Mineralization of organic matter and nitrogen transformations in the Gotland Sea deep water

Abstract

Measurements of the concentrations of total CO 2 (C T), O 2, H 2S, NO 2 −, NO 3 −, and NH 4 + were performed between 2003 and 2007 during intervals of 2–3 months in the deep water of the Gotland Basin (Baltic Sea). The data were used to determine mineralization and denitrification rates in four sublayers below 150 m during a period of stagnation. To account for the vertical mixing at the boundaries of the sublayers, mixing coefficients were determined based on the temporal changes of the salinity profiles. Mass balances were established for each measured variable and yielded the sink/source rates in the individual sublayers for each time interval. The seasonality of mineralization rates was confined to water layers that were in close contact with the sediment surface, indicating that the mineralization took place mainly at the sediment surface. Moreover, it was shown that the mineralization rates did not depend on the redox conditions in the water column. The total mineralization rate in the basin was found to be 2.0 mol m –2 yr –1, thereby implying that about 80% of the particulate organic carbon input occurred by lateral transport. A redox balance indicated that manganese dioxide and possibly iron oxides contributed to the oxidation of organic matter by about 13%. Intense denitrification was observed during the transition from oxic to anoxic conditions. A value of 456 mmol–N m –2 yr –1 was found, which included a possible anammox contribution of about 5%.