Temperature, Transparency, and Phytoplankton Productivity in Crater Lake, Oregon1

Maximum productivity and the largest concentration of chlorophyll a in Crater Lake during summer 1968 and 1969 occurred at depths where temperatures were near 4C and measurable light <4% of surface illumination. It is suggested that the phytoplankton in the lake consists mostly of oligothermal‐oligophotic populations that are limited to depths greater than 70 m in summer, but occupy the 0–70‐m stratum in winter, so that a rather constant rate of production per unit area is maintained throughout the year.