Phytoplankton assemblage changes during decadal decreases in nitrogen loadings to the urbanized Long Island Sound estuary, USA

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Despite reductions in nitrogen loadings from wastewater treatment plants (WWTPs) discharging into Long Island Sound (LIS) over the last 15 yr, eutrophication and hypoxia remain a severe problem. Here we used time series of hydrography, meteorology, nutrients, and phyto- plankton pigments to explore the relationships between planktonic biomass, nutrient stocks, and physical regimes in LIS. With the exception of the most eutrophied station in the west, dissolved inorganic nitrogen (DIN) decreased between 1995 and 2009, likely resulting from WWTP upgrades. However, total dissolved nitrogen increased during this period, primarily driven by ris- ing organic nitrogen pools. Simultaneous increases in inorganic phosphorus, silicate, and chloro- phyll a (chl a) were also observed. Starting in 2002, pigment-based phytoplankton community composition revealed systematic declines in diatom abundances coincident with increases in dinoflagellates and other flagellated phytoplankton groups. Despite this, bottom water dissolved oxygen concentrations did not improve. The apparent paradox between increasing DIN limitation and escalating chl a concentrations in LIS suggests a shifting nutrient stoichiometry and an altered phytoplankton community in which phytoflagellates have increased in abundance relative to diatoms. Despite these changes, diatoms remained the most abundant algal group by the end of the study. In addition, a shift in chl a stocks in the year 2000 coincided with decreases in temper- ature, increases in salinity, and the proliferation of several algal groups. These results reveal the complex nature of eutrophied estuaries and indicate that policies targeting only inorganic nitro- gen loadings may be insufficient to mitigate eutrophication in systems such as LIS.