The summer of 1967 was exceptionally warm and calm, creating conditions favorable to thermal stratification. On August 22, 1967, the thermal gradient was unusually pronounced for Crater Lake (Figure 10). The onset of thermal stratification was apparent on July 22, 1968, (Figure l l ), but further stratification was destroyed by a month-long period of extremely cold and windy weather. The thermal gradient had lessened by the last sampling period in late August (Figure 12).
Figure 10. Thermal profile and Secchi disc depth on August 22, 1967 in Crater Lake, Oregon.
Figure 11. Thermal profile and Secchi disk depth 0″ July 22, 1968, in Crater Lake, Oregon
Variations between Secchi disk readings were affected more by weather conditions than optical properties of the lake. Estimates of primary production showed a maximum 14c uptake at 80 m on June 14, 1968 (Figure 13), with very little uptake of 14C above 30 m. On July 25, and August 27, 1968, this difference in primary production with depth had disappeared. An increasing uptake of 14c in the shallower depth strata and a decreasing uptake between 60 to 100 m created an apparent orthograde condition.
It is likely that conditions do exist when food is absent from the surface waters. The C data recorded on July 24, 1967, appeared similar to the data of June 14, 1968. Utterback & (1942) in their study of the distribution of phytoplankton in Crater Lake found very few cells above 30 m. Their reported maximum concentration was at 75 m with large numbers occurring down to 200 m.
Figure 12. Thermal profile and Secchi disc depth on August 27, 1968, in Crater Lake, Oregon. *Read during extremely rough weather: cold, windy, rain.
Figure 13. Primary productivity (carbon-14) in Crater Lake, 1968, represented in total counts per minute. (Courtesy of D. W. Larson)
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