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

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posted on 21.12.2017 by Andrew Pledger, Stephen Rice, Jonathan Millett
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.

Funding

The Barbel Society

History

School

  • Social Sciences

Department

  • Geography and Environment

Published in

Journal of Geophysical Research: Earth Surface

Citation

PLEDGER, A.G., RICE, S.P. and MILLETT, J., 2017. Foraging fish as zoogeomorphic agents: An assessment of fish impacts at patch, barform, and reach scales. Journal of Geophysical Research: Earth Surface, 122 (11), pp. 2105–2123.

Publisher

© American Geophysical Union

Version

VoR (Version of Record)

Publisher statement

This work is made available according to the conditions of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) licence. Full details of this licence are available at: https://creativecommons.org/licenses/by-nc-nd/4.0/

Publication date

2017-10-18

Notes

This paper was published in the journal Journal of Geophysical Research: Earth Surface and is also available at https://doi.org/10.1002/2017JF004362.

ISSN

2169-9003

Language

en

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