TABLE 2 Relationship between Variable DO(at 1.5 m) and WCDO Rating

Minimum DO at 1.5 m

WCDO Rating Change

<5 mg/l

Decreased one unit (e.g., 5 to 4)

<4 mg/l

Decreased two units

<3 mg/l

Decreased three units



tors of poor ecological health. The large amounts of algal material produced under these conditions also deplete oxygen concentrations as the algae die and decompose. This can cause or aggravate problems of low DO in bottom waters. TVA results indicate that a mean summer chlorophyll-a concentration greater than 15 μg/l is a threshold above which these undesirable conditions are likely.

These threshold levels are incorporated into the chlorophyll-a ratings at each sampling location (Table 3). The average summer chlorophyll-a concentration of monthly photic zone samples collected from April through September (or October) is compared with these criteria and rated accordingly.

Sediment Quality

Contaminated bottom sediments can have direct adverse impacts on bottom fauna and can often be long-term sources of toxic substances in the aquatic environment. Wildlife and humans may be affected by these contaminants by ingestion or through direct contact. These effects may occur even though the water

TABLE 3 Threshold Chlorophyll-a Concentrations and Corresponding Ratings

Average Concentrationa


<3 μg/l

3 (fair)b

3 to 10 μg/l

5 (good)

>10 to 15 μg/l

3 (fair)

>15 μg/l

1 (poor)

a If any single chlorophyll-a sample exceeds 30 mg/l, the value is not included in calculating the average, but the rating is decreased 1 unit (e.g., 5 to 4) for each sample that exceeded this value.

b If nutrients are sufficient (e.g., nitrate plus nitrite greater than 0.05 mg/l and total phosphorus greater than 0.01 mg/l) but chlorophyll-a concentrations are low (e.g., less than or equal to 2 μg/l), some other limiting or inhibiting factor such as toxicity is likely. When these conditions exist, chlorophyll-a is rated 2 (poor).

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