Sediment-water oxygen and nutrient exchanges along a depth gradient in the Baltic Sea

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Abstract
Bottom water and sediment characteristics and net sediment-water fluxes of oxygen, nitrogen, phosphorus and hydrogen sulphide were measured under oxic and experimentally induced anoxic conditions along a depth gradient (47 to 130 m) during summer in the Baltic proper. Temperature, salinity and dissolved nutrient concentrations (particularly phosphorus and nitrate) in bottom waters increased with depth while oxygen concentrations decreased sharply. Sediment organic content was much higher in sediments located beneath the permanent pycnocline (ca 65 m). Sediments at all stations were somewhat depleted in total N relative to total C (C:N .apprxeq. 10). Sediments at the shallow station (47 m) were highly enriched with total P relative to C or N (C:N:P .apprxeq. 18:2:1) but were somewhat depleted in P at the deeper stations (C:N:P .apprxeq. 120:12:1). Under oxic conditions oxygen fluxes ranged from 214 to 777 .mu.mol O m-2 h-1 and decreased with depth. Phosphorus and nitrite fluxes were always very small and were directed either into or out of the sediments. Ammonium fluxes were small (1 to 30 .mu.mol N m-2 h-1) at all stations and did not exhibit a clear pattern with depth; O:N-NH4 flux ratios were close to expected ''Redfield ratios'' at the deep station (16:1) but were much higher (> 55:1) at the shallower sites indicating that processes other than only ammonification were taking place in sediments. Nitrate was always consumed by sediments (ca 1 to 16 .mu.mol N m-2 h-1) and fluxes were proportional to nitrate concentrations in the overlying water. Under anoxic conditions there was a dramatic increase in P-PO4 fluxes (2 to 40 .mu.mol P m-2 h-1) and a smaller increase in N-NH4 fluxes (14 to 35 .mu.mol N m-2 h-1). Large hydrogen sulphide fluxes (> 40 .mu.mol S m-2 h-1) were observed in sediments from the deepest station only. Under oxic conditions sediment recycling of N and particularly of P were small compared to estimated rates of burial of P and burial plus denitrification of N. However, under anoxic conditions, sediment recycling of both N and P were similar to or much greater than sediment loss terms of burial and denitrification. Sediment regeneration under oxic conditions could supply 1 to 8% and 0 to 2% of estimated phytoplankton demand of N and P, respectively; under anoxic conditions 12% of N and up to 200% of P demand could be met via sediment recycling.