Spatial distribution of sediment particulate organic matter on two estuarine intertidal mudflats: a comparison between Marennes-Oléron Bay (France) and the Humber Estuary (UK)

A comparison based on the bulk composition of surficial sediment particulate organic matter (POM) has been carried out on mudflats representative of two large European macrotidal estuarine zones: the Marennes-Oléron Bay and the Humber Estuary. Samples were collected in early summer, at low tide, during medium spring tide. Six sites were investigated along a 2500 m cross-shore transect with 12 cores collected at each site. Elemental analyses (C, N) and proximate analyses (chloropigments, carbohydrates and lipids) were performed on the fraction <125 μm at 4 levels: 0–1, 1–3, 3–6 and 6–10 cm. The POM was largely detrital, especially in the Humber mudflat. No clear decrease with depth in sediment was recorded for particulate organic carbon, total nitrogen and lipids, which emphasized the importance of sediment reworking by bioturbation and resuspension processes. However, decreases were observed for chloropigments and carbohydrates. Spatially, the general seaward decrease in the concentration of most compounds suggested a preferential accumulation of POM in the sheltered sites close to shore. Multivariate analysis showed that the POM collected at the sediment surface is similar between Marennes-Oléron and the Humber: it is mainly fresh and labile POM, probably largely derived from microphytobenthos. By contrast, deeper layers develop a signature which depends on the wider environment of the area studied: the significance of nitrogen and carbohydrates in Marennes-Oléron samples may reflect the influence of bivalve biodeposits rich in polysaccharides, whereas the importance of carbon and lipids in Humber samples may be partly related to pollution. This study demonstrated that it was possible to discriminate between different mudflats by using elemental and proximate analyses.