Organic matter degradation and biogenic element cycling in a nearshore sediment (Kiel Bight)1

Abstract

Benthic degradation of organic matter and the cycling of its C, N, P components was investigated at a coastal station (20 m) in Kiel Bight (western Baltic). Annual rates of carbon combustion and nutrient release from the sediment were derived from a comparison of input and burial of organic matter in the top sediment layers and from sediment oxygen consumption and nutrient release rates as determined by in situ enclosure experiments (2.9 mol O₂·m−² yr ¹, 0.16 mol N·m⁻²·yr⁻¹, 0.020 mol P·m⁻²·yr⁻¹, and 0.58 mol Si·m⁻²·yr⁻¹). A long term incubation was used to simulate chemical changes during the transition to anoxic conditions. The comparison of diffusive fluxes calculated from porewater profiles with total release rates indicates that the sediment surface is the major site of organic matter degradation, as a consequence of low sedimentation rate. From the organic matter reaching the bottom via sedimentation only 22% of the organic carbon (13% N_org and 15% P_org) is accumulated below 10 cm as sedimentary carbon and its reduced sulfur equivalent. While 75% of the carbon and 66% of the organic phosphorus input are returned to the water column, only half of the nitrogen input is released from the bottom; the rest is probably lost due to denitrification. Benthic flux contributes <20% to the annual P and N requirements for the pelagic primary producers.