A plume river across the southern North Sea

Martien A. Baars, Dep. Biological Oceanography, NIOZ, Texel, The Netherlands

Satellite images show a marked plume of suspended matter from Norfolk stretched north-eastwards in the direction of the residual currents into the open Southern Bight. This East Anglian Silt Plume temporarily sediments at the transition on the Dutch Continental Shelf between the shallow (25 m) Texel and Vlieland Grounds and the deeper (50 m) Oyster Ground: the Frisian Front. The new program 'Plume & Bloom' studies the hydrography and plankton dynamics over this whole central area of the Southern Bight in order to test the hypothesis that English Coastal Water could be a nutrient source at the Frisian Front late in the productive season. Results of a series of cruises during July-December 2000, showed a robust hydrographic structure. In between the Central North Sea Water in the north and the tongue of Channel Water in the south, there was a marked 'river' of English Coastal Water. The 'river' was largely situated within the East Anglian Silt Plume. Turbidity in the Plume during summer was relatively low, and underwater light conditions were good enough for phytoplankton production. During moderate wind, diatoms grown in the English coastal waters became mineralised in the Silt Plume, as ammonia and silicate concentrations increased from west to east. In addition, periods with strong western winds, and consequently fast residual currents, seemed to generate pulses of UK river nutrients into the Silt Plume. The visual effect of this load of regenerated and/or new nutrients seem to depend on the wind regime. With strong SW winds, the 'river' ended well north in the Oyster Ground, where water column depth was too large to give dense blooms. With strong NW winds, the 'river' was pushed on the upper slope and the shallow sands south of the Frisian Front, and blooms developed there. Not at the Frisian Front proper, since turbidity there was generally too high, due to the tidal resuspension of the mud-rich slope zone.