Oyster regulation of biogeochemical cycling in temperate estuaries

Abstract

Of the many changes humans have caused in coastal systems, excess nutrient loading is perhaps the most dramatic. Specifically, excess nitrogen (N) can lead to a series of negative consequences such as eutrophication, low oxygen conditions, and decreased biodiversity. Concurrent with changes in nutrient loading, coastal shellfish populations have been devastated through overharvesting, disease, and pollution. For example, oyster reefs – once a dominant feature along many coastlines – have been reduced by 85% of their historic range globally. Today, oysters are returning to coastal systems through restoration projects and a boom in aquaculture. Yet the impact of returning oysters to coastal systems is unknown. My dissertation helps to fill this major knowledge gap. Specifically, this dissertation focuses on the role oysters play in regulating coastal nutrient cycling and greenhouse gas (GHG) emissions. In chapter one, I estimated the GHG cost of protein production using oyster aquaculture. Using a combined field and laboratory approach, I quantified rates of N2O, CH4, and CO2 release from cultured oysters, and changes in sediment fluxes of these GHGs. On a kg CO2-equivalent kg-1 protein produced, oyster aquaculture has less than 0.5% of the GHG cost of terrestrial livestock production. In chapter two, I took advantage of an oyster aquaculture chronosequence to examine how organic matter loading from oysters altered sediment N cycling processes over time. I found that sediment fluxes under oyster aquaculture oscillated over time, shifting between N removal (N2) and recycling (NH4+) processes, demonstrating non-linear dynamics. In chapter three, I demonstrate that sediment N cycling processes in oyster habitats follow seasonal patterns of water column productivity, recording net denitrification in the spring following a phytoplankton bloom and net nitrogen-fixation in the fall. In chapter four, I use a meta-analysis approach to describe the role of oysters in regulating coastal nutrient recycling, removal of excess N, and GHG footprint. I show that in a biogeochemical context oyster reefs and aquaculture are interchangeable habitat that stimulate both N removal and recycling, with only a small GHG footprint.