Temporal variations in microbial activities and carbon turnover in subtidal sandy sediments

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 5 were mostly related to seasonal temperature change, but also to changes in substrate availability. These temporal patterns in activity were barely reflected in bacterial (200–400mmolCm−2) and microphytobenthic biomass (800–1500mmolCm−2) or the sedimentary carbohydrate inventory (1300–2900mmolCm−2), suggesting that grazing controls the standing stocks of the microphytobenthic and bacterial assemblages. 10 Despite their exposure to strong hydrodynamic forces such as tidal currents and windinduced wave surge, the subtidal sandy sediments showed persistent vertical gradients in bacterial abundances, bacterial 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. Despite the low organic carbon 15 content typical for surge-exposed sandy sediments, high extracellular enzymatic activities and bacterial carbon production rates indicate a very active heterotrophic bacterial community, with a gross secondary productivity of 30–180mmolCm−2, and a biomass turnover time of 2–18 days. Our data suggest that this high activity is supported by the rapid flux of carbohydrates from microphytobenthic primary productivity. Accordingly, 20 the potential activities of enzymes hydrolyzing carbohydrates cover most of the total bacterial carbon demand during all seasons.