Distribution of extracellular carbohydrates in three intertidal mudflats in Western Europe

In this study, the spatial distribution of two operationally defined extracellular carbohydrate fractions (water- and EDTAextractable carbohydrates) were examined in three intertidal mudflats in Western Europe (Dollard, the Netherlands; Marennes, France; Humber, UK). The three mudflats were sampled along cross-shore transects and sediment cores were sliced to a depth of 5 cm. In these mudflats diatoms were the dominant component of the microphytobenthos. Carbohydrate content showed little variation with depth but varied along transects within each mudflat. Carbohydrate contents were also significantly different between mudflats and the carbohydrate contents of the stations within a mudflat grouped together resulting in separate clusters. This was also observed when the Marennes mudflat was investigated on a temporal scale. These results suggest that processes that act on the scale of whole mudflats determine the variations in extracellular carbohydrate contents. In the surface 0.5 cm of the sediment, waterextractable carbohydrates showed a correlation with both chlorophyll a content and median grain size while EDTA-extractable carbohydrates were only correlated with median grain size. Incubation experiments also showed the importance of microphytobenthos as a source of extracellular carbohydrate especially when subjected to the light. Analyses of the monosaccharide distribution of the carbohydrate fractions revealed that the carbohydrate composition was largely similar between the areas investigated. Structurally, the carbohydrates found in these sediments seem to represent a biorefractory part of the freshly produced carbohydrates that remained after rapid degradation of the more labile component.