Inorganic carbon limitation of photosynthesis in lake phytoplankton

1. Inorganic carbon availability influences species composition of phytoplankton in acidic and highly alkaline lakes, whereas the overall influence on community photosynthesis and growth is subject to debate. 2. The influence of total dissolved inorganic carbon (DIC) and free CO2 on community photosynthesis was studied in six Danish lakes during the summer of 1995. The lakes were selected to ensure a wide range of chlorophyll a concentrations (1-120 mu g l(-1)), pH (5.6-9.6) and DIC concentration (0.02-2.5 mM). Photosynthesis experiments were performed using the C-14 technique in CO2-manipulated water samples, either by changing the pH or by adding/removing CO2. 3. Lake waters were naturally CO2 supersaturated during most of the experimental period and inorganic carbon limitation of photosynthetic rates did not occur under ambient conditions. However, photosynthesis by phytoplankton in lakes with low and intermediate DIC concentrations was seriously restricted when CO2 concentrations declined. Similarly photosynthesis was limited by low CO2 concentrations during phytoplankton blooms in the hardwater alkaline lakes.