The importance of biotic entrainment for base flow fluvial sediment transport

Stephen P Rice,Chris Extence,Matthew F Johnson,Jake Reeds,Kate Mathers

doi:10.1002/2015jf003726

Abstract

AbstractSediment transport is regarded as an abiotic process driven by geophysical energy, but zoogeomorphological activity indicates that biological energy can also fuel sediment movements. It is therefore prudent to measure the contribution that biota make to sediment transport, but comparisons of abiotic and biotic sediment fluxes are rare. For a stream in the UK, the contribution of crayfish bioturbation to suspended sediment flux was compared with the amount of sediment moved by hydraulic forcing. During base flow periods, biotic fluxes can be isolated because nocturnal crayfish activity drives diel turbidity cycles, such that nighttime increases above daytime lows are attributable to sediment suspension by crayfish. On average, crayfish bioturbation contributed at least 32% (474 kg) to monthly base flow suspended sediment loads; this biotic surcharge added between 5.1 and 16.1 t (0.21 to 0.66 t km−2 yr−1) to the annual sediment yield. As anticipated, most sediment was moved by hydraulic forcing during floods and the biotic contribution from baseflow periods represented between 0.46 and 1.46% of the annual load. Crayfish activity is nonetheless an important impact during baseflow periods and the measured annual contribution may be a conservative estimate because of unusually prolonged flooding during the measurement period. In addition to direct sediment entrainment by bioturbation, crayfish burrowing supplies sediment to the channel for mobilization during floods so that the total biotic effect of crayfish is potentially greater than documented in this study. These results suggest that in rivers, during base flow periods, bioturbation can entrain significant quantities of fine sediment into suspension with implications for the aquatic ecosystem and base flow sediment fluxes. Energy from life rather than from elevation can make significant contributions to sediment fluxes.

Highlights

Sediment transport is regarded as an abiotic process driven by geophysical energy, but zoogeomorphological activity indicates that biological energy can fuel sediment movements
Sediment transport continues to be predominantly regarded as an abiotic process driven by the conversion of potential energy derived from relief to kinetic energy across elevation gradients
This might be interpreted as indicating that the biotic contribution to total sediment flux is geomorphologically inconsequential, but two arguments suggest that such a conclusion would be premature

Summary

Introduction

Animals play a significant role in geomorphological systems [Viles, 1988; Butler, 1995; Butler and Sawyer, 2012; Johnson and Rice, 2014; Holtmeier, 2015; Albertson and Allen, 2015] often via complex ecogeomorphological feedbacks [Naiman et al, 2000; Hall and Lamont, 2003; Wheaton et al, 2011; Beschta and Ripple, 2012] that have implications for the responsible organisms and the wider ecosystem (ecosystem engineering: Jones et al [1994], Wright and Jones [2006], Moore [2006], and Jones [2012]). Despite increasing recognition of zoogeomorphological activity there is a pervasive but untested assumption that the impact of animals on sediment flux is minor relative to geophysical forcing. Sediment transport continues to be predominantly regarded as an abiotic process driven by the conversion of potential energy derived from relief to kinetic energy across elevation gradients. With only a few exceptions in fluvial geomorphology [Tashiro and Tsujimoto, 2006; Albertson et al, 2014] and rare occurrences in other domains [Borsje et al, 2008], sediment transport formulations do not recognize animal activity or the potential contributions of biological energy. In the absence of clear supporting evidence it is prudent to test this orthodoxy by investigating what relative contribution fauna make to the movement of sediment at the Earth’s surface. A recent report from the U.S National Academy of Sciences [National Research Council, 2010] pointed out the need for such research, because understanding of Earth surface geophysical processes is severely constrained without a fuller appreciation of interactions with biota

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