Otsego is dimictic, as is typical of moderate-sized lakes in the northern United States. Figure 20 illustrates temperature isopleths taken in central portions of the lake during 1969 (Harman et al., 1980). Monitoring continued sporadically from that time through 1987. Throughout 1988, Iannuzzi (1991a; b) conducted a study monitoring the water column weekly at 11 stations (Figure 20a). Thirteen stations were monitored for 8 weeks during summer stratification.
Water temperature throughout Otsego Lake ranged from 0.0o C to 26.8o C in 1988, with a mean water column temperature of 11.4o C for the 13 sample locations (Iannuzzi, 1991a; b). Maximum epilimnion and shoreline water column temperatures at all sites were recorded during the first two weeks of August. Minimum temperatures for the winter stations were intermittently recorded from January through the first week in March. Maximum hypolimnion/profundal bottom temperatures were recorded on 10/22 and 11/29 for TR6-C (average recorded depth = 17.0 m) and TR4-C (average recorded depth 48.2 m), respectively, and on 9/2 for TR1-C and TR2-C (average recorded depths = 9.2 and 11.8 m, respectively).
Based on temperature and dissolved oxygen data, these dates represented the
elimination of thermal stratification at each site, with the time difference
attributed to the large variation in average depths. Surface temperatures
throughout the lake are spatially and temporally homogeneous (Figures 21, 22)
(Iannuzzi, 1991a; b). Winter temperatures fluctuated between 0.0o C
and 4.0o C with
the average for each site remaining less than
2.0o C until the first week of April when ice breakup began. Surface
temperatures then steadily increased from a range of 4.0o C to
5.1o C during spring turnover, to the summer maxima range of
24.4o C to 26.8o C during the first two weeks of August.
Temperatures then steadily declined into the fourth week of November when fall
turnover occurred. The temperature range during this period was 4.6o
C to 6.8o C. Weekly temperature increases from spring turnover until
the summer maxima, and alternate decreases through to fall turnover, occurred
at an average rate of approximately 1.5o C per week.
Ice coverage of littoral areas, as observed in Rat Cove, Blackbird Bay, the
Cooperstown waterfront at the south end of the lake, and Hyde Bay at the north
end of the lake, began during the first week of January in 1988. Complete ice
coverage throughout the lake was firs
t observed on 1/8 and lasted through the
first week of April, when mid-lake breakup began. Ice cover remained in the
aforementioned littoral areas through 4/6 with the exception of Hyde Bay, in
which coverage lasted until 4/10. On 4/12 no ice cover remained on the lake
(Iannuzzi, 1991a; b).
Spring turnover ensued immediately after ice breakup and lasted for a period of
two to three weeks. Summer stratification began during the first week of May
(sample dates 5/5-6). Maximum stratification, he
re defined as the largest
difference between surface and bottom temperatures at the mid-lake sites, was
reached during the first week of August (sample dates 8/4-5), typically
coinciding with surface temperature maxima throughout the lake. Erosion of the
thermocline ensued with the cooling of surface waters, and lasted approximately
through the third week of November (sample dates 11/18-19). Fall turnover
occurred during either the latter half of the third week, or the beginning of
the fourth week of
November (sample dates 11/27-29), and lasted through the
onset of ice cover (Iannuzzi, 1991a; b).
Ice coverage of littoral areas began early in the 1988-89 winter season, with
most such areas covered during the third week of December, coinciding with the
final sample date (12/17).
Iannuzzi's work documented the spatially homogeneous nature of the physical and
chemical water quality characteristics of Otsego Lake that are well represented
by analysis of data collected at the center of
the lake (TR4-C). This
information led to a systematic year-round monitoring program at that site that
began in 1992 and continues to the present (Homburger and Buttigieg, 1992;
Groff et al.,1993; Harman, 1994a).
The summer heat income (wind distributed heat and surface radiation) is
21,170gm-cal, according to Tressler and Bere (1936). In the hypolimnion,
15,120gm-cal, or over 60%, are stored.
The lake is usually covered with ice in January, and breakup occurs in
Marc
h or April. In most years, as in 1988, the last area to freeze is north of
the center of the basin, where Hyde Bay joins with the lake proper. The
shortest period of ice cover ever recorded was in the winter of 1912-1913 when
the lake did not freeze until February 18, 1913, with the breakup occurring on
March 21, 1913. Data from Pack and Hollis (1973) and Hollis (1994) indicating
periods of ice cover since 1842 are illustrated in Figure 23.
Information on the approximate thickness atta
ined by the ice each winter
was collected from 1971 to 1975 in Rat Cove, and sporadically to date at the
Biological Field Station. In 1970-1971, it reached a thickness of about 30cm,
the thickest recorded. In mild winters, under snow cover, the thickness often
is much less (24 cm in 1974-75).
Shoreline alteration and damage of artificial structures on the shore (e.g.
breakwaters) due to lake ice occurs in two ways: 1. by expansion and
contraction associated with temperature changes throughout
the winter and
spring before breakup and, 2. by moving ice during the meteorological events
responsible for breakup of ice cover.
Most ice damage on Otsego Lake can be attributed to the former, which heaves
rip-rap and breakwaters and often pushes natural unconsolidated beach materials
into large berms parallel with the water. Ice breakup is usually not
accompanied by extensive catastrophic change in the eulittoral environment
because the ice is not often moved by wind until it is structura
lly weakened by
warm spring weather. Upon coming in contact with the shore or any solid object,
ice 12cm or more in thickness will typically break up easily into pencil-shaped
columnar crystals. If, however, the ice starts to move before its structural
integrity has been weakened, extensive damage may occur in areas exposed to the
prevailing winds.