The Interdependence of Photon Irradiance and Free Carbon Dioxide or Bicarbonate Concentration on the Photosynthetic Compensation Points of Freshwater Plants

Summary The free CO2 (CO2*) and HCO3− compensation points of eight freshwater macrophytes and four phytoplankters were measured using a pH‐drift technique developed by Allen and Spence (1981). Below a critical photon irradiance, which was about that at which net photosynthesis was half‐maximal, CO2* and HCO3− compensation points increased with decreasing photon irradiance. However, the CO2* compensation point of two species that were able to use HCO3− were little or not affected by photon irradiance. The light compensation point increased at low CO2* or HCO3− concentrations. The simultaneous generation by phytoplankton of light and carbon depletion could have a large deterimental effect on macrophyte performance.