Temporal variations in carbon isotope ratio of phytoplankton in a eutrophic lake

Temporal variations in carbon isotope ratio of phytoplanktonand dissolved inorganic carbon (DIC) in Lake Suwa were reported.In summer, blooming of Microcystis spp. resulted in low concentrationsof DIC and high pH, and HCO₃^– was the prominent speciesof DIC. Chlorophyll-specific rates of photosynthesis were relativelyconstant irrespective of the algal biomass during summer. Carboxylationin photosynthesis of Microcystis spp. was mainly catalyzed byribulose bisphosphate carboxylase (RuBPCase). Carbon isotopediscrimination between ¹³C of phytoplankton and DIC was considerablysmall in early summer and appeared to be negatively correlatedto DIC concentration. We concluded that carbon fixation by phytoplanktonin Lake Suwa is controlled not by the switch of photosyntheticpathways, but by low DIC concentration and high pH, suggestingthat photosynthesis of Microcystis spp. in Lake Suwa is governedby uptake kinetics other than the carboxylation step.