Abstract
Estuaries worldwide are increasingly degraded by anthropogenic nitrogen (N) inputs, primarily from their watersheds. Several municipalities in southeastern Massachusetts (MA) are implementing suspended oyster aquaculture in their estuaries as a bioremediation method to increase N removal. Floating oyster aquaculture removes N through the processes of assimilation, sediment burial, and enhanced denitrification (microbial conversion of nitrate to N2). However, municipalities require quantitative data to determine the effectiveness of suspended oyster aquaculture before it can be incorporated into a regulatory framework. While many studies have looked at the impacts of oyster cultivation on nitrogen cycling, none have looked at the combined effects of filtration, biodeposition, sediment denitrification, and assimilation on nitrogen removal from coastal embayments typical of southeastern MA. The goal of this dissertation is to quantify the mass of N that suspended oyster culture removes from shallow water coastal embayments typical of southeastern MA. (2); and (3) determining the Ncontent of aquaculture oysters raised in southeastern MA embayments with varying water quality and the total nitrogen removed from a system through oyster aquaculture and subsequent oyster harvest. Chapter 1 addresses biodeposition rates, typical settling velocities of biodeposits through the water column, and the area over which biodeposits from suspended oyster culture settles. I developed and validated a model to predict biodeposition areas to further define rates of enhanced denitrification associated with oysters deployed in floating bags. The modeling approach can be implemented in systems with a low probability of biodeposit erosion to predict biodeposition intensity relative to the model grid. Chapter 2 determines sediment-water nutrient fluxes (i.e., nitrification and denitrification rates) and the enhancement of natural processes resulting from oyster culture, quantifies the impact of suspendedoyster culture on nitrogen cycling and determining oyster impact on nitrogen cycling by separately measuring nitrogen species, and then combines the data for an overall look at the “oyster effect”. The biodeposition model developed in Chapter 1 is used to predict N deposition, and collected and incubated sediment cores collection to determine nutrient and N2-N fluxes. Denitrification was enhanced in sediments affected by oyster biodeposition compared to the background; the level of enhancement depended on the biodeposition rate and oxygen availability. Chapter 3 determined relationships between oyster nitrogen content and oyster size and nitrogen-related water quality metrics. There are significant relationships between oyster size and the mass of nitrogen. The percentage of N in a whole dry oyster is influenced by site-specific water quality constituents, oyster size (relative size of tissue and shell), and deployment depth. By combining the results from each of the studies, the effect of oyster culture in removing nitrogen from estuarine waters through harvest and alterations to the nitrogen cycle across various estuaries along a nutrient-related water quality gradient was quantified. This dissertation provides data necessary to assess the efficacy of using suspended oyster aquaculture to reach N reduction goals within eutrophic coastal embayments typical of southeastern MA.