Influence of particle mixing on vertical profiles of chlorophyll a and bacterial biomass in sediments of the German Bight, Oyster Ground and Dogger Bank (North Sea)

In May and September 1999 11 stations were sampled in the southern and cent ral North Sea, located in the German Bight, eastern Oyster Ground and Dogge r Bank. The study focused on the influence of particle mixing on transport of chlorophyll a to deeper sediment layers and vertical bacterial distribut ion (max. depth = 10 cm). The sampling stations were chosen to reflect a gr adient in environmental conditions in the North Sea. The sampling stations differed in respect to redox potential (eH up to -243 mV in the German Bigh t and up to 274 mV in the offshore regions), silt content (up to 54% in the German Bight and 0.34% at the northern Dogger Bank) and different proporti on of fresh organic material on total organic matter content (C/N ratios ra nging from 9.27 in the German Bight up to 1.72 in the offshore sediments). Although bacterial densities (8.55 x 10(9) g(-1) in the German Bight up to 0.35 x 10(9) g(-1) in offshore sediments) were significantly correlated to chlorophyll a content in the sediment (P < 0.01), inconsistencies in the te mporal pattern of both variables in the surficial sediment layer suggested, that the dynamics of bacterial densities is generally controlled by food s upply but also by other variables. The chlorophyll a content in the surfici al sediments of the German Bight (up to 1.84 <mu>g g(-1)) was significantly higher than in the Oyster Ground (up to 0.58 mug g(-1)) and the Dogger Ban k area (up to 0.68 mug g(-1)). With increasing chlorophyll a input to the b enthic realm a subsequent enhanced burial of this compound into deeper sedi ment layers was expected either by biological (bioturbation) or by physical sediment mixing. However, the vertical profile of chlorophyll a decreased steeply in the sediments of the German Bight. Contrary, subsurface peaks we re measured in the offshore areas. It was concluded from these results, tha t the vertical distribution of organic matter in sediments is less limited by the quantitative input from the water column but concomitant with partic le mixing itself. The extent and possible mechanisms of particle mixing in the different study areas in relation to specific environmental factors is discussed. (C) 2001 Academic Press.