Sources of nitrogen for the vertical particle flux in the Gotland Sea (Baltic Proper)—results from sediment trap studies

Abstract

Between summer of 1996 and 1997, a continuous set of samples was collected in a sediment trap at 140-m water depth at a station in the central Gotland Sea. Fluxes of total sedimented matter, particulate organic carbon and nitrogen, biogenic silica and particulate phosphorus as well as the isotopic composition of organic carbon and nitrogen were estimated. The total vertical particles flux was found to be 35 g m −2 year −1 and fluxes of organic carbon, total nitrogen and phosphorus and biogenic silica amounted to 6.0, 0.8, 0.19 and 6.2 g m −2 year −1, respectively. Highest rates of vertical particle-transport for all compounds occurred in September 1996 and lowest fluxes during October–February. Based on the isotopic signature in the particulate nitrogen, fixation of atmospheric nitrogen by pelagic cyanobacteria could be identified as the major source for export production during the summer season. The isotopic composition of the spring sedimentation pulse indicates uptake of a mixture of laterally introduced and locally produced nitrate accumulated in the winter mixed layer during the non-growth season. The contribution of nitrogen fixation to the sedimentary nitrogen inventory in the central Gotland Sea is estimated to be in the range of 45%. This is consistent with nitrogen isotope data of surface sediments and budged calculations. Annual accumulation rates of organic carbon, nitrogen and phosphorus in the underlying sediments (at 241-m water depth) are two- to sevenfold higher than the measured vertical particle flux indicating laterally advected material as the main source for sediment formation in this depositional area. The stable isotope composition of organic carbon and nitrogen of surface sediments was comparable to a weighted annual mean from the sediment trap samples, implying the source of bulk sedimentary organic matter originates from the pelagial of the central Baltic Sea.