As part of its monitoring program, the Tennessee Valley Authority (TVA) developed a chlorophyll and nutrients rating scheme to represent background or natural conditions within each watershed. Researchers used chlorophyll a concentrations to estimate algal productivity and trophic conditions of lakes and reservoirs. Professional judgment was used to identify concentration ranges indicative of good, fair, and poor conditions. The classification scheme for evaluating chlorophyll concentrations in TVA reservoirs is based on expected natural nutrient levels for each watershed.
That approach separates TVA reservoirs into two classes for chlorophyll expectations: those expected to be naturally oligotrophic because they are in watersheds with naturally low nutrient concentrations and those expected to be naturally mesotrophic. The primary concern for naturally oligotrophic reservoirs is early identification of cultural eutrophication. For the reservoirs expected to be mesotrophic, the principal concern is the potential for algal productivity (and chlorophyll levels) to increase to the point of creating the undesirable characteristics associated with eutrophic lakes (e.g., dense algal blooms, poor water clarity, low DO, and predominance of noxious blue-green algae).
Composite chlorophyll a samples were collected along with algae and zooplankton samples. In situ water column profiles also were measured to support trophic state assessments. Chlorophyll ratings at each sampling location are based on the average summer concentration of monthly composite photic zone samples collected from April through October (or September). If nutrients are present (e.g., TP greater than 0.01 mg/L and nitrate+nitrite-nitrogen greater than 0.05 mg/L) but chlorophyll a concentrations are generally low (e.g., < 3 µg/L), other limiting or inhibiting factors (e.g., high stream flows, turbidity, toxicity) are considered to be present and the chlorophyll a rating is decreased by 1 unit (USEPA 2000a).
Reference:
USEPA. 2000a. Nutrient Criteria Technical Guidance Manual: Lakes and Reservoirs. EPA-822-B-00-001. U.S. Environmental Protection Agency, Office of Water, Office of Science and Technology, Washington, DC. Accessed October 2016. https://nepis.epa.gov/Exe/ZyPDF.cgi/20003COV.PDF?Dockey=20003COV.PDF.