The dissolved oxygen content of Otsego Lake surface waters usually exhibits saturated or near-saturated cond itions; these persist under ice cover as well as in open water (Figure 28 and Tables 18,19). Since autumn turnover normally extends for a period of at least one month, and spring overturn usually continues longer, the lake waters become well aerated from surface to bottom. Measurements made during both periods of overturn indicate that 90% saturation is consistently established with each mixing; during the spring, supersaturated conditions normally prevail at all depths. Over the course of summer str atification, metalimnetic maxima and minima are normally exhibited in the oxygen curves. A positive heterograde curve develops during early stages of stratification, whereas in late summer, a negative heterograde curve may be manifest. By early fall of recent years, this phenomenon often has extended below the metalimnion, resulting in poorly oxygenated conditions in the upper hypolimnion. Figures 29a and b illustrate this situation as observed in 1969, 1988, and 1993. Figures 29c and d and Figures 29 e and f provide corresponding profiles of chlorophyll a and total phosphorus-P, respectively (note earliest year varies among parameters due to limitations in the available data set).
Dissolved-oxygen levels in the hypolimnion exhibit trends that are characteristic of a mesotrophic condition (Figures 30a-d). These are hypolimnetic decreases exhibited in late summer and fall. In recent years these reductions have been extensive enough to render the lower lake waters uninhabitable fo r lake trout (Figures 30b-d). Dissolved-oxygen levels of below 5 (Piper et al., 1982) or 6 mg/l (Doudoroff and Shumway, 1967), which presumably pose a danger to such fish, have not been reached throughout the entire hypolimnion until late fall. By that time, the surface waters have cooled enough to allow for migration into epilimnetic waters.
Figure 28 illustrates the conditions occurring in 1969 (Harman et al., 1980). During that fall a 25 m layer of hypolimnetic water from 25 m in depth to the bottom at 50 m exhibited oxygen concentrations less than 6 mg/l. This provided a zone approximately 12 m in depth having both temperature and oxygen values in the optimal range for lake trout. Bottom oxygen concentrations approximated 1 mg/l. In 1988, after several years of increased water clarity, waters having less than 6 mg/l of oxygen were only found below 45 m (a 5 m stratum). Bottom waters exhibited a minima of more than 4 mg/l. Figure 30b shows the situation in 1993 when the entire hypolimnion (the water column 12 m from the surface to the bottom), a 40 m stratum, exhibited oxygen concentrations of less than 6 mg/l. A 30 m layer of hypolimnetic water showed less than 5 mg/l of oxygen. Such conditions are likely to be stressful to cold-water fishes and, if allowed to worsen, may preclude their survival. McMurtry et al. (1995) provide data indicating that less than 7 mg/l oxygen stresses salmonids physiologically and 3 mg/l is the lethal threshold for these fish. In 1993, the bottom waters possessed less than 0.3 mg/l dissolved oxygen in solution. Table 57 ("Trophic Status" section) shows areal hypolimnetic oxygen deficits for several years illustrating the above mentioned situation.
Subtle variations in nutrient loading and weather conditions have definite impacts on the extent of oxygen depletion in bottom waters. High algal standing crops in the metalimnion, indicated by increasing chlorophyll a and total phosphorus in the water column, are co rrelated with increased dissolved oxygen depletion in bottom waters Figures 29a-f; see Figure 121 in "Trophic Status section"). In 1993 and 94 algal standing crops were large, corresponding with increased populations of the introduced zooplankton grazer, the alewife (Alosa pseudoharengus).
Hypolimnetic decreases in oxygen concentrations during winter stratification are not nearly as severe as those brought about by summer stagnation. The situation as illustrated for 1988 (Figure 30a) i s the most extreme that has been observed. Figures 28, 30b-d are more representative.