The influences of phenology, spatial distribution, and nitrogen form on Long Island Sound phytoplankton biomass and taxonomic composition

Long Island Sound (LIS), USA, is an urban estuary that receives excessive nitrogen (N) inputs resulting in ecosystem impairments, such as hypoxia and algal blooms. Despite established linkages between N concentrations and algal blooms worldwide, connections between N forms and phytoplankton assemblages within LIS remain less characterized, representing a critical information gap for water quality management. This study assessed the spatio-temporal distributions of phytoplankton biomass and abundances, N, and other key water quality parameters Mar-2020 to Oct-2021 in Western (WLIS), Central (CLIS), and Eastern LIS (ELIS). During both study years, mean dissolved inorganic N (DIN) concentrations were significantly higher during fall than other seasons (ANOVA: p < 0.004), coincident with diatom blooms. Diatoms were abundant throughout the study, though WLIS flagellate numbers rose during Jul and Aug both years. Smaller (<5 mu m diameter) cells generally contributed most to overall chlorophyll a (Chl a) and with significantly higher summer biomass in WLIS and CLIS than other seasons (ANOVA; WLIS: p < 0.004; CLIS: p < 0.006). Across LIS, there was a clear west to east decline in levels of Chl a, DIN, and the proportional contribution of picoplankton to overall phytoplankton biomass; however, seasonal mean concentrations of Chl a, nutrients, and other water quality parameters did not significantly differ between sampling (0.5 and 2 m) depths except for ELIS summer temperature and Chl a (T-test; both p = 0.002). Given climate change challenges, such as warmer temperatures enhancing blooms particularly in systems enriched with anthropogenic N, findings indicate that N-management strategies should re-evaluate targets spanning LIS. Study results are likely applicable to other eutrophic urban estuaries worldwide.