Photosynthetic Inorganic Carbon use by Freshwater Plants

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Abstract
(1) A pH-drift technique was used to calculate free CO2 (CO* 2) and bicarbonate (HCO- 3) compensation points for a range of freshwater macrophytes and phytoplankters. Some half-saturation constants for CO2 uptake were also estimated. The validity of the technique and interpretation is discussed and generally confirmed. (2) Variation in the CO* 2 compensation point of Hippuris vulgaris was related to the oxygen concentration of the bathing medium, and seasonal changes in (HCO- 3) compensation point were found in Potamogeton filiformis and in HCO- 3 and CO* 2 compensation points in Elodea canadensis. The performance of these two macrophytes is probably affected more by seasonal changes in HCO- 3 compensation points than by CO* 2 compensation points. (3) The quotient of total carbon remaining at the end of a pH-drift experiment to the constant alkalinity of the solution (CT/Alk) was used as a measure of the uptake ability of a species. Ranked by this criterion, species showed a fairly continuous range in uptake ability with a nearly fifteen-fold difference between the most efficient (low quotient) and least efficient (high quotient) species. (4) Although the uptake abilities of macrophytes and phytoplankters overlapped, the four most efficient carbon extractors were phytoplankters, and the nine least efficient were macrophytes. Anabaena cylindrica, Microcystis aeruginosa and Scenedesmus quadricauda removed more than 90% of the available carbon compared with 70% for the most efficient macrophytes tested (Myriophyllum spicatum and Potamogeton perfoliatus). (5) Those species which remove 5% or less of the available carbon are taken to be unable or poorly able to use HCO- 3. Such macrophytes may benefit from: (i) growing in unproductive lakes that do not experience carbon depletion; (ii) seasonal access to aerial CO2; (iii) high CO* 2 concentrations in and close to the sediment. (6) Competition for inorganic carbon may be an important factor determining the outcome of seasonal and longer-term changes in the species composition of productive freshwaters. The reduction or loss of submerged macrophytes in certain recently enriched lakes may result partly from carbon competition with phytoplankton.