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

Abstract

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 warranted. Copyright © 2014 John Wiley & Sons, Ltd.

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

“…The protocol pertaining to fish husbandry was consistent with that described in Pledger et al (2014Pledger et al ( , p. 1501, with two modifications. First, water in two 1000-l holding tanks was cooled and maintained at a constant temperature of 16.70°C ± 0.003 (±1 standard deviation).…”

“…These findings emphasise the role of biotic factors in controlling geomorphic impact. Third, Pledger et al (2014) found that foraging juvenile Barbel modified water-worked surface gravels, undoing stable imbricate structures and increasing microtopographic roughness. These changes coincided with an average increase in initial bedload flux and overall sediment yield of 60% and 82%, respectively, under entrainment flows.…”

“…The sediment mixture in the observation area was slightly coarser than the mixture used in Pledger et al (2014) and consisted of a log-normal grain-size distribution of 5.6-22.6 mm gravels (D 5 = 6.1 mm, D 50 = 10.0 mm, D 95 = 19.3 mm). Marine gravels were used that were predominantly bladed (Sneed and Folk, 1958) and well rounded (Krumbein, 1941).…”

“…To investigate the role of species, the foraging effects of like-sized Barbel and Chub, 8-10" in length, on bed material disturbance and subsequent transport were compared (component 2). Barbel and Chub were chosen for comparison in component 2 because they are two common, benthic-feeding fish that occupy similar habitats but have different physiologies and specific feeding habits, as illustrated by Pledger et al (2014;their Table IV), and therefore potentially different zoogeomorphic capabilities and impacts. Expectations are that larger fish will have a greater impact and that Barbel being a benthic foraging specialist will have a greater geomorphic impact than Chub, a renowned opportunistic forager.…”

“…The flume setup (Fig. 1) was almost identical to that used and described by Pledger et al (2014), where additional details can be found.…”