The temperature optimum for photosynthesis of natural populations of bluegreen
algae (cyanobacteria) from Lake Mendota was determined during the
period of June to November 1976. In the spring, when temperatures ranged from
0 to 200C, there were insignificant amounts of blue-green algae in the lake (less
than 1% of the biomass). During the summer and fall, when the dominant
phytoplankton was blue-green algae, the optimum temperature for photosynthesis
was usually between 20 and 300C, whereas the environmental temperatures
during this period ranged from 24°C in August to 120C in November. In general,
the optimum temperature for photosynthesis was higher than the environmental
temperature. More importantly, significant photosynthesis also occurred at low
temperature in these samples, which suggests that the low temperature alone is
not responsible for the absence of blue-green algae in Lake Mendota during the
spring. Temperature optima for growth and photosynthesis of laboratory cultures
of the three dominant blue-green algae in Lake Mendota were determined. The
responses of the two parameters to changes in temperature were similar; thus,
photosynthesis appears to be a valid index of growth. However, there was little
photosynthesis by laboratory cultures at low temperatures, in contrast to the
natural samples. Evidence for an interaction between temperature and low light
intensities in their effect on photosynthesis of natural samples is presented.