Sediment mineralization, nutrient fluxes, denitrification and dissimilatory nitrate reduction to ammonium in an estuarine fjord with sea cage trout farms

Abstract

Sediment oxygen consumption, nutrient fluxes across the sediment-water interface, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) were measured in sediment below the cages of a rainbow trout farm in the estuary of Horsens Fjord, Denmark, and compared to that in the sediment at reference stations elsewhere in the fjord. Denitrification and DNRA were measured by the isotope pairing technique and a 15N technique, respectively. The measurements are the first published data of in situ activities in sediment below marine fish farms. Oxygen consumption by the sediment and effluxes of NH4+ and PO43- were markedly stimulated by organic matter loading from the overlying fish cages. Denitrification of nitrate from the water column (Dw) was only of significance during autumn and winter when runoff from land resulted in high water column nitrate concentrations. In contrast, coupled nitrification-denitrification (Dn) occurred in the sediment throughout the summer, with the rates in the sediment below the fish cages surprisingly being of the same magnitude as those at the reference stations. Competition for inorganic nitrogen from benthic microalgae probably inhibited the activity of both nitrifiers and denitrifiers at the reference stations, while a mat of Beggiatoa spp. on the sediment surface below the fish cages in late summer indicated the presence of sulfide in the uppermost layers of the sediment, which almost ceased Dn. While DNRA was absent in sediment unaffected by the trout farms, it was of quantitative importance in the reduced sediment below the fish cages, DNRA activity being up to 7-fold greater than denitrification activity. The ecological consequence of this shift in the relative importance of the processes in response to organic matter loading was a reduced nitrogen removal by denitrification and an increased efflux of NH4+ to the water column, resulting in stimulation of pelagic primary production and increased nitrogen retention by the ecosystem. The excess nitrogen input to the fjord from the trout farms corresponded to approximately 12% of the total nitrogen load from land during the summer months. Only 0.1% of this additional nitrogen input was removed by denitrification in the underlying sediments during the fish production season. In general, shallow estuarine ecosystems hosting fish farms may therefore face increasing eutrophication problems due to the direct supply of nutrients in the fish food and to shifts in the dominant microbial processes in the sediment below the fish cages.