MEPS 560:173-183 (2016)  -  DOI: https://doi.org/10.3354/meps11922

ABSTRACT: Oysters are estuarine ecosystem engineers, in that their physical structure and biological function affect ecosystem processes such as organic matter and nutrient cycling. Oysters deliver material to the sediments through biodeposition and sedimentation caused by modification of flow around the reef. We conducted an experiment to distinguish between biotic effects and physical structure of oyster reefs on sediment nitrogen cycling. Experimental reefs consisting of live oysters, oyster shells alone and mudflats (controls) were sampled for a period of 4 wk for sediment organic matter, C and N content and fluxes of nitrogen (NH₄⁺, NO_X and N₂) and oxygen (O₂). We hypothesized that the biological activity of the oyster would deposit more, higher quality organic matter compared to deposition from flow modification alone, thus facilitating denitrification and having a larger impact on sediment nitrogen cycling. Compared to the controls, the live oyster experimental reefs increased sediment denitrification by 61% and the shell experimental reefs showed a 24% increase. The live oyster experimental reef also had the largest O₂ demand and NH₄⁺ production. Reef structure likely increased organic matter deposition, but the higher quality and larger quantity of organic matter associated with live oysters increased denitrification and microbial respiration. This experiment shows that the ecosystem service of nitrogen removal provided by oysters is primarily driven by the biological function of the oysters and secondarily from the physical structure of the reef. Our increased understanding of how oysters engineer ecosystems and modify nutrient cycling can help guide future oyster restoration efforts.

KEY WORDS: Crassostrea virginica · Nitrogen · Denitrification · Oyster reefs · Biogenic habitat · Ecosystem engineers