Diel and Annual Cycles of Net Plankton Photosynthesis in Lake Ontario

Abstract

Photosynthesis measurements on net plankton have been carried out for the period April, 1972 to January, 1973 in Lake Ontario. The rising and falling light regimes used on the net phytoplankton (> 64 μ) give marked photosynthetic hysteresis effects in winter and in spring for diatom-dominated populations. Diel fluctuations in photosynthesis and "sun" or "shade" adaptations showed a close interaction between the phytoplankton and the surface light intensity. Systematic seasonal changes have been recorded in the magnitude of the photosynthetic hysteresis effect, saturation light intensities, compensation points, maximum photosynthesis, and respiration rates. An inverse correlation has been found between net plankton mean maximum photosynthesis rates and the rate of change of water temperature. Also phytoplankton respiration rate is largely a function of temperature.The data have been unified to give a picture of daily photosynthesis patterns at different depths in a static water column. The data compare well with ¹⁴C moored bottle experiments. Predictive equations have been calculated for maximum photosynthesis rates at different times of the year.