parrett, Somerset. Hydromorphological and ecological sampling occurred at a control site (upstream of the injection site), within the managed area (injected) and downstream of the dredge site on multiple occasions before and after dredging. Additionally, during dredging, the sediment plume was surveyed via pelagic trawling and captured fish were inspected for mortality or signs of stress or ill health. Approximately 33,900 cubic metres of sediment were mobilised from the injection site, with management activities coinciding with a 39% increase in channel roughness. With regard fish, all 236 individuals captured during the dredging operation were alive, showed no obvious signs of dredging-induced stress and were returned unharmed post-processing. There was a significant decrease in fish abundance following operations at the injection site, with mean values declining from 15 to 3, possibly due to fish avoidance of the injection site. No other parameters or biotic indices were significantly affected at the dredge site. Studies such as these that utilise robust experimental designs and consider a range of aquatic organisms are integral for understanding the factors that contribute to successful restoration and flood risk management and form an important bridge between academic research to underpin practice

Characterising the geomorphological and physicochemical effects of water injection dredging on estuarine systems

Dredging is a globally important aquatic system management activity, used for navigation improvement, contamination removal, aggregate production and/or flood risk mitigation. Despite widespread application, understanding of the environmental effects of some dredging types remains limited. Field campaigns in 2016 and 2017 in the River Parrett estuary, UK, therefore investigated the geomorphic and physicochemical effects of Water Injection Dredging (WID), a poorly studied hydrodynamic dredging technology. WID, applied to restore channel capacity for the maintenance of flood water conveyance in the tidal River Parrett, influenced surface elevations but not grain-size characteristics of dredged bed sediments. Topographic alterations due to the 2016 WID operation were short-lived, lasting less than 10 months, although benefits of the 2017 WID operation, in terms of volumetric change, outlasted the ≈12-month study period. Dredging had a significant impact on water physicochemistry (pH, dissolved oxygen, total suspended solids and turbidity) when comparing pre- and during-dredging conditions within the dredge reach, although time-series analysis found dredging effects were comparable in magnitude to tidal effects for some parameters. WID is typically targeted at the thalweg and not the banks, rendering the geomorphic signature of the method different to those of other, often more invasive dredging technologies (e.g. mechanical dredging methods). Further, thalweg not bankside dredging may have potential positive ecological implications, particularly where the majority of biomass is located within the channel margins, as in the tidal River Parrett. Collectively, data suggest WID can be an effective method for sediment dispersal within tidal systems although regular application may be required to maintain cross sectional areas, particularly where management precedes periods of low flows and/or high rates of sediment accumulation. In future, more work is required to better understand both the physical and ecological implications of WID as a flood risk management tool in estuaries and rivers

Hydromorphological and ecological impacts of water injection dredging in the River Parrett, Somerset Levels, Somerset, UK [Conference presentation]

Hydromorphological and ecological impacts of water injection dredging in the River Parrett, Somerset Levels, Somerset, UK [Conference presentation