Bream (Abramis brama L.) as zoogeomorphic agents and ecosystem engineers: implications for fine sediment transport in lowland rivers

It is becoming increasingly clear that both plants and animals can act as zoogeomorphic agents and ecosystem engineers within riverine environments by modifying the nutrient, sediment and hydraulic dynamics. However, although there is more interest in the relationships between the biotic and abiotic environments, the role of zoogeomorphic agents and ecosystem engineers are yet to be fully recognised or established in either geomorphological or ecological models. One area which has received little attention is foraging by benthic fish, even though these species inhabit rivers worldwide and collectively can impart significant amounts of energy onto river beds. This is particularly important amongst shoaling species that live in fine sediment environments as the cumulative effect of their foraging upon sediment dynamics is likely to be high given how little energy is required to entrain sediment transport. To address this research gap, a series of experiments were conducted to investigate the effect of bioturbation by bream (Abramis brama), a common European benthic fish, and their impacts on sediment dynamics.

Mesocosm experiments were designed to explicitly examine the drivers and assess the potential environmental effects of bream bioturbation. The results suggest that sediment suspension caused by bream foraging increased with fish size, fish number (intraspecific competition) and food availability (p = < 0.001). These results highlighted that significant levels of turbidity (as a measure of suspended sediment) were created by bream when more than 1 fish was present, at any fish size and at natural food densities. Importantly, these findings imply that significant levels of turbidity will be created under natural conditions in the field, suggesting that bioturbation effects may be widespread.

Additional mesocosm and flume experiments were used to explore the effect of interspecific interactions on bream bioturbation. Here, the bioturbation associated with feeding by roach (Rutilus rutilus; a functionally similar species to bream) was established. Then, bioturbation was measured when roach and equal numbers of bream were placed in the mesocosm. Turbidity increased significantly when the bream had interspecific competition (p = < 0.001). This relationship was particularly evident as fish number increased (p = < 0.001); during the three-fish experiments, mean turbidity levels increased by 388% (56.86 NTU) and the 95th percentile turbidity by 407% (101.95 NTU) when compared with the turbidity created without interspecific competition. The turbidity levels from these experiments were then used to assess the impact of fish induced bioturbation upon the feeding efficiency (the rate of capture of drifting insect larvae) of roach by replicating a riverine environment within a circulating

i flume. Feeding efficiencies were significantly reduced at mean experimental turbidity values (~15 NTU; p = < 0.001) and no feeding was recorded at higher turbidity levels. These results show, for the first time, that the level of suspended sediment created by bream cause detrimental effects to other species around them.

A field campaign was used to assess the feeding of bream at a patch scale. Here surveys using underwater sonars displayed, for the first time, areas containing large numbers of bream feeding pits. These pits were surveyed at six locations within three different aquatic environments to provide both spatial and morphological characteristics. Pit morphology, density and total feeding areas were statistically different between rivers, drains and the Norfolk Broads (p = < 0.001). Feeding pit size and the extent of the feeding areas was greatest in the Norfolk Broads which is speculated to be the result of the lack of hydraulic regime, but this was not formally tested. Using these measures, estimations for the total volume of sediment displaced, total sediment surface disturbed and total volume of sediment per mean feeding area were established. These measurements provided the first estimations of fine sediment displacement by foraging fish in lowland rivers and act as an important foundation for lowland zoogeomorphic research.

The feeding pits were investigated further by assessing their impact on near-bed hydraulics. Here, riverine pits and their respective physical parameters were scaled and modelled within a recirculating flume. At different natural pit densities, a vertical profile of ADV measurements was used to obtain orthogonal velocities and TKE at discrete positions above the bed surface. Stream wise velocity and, to a lesser extent, vertical velocity, both increased in the presence of pits, especially higher in the flow and at the highest feeding pit density. However, the presence of pits appeared to depress TKE very close to the bed which remains unexplained. Importantly, the results from the scaled hydraulic experiments have measured, for the first time, the impact of biogenic depressions on lotic hydraulics and turbulence parameters.

Together, results from these experiments indicate that bream are significant zoogeomorphic agents and ecosystem engineers through their foraging promoting an increase in suspended sediment. The implication of this result is that bream have the potential to be an impactful zoogeomorphic agents in freshwater environments, in large part because the fine sediment environments they inhabit require the application of relatively little energy to induce sediment transport via bed material suspension and displacement. Therefore, the cumulative effect of bream activities on sediment transport is possibly relatively high and this warrants additional research effort.