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In intertidal sediments the presence of microphytobenthos in and on the surface affects the sediment biogeochemistry, can influence the flux of substances across the sediment interface and changes the physical surface structure and sediment stability. In this paper new methods of sampling and for examining the microphytobenthos assemblages of sediments have been implemented. This included the new Cryolander method for the fine scale (0.1 mm) sampling of intertidal sediments. Algal pigments in sediments were determined with a new HPLC method on a very fine-scale ( mu m) with depth. Such finely resolved pigment profiles are combined with electron micrographs of surface sediments to provide a new and radically altered understanding of the concentration of algal biomass in and on sediment surface layers. It is shown that benthic algal populations can be highly stratified in the top millimeters of sediments, with most of the algal biomass occurring in the top 200 mu m (up to 700 mu g chlorophyll a/g). Therefore, in order to determine vertical algal distribution accurately, in all investigations where the absolute pigment concentrations at the surface of sediments are important, sediments should be sampled and analysed using microtechniques. This can prevent underestimation of chlorophyll at the surface of sediments of a factor as high as 6, as sediment sampling techniques based on cores with a depth of mm/cm can give results representing a dilution of the actual surface algal populations and even water contents. If the extent of this dilution is unknown, this can cause problems in algal estimations in general. Specifically, this is of great relevance to ground truth data for remote sensing and to diffusive boundary layer and porosity assumptions in biogeochemical considerations
