Inorganic carbon can be in short-supply in freshwater relative to that needed by freshwater plants for photosynthesis because of a large external transport limitation coupled with frequent depleted concentrations of CO2 and elevated concentrations of O2. Freshwater plants have evolved a host of avoidance, exploitation and amelioration strategies to cope with the low and variable supply of inorganic carbon in water. Avoidance strategies rely on the spatial variation in CO2 concentrations within and among lakes. Exploitation strategies involve anatomical and morphological features that take advantage of sources of CO2 outside of the water column such as the atmosphere or sediment. Amelioration strategies involve carbon concentrating mechanisms (CCM) based on uptake of bicarbonate, which is widespread, C4-fixation which is infrequent and Crassulacean Acid Metabolism (CAM) which is of intermediate frequency. CAM enables aquatic plants to take up inorganic carbon in the night. Furthermore, daytime inorganic carbon uptake is generally not inhibited and therefore CAM is considered to be a carbon conserving mechanism. CAM in aquatic plants is a plastic mechanism regulated by environmental variables and is generally down-regulated when inorganic carbon does not limit photosynthesis. CAM is regulated in the long term (acclimation during growth), but is also affected by environmental conditions in the short term (response on a daily basis). In aquatic plants CAM appears to be an ecologically important mechanism for increasing inorganic carbon uptake, since the in situ contribution from CAM to the C-budget generally is high (18-55%)
