Citation:

Humphries, A., S. AYVAZIAN, J. CAREY, B. Hancock, S. Grabbert, D. J. COBB, C. J. STROBEL, AND R. Fulweiler. Directly measured denitrification reveals oyster aquaculture and restored oyster reefs remove nitrogen at comparable high rates. Frontiers in Marine Science. Frontiers, Lausanne, Switzerland, 3(74):doi: 10.3389/fmars.2, (2016).

Impact/Purpose:

Coastal systems are increasingly impacted by over-enrichment of nutrients which has cascading effects for ecosystem functioning. Oyster aquaculture and restoration are hypothesized to mitigate excessive nutrients via assimilation, burial or benthic denitrification studies. The values provided by this experimental study in Ninigret Pond, RI will be important for managers as they incorporate oysters into nutrient reduction strategies.

Description:

Coastal systems are increasingly impacted by over-enrichment of nutrients, which has cascading effects for ecosystem functioning. Oyster restoration and aquaculture are both hypothesized to mitigate excessive nitrogen (N) loads via benthic denitrification (DNF). However, this has not been extensively examined in New England, a place where nutrient runoff is high and increasing oyster (Crassostrea virginica) restoration and aquaculture activity is taking place, oftentimes in close proximity to one another. In this study, we develop a novel in situ methodology and quantify net N2 and O2 fluxes across the sediment-water interface in a shallow (~1 m) coastal pond in southern Rhode Island. We sampled seasonally during 2013 and 2014 at restored oyster reefs, oyster aquaculture, oyster cultch (shell), and reference (control) sediment. Restored oyster reefs and aquaculture enhanced sediment DNF rates by 3638 and 2723 mmol N2 - N m-2 y-1, respectively, but the placement of oyster cultch only enhanced sediment DNF rates by 271 mmol N2 - N m-2 y-1. The increase in DNF rates was dependent on season, with the greatest rates measured in the fall. Sediment oxygen demand (SOD) followed similar patterns but with greater rates in the summer, and displayed a strong linear relationship with DNF (slope = 0.019). Overall, our results demonstrate that oyster reef restoration and oyster aquaculture can remove N in comparable ways via DNF, but that nutrient processing may be environmentally dependent and tied to the metabolism of oysters via biodeposition. These results will be important for managers as they incorporate oysters and associated management activities into nutrient reduction strategies and consider trade-offs in ecosystem functioning between restoration and aquaculture activities.

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