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Tidal resuspension and deposition of particulate matter in the Oyster Ground, North Sea
Van Raaphorst, W.; Malschaert, J.F.P.; van Haren, J.J.M. (1996). Tidal resuspension and deposition of particulate matter in the Oyster Ground, North Sea, in: (1991). Proceedings of the International Symposium on Hydro- and Aerodynamics in Marine Engineering HADMAR '91, Varna, Bulgaria, 28 October - 1 November 1991. pp. 1-49
In: (1991). Proceedings of the International Symposium on Hydro- and Aerodynamics in Marine Engineering HADMAR '91, Varna, Bulgaria, 28 October - 1 November 1991. Bulgarian Ship Hydrodynamics Centre: Varna. 2 vol. pp., more

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Document type: Project report

Keyword
    Marine

Authors  Top 
  • Van Raaphorst, W.
  • Malschaert, J.F.P.
  • van Haren, J.J.M.

Abstract
    Moored instruments (current meters, fluorometers and transmisson meter) were used in combination with sediment traps (aspect ratio >4) operating at hourly opening intervals and shipborn sampling to determine the fluxes of deposition and resuspension of Total Suspended Matter(TSM) under influence of tidal action in the 45 m deep Oyster Ground, North Sea. Here, we present data from the mixed layer below the major thermocline at 20 m above the bottom (mab) obtained during a 14 days period with calm weather in July 1994. Around neap tide near bottom current velocity remained <0.15 m.s-1 and sea bed spring tide current speed increased with maximum values >0.20 m.s-1 and TSM was dominated by slow settling of particles advected from a relatively turbid area south-east of the study site. At the onset of spring tide current speed increased with maximum values > 0.20 m.s-1 and sea bed friction velocities exceeding the threshold 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 dominating source to the mass fluxes in the sediment trap (3.2 mab), which showed a distinct tidal cycle with highest fluxes directly after low matter 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 forming at periods in the tidal cycle with low current speed and relatively high concentrations of both chlorophyll-a and TSM. 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 days study period, being the difference between simultaneously operating gross fluxes of deposition and resuspension. Upon deposition the average retention time of particles until their next resuspension and further transport across the shelf is calculated at 1-2 weeks, which may be sufficient for substantial decomposition of organic matter associated with TSM. The model showed that the saltative particle exchange between the sea and the water column is intensified by storms, resulting in net erosion during the storms and bulk resettling thereafter. It is concluded that the Oyster Ground serves as a mid-shelf temporary depocentre and that mineralisation at this and similar areas may pay a crucial role in the carbon budget of the North Sea.

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