Abstract
Shellfish aquaculture has undergone substantial growth in New England, yielding environmental and economic benefits. However, the industry faces a significant challenge posed by harmful algal blooms (HABs). Toxin-producing phytoplankton can endanger human health, ecosystem health, and lead to the closure of shellfish harvesting. This thesis focuses on monitoring HABs and their impact on farmed oysters in Nasketucket Bay, Massachusetts. A year-long phytoplankton survey was conducted from 2022-2023 at the Blue Stream Shellfish oyster farms, involving weekly collection of water samples to analyze the phytoplankton community, and monthly measurements of the growth of eastern oysters (Crassostrea virginica) during the summer and fall growing season. A comparison to the nearest Massachusetts Division of Marine Fisheries (MA DMF) HAB monitoring station in Mattapoisett Harbor revealed that the MA DMF site effectively captured the presence of phytoplankton taxa that raise public health concerns in Nasketucket Bay. The most-common harmful alga throughout the survey was Pseudo-nitzschia spp., the causative agent of Amnesic Shellfish Poisoning, although abundance levels were not sufficient to trigger a shellfish closure. In addition, the ichthyotoxic phytoplankton Margalefidinium polykrikoides and Karenia mikimotoi were present at notable levels. The timing and size of these blooms had little impact on oyster growth, and growth was best during periods of high phytoplankton biodiversity. In addition, the oyster measurements obtained quantitative differences in shell growth, mortality, and condition index in sunken vs. floating aquaculture gear, with floating oysters maintaining better health and uniform shell dimensions. Furthermore, the deeper waters located farther out in Nasketucket Bay exhibited compromised oyster health and elevated mortality rates, possibly attributed to increased predation on oysters. These findings have implications for informing future aquaculture farms operating in the bay, aiding in the development of more effective strategies for growing oysters.