If only one indicator of reservoir health could be measured, dissolved oxygen (DO) would likely be the indicator of choice. Hutchinson (1975) states that a series of oxygen measurements probably provides more information about the nature of a lake than any other kind of data. The presence, absence, and levels of DO in a lake or reservoir both control and are controlled by many physical, chemical, and biological processes (e.g., photosynthesis, respiration, oxidation-reduction reactions, bacterial decomposition). DO measurements, coupled with observations of water clarity (Secchi depth), temperature, nutrients, and some basic hydrologic and morphometric information, yield substantial information about the ecological health of a reservoir.
Ideally, a reservoir has near-saturation concentrations of DO available to fish, insects, and zooplankton throughout the water column. This is usually the case during winter and spring when most reservoirs are well mixed. However, summer brings more sunlight, warmer water, and lower flows. This causes thermal stratification and increased biological activity; these combine to produce a greater biochemical demand for oxygen, particularly in the deeper portions of the reservoir. As a result, summer levels of DO often are low in the metalimnion and hypolimnion (intermediate and deepest regions). Hypolimnetic and metalimnetic oxygen depletion are common but undesirable occurrences in many reservoirs, especially storage impoundments. If DO concentrations are low enough, or low concentrations are sustained long enough, the health and diversity of the fish and benthic communities suffer. Sustained near-bottom anoxia also promotes release of ammonia, sulfide, and dissolved metals into the interstitial pore water and near-bottom waters. If this phenomenon persists long enough, these chemicals can cause chronic or acute toxicity to bottom-dwelling animals.
Historic information for reservoirs in the Tennessee Valley reveals that the burrowing mayfly (Hexagenia sp.) disappears from the benthic community at DO concentrations of 2 mg/l and below (Masters and McDonough, personal communication, 1993). Most fish species avoid areas with DO concentrations below 2 mg/l; fish growth and reproduction decrease at these levels, and many highly desirable species such as sauger and walleye simply cannot survive at such low levels of DO. Consequently, TVA considers 2 mg/l a critical level for evaluating ecological health and has incorporated it into the scheme for rating DO.
The rating scheme considers oxygen concentrations both in the water column (WCDO) and near the bottom of the reservoir (BDO). The DO rating at each sampling location (ranging from 1, "poor," to 5, "good") weights equally the average monthly summer water column and bottom water DO concentrations. Summer is defined as a 6-month period when maximum thermal stratification and maximum hypolimnetic anoxia are expected to occur (April through Septem-