Heterogeneity in Nantucket Harbor: A Case Study in Spatial Variation, Temporal Exposure, and Coastal Habitat Assessment

Elizabeth Sherman

Nantucket Harbor has experienced substantial changes in water quality, together with losses of important biological resources that have long sustained the island. Both local and global effects have been linked to changes in nutrient concentration, water clarity, temperature and dissolved oxygen, which in turn have been associated with a decrease in eelgrass coverage and variation in bay scallop abundance. In this study I analyze data from two complementary water quality monitoring programs: the Nantucket Natural Resources Department record from 2010-2025 at four different sites across the harbor, and the continuous record from the Maria Mitchell Association monitoring buoy in 2025 at one lower harbor location. The NRD discrete sampling record revealed spatial variation and long-term temporal patterns in total nitrogen (TN), water clarity and chlorophyll-a concentration, with more enclosed sites characterized by more stressful values. There has been no significant change in water clarity and chlorophyll-a over the 15-year study period, while TN values appear to have been decreasing at some of the sites. Nevertheless, even in sites with apparent improvement, TN values frequently exceeded the 25 µM threshold established by the Massachusetts Estuary Project. The continuous data from the MMA buoy revealed that organisms in the harbor were often exposed to temperatures above the 23 °C sublethal stress threshold for eelgrass. Moreover, dissolved oxygen levels below 6 mg/L sometimes co-occurred with elevated temperatures, potentially exacerbating the impact of each stressor. There is urgency in understanding the relationship between harbor water conditions and the changes in eelgrass coverage and bay scallop abundance reported in separate Town studies. The two analyses described in this study provide complementary information needed to understand those relationships: the NRD record reveals spatial variation and long-term trends, while the continuous buoy record reveals short-term variation and co-occurring stressors that discrete sampling cannot resolve.