Oxygenation of an anoxic fjord basin strongly stimulates benthic denitrification and DNRA
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Hypoxia hampers eutrophication reduction efforts by enabling high nutrient fluxes from sediment to bottom waters. Oxygenation of hypoxic water bodies is often proposed to reduce benthic ammonium and phosphate release. This study investigates the functional response of benthic nitrate-reducing processes to a long-term engineered oxygenation effort in a density-stratified fjord with euxinic bottom waters. Oxygenation was achieved by mixing surface water with deep, euxinic water, which increased oxygen and nitrate concentrations in the deep water column. The presence of nitrate instigated benthic nitrate reduction in the newly oxidized sediments by equally stimulating denitrification and dissimilatory nitrate reduction to ammonium (DNRA). DNRA and total nitrate reduction rates, as well as the contribution of DNRA to total nitrate reduction, decreased with increasing exposure time of the sediments to oxygen. The relative importance of DNRA as a nitrate sink was correlated to nitrate concentrations, with more nitrate being reduced to ammonium at higher bottom water nitrate concentrations. Overall, engineered oxygenation decreased the net efflux of dissolved inorganic nitrogen from the sediments by stimulating net nitrate removal through denitrification.
KeywordsDissimilatory nitrate reduction Denitrification DNRA Anammox Engineered oxygenation Hypoxia By Fjord
We thank Anders Sjösten (Department of Ecology, Environment and Plant Sciences, Stockholm University) and Lars Ljungqvist (Sven Lovén Centre for Marine Sciences, University of Gothenburg) for nutrient analyses; Dariia Atamanchuk and Madeleine Nilsson for help during cruises; and the crew of the R/V Skagerak (University of Gothenburg) for technical and logistic assistance at sea. Constructive criticism from three anonymous reviewers helped us improve the submitted manuscript. Financial support for this study came from the Swedish Environmental Protection Agency (NV 08/307 F-255-08) through the project BOX (A pilot study to evaluate effects of possible Baltic deep water OXygenation), the Agouron Institute, and the Danish National Research Foundation (DNRF53).
This study was funded by the Swedish Environmental Protection Agency (Grant Number NV 08/307 F-255-08).
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Statement of human and animal rights
This research did not involve human participants or animals.
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