Photosynthetic-irradiance (P-I) curves and partitioning of photosynthate into major end-products (protein, lipids, polysaccharides, and low molecular weight [LMW] metabolites) were examined for phytoplankton communities from Lakes Huron and Michigan. The mean and variance of P-I parameters and photosynthetic end-products were similar in both lakes. Mean PMB (maximum light saturated rate) and α (initial linear slope) values were 2.3 mg C·mg Chl−1·h−1 and 5.5 mg C·mg Chl−1·Einst−1·m2 for Lake Huron communities, and 2.4 mg C·mg Chl−1·h−1 and 7.0 mg C·mg Chl−1·Einst−1·m2 for Lake Michigan communities. The mean percent incorporation of 14CO2 into proteins, lipids, polysaccharides, and LMW metabolites from short-term experiments (2-4 h) were 32.4, 21.3, 28.0, and 18.9, respectively, for Lake Huron communities and 34.8, 24.7, 24.5, and 15.8, respectively, for Lake Michigan communities. Over longer incubations the activity in each end-product increased linearly during the day; during the night the activity in the LMW and polysaccharide fractions decreased and the activity in the protein fraction increased. There were significant seasonal variation in P-I parameters and the photosynthetic end-products. In both lakes, phytoplankton communities from the late winter-spring isothermal period were characterized by lower PMB values, higher α values, significant susceptibility to photoinhibition, and less incorporation into protein, as compared to communities from periods when the lakes were thermally stratified.
- Primary productivity
- light intensity