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MEPS 274:41-56 (2004)  -  doi:10.3354/meps274041

Role of physical and biological processes in sediment dynamics of a tidal flat in Westerschelde Estuary, SW Netherlands

J. Widdows1,*, A. Blauw2, C. H. R. Heip3, P. M. J. Herman3, C. H. Lucas4, J. J. Middelburg3, S. Schmidt5, M. D. Brinsley1, F. Twisk6, H. Verbeek6

1Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth PL1 3DH, UK
2WL | Delft Hydraulics, PO Box 177, 2600 MH Delft, The Netherlands
3Netherlands Institute of Ecology, Centre for Estuarine and Marine Ecology (NIOO-KNAW), Korringaweg 7, PO Box 140, 4401 AC Yerseke, The Netherlands
4School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK
5Laboratoire des Sciences du Climat et de l¹Environnement (CEA-CNRS), Domaine du CNRS, Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France
6National Institute for Coastal and Marine Management, Grenadresweg 31, PO Box 8039, 4330 EA Middelburg, The Netherlands

ABSTRACT: This article synthesises a series of studies concerned with physical, chemical and biological processes involved in sediment dynamics (sedimentation, erosion and mixing) of the Molenplaat tidal flat in the Westerschelde (SW Netherlands). Total sediment accretion rate on the flat (sand to muddy sand) was estimated to be ~2 cm yr-1, based on 210Pb and 137Cs profiles. 7Be showed maximum activity in the surface sediments during summer, reflecting accretion of fine silt at this time of year, and total vertical mixing of sediment to be in the order of 50 cm2 yr-1. The extent to which different physical and biological processes (tidal currents, air exposure, bio-stabilisation, biodeposition and bioturbation) contributed towards sediment dynamics was estimated. A sediment transport model based on physical factors estimated sedimentation rates of 1.2 cm yr-1, but did not account for tidal or seasonal variation in suspended particulate matter (SPM), wind or effects of spring-neap tidal cycles. When the model was run with an increased critical bed shear stress due to the microphytobenthos, net sedimentation rates increased 2-fold. These higher rates were in closer agreement with the rates derived from the depth profiles of radionuclides for the central region of the tidal flat (2.0 to 2.4 cm yr-1). Therefore a significant part of the sedimentation rate (~50%) may be explained by spatial-temporal changes in biological processes, including Œbio-stabilisation¹ by microphytobenthos, together with the enhanced biodeposition of silt by suspension feeders, and offset by processes of Œbio-destabilisation¹ by grazers and bioturbators. In the centre of the tidal flat there was a shift from high sediment stability in spring-summer 1996 to low sediment stability in autumn 1997, quantified by a significant reduction in critical erosion velocity of 0.12 to 0.15 m s-1, and accompanied by a 30- to 50-fold increase in sediment erosion rate. The change was associated with a shift from a tidal flat dominated by benthic diatoms and a low biomass of bioturbating clams (Macoma balthica), to a more erodable sediment with a lower microphytobenthos density and a higher biomass of M. balthica. Vertical mixing of sediment and organic matter, studied using a variety of tracers, was rapid and enhanced by advective water flow at sandy sites and by burrowing polychaetes and bivalves at silty sites.

KEY WORDS: Sediment dynamics · Erosion · Sedimentation · Mixing · Microphytobenthos · Biostabilisation · Bioturbation · Modelling

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