Impacts of Culture Technique and Estuarine Position on Oyster Mariculture-mediated Nitrogen Removal in Temperate Estuaries

Abstract

Excess nitrogen is a driver of eutrophication and a threat to coastal water quality, ecosystems, and economies. Oyster reef habitat has been shown to enhance nitrogen removal processes, including denitrification (DNF), in adjacent sediments via nutrient-rich biodepositions. However, little is known regarding DNF within oyster mariculture leases, including how DNF varies among methods (i.e., gear types) or locations (i.e. between and within estuaries). Here, the effects of three common mariculture methods (i.e., floating bags, bottom cages, loose-on-bottom) on nitrogen cycling in three temperate North Carolinian waterbodies (Newport River, Stump Sound, Core Sound) and the effect of position along salinity (22–33 ppt) and sediment organic matter (SOM) (9.0–0.8%) gradients on nitrogen cycling in one estuary were measured. Oyster mariculture can match natural reefs’ DNF rates (floating bags 44.8 ± 11.7, bottom cages 38.6 ± 26.2, loose-on-bottom 104.8 ± 81.9, reef 103.2 ± 55.9 µmol N2-N m−2h− 1), but this service varies by method, waterbody, and over time. Loose-on-bottom DNF was ~ 10x higher than other methods in Newport River, bottom cages were highest (by 1.5x) in Stump Sound, and no culture method surpassed another in Core Sound. The following year, DNF on the Newport River loose-on-bottom lease decreased 10x, yielding no culture differences. Along an estuarine salinity and SOM gradient, oyster culture enhanced DNF relative to bare flats only up-estuary, by 6.5x. Overall, up to order-of-magnitude DNF variability among culture methods, locations, and over time suggests that quantifying an oyster lease’s nitrogen removal requires rigorous site-specific analysis rather than generalized estimation.