Influence of biota on spatial and temporal variation in sediment erodability and material flux on a tidal flat (Westerschelde, The Netherlands)

Annular flumes were used to quantify benthic-pelagic exchanges in relation to changes in current velocity and sediment biota. Various physical, chemical and biological parameters were determined for 4 sites on the Molenplaat tidal flat in the Westerschelde (The Netherlands) during 2 field campaigns in May-June and August-September 1996. These included: the mass of sediment eroded, maximum and mean erosion rates, critical erosion velocity ((U) over bar(crit)), suspension feeding/biodeposition rate, oxygen and ammonium fluxes, density of macrofauna species, chlorophyll a (chl a), colloidal carbohydrate, and physical properties of the sediment. The study showed marked and statistically significant spatial and temporal differences in the potential for sediment erosion. Sites in the centre of the tidal flat were less erodable than those on the edge. All sites on the tidal flat had a significantly (p < 0.001) lower erosion potential in June (i.e, higher (U) over bar(crit) and lower erosion rates) compared to September. The presence of a well-developed microphytobenthos community in June resulted in a statistically significant relationship between (U) over bar(crit) and chl a/colloidal carbohydrate (r = +0.85, p = 0.01). There was a significant relationship (r = +0.88, p = 0.005) between sediment erodability (mass of sediment eroded and erosion rate) and the density of the clam Macoma balthica, a major bioturbator of the surface sediments. Analysis of the data also demonstrated significant relationships between clearance rate and Cerastoderma edule biomass (r = +0.91, p < 0.001), and between sediment chi a/colloidal carbohydrate and C. edule biomass (r = +0.92; p < 0.001), reflecting the influence of suspension feeders on both the water column and the sediment, through their biodeposits. The significant increase in sediment erodability between June and September reflected the shift in the overall balance between the major 'biostabilisers' (microphytobenthos) and 'bio-destabilisers' (bioturbating bivalves). This interpretation of field-derived correlations is consistent with previously established relationships and mechanistic understanding derived from controlled flume experiments.