Water clarity varies considerably both seasonally and spatially. The waters with the greatest transparency generally occur in the central portions of the main basin where clarity is generally a function of algal densities (see "Phytoplankton" section). The north and south ends of the lake and Hyde Bay are the most turbid. In addition to algal populations in these regions, there are high suspended silt concentrations derived from tributary streams and re-suspended surficial sediments in shallow waters. Figure 24 illustrates this pattern as observed on 8/8/82 (Harman, 1983). A study utilizing nephelometric data was conducted in 1993 by Groff (1994) reflected the same pattern (Figure 25a-c).
Figure 26 represents average annual Secchi disk transparency at center lake (TR4-C) in 1935, 1968-73, 1975-82, 1984-87, 1988, and 1992-94 (Harman et al., 1980; Harman, 1983; Iannuzzi, 1991a; b; Homburger and Buttigieg, 1992; Groff et al., 1993; Harman, 1994a). Variations in the number and timing of yearly samples prior to 1988 may limit the comparability of these data. Mean annual transparency values since 1992 include biweekly observations throughout the ice free season and monthly observations during ice cover. The minimum annual transparency in the 1970s and 1980s typically occurred in mid-summer (Harman et al., 1980), as illustrated in 1973 during a bloom of Anabaena sp. in August. In more recent years, times of minimum clarity have varied considerably (Figure 27); in 1993 a bloom of Oscillatoria rubescens (Ramsey, Unpubl.) was correlated with Secchi disk transparen cies of 2.4 m in April. In 1994 transparencies of 2.5-2.6 m persisted through June and July associated with a bloom of Chlorella vulgaris (Ramsey, Unpbl.).
The maximum transparency reported at TR4-C was 10 m in April, 1969 and again in August, 1982. Transparency increased dramatically for several weeks following tropical storm Agnes in June, 1972 (Harman et al., 1980). This increase may reflect ecosystem-wide disruptions caused by the storm. Improvements in transparency were c oncurrently noted for Skaneateles, Owasco, and Hemlock Lakes (Mills, 1975) but not for Cayuga (Godfrey, 1977b) or Conesus (Mills, 1975). In 1993, low Secchi disk values were generally attributed to extraordinarily large spring runoff (Albright, 1996) and the ensuing high water levels (Harman and Ernst, 1994); however, Oscillatoria (mentioned above) was present in pre-bloom concentrations in February, well before the spring thaw. Average annual Secchi disk transparencies in the years 1968-69 wer e quite similar to 1935 values reported by Tressler and Bere (1936) (see Figure 26). Those values are typical of the mesotrophic Finger Lakes on the northern edge of the Appalachian plateau (Berg, 1963).
A trend towards decreasing transparencies occurred from 1970-73 after the opening of Glimmerglass State Park which drained effluent from a sand-filter, waste-water treatment system into the lake via Shadow Brook (Figure 26, 1970-73). Concurrent with the change of that system to sub-surface di sposal, and the passage of regulations preventing the widespread use of high phosphate detergents, transparencies tended to increase (Figure 26, 1975-82).
A second, more recent, trend towards decreasing clarity that first appeared during the middle 1980s and continues to date (Figure 26, 1984-95), has been paralleled by increasing concentrations of total phosphorus in the water column (Figure 27a). This was assumed to be largely due to the only recognized potential change in external phosphor us loading in the watershed, which was associated with the sub-division of forested and agricultural land for residential development. In the late 1960s, settlements were estimated to account for less than 0.01% of the watershed (Shelton et al., 1968). By 1990, 0.86% of the watershed fell into this category (Baumann, 1990). Further decreases in transparency, recognized in 1992 and continuing to the present, correlate with the introduction and population irruption of the alewife (Alosa pseud oharengus) (Foster, 1990; 1993; Foster and Wigin, 1990; Foster and Gallup, 1991; France and Taylor, 1994; Harman and Toner, 1993; Wigin, 1991a; b).
Vertical extinction and absorption coefficients are tabulated for 1972, 88, and 92-95 (Table 17). They describe the rate of decrease of light intensity with depth, and normally approximate a constant describing the light-absorbency qualities of a given body of water. The compensation point (defined physically as the depth at which 1% of the s urface ambient light is transmitted) is directly correlated with Secchi disk readings (r=0.85), indicating that Secchi readings are a good indicator of light conditions in Otsego.
Otsego Lake has the blue-green color typical of oligotrophic lakes possessing high concentrations of calcium carbonate. The algal blooms typical of the mid-1970s ephemerally turned the water a translucent pea green. Those blooms reappeared during much of the spring and early summer seasons in 1993-94. During the interim these conditions were not typically observed.