The biogeochemistry of oyster restoration: Initial conditions determine potential mitigation

Background/Question/Methods

Oyster restoration projects in lower Alabama may have ancillary benefits by indirectly decreasing nitrogen (N) loading in near shore areas through biodeposits to the sediments that stimulate microbial denitrification.  The overall objective of the study was to quantify the ability of the eastern oyster (Crassostrea virginica) to mitigate eutrophication by indirectly removing excess N from the system.  The potential of oysters at typical restoration densities to act as bioremediators was assessed from triplicate sediment cores from three treatments (control, juvenile, and adult) at two sites in Bon Secour Bay, Alabama.  Sediments were analyzed for potential nitrification, denitrification, and N₂ fixation activity using slurry incubations and net N₂ flux using a flow-through system and membrane inlet mass spectrometer (MIMS) at the beginning of the experiment and again after 3 months of oyster deployment.  Hydrogen sulfide (HS^-) and oxygen (O₂) profiles were made from additional cores using microelectrodes and a UniSense multimeter.  The relationship between oyster treatments and N removal was assessed using ANOVA statistics.  

Results/Conclusions

At the East site, the juvenile sediment N₂ uptake (-41.6 ± 0.44 μmol N m^-2 hr^-1) had increased 300%, relative to the control (-10.4 ± 0.77 μmol N m^-2 hr^-1), while the adult uptake (-1.6 ± 0.39 μmol N m^-2 hr^-1) had decreased 85%.  At the West site, the juvenile treatment had 32% higher N₂ efflux (11.6 ± 0.25 μmol N m^-2 hr^-1) relative to the control (8.8 ± 0.38 μmol N m^-2 hr^-1) but the adult efflux was 94% less (0.5 ± 0.25 μmol N m^-2 hr^-1).  The contrasting pattern was caused by different initial redox conditions.  The juvenile treatment at the East site had higher HS^- relative to the control treatment (391 ± 0.81 and 232 ± 0.44 μM, respectively) while in the adult treatment HS^- was undetectable.  In contrast, at the West site, HS^- was not detectable by the study end.  Inhibition of nitrification by HS^- and N₂ fixation by sulfate reducers at the East site caused net N₂ uptake regardless of treatment.  These results indicate that when not HS^- inhibited, associated oyster biodeposits stimulated N removal.  This study suggests that the potential for oyster restoration to remediate excess N depends on initial redox conditions.