Dynamics of nutrients and oxygen/hydrogen sulfide in the Baltic Sea deep water
Bo G. Gustafsson, Anders Stigebrandt
Journal of Geophysical Research: Biogeosciences·2007·48 citations
The water in the deepest basins of the Baltic Sea is only intermittently renewed. The interval between water renewals, so‐called stagnation periods, are typically several years during which only turbulent diffusive exchange of matter occurs with overlying water. The diffusive fluxes of nutrients, oxygen, and hydrogen sulfide through the 150‐m level in the Eastern Gotland deep basin were computed from high‐quality hydrographic and hydrochemical observations. Comparing diffusive fluxes with observed storage changes in the basin water, the residual corresponding to the net source or sink due to biogeochemical transformations could be obtained. It is found that decomposition rates of organic matter vary by a factor of about 3.5 mostly owing to variations in redox conditions. On average, 15.8 gC m−2 yr−1 are decomposed in the deep basin. No trend is found in the investigated period 1965–2005. The variability of the phosphate source is dominated by release of dissolved phosphate from particulate iron‐bound phosphate at oxic to anoxic transitions. It is found that the deep sediments release about 3 gP m−2 when overlying water turns anoxic. Denitrification rates are on average 2.6 gN m−2 yr−1, but also these are highly variable depending on redox conditions. By adding observed organic matter accumulation in the sediments to the estimated organic matter decomposition, we conclude that the supply of organic matter through the 150 m horizon is about 30.6 gC m−2 yr−1. Further, the results corroborate that the nutrient content of the organic matter supply follows approximately the Redfield ratio.