Moored current meters, fluorom eters and transmissometers were used in combination with sediment traps (aspect ratio >4) and shipborne sampling to determine fluxes of deposition and resuspension of total suspended matter (TSM) under tidal action in the 45 m deep Oyster Grounds, North Sea. Here, we present data from the mixed layer below the major thermocline at about 20 m above the bottom (mab) as obtained during a 14-day period of calm weather in July 1994. Around neap tide near-bottom current velocities remained smaller than 0.15 m s-1 and TSM was dominated by particles advected from a relatively turbid area to the southeast of the study site. At the onset of spring tide, current speeds increased with maximum values greater than 0.20 m s-1 and seabed friction velocities exceeding the threshold value for resuspension. Particles resuspended were strongly enriched with organic carbon compared to the bulk sediment, suggesting that not the bed proper but a fine-grained fluff fraction was eroded. This resuspended fluff was by far the dominant source for the mass fluxes in the sediment trap (at 3.2 mab), which showed a distinct tidal cycle with highest fluxes directly after low water slack tide and lowest fluxes during maximum ebb current. This pattern was caused by variations in apparent settling velocity of TSM, presumably due to floc formation during periods in the tidal cycle when current speeds were low and relatively high concentrations of both chlorophyll-a and TSM were found. From a simple model on advection, deposition and resuspension of TSM, we calculated a net accumulation on the sediment of 75 g m-2 during the 14-day study period, which is the difference between gross fluxes of deposition and resuspension. Upon deposition, the average retention time of particles until their next resuspension is calculated at 1-2 weeks, which may be sufficient for substantial decomposition of organic matter associated with TSM. This implies that, upon resuspension, particles transported further along the shelf are relatively poor in organic carbon. It is concluded that the Oyster Grounds serve as a mid-shelf temporary depocenter and that mineralization in this and similar areas may play a crucial role in the carbon budget of the North Sea.
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