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PO4 release at the sediment surface under anoxic conditions: a contribution to the eutrophication of the Baltic Sea?

Schneider B.

Oceanologia

2011

No. 53 (1-TI)

415-429

The vertical profiles of phosphate, total CO2 and oxygen/hydrogen sulphide were determined in the deep water of the Gotland Sea during March 2003 to July 2006 with a temporal resolution of 2-3 months. This time span included the shift from anoxic to oxic conditions resulting from a water renewal event, as well as the transition back to anoxic waters during the subsequent two-year stagnation period. The data from depths below 150 m were used to identify and quantify phosphate release and removal processes. The relationship between the total CO_2 generated by mineralization (CT, min) and the PO_4 concentrations indicated that the initial decrease in the phosphate concentrations after the inflow of oxygen-rich water was mainly a dilution effect. Only about one third of the PO_4 removal was a consequence of the precipitation of insoluble iron-3-hydroxo-phosphates (Fe-P), which occurred slowly at the sediment surface under oxic conditions. From the CT, min/PO4 ratios it was also concluded that the formation of Fe-P was reversed during the later phase of the stagnation, when the redoxcline approached a depth of 150 m. A phosphate mass balance was performed for four deep water sub-layers in order to quantify the dissolution of Fe-P during the stagnation period and thus to estimate the amount of Fe-P deposited during the last inflow of oxygen-rich water. A value of about 50 mmol-P m-2 was found, which refers to the specific biogeochemical conditions during the change from anoxic to oxic conditions that preceded the stagnation period.