Nitrate Loads and Concentrations From Forested Watersheds and Implications for Long Island Sound

• Published in: Journal of geophysical research. Biogeosciences Vol. 130; no. 4
• Main Authors: Spaetzel, Alana B., Shanley, James B., DeSimone, Leslie A., Mullaney, John R.
• Format: Journal Article
• Language: English
• Published: 01.04.2025
• Subjects: Connecticut river; long island sound; nitrate trends; stream nitrate; TMDL
• ISSN: 2169-8953; 2169-8961
• DOI: 10.1029/2024JG008489

Reduction in point sources of nitrogen has led to improvement in water quality of the Long Island Sound (LIS) since 2000, but changes in nonpoint sources are less clear. A significant yet poorly quantified nonpoint nitrogen source is the forested landscape. Because a large proportion of the LIS basin is forested, even small areal inputs from the forested landscape have a large cumulative effect on nitrogen loading to LIS. Atmospheric nitrogen deposition, the primary source of nitrogen to forested landscapes in LIS basin, has been declining for several decades. However, nitrogen export in streams does not necessarily mirror nitrogen deposition. To assess forest nitrogen export to LIS, we estimated annual average concentrations and fluxes of nitrate in 17 forested watersheds in and near the LIS basin. Average flow‐normalized nitrate‐nitrogen concentrations ranged from less than 0.05–0.43 mg per liter among all sites; annual flow‐normalized yields ranged from 0.45 to 4.3 kg per hectare. Flow‐normalized annual average concentrations and yields of nitrate between water years 1991–2021 did not monotonically increase or decrease at most watersheds. Where determined, the other major N species generally had comparable magnitude and trends. Based on the watersheds analyzed in this study, forested areas are not responding uniformly to the continued decline of atmospheric nitrogen deposition. The variability among sites may indicate that local‐scale factors exert substantial influence over the magnitude and trends in nitrogen exports. One watershed that had increasing development showed an increasing trend in nitrate, but not in dissolved organic nitrogen. Plain Language Summary Water quality has substantially improved in Long Island Sound since 2000 largely through reduction of nitrogen input from human sources such as wastewater treatment facilities. However, less is known about changes in nitrogen export from forested areas, which make up more than half of the land that drains to the Sound. The primary source of nitrogen in the forested areas is from the atmosphere, and although the amount of nitrogen coming from the atmosphere has been decreasing, streams draining forested areas may not directly reflect these changes. In this study we looked at data from forested streams in the states of Connecticut, Massachusetts, New Hampshire, New York, and Vermont to estimate how much nitrogen is being carried off by streams in these forested watersheds and whether this amount has changed through time. We found that the amount of nitrate (a common form of nitrogen) in nearly all the forested streams we studied was not increasing or decreasing year to year and that there were considerable differences in the patterns over time. We did see that in one stream, the amount of nitrate increased greatly over time, which may be related to the loss of forested area to residential development. Key Points Even with reduced atmospheric nitrogen (N) deposition in the Long Island Sound basin, forest stream N export seems to have changed little Nitrogen exports likely increase where forested land is subject to natural or human‐caused disturbance, including loss to development Point sources of N have decreased since 2000; forested land yields a growing share of N to LIS and could be targeted for management