Impact of marine fish cage farming on metabolism and sulfate reduction of underlying sediments

Total sediment metabolism (measured CO2 production across the sediment-water interface) and sulfate reduction (measured by 35S technique) was examined in the organic rich sediments around a marine fish cage farm in shallow Danish waters Sediment metabolism beneath the net cages was about 10 times higher during the farming period (525 to 619 mm01 CO2 m-' dl ) than at an unaffected control station (24 to 70 mm01 CO2 m-2 d-l ) . Depth-integrated sulfate reduction rates (0 to 10 cm) beneath the net cages (234 to 310 mm01 m-2 d' ) could support 75 to 118 O/O of the measured CO2 production across the sediment-water interface. At the end of the farming period and during winter (no fish farming), sediment metabolism and sulfate reduction rates decreased considerably (33 to 77 mm01 CO2 m-"-'), but both rates were still elevated compared to the control station, indicating that the impact of fish farming on the anaerobic mineralization was prolonged. Nearly all reduced 3 5 ~ label was recovered in the acid volatile fraction (AVS). During decreasing sulfate reduction rates, however, the chromium reducible fraction (CRS) became more important in the upper oxidized sediment layers. Pore water profiles of mineralization products (HC03and NH,') reflected the rapid decomposition and showed a preferential regeneration of nitrogen throughout fall and early spring.

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