How has the concentration of nitrogen in the waters of Great Bay Estuary changed over time?
Nitrogen concentration varies by location and type of nitrogen. Total nitrogen (TN), which is less variable in space and time than dissolved inorganic nitrogen (DIN), shows a statistically significant decreasing trend at Adams Point. TN shows a statistically significant increasing trend at the Chapman’s Landing and Lamprey River stations. No other stations indicate TN trends. For DIN, the Oyster River and Upper Piscataqua River stations indicate statistically significant decreasing trends while Chapman’s Landing indicates a statistically significant increasing trend.
Total Nitrogen (TN): Includes both dissolved inorganic nitrogen (DIN) and nitrogen contained in particulate and dissolved organic matter, and is considered to be a more accurate measure of the nitrogen status of an estuary than DIN alone. TN at Adams Point shows a significant decreasing trend (Figure 4.1), but it is important to note that the time series begins relatively recently, in 2003. Since 2012, median values ranged from 0.23mg/L to 0.30mg/L over the sample season for TN at Adams Point. Figure 4.1 indicates that the years 2005, 2008 and 2015 experienced TN concentrations above 0.6 mg/L.
TN values at the Lamprey River and Chapman’s Landing stations (see Monitoring Map p.46) show a significantly increasing trend, with average values over the last reporting period (2009-2011) of 0.52 and 0.90 mg/L, respectively. Average values for other stations were: 0.77 mg/L (Squamscott River), 0.35 mg/L (Great Bay), 0.52 mg/L (Oyster River), 0.44 mg/L (Upper Piscataqua) and 0.24 mg/L (the Coastal Marine Laboratory in Portsmouth Harbor).
Dissolved Inorganic Nitrogen (DIN): At Adams Point, median values for DIN for 2012 to 2015 ranged from 0.04 to 0.1 mg/L comparable to median values for the years 1974 to 1981 (Figure 4.2). For reference, the EPA National Coastal Assessment Condition Report categorizes values less than 0.1 as “good.” Other categories include “fair” (0.1 to 0.5 mg/L) and “poor.” (greater than 0.5 mg/L).28, 29
The Oyster River and Upper Piscataqua River stations both showed statistically significant decreasing trends for DIN, with average values since 2012 at 0.18 and 0.04 mg/L, respectively. In contrast, Chapman’s Landing showed a statistically significant increasing trend with average values since 2012 at 0.48 mg/L. Average values for other stations were: 0.37 mg/L (Squamscott River), 0.21 mg/L (Lamprey River), 0.08 mg/L (Great Bay) and 0.09 mg/L (Coastal Marine Lab).
Nutrient concentrations in the water are affected by nutrient loading from the watershed. As noted in the Nutrient Loading section, loadings since 2012 have been reduced in part due to reductions at municipal wastewater treatment facilities. Additionally, loading has been reduced due to consecutive years of low annual rainfall amounts and low occurrence of extreme rainfall events, which equate to less non-point source loading from run-off.