Chlorophyll-a is the green pigment every photosynthetic alga carries, so measuring it measures algal biomass directly. Where phosphorus tells you how much fuel your lake has available, chlorophyll-a tells you how much of that fuel has already been converted into algae. It is the standard biomass metric in limnology and one of the three variables — with total phosphorus and water clarity — used worldwide to classify lake trophic condition. Method EPA 446.0. Reporting limit 1 mg/m3, which is the same value as 1 ug/L.