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 tempora l variation in the supply of fresh organic matter to the benthos and its su bsequent metabolic reaction. Stations were chosen on the basis of differenc es in sediment type, depth and depositional regime. Sediments were sampled, sliced and analysed for phytopigments, fatty acids and nucleic acids. On-d eck sediment core incubations provided the sediment oxygen demand (SOD). Fa tty acids as well as chlorophyll a indicated a seasonal variation of phytod etritus sedimentation, with highest values in May, reflecting the spring bl oom sedimentation. The two stations in the depositional areas German Eight and Skagerrak had the highest input of algal detritus into the sediment. Th ese stations also contained higher concentrations of bacterial and long-cha in fatty acids, probably related to the relatively higher proportion of fin e 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 th e benthic metabolic reaction to the labile organic matter supply. The three stations in the southern and south-eastern North Sea had a SOD congruent w ith the phytodetrital inventories, with highest SOD measured at the German Eight. Surprisingly, the Skagerrak sediment did not have a high oxygen dema nd, although it contained the highest inventory of labile organic matter an d high RNA concentrations. The fluxes of carbon to the sediment at station SK, calculated from the chlorophyll a inventory, were about four times high er than the sediment oxygen demand. We hypothesise two mechanisms for this discrepancy; one is the decreased mineralisation rate of labile organic mat ter due to sorption to clayey sediment particles, the other is advection of labile organic matter from the shallower coastal waters of northern Denmar k to and subsequent removal from the deeper Skagerrak sediments by periodic ally occurring high near-bottom water currents. (C) 1999 Academic Press.