Total Nitrogen (TN)

Status and Trends

The total nitrogen in water is comprised of dissolved inorganic and organic nitrogen and particulate organic and inorganic nitrogen, minus N₂ gas. Phytoplankton and bacteria contribute to the amount of dissolved inorganic nitrogen content. Decomposition of aquatic life adds both dissolved organic and particulate organic nitrogen to water; while sewage runoff, erosion, and overland flow increases particulate inorganic nitrogen levels in water. Bacterial denitrification converts nitrate to N₂ gas, hence the loss of some of the water's nitrogen.

Median TN concentration ranged from 0.5 mg/L at the station on Cherry Creek Cherry Creek near Meadow Mountain Run Road to 5.2 mg/L on Antietam Creek at Poffenberger Road below Funkstown, MD. Highest median concentrations of TN (TN > 2.64 mg/L) occurred at 26% of all stations. Stations with high median TN concentrations were located in agricultural regions in the eastern portion of the Upper Potomac Basin and western portions of the Middle Potomac Basin. Deer Creek in the eastern portion of the upper Western Shore Basin, also, had median TN concentration in the upper 1/3 of the benchmark data set.

Lowest median concentrations (i.e., TN < 1.68 mg/L) occurred at 50% of all stations. Stations on the Susquehanna River, in suburban Washington, the Patuxent below Rocky Gorge Dam and along the Potomac River had the lowest median TN concentrations. Low median concentrations are particularly evident at stations west of Hancock, MD

Trends of decreasing concentration occurred at 76% of the stations. None of the stations exhibited increasing trends. The relative frequency of stations with decreasing trends was inversely related to the median concentration (81%, 77% and 64% for the lower, mid and upper tercile respectively).

Strongest correlations between TN and TP occurred at Deer Creek, the Little Youghiogheny River and the lowest station on the Patuxent River. Significant positive correlations occurred at 42% of all stations suggesting a common source for nitrogen and phosphorus or delivery conditions at these stations that bring in more nitrogen also bring in more phosphorus. Phosphorus was inversely related to nitrogen at the station on the North Branch of the Patapsco at MD Route 91, which implies that the conditions that suppress TP concentrations with increasing TN concentrations have been stable over time. The slope parameter on the regression line of those stations with a positive relationship representing the ratio of TP:TN ranged from 0.026 to 0.124. For comparison, if one looks at the elemental composition of individual species of phytoplankton grown under nitrogen or phosphorus limitation, the P:N ratio can vary from approximately 1:60 to 1:6 or, in decimal terms, 0.017 - 0.167. Thus, algal growth at any of the stations in Figure 1 would not be limited.

Total Nitrogen (TN) data for sampled locations are summarized in these plots.

• Land use/site map

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This page was updated on 12/5/2005