Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
As the bivalve aquaculture industry expands worldwide, there is growing interest in its use to mitigate coastal eutrophication, the increased supply of organic matter to an ecosystem. Bivalves influence eutrophication by exerting `top-down' control on primary production through feeding while simultaneously influencing local `bottom-up' effects by increasing nutrient recycling. Additionally, nitrogen (N) is removed via harvest and potentially enhanced denitrification (DNF); however, DNF competes for nitrate (NO3-) with dissimilatory nitrate reduction to ammonium (DNRA), an N retention process. Seasonal in situ flux measurements in Cherrystone Inlet, VA, demonstrated that clam aquaculture sediments are a source of ammonium (NH4+), derived from clam excretion and microbial mineralization of clam biodeposits. Macroalgae, which proliferate on predator-exclusion nets utilized by the US clam industry temporarily sequester this regenerated N. Clam cultivation influences eutrophication locally by providing N in excess of macroalgal N demand, facilitating increased macroalgal production. Experiments investigated the competition between DNF and DNRA within clam sediments. at clam beds in Cherrystone Inlet, DNRA was more favored over DNF than at uncultivated sediments, likely due to the availability of labile organic carbon supplied by clams, low nitrate availability, and sulfidic sediments. However, a comparative study across clam aquaculture sites in the Sacca di Goro, Italy, where Ruditapes philippinarum are cultured, and on the Eastern Shore, VA, where Mercenaria mercenaria are cultured, revealed that the competition between DNF and DNRA is highly dependent on the environment and particularly the relative availability of labile carbon to NO3-. DNF exceeded DNRA at sites in the Sacca di Goro with elevated water column NO3-, concurrent with high abundances of a burrowing amphipod (Corophium sp.) that promoted nitrification. DNRA exceeded DNF at the VA sites and in the eastern region of the Sacca di Goro, where clam biomass was high, water column NO3" low, and sediments were generally reduced. Variability in rates across sites highlights the challenge in generalizing about the role of DNF in enhancing N removal across all clam aquaculture locations. An ecosystem-scale C and N budget was constructed for Cherrystone Inlet to understand the influence of clam cultivation on energy flow and eutrophication at a basin-wide scale. Although clam cultivation occupied only 3% of the Inlet's surface area, the clams filtered a volume equivalent to 7-44% of the system daily. Annually, N regeneration at the clam beds was ~3-fold higher than N removed by harvest. Due to the short water residence time, low watershed N load, and close vicinity of clam beds to the mouth of the Inlet, cultivated clams are likely subsidized by phytoplankton from the Chesapeake Bay. Thus, the N regenerated at the clam beds, which fuels macroalgal production would not be present in the system without facilitation by the cultured clams. This study demonstrates that although clams may dampen eutrophication by removing phytoplankton from the water column, high densities of clams can facilitate rapid N turnover through excretion and DNRA, fueling macroalgae, a form of eutrophication. The effect of clam aquaculture on N removal and subsequently organic matter supply is highly dependent on environmental conditions and clam cultivation practices, as well as the scale considered. at a large-scale (e.g. Chesapeake Bay) clam aquaculture is a net sink for N through harvest, however this study suggests that clam aquaculture may increase N and organic matter supply (i.e. macroalgae) on a basin-wide scale (e.g. Cherrystone Inlet).
© The Author
Murphy, Anna Elizabeth, "Effects of commercial clam aquaculture on biogeochemical cycling in shallow coastal ecosystems" (2015). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1539616787.