Microspatial variation in carbohydrate concentrations with depth in the upper millimetres of intertidal cohesive sediments

The carbohydrates present in surface sediments represent a readily available carbon source and their polymeric components increase sediment stability. Information regarding the occurrence of these carbohydrates is therefore important in the consideration of biogeochemical cycling, heterotrophic metabolism and sediment transport. The distributions of carbohydrates within the surface sediments of three intertidal mud flats were examined on a microscale (200 mu m) The carbohydrates were operationally separated into two fractions and these differed in their distribution with depth. Dry mass concentration increased significantly with depth in the upper 2 mm and the structure of surface sediment changed visibly within a short distance of the surface. The upper 300 mu m of the sediment was highly porous but became compact by 4 mm. Epipelic diatoms and cyanobacteria were observed at high densities in the upper 300 mu m. In addition, the concentration of colloidal carbohydrates increased significantly in a landwards direction along a short transect. Furthermore, the distribution of sediment carbohydrates was apparently influenced by sediment bed morphology and these findings are discussed. (C) 1998 Academic Press Limited.