Coastal Bivalve Aquaculture Carbon Cycling, Spatial Distribution and Resource Use in Virginia, USA and Baja California, Mexico

Aquaculture is a global industry responsible for over half of the world’s fisheries production. A major component of this industry is shellfish aquaculture. Shellfish aquaculture is prevalent in many nearshore marine ecosystems and has effects on both the system structure and processes. These effects on coastal environments have both ecological and economic impacts with policy and management implications. This thesis explores several aspects of shellfish aquaculture in coastal Virginia, USA and Baja California, Mexico using carbon budgeting, geographic information systems (GIS) analysis and stable isotope analysis.

The impact of hard clam (Mercenaria mercenaria) aquaculture on carbon cycling in a tidal inlet was evaluated for Cherrystone Creek, a small tributary of the Chesapeake Bay. The fluxes and pools of organic and inorganic carbon driven by clam aquaculture were of a similar magnitude to system processes such as water column production and carbon burial. Clam consumption is likely supported by production from outside of the system imported daily through tidal exchange. CO2 production is also enhanced through clam respiration and calcification. A large amount of carbon (135 Mg C yr-1) is removed annually through harvests in the clam shell and tissue material. The carbon associated with these withdrawals is generally not returned to the system. Intensive shellfish aquaculture alters coastal carbon cycling through the addition of large fluxes and pools of organic and inorganic carbon.

An analysis of annual aerial images of the ocean side lagoons of the Virginia Coastal Reserve (VCR) in GIS was conducted to identify the temporal and spatial trends of clam aquaculture for the period 2002 – 2012. Aquaculture of the hard clam has increased annually, even while state harvest numbers have remained relatively stable. The number of clam beds has grown by about 250 beds per year from 1,180 in 2002 to 4,430 in 2012. This increase corresponds to over 1 km2 of new clam farms in the VCR. Clam farms were not randomly located or based solely on bottom area available for leases that allow aquaculture. They were located in shallow water adjacent to inlets and channels. Using spatially explicit data for the VCR, constraints related to bathymetry, water residence time and the sediment grain size of clam farms were determined and used to predict potential areas for future aquaculture expansion. Clam farms are most likely to be found in locations with shallow depths (0 - 2.5 m below mean sea level), short water residence times (<0.5 hours - 108 hours) and sandy sediments (40 - 90%). Clam aquaculture in the VCR currently occupies 1.8 km2 with a potential habitable zone of 120.9 km2 remaining, indicating the potential for future expansion with an unlikely spatial limitation. Given the good water quality of these coastal lagoons and the high flushing rate, it is also unlikely that aquaculture will encounter resource limitations in the near future.

Lastly, shellfish aquaculture resource use was explored with a stable isotope analysis of Pacific oysters (Crassostrea gigas) and their potential food resources in Bahía San Quintín, Baja California, Mexico. The stable isotopes ratios of hydrogen (2H/1H) and carbon (13C/12C) were measured for oysters, seagrass (Zostera marina) and macroalgae (Ulva spp.) and calculated for phytoplankton. These values were used in a Bayesian mixing model to estimate a posterior distribution of resource use. There was no seasonal effect on resource use as upwelling conditions typical of the region were below average prior to sampling events. However, there was a strong spatial gradient in the system. Phytoplankton were the primary resource (median values 67 and 79%) for oysters nearest to the mouth of the bay while macroalgal importance increased (43 and 56%) for oysters in the upper reaches of the system. The mixed resource use of the oysters highlights their ability to adapt to different locations and resource availability, potentially allowing for a higher system carrying capacity.