Interactions of inorganic carbon and light availability as controlling factors in aquatic macrophyte distribution and productivity

The roles of inorganic carbon and light availability in the quantitative distribution and photosynthetic productivity of submersed aquatic macrophytes were investigated in two lakes in a stream‐connected hard‐water lake chain where light availability and pH increase and total inorganic carbon decreases as water flows from a turbid productive lake through progressively clearer less productive lakes. Bicarbonate‐using species dominated the macrophyte communities of both lakes, but species requiring free CO 2 were present, primarily in the more turbid lake. In photosynthesis experiments, CO 2 ‐requirer Najas flexilis was significantly competitive when light limitation reduced the efficiency of HCO 3 − ‐user Potamogeton pectinatus. The results suggest that inorganic carbon availability can be important in macrophyte growth even within relatively small ranges of total alkalinity and that light availability is a potentially important interacting factor.