Table of Contents

• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Parameters
• Instruments
• Project Information
• Funding

Closed Oyster Mesocosm 15N Tracer

¹⁵N labeled phytoplankton was continuously pumped for three days into 30.5 cm diameter mesocosms  containing  10 cm of sediment and 20 liters of overlying water.   Three mesocosms contained twelve 7-cm long oysters (O1, O2, O2) and three mesocosms had no oysters and served as controls (C1,C2,C3). Mesoscosms were flow through with a 0.3 day residence time and open to the atmosphere. After phytoplankton feeding period, oysters were removed and mesocosms allowed to flush for 3 days.  Tank feed water was then stopped and mesocosms sealed. Time series N₂O, ¹⁵N₂ , and sediment ¹⁵N were measured during the period when the mesocom was sealed until the dissolved oxygen reached 2 mgL-1.  Then the mesocosms were reopened and the feed water restarted. This first incubation ID is CS1.  The mesocosms functioned in flow through mode for several days (rest period) and then the incubation / measurement procedure was repeated.  This second incubation period ID is CS2.  After CS2, the mesocosms were reopened and put into flow through.  In total there were 5 closed system incubation periods.  The following ‘rest periods’ (hours) were established between CS1-2, 2-3, 3-4, 4-5: 49 hours, 90 hours, 110.5 hours, 138.5 hours.

Water samples for N₂O analysis were collected with a peristaltic pump through a syringe needle directly into 12 ml exetainer that had been flushed with N₂ and preserved with KOH to a pH above 12. Approximately six ml sample was collected. N₂O concentrations in the headspace were measured on a GC-ECD.  Water samples for 15N2 samples were collected with a peristaltic pump through a syringe needle directly into 30 ml serum bottles that had been flushed with He and preserved with KOH to a pH above 12.   Approximately eight ml sample was collected. ¹⁵N₂ was analyzed by GC - Isotope Ratio Mass Spectrometry (IRMS). Sediment ¹⁵N samples were collected from the mesocoms using a 2cm diameter core.  The top 3 cm was retained for analysis.

N₂O concentrations were calculated from N₂O calibration curve and corrected for N₂O solubility in the aqueous phase using the Bunsen coefficient. ¹⁵N₂ was normalized to air and air saturated water standards and reported in the delta notation. Sediment ¹⁵N values were normalized to reference materials and reported in the delta notation.

BCO-DMO Processing:

• added conventional header with dataset name, PI name, version date
• modified parameter names to conform with BCO-DMO naming conventions

Extracted from the NSF award abstract:

Oyster reefs are biogeochemical hot spots and prominent estuarine habitats that provide disproportionate ecological function. Suspension-feeding eastern oysters, Crassostrea virginica, are capable of improving water quality and diminishing eutrophication by filtering nutrients and particles from the water and depositing them in the sediments. Remineralization of these deposits may enhance sedimentary denitrification that facilitates nitrogen removal in tidal estuaries. However, the scientific underpinning of oyster reef function has been challenged in various studies. In addition, recent studies of filter feeding invertebrates reported the production of nitrous oxide (N2O), a greenhouse gas, as an end product of incomplete denitrification by gut microbes. C. virginica could be another source of N2O flux from intertidal habitats. Preliminary work indicated substantial N2O production from individual oysters. The estimated N2O production from high density oyster reefs may exceed the N2O flux measured from some estuaries. With the new discovery of N2O emission and uncertainty regarding eutrophication control, the ecological value of oyster reef restoration may become equivocal.

This project will quantify N2O fluxes to understand the factors controlling N2O emission from oyster reefs. Sedimentary N processes will be examined to develop an oyster reef N model to estimate N2O emission from tidal creek estuaries relative to other N cycling processes. The PIs hypothesize that intertidal oyster reefs are a substantial source of N2O emission from estuarine ecosystems and the magnitude of emission may be linked to water quality. If substantial N2O flux from oyster reefs is validated, ecological benefits of oyster reef restoration should be reevaluated. This interdisciplinary research team includes a microbial ecologist, a biogeochemist, an ecologist and an ecosystem modeler. They will utilize stable isotope and molecular microbiological techniques to quantify oyster N2O production, elucidate microbial sources of N2O emission from oysters and sediments, and estimate seasonal variation of N2O fluxes from oyster reefs. Measurements from this study will be integrated into a coupled oyster bioenergetics-sediment biogeochemistry model to compare system level rates of N cycling on oyster reefs as a function of oyster density and water quality. Modeling results will be used to assess the relative trade-­offs of oyster restoration associated with N cycling. They expect to deliver the following end products:1) estimation of annual N2O flux from oyster reefs as an additional source of greenhouse gases from estuaries, 2) a better understanding of the environmental and microbial factors influencing N2O and N2 fluxes in tidal estuaries, 3) transformative knowledge for the effect of oyster restoration on water quality enhancement and ecosystem function, 4) direct guidance for oyster restoration projects whose goals include water quality enhancement, and 5) a modeling tool for use in research and restoration planning.