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Two field studies were conducted to study the in situ net production of extracellular carbohydrates and the distribution of benthic diatoms over a period of 24 h per study. A comparison was made between a situation where a clear surface biofilm of diatoms had developed and a situation where this was not the case, Vertical profiles were made by sampling the top 2 mm of the sediment at depth intervals of 0.2 mm using the 'cryolander' technique. In the presence of a biofilm, diatom distribution showed a consistent pattern when the sediment was emersed. In the light, most of the diatoms were present in the top 0.2 mm while in the dark, diatoms were homogeneously distributed in the upper 2 mm of the sediment. When a biofilm was absent, no clear patterns were observed. Extracellular carbohydrates were extracted from the sediment and separated in 2 operationally defined fractions (colloidal and EDTA- extractable). The 2 carbohydrate fractions showed a different dynamic behaviour. The colloidal carbohydrate fraction was highly variable while the EDTA-extractable fraction behaved more conservatively. Only in the light and in the presence of a diatom biofilm, was production of extracellular carbohydrates observed, The maximum rate of chlorophyll-normalized production of extracellular carbohydrates, expressed in glucose equivalents (g g(-1)), amounted to 20 h(-1) in the upper 0.2 mm. The molecular size distribution of both carbohydrate fractions was similar. The monosaccharide composition was also similar, except that the EDTA-extractable fraction contained a higher percentage of uronic acids, Carbohydrates produced during tidal emersion were rich in glucose and were rapidly turned over. [KEYWORDS: extracellular polymeric substances (EPS); polysaccharides; microphytobenthos; mud flat; monosaccharide composition; cryolander Polysaccharide hydrolysis rates; benthic microalgae; marine-sediments; epipelic diatoms; cohesive sediments; organic-carbon; chlorophyll-a; environments; assemblages; irradiance

Short-term dynamics in microphytobenthos distribution and associated extracellular carbohydrates in surface sediments of an intertidal mudflat