Impact of Anthropogenic Nitrogen Input on Long Island Sound Water Quality

Point source and non-point source (NPS) nitrogen loads in the past 26 years from 11 monitored areas along the coast of Long Island Sound were related to water quality parameters to assess the impact of nitrogen input on water quality. Of the 11 areas, Areas 2, 4, 8, and 9 supply more than 80% of the total nitrogen to the Sound. Nitrogen input from Areas 2 (along Connecticut River) and 4 (along Housatonic River) is dominated by non-point sources (riverine and coastal NPS), whereas nitrogen input from Areas 8 and 9 (along East River) is dominated by point sources (sewage treatment plants, STPs). Time series data of the nitrogen loads along the coast and water quality parameters at 18 monitoring stations within the Sound were deseasonalized (yearly cycles removed) to assess long-term trends and cyclical variations around the trend line. There is no clear long-term trend for the total nitrogen loads from the two major riverine sources (Areas 2 and 4). In contrast, the total nitrogen loads from Area 8 (dominated by STPs) showed a slight decrease trend between 1991 and 1996, a significant drop from 1997 to 1999 (likely as a result of much improved nitrogen removal efficiencies), and a rebound after 2003. Total nitrogen loads from Area 9 dropped significantly from 1994 to 1995 and slightly from 1995 to 1999, and then leveled off. Chlorophyll a (chl-a) levels at all stations decreased gradually from 1991 to 1999, rebounded quickly around 2000-2002, and then leveled off thereafter. Total dissolved nitrogen (TDN) concentrations showed a similar long-term trend. These long-term trends for Chl-a and TDN appeared to reflect the trend of the total nitrogen loads from areas 8 and 9, indicating that the reduction of nitrogen at STPs indeed had a positive impact on LIS water quality.