ABSTRACT:

Oysters enhance benthic-pelagic coupling in coastal systems by moving large quantities of suspended particulates to the sediments, stimulating biogeochemical processes. Recent research efforts have focused on quantifying the impact of oysters on coastal biogeochemical cycling, yet there is little consensus on how oysters influence processes across systems. A potential driver of this variance is availability of organic material suspended in the water column and subsequent loading to sediment by oysters. Here, we measured fluxes of sediment di-nitrogen (N₂-N), ammonium (NH₄⁺), combined nitrate-nitrite (NO_x), and phosphate (PO₄^3-) in spring, summer, and fall at 2 oyster reefs and 1 farm in a temperate estuary (Narragansett Bay, Rhode Island). We then linked these fluxes with patterns of water column primary production. Nitrogen removal from the system was highest in spring, when we detected net sediment denitrification (48.8 µmol N₂-N m^-2 h^-1) following a winter-spring diatom bloom. In contrast, we measured sediment N₂ fixation in fall (-44.8 µmol N₂-N m^-2 h^-1) at rates nearly equivalent to spring denitrification. In the summer, we measured a nearly net zero sediment N₂-N flux (-2.7 µmol N₂-N m^-2 h^-1). Recycling of nitrogen to the water column was consistent across seasons, composed almost exclusively of NH₄⁺. These results demonstrate that sediment nitrogen cycling in oyster habitats is dynamic and can change rapidly based on seasonal patterns of productivity. At carrying capacity, the impact of oysters on nitrogen cycling is large and should be considered during efforts to increase oyster populations through aquaculture or reef restoration.