Nitrogen doesn't exist in a lake as one thing. It cycles through several chemical forms, and which form dominates tells you where it came from and how available it is to algae. This panel measures the three that matter most: nitrate, ammonia, and total Kjeldahl nitrogen. Read together they are a sequence, not three separate numbers — hence the name. Methods EPA 300.0 for nitrate, EPA 350.1 for ammonia, and EPA 351.2 for TKN, all reported in parts per million.