Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 6 7 2009 10.5194/bg-6-1149-2009 http://www.biogeosciences.net/6/1149/2009/ http://www.biogeosciences.net/6/1149/2009/bg-6-1149-2009.html http://www.biogeosciences.net/6/1149/2009/bg-6-1149-2009.pdf 1149 1165 2009-07-10 Temporal variations in microbial activities and carbon turnover in subtidal sandy sediments S. I. Böer sboeer@mpi-bremen.de C. Arnosti J. E. E. van Beusekom A. Boetius Microbial Habitat Group, Max Planck Institute for Marine Microbiology, Bremen, Germany Dept. of Marine Sciences, University of North Carolina-Chapel Hill, Chapel Hill, NC, USA Wadden Sea Station Sylt, Alfred Wegener Institute for Polar and Marine Research, List/Sylt, Germany School of Engineering and Science, Jacobs University Bremen, Germany Temporal dynamics and vertical patterns in bacterial abundances and activities were studied in a shallow subtidal sand flat in the Sylt-Rømø Basin (North Frisian Wadden Sea, Germany). Extracellular enzymatic activities, bacterial carbon production and community respiration showed strong (factor of 4–5) temporal variations that were mostly related to seasonal temperature change and to changes in substrate availability. These temporal patterns in enzymatic activity were barely reflected in bacterial (200–400 mmol C m^−2) and microphytobenthic biomass (800–1500 mmol C m^−2) or the sedimentary carbohydrate inventory (1300–2900 mmol C m^−2), suggesting that grazing controls the standing stocks of the microphytobenthic and bacterial assemblages. Despite their exposure to strong hydrodynamic forces such as tidal currents and wind-induced wave surge, the subtidal sandy sediments showed persistent vertical gradients in bacterial abundances, carbon production and extracellular enzymatic activities at all times. The vertical distribution of these parameters was tightly coupled to that of the microphytobenthos, dominated by diatoms. 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