DENITRIFICATION IN THE CENTRAL BALTIC - EVIDENCE FOR H2S-OXIDATION AS MOTOR OF DENITRIFICATION AT THE OXIC-ANOXIC INTERFACE

Denitrification was investigated in August 1986 and July 1987 in the water column of the Gotland Deep, an anoxic basin in the Baltic Proper. Denitrification rates were determined by means of the acetylene blockage method in the whole water column with emphasis on the low-oxygen water above the H2S-containing deep water. Denitrification was restricted to a layer of about 10 m at the oxic-anoxic interface (130 m depth), where NO3- and H2S coexisted. Rates in 1986 and 1987 were 110 and 44 nmol Nl-1 d-1 respectively. Samples from above the interface never showed denitrification, even after incubation periods of up to 12 d, obviously due to the combined effect of too high oxygen and too low carbon concentrations. Addition of nitrate to all samples enhanced denitrification only for H2S-containing water samples from the anoxic deep water. The coincidence of electron donor availability and H2S-concentration substantiated the hypothesis that reduced sulfur compounds could be valuable electron donors for denitrification by the bacterial community at the oxic-anoxic interface. Addition of sulfide or thiosulfate to water samples from just below the interface resulted in immediate denitrification of the added nitrate, with concomitant growth of the bacterial community. This indicates that denitrification in the oxic-anoxic interface could be efficiently driven by oxidation of reduced sulfur compounds provided by the anoxic deep water. Conceivable ecological implications of these findings are uncoupling of denitrification from carbon flux derived from phytoplankton primary production and strong dependence on mixing processes at the oxic-anoxic interface.