Abstract

Phytoplankton play a major role in marine ecosystem health. They form the base of aquatic food webs, but under conditions of nutrient loading and high stratification, they can develop into harmful algal blooms that produce toxins harmful to humans and wildlife. Ongoing phytoplankton blooms have been observed in Long Island’s (LI) coastal waters for the past half-century, but there is a lack of a comprehensive view of phytoplankton spatiotemporal distribution and their driving factors due to the analysis of specific sampling sites, species, and years. Thus, this study obtained 20 years of chlorophyll-a, climate, and nutrient remote sensing and in situ data from the ERDDAP data server and the CTDEEP Long Island Sound Water Monitoring Program to establish phytoplankton phenology using the threshold criterion and cumulative sum of anomalies methods and to investigate regional differences, influencing factors, and interannual trends using correlation and linear trend analysis. The phenology of summer-autumn blooms in Long Island Sound (LIS) was associated with high sea surface temperatures (r = -0.46, p < 0.01). In contrast, winter-spring blooms were most strongly correlated with low salinity (r = -0.52, p < 0.01), indicating P-rich Connecticut river discharges as the dominant nutrient source. However, phytoplankton in LI’s southern shores lack access to river outlets, so phytoplankton production was driven by deep winter mixings from low SST that replenish surface water nutrient levels (r = -0.71, p < 0.05). Finally, our results showed strong decreasing interannual trends of autumn chlorophyll-a levels from 2003-2022 (r < -0.66, p < 0.05), potentially due to heightened N-limitation. Hence, the effects of declining phytoplankton productivity on LI’s fisheries and marine ecosystems should be further investigated.