A model of sediment transport over an intertidal transect, comparing the influences of biological and physical factors

Abstract

This paper compares modeled biotic and physical effects on intertidal sediment transport, using parameterizations that are based on laboratory and field experiments. A one-dimensional model of an intertidal transect is constructed. The model is aligned cross shore and includes movement of water and suspended sediment. Within the model, tidal currents cause erosion, and bioturbation by the clam, Macoma balthica, alters the erodability of the bed sediment. The concentration of chlorophyll a in the surface sediment (which is an indicator of microphytobenthos density) alters the critical erosion velocity. External sediment supply is specified as an offshore suspended matter concentration. The model is applied within Spurn Bight (Humber Estuary, UK). The effects of various tide heights, biota densities, and external suspended sediment concentrations are investigated. Offshore sediment supply dominates the net deposition below midtide level, but factors affecting intertidal sediment erosion and deposition become important at higher shore levels. Changes in erosion or deposition caused by natural variation in biota densities are as large as those caused by changes in tidal range and currents over a spring-neap cycle, or by doubling external supply. Seasonal variations in densities of stabilizing microphytobenthos can alter the magnitude of net deposition on the upper shore by a factor of two. Interannual variation in numbers of bioturbating clams can change net deposition by a factor of five. These results show that biotic influences on transport of sediment within the intertidal zone are significant and will play a role in determining sediment budgets over tidal to monthly timescales.Sheltered estuaries often have extensive intertidal areas. These are important as both sources and sinks for fine sediment within the estuary. As such, they have a major role in the storage and cycling of those pollutants that preferentially bind to small particles. Mudflats and salt marshes in the U.K. are an important habitat for wintering birds and their prey species. Accretion of mudflats and salt marshes provides a valuable natural coastal defense in a regime of rising sea level, serving to dissipate energy from high tides and storm waves.The shallow coverage of water on the intertidal zones allows wave energy to penetrate to the bed, increasing the effect of small waves on sediment erosion. Rapid tidal flooding and drying of large, low gradient areas can also cause erosion by raising current speeds above the critical value. Turbid water is advected across the flats as the tide rises and is deposited during the long period of low current speeds near high water. Populations of benthic animals can affect erosion and deposition of particles actively (through bioturbation and biodeposition) and passively (by altering sediment characteristics). Algal mats and macroalgae can physically shield sediment, alter cohesiveness, and influence current velocity profiles. This paper addresses the question of how important biotic effects on s...

Benthic community-mediated sediment dynamics

“…Such an approach has been useful in explaining salt marsh development (Temmerman et al 2007). Studies by Le Hir et al (2007) and by Wood & Widdows (2002) have included the temporal and seasonal dynamics of both MPB and macrofauna effects and arrive at similar qualitative and quantitative conclusions. Both studies conclude that MPB effects are fairly well understood, relatively easy to model and overall stabilising.…”

Top-down control inhibits spatial self-organization of a patterned landscape

“…Observations from this system suggest that the decline is potentially caused by increased grazing pressure by benthic animals . The effects that macrobenthic grazers have on the interaction between diatoms and sedimentation have been investigated in a number of field and model studies (Wood and Widdows 2002, Paarlberg et al 2005, Le Hir et al 2007, revealing that herbivore grazing can influence sediment dynamics both directly by feeding and indirectly through bioturbation. Hence, intertidal mudflats provide a unique opportunity to investigate the effects of increasing top-down control in spatially self-organized ecosystems and to test the effects of grazing on selforganization and landscape formation in manipulative experiments.…”

Long-term and high-resolution measurements of bed level changes in a temperate, microtidal coastal lagoon

“…The mediation of the erodibility of fine-grained sediments by both microzoobenthos and macrozoobenthos has for example now been described for several species and field sites (e.g., Nowell et al, 1981;Paterson, 1989;Yallop et al, 1994;Widdows and Brinsley, 2002). In spite of these studies, little is still known about the net-effect of biotic processes with respect to mudflat sedimentation and budgets of fine-grained sediments although a few modelling studies have indicated that this effect may potentially be large (Wood and Widdows, 2002;Widdows et al, 2004;Lumborg et al, in press).…”