Quantifying the Effects of Floating Oyster Aquaculture on Nitrogen Cycling in a Temperate Coastal Embayment

Estuaries worldwide are increasingly degraded by anthropogenic nitrogen (N) inputs, primarily from their watersheds. In southeastern Massachusetts, several municipalities are implementing floating oyster aquaculture in their estuaries as a means of increasing N removal through assimilation into oyster biomass, sediment burial, and enhanced sediment denitrification. Denitrification associated with floating oyster aquaculture was quantified in a Cape Cod tidal salt pond to determine the effectiveness of this non-traditional approach. Exchange of dinitrogen gas, oxygen, and nutrients between water column and sediments was measured inside and outside of sediment areas affected by oyster biodeposits. Enhanced sediment denitrification was observed during each of the 3 years of the study, with the degree of enhancement varying between seasons and years; enhanced denitrification totaled 2.8, 2.4, and 3.3 g N 2 -N m −2 (265%, 309%, and 388% of background) for summer and fall months of 2016, 2017, and 2018, respectively. Continued enhanced denitrification each following spring contributed additional N removal, averaging 1.4 g N 2 -N m −2 (449% of background) beyond previous summer and fall enhancements. Differences in enhanced denitrification were attributable to interannual and seasonal differences in N deposition, bottom water dissolved oxygen, and nitrate + nitrite concentrations. Oyster biomass within each bag and the pond varied by year resulting in differences in the spatial extent and intensity of N biodeposition to surficial sediments, and level of enhanced denitrification. This shows that such information is necessary to assess the efficacy of using floating oyster aquaculture to reach N reduction goals within eutrophic coastal embayments typical of southeastern Massachusetts.