Benthic variability in intertidal soft-sediments in the mesohaline part of the Schelde estuary

The benthic (zoobenthos and microphytobenthos) and physical characteristics of intertidal sediments were studied in April and September 1997 on 10 locations, differing in elevation and exposure to tidal currents, situated on three mudflats in the mesohaline part of the Schelde estuary. Sediment characteristics were spatially and temporally relatively homogeneous among the sampling locations, all characterized by a high proportion of mud. It emerged that the nature of the sediment (in terms of granulometric characterization, organic matter) did not fully explain the substantial spatial and temporal (seasonal) variability in the benthos (pigment contents and benthic fauna), but that other environmental conditions explained the observed variability. Chlorophyll a and fucoxanthin contents in the top layer (0–0.5 cm) of the sediment were about 7 times higher in April than in September, but a large spatial variation was observed, especially in April, which was mainly attributed to a difference in elevation of the mudflats. The high pigment contents in April were accompanied by a relative low abundance of the benthic fauna. In April sub-surface deposit feeders, mainly Oligochaeta, were numerically most important in the 1000 and 500 m fraction of the zoobenthos, and Nematodes in the 250 m fraction. The surface deposit feeders Manayunkia aestuarina and Copepoda were locally very abundant, showing a strong positive correlation with elevation and pigment contents. The critical shear stress for erosion, measured in situ with the SedErode device, varied between 0.26 and 0.43 N m^–2 in April. The critical shear stress for erosion was lower in September when pigment contents were significantly lower and benthic fauna abundance was higher. In September Corophium volutator, Heteromastus filiformis, Nereis diversicolor and Macoma balthica dominated the 1000 m fraction, Oligochaeta and C. volutator the 500 m fraction, and Nematodes, M. aestuarina, Oligochaeta and Copepoda the 250 m fraction. It is argued that in estuarine systems with high seasonal variability in river flow rate and therefore salinity, the upper-middle estuarine fauna may switch each year between an oligohaline and a mesohaline fauna, which result in communities that seldom progress beyond early benthic-community succession. The majority of the benthic infauna was found in the upper 3 cm in both seasons, with surface deposit feeders (e.g. C. volutator, M. aestuarina) mainly occurring in the top 1 cm of the sediment, showing a strong correlation with both elevation and pigment contents and sub-surface deposit feeders (e.g. H. filiformis, Oligochaeta) occurring deeper in the sediment, showing a positive correlation with the mud content of the sediment.