Accounting for the power of nature: Using flume and field studies to compare the capacities of bio‐energy and fluvial energy to move surficial gravels

Abstract

River channels, riparian and floodplain forms and dynamics are all influenced strongly by biological processes. However, the influence of macroinvertebrates on entrainment and transport of river sediments remains poorly understood. We use an energy‐based approach to explore the capacity of benthic animals to move surficial, gravel‐bed particles in field and laboratory settings and use the results to assess the relative significance of biological and physical benthic processes. Our results showed that in 11 British gravel‐bed rivers, the maximum energy content (i.e., calorific content) of macroinvertebrate communities generally matched the flow energy associated with median discharges and, at multiple sites, exceeded that of the 10‐year return interval flood. A series of laboratory experiments used to estimate the minimum energy expended by signal crayfish ( Pacifastacus leniusculus ) when performing geomorphic work established that crayfish move gravel particles at energy levels below that expected of the flow, complicating direct comparisons of the capacity for macroinvertebrates and fluvial flows to influence bed mobility. Our findings suggest that the influence of macroinvertebrate communities in either promoting or suppressing, the mobilisation of the bed may be large compared to equivalent values of fluvial energy. Based on these findings, we conclude that in the gravel‐bed rivers studied, the macroinvertebrate community's potential to perform geomorphic work matches or exceeds the stream power during most of the year. Although our study examined biological and fluvial energy systems separately, it is important to recognise that in nature, these systems are highly interactive. It follows that utilising the energy framework presented in this paper could lead to rapid advances in both fluvial biogeomorphology and river management and restoration.