Benthic organic matter supply and metabolism at depositional and non-depositional areas in the North Sea

In 1994, four stations in the southern North Sea, the German Eight and the Skagerrak were visited with the aim to get insight into spatial and temporal variation in the supply of fresh organic matter to the benthos and its subsequent metabolic reaction. Stations were chosen on the basis of differences in sediment type, depth and depositional regime. Sediments were sampled, sliced and analysed for phytopigments, fatty acids and nucleic acids. On-deck sediment core incubations provided the sediment oxygen demand (SOD). Fatty acids as well as chlorophyll a indicated a seasonal variation of phytodetritus sedimentation, with highest values in May, reflecting the spring bloom sedimentation. The two stations in the depositional areas German Eight and Skagerrak had the highest input of algal detritus into the sediment. These stations also contained higher concentrations of bacterial and long-chain fatty acids, probably related to the relatively higher proportion of fine particles in the sediment and to the input of terrestrial organic matter and/or refractory marine organic matter, respectively. The metabolic indices, viz. the SOD and the RNA concentrations reflected the benthic metabolic reaction to the labile organic matter supply. The three stations in the southern and south-eastern North Sea had a SOD congruent with the phytodetrital inventories, with highest SOD measured at the German Eight. Surprisingly, the Skagerrak sediment did not have a high oxygen demand, although it contained the highest inventory of labile organic matter and high RNA concentrations. The fluxes of carbon to the sediment at station SK, calculated from the chlorophyll a inventory, were about four times higher than the sediment oxygen demand. We hypothesise two mechanisms for this discrepancy; one is the decreased mineralisation rate of labile organic matter due to sorption to clayey sediment particles, the other is advection of labile organic matter from the shallower coastal waters of northern Denmark to and subsequent removal from the deeper Skagerrak sediments by periodically occurring high near-bottom water currents. (C) 1999 Academic Press.