Estimation of in situ primary product ion from parameters of the photosynthesis-light curve obtained in laboratory incubators

Photosynthesis in Lake Constance phytoplankton was assessed in situ at the depths of sample collection, and in parallel in incubators in white and green light, using phytoplankton from two sampling depths. Green light (540-560 nm) is best transmit­ted by Lake Constance water. Light-saturated photosynthetic rates as measured in incubators can be corrected to in situ temperatures with reasonable accuracy by using a Qio of 2.3. This quotient was determined empirically for Lake Constance phyto­plankton by year-round comparisons of in situ maximum assimilation numbers with values obtained in incubators at one single water temperature. Light-limited rates of photosynthesis are dependent on the spectral properties of the light. In the incubator with green light, the initial slope during most experiments was smaller than in concomitant laboratory incubations using white light. These differences in slope between green and white light can be attributed to differences in the light absorption cross-sections of the algae, which should be smaller in green than in white light. The initial slope of the photosynthesis versus irradiance curve as measured in situ frequently was greater than in the incubator with white light, although the light absorption cross-section should be smaller in the lake whose underwater light is predominantly green. Enhanced initial slopes observed in situ were probably due to vertical variations in photo-acclimation of the phytoplankton. Both light-saturated and light-limited photosynthetic rates vary between phytoplankton collected at different water depths, probably due to differences in photo-acclimation. For predic­tions of phytoplankton productivity from laboratory incubations, temperature correc­tions of light-saturated photosynthetic rates are recommended. The range of light intensities in the incubator should match the range encountered in situ. For realistic predictions of light-limited photosynthesis, the spectral composition of the light in the incubator should resemble the spectral features of the underwater light field. It is suggested, moreover, that shallow phytoplankton be used to simulate near-surface photosynthesis, and deep-living algae to estimate light-limited photosynthesis.

Article from Marine Science Symposia Vol. 197 - "Measurement of Primary Production from the Molecular to the Global Scale". Symposium held in La Rochelle, 21-24 April 1992. To access the remaining articles please click on the keyword "MSS Vol. 197".