Journal
ESTUARIES AND COASTS
Volume 41, Issue 4, Pages 1130-1146Publisher
SPRINGER
DOI: 10.1007/s12237-017-0327-x
Keywords
Denitrification; DNRA; Oyster aquaculture; Crassostrea virginica; Eutrophication
Funding
- National Oceanic and Atmospheric Administration, U.S. Department of Commerce [NA14OAR4170093]
- National Science Foundation (Virginia Coast Reserve Long Term Ecological Research grant) [DEB-1237733]
- Virginia Water Resources Research Center
- David H. Smith Conservation Research Postdoctoral Fellowship Program
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Filter-feeding bivalves, like oysters, couple pelagic primary production with benthic microbial processes by consuming plankton from the water column and depositing unassimilated material on sediment. Conceptual models suggest that at low to moderate oyster densities, this deposition can stimulate benthic denitrification by providing denitrifying bacteria with organic carbon and nitrogen (N). While enhanced denitrification has been found at oyster reefs, data from oyster aquaculture are limited and equivocal. This study measured seasonal rates of denitrification, as well as dissimilatory nitrate reduction to ammonium (DNRA), and dissolved inorganic N fluxes at a rack and bag eastern oyster (Crassostrea virginica) aquaculture farm. Consistent with models, denitrification was enhanced within the farm, with an average annual increase of 350% compared to a reference site. However, absolute denitrification rates were low relative to other coastal systems, reaching a maximum of 19.2 mu mol m(-2) h(-1). Denitrification appeared to be nitrate (NO3-) limited, likely due to inhibited nitrification caused by sediment anoxia. Denitrification may also have been limited by competition for NO3- with DNRA, which accounted for an average of 76% of NO3- reduction. Consequently, direct release of ammonium (NH4+) from mineralization to the water column was the most significant benthic N pathway, with seasonal rates exceeding 900 mu mol m(-2) h(-1) within the farm. The enhanced N processes were spatially limited however, with significantly higher rates directly under oysters, compared to in between oyster racks. For commercial aquaculture farms like this, with moderate oyster densities (100-200 oysters m(-2)), denitrification may be enhanced, but nonetheless limited by biodeposition-induced sediment anoxia. The resulting shift in the sediment N balance toward processes that regenerate reactive N to the water column rather than remove N is an important consideration for water quality.
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