Nitrogen

Nitrogen is important to all living things. Nutrients such as nitrogen occur naturally in both freshwater and saltwater. Plants and animals need nutrients to grow and survive. But when too much nitrogen enters the water it can fuel the growth of algae, creating dense blooms that block sunlight and reduce oxygen for fish and other organisms. Nitrogen runs off the land during rain events. Atmospheric nitrogen from industry settles on the water.

Excess nitrogen can fuel the algal blooms, like this one in the Tred Avon River.

How is it measured?

Chesapeake Bay

Total nitrogen was measured at approximately 159 stations up to 8 times during the periods of interest (April to October). Total nitrogen is the amount of nitrate, nitrite, and ammonia that is dissolved in the water. The proportion of time that total nitrogen was above the threshold at each station was calculated.

Chesapeake Watershed

Total nitrogen was measured year round from 2012-2017. Total nitrogen is the amount of nitrate, nitrite, and ammonia that is dissolved in the water. The proportion of time that total nitrogen was above the threshold at each station was calculated. Station scores were averaged to HUC12. HUC12 scores were area-weighted to the reporting region scores.

Threshold Levels

Chesapeake Bay

Thresholds were determined by salinity regime.

Salinity Regime Reference Community Thresholds (mg L-1)*
Tidal Fresh ≤0.9
Oligohaline ≤0.9
Mesohaline ≤0.6
Polyhaline ≤0.5

 *EcoCheck (2011)

Chesapeake Watershed

Thresholds were determined by bioregion.*

Score Piedmont, Ridges, and Valleys Thresholds (mg L-1) Coastal Plain thresholds (mg L-1)
5 <0.64 <0.82
4 ≥0.64 - <1.65 ≥0.82 - <1.52
3 ≥1.65 - <2.15 ≥1.52 - <2.22
2 ≥2.15 - <2.65 ≥2.22 - <2.66
1 ≥2.65 - <3.66 ≥2.66 - <3.61
0 ≥3.66 ≥3.61

*EcoCheck (2013)