- water
- water-quality
Total nitrogen is a reporting aggregate of a sample's nitrogen forms — derived, depending on what a lab measures, either as a direct TN result or by summing nitrate, nitrite, ammonia, and organic nitrogen (often via total Kjeldahl nitrogen plus nitrate and nitrite). Like total phosphorus, it is a nutrient that fuels algal growth; while phosphorus usually limits freshwater blooms, nitrogen co-drives them and governs the toxicity of some cyanobacteria, so a nutrient total maximum daily load typically tracks both.
Nitrogen also matters directly to permit compliance: ammonia is toxic to aquatic life at low concentrations and its decay consumes oxygen, so a discharge's nitrogen load ties straight into the receiving water's dissolved oxygen budget. The platform reads total nitrogen as one of the two nutrient loads that decide whether a reach can absorb a new discharge, not as a stand-alone number.