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

Abstract

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 phytoplankton 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 rising organic nitrogen pools. Simultaneous increases in inorganic phosphorus, silicate, and chlorophyll 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 temperature, 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 nitrogen loadings may be insufficient to mitigate eutrophication in systems such as LIS.

Formation of Low-Molecular-Weight Dissolved Organic Nitrogen in Predenitrification Biological Nutrient Removal Systems and Its Impact on Eutrophication in Coastal Waters

“…Our lab reactor studies and field effluent analyses therefore suggest that there is a high likelihood that upgrading WWTPs from CAS to predenitrification BNR leads to larger amounts of DON, particularly in the form of LMW-DON, in the effluent, although it decreases DTN by reducing DIN. Our findings may also provide one possible explanation for the observation that the level of DON in LIS has actually increased between 1995 and 2009, although total N loads to LIS have significantly decreased mainly due to regional WWTP upgrades …”

“…Under these conditions, potentially toxic phytoplankton blooms can negatively impact ecosystem function by disrupting food webs, causing seasonal hypoxia, and threatening resident aquatic plants and animals. In estuaries and coastal oceans, nitrogen (N) is typically the nutrient controlling primary production and phytoplankton bloom formation. , Consequently, substantial efforts have been made to decrease N loads to these environments, and reducing N discharge from wastewater treatment plants (WWTPs) has been a primary target supported by legislation.…”

“…WWTPs upgraded to BNR have significantly decreased total N loads to their receiving coastal waters. For example, a total maximum daily load (TMDL) was established for the Long Island Sound (LIS) in 1995; the goal was to reduce anthropogenic N loads by 58.5% by 2014 (NYSED and CTDEEP, 2000), and 70% of this goal was achieved by 2010, primarily through upgrades of regional WWTPs. , However, despite the successful reduction of N loads, accelerating eutrophication and hypoxia continue to plague the LIS. , These observations are not limited to the LIS; many other urbanized coastal systems are continuing to experience eutrophication, even after a significant decrease of N loads through upgrades of WWTPs. , It is also worth noting that the phytoplankton community composition in these coastal environments has shifted toward an increase in harmful taxa, including dinoflagellates and cyanobacteria (most commonly non-N 2 fixing cyanobacteria, such as Microcystis ). ,− Multiple factors, such as changes in climate and input of N from nonpoint sources, ,, should be considered together for this “paradox” (i.e., decrease in total N input but increase in phytoplankton bloom) . However, these unexpected observations also raise the possibility that some aspects of BNR, especially predenitrification BNR, yield environmental responses, as yet overlooked.…”

Comparative assessment on the influences of effluents from conventional activated sludge and biological nutrient removal processes on algal bloom in receiving waters

“…Both Connecticut River and LIS water showed very similar DTN at around 0.3 mg/L-N. The PO 4 3concentrations were similar between the two effluents, 0.72 mg/L-P for CAS and 1.04 mg/L-P for BNR.…”

“…As of approximately ten years after the initial implementation of the plan, around $650 million had been spent on reducing N loadings from point sources [1]. Despite these efforts the LIS area affected by hypoxia in 2006 was actually larger than the area affected in 1991 [3], and eutrophication and hypoxia problems have continued to occur in LIS [4].…”