Abstract: | Rates of photosynthesis of four submerged stream macrophyteswere examined under varying pH and composition of inorganiccarbon species. Callitriche stagnatis and Sparganium simplexused only CO2 for photosynthesis. Potamogeton crispus and P.pectinatus used HCO3 in addition to CO2, but with much lowerefficiency. The photosynthetic rates at air equilibrium anda total inorganic carbon concentration of 5.0 mM were 23times lower than maximum rates at CO2 saturation for the HCO3users and 1014 times lower for the CO2 users. The CO2compensation point of entire plants of Callitriche (2.5 µM)and Sparganium (6.0µM) was well below the equilibriumconcentration (15 µM). and the low saturation points (250500µM) also pointed to efficient use of CO2. Callitricheand Sparganium compete successfully with HCO3 users inhardwater streams, which have a higher exchange and generationcapacity of CO2 than stagnant and more soft waters. Rates ofphotosynthesis of Potamogeton crispus and P. pectinatus decreasedat high pH. Depending on the two alternative hypotheses forHCO3use, this decline can be explained by CO3inhibition of HCO3 uptake or by increasing capacity tobuffer H+efflux from the plant. Habitats subject to high pH,e. g. small ponds with dense vegetation, may have a strong selectionfor efficient mechanisms of HCO3 use. Key words: Photosynthesis, Macrophytes, Carbon-source |