Measuring carbon and nitrogen bioassimilation, burial, and denitrification contributions of oyster reefs in Gulf coast estuaries
The eastern oyster (Crassostrea virginica) and the reefs they create provide significant ecosystem services. This study measured their possible role in nutrient mitigation through bioassimilation, burial, and oyster-mediated sediment denitrification in near-shore shallow water (< 1 m water depth) and deep-water (> 1 m water depth) oyster reefs in Louisiana. Nitrogen (N) and carbon (C) in shell and tissue differed by oyster reproductive status, size, and habitat type. Changes in tissue percent N and C post-spawning combined with significant reductions in tissue dry weight from the release of gametes, resulted in 20 and 46% reductions in tissue N and C load (mg), respectively, for a 100-mm oyster. Oyster reefs did not enhance burial rates, with burial range rates estimated at 1.4–2.6 g N m−2 year−1, and 26.9–43.8 g C m−2 year−1. Closed-system ex situ incubations indicated net denitrification in all habitat types studied, with the highest rates exceeding 600 µmol N m−2 h−1 during the summer, but no enhancement attributable to oyster reefs specifically. Within the highly productive, organic-rich wetland complex systems of coastal Louisiana, oyster reefs were not associated with enhanced denitrification, likely due to the organic-rich setting, and redundant supplies of organic nitrogen and carbon from adjacent marshes. Context remains critical in determining ecosystem provision of habitats, and efforts to extrapolate and predict nitrogen removal across locations necessitates consideration of local conditions. Considering the large extent of reefs and oyster production across coastal Louisiana, oyster habitats may still contribute to N and C mitigation, but their unique contribution likely comes from bioassimilation, and removal of the oysters from the system.
This work was funded through the Coastal Science Assistantship Program, through Louisiana Sea Grant and the Coastal Protection and Restoration Authority of Louisiana, as well as the Louisiana Department of Wildlife and Fisheries through the U.S. Geological Survey Louisiana Fish and Wildlife Cooperative Research Unit. Thanks to Dr. Chris Swarzenski for comments on an earlier version of this manuscript. Early discussion with Drs. Bryan Piazza and Michael Piehler contributed to the development of this project. Input and discussion with Drs. Austin Humphries, S. Ayvazian and Boze Hancock informed this work. Many thanks to Robert Twilley, Edward Castañeda, Andy Nyman, Jerome La Peyre, John White, Mike Kaller, Brett Collier, Kristy Capelle, Romain Lavaud, Christian Flucke, Songjie He, Scott Harlamert, Steven Madere, Jay Curole, and Tommy Blanchard for their help and expertise. Data are available through MKL. Reviews from three anonymous reviewers significantly improved this manuscript. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable international, national and/or institutional guidelines for sampling, care and experimental use of organisms for the study have been followed.
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