Photosynthetic response of two submersed macrophytes to dissolved inorganic carbon concentration and pH1

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Abstract
The photosynthetic responses of Myriophyllum spicatum and Vallisneria americana to total dissolved inorganic carbon (DIC) concentration (pH 8.0; 25°C; saturating light) indicate that both species are sensitive to changes in DIC concentrations at natural lake levels, and that M. spicatum has a somewhat lower half‐saturation constant than V. americana for that relationship. This difference is probably not ecologically significant.Carbon uptake rates of V. americana decline by 61% from pH 7 to 8, but change little from pH 8 to 9, whereas those of M. spicatum decline by 35% from pH 7 to 8 and by a further 58% from pH 8 to 9. These decreases do not parallel the corresponding decline in free CO2 availability over this pH range.Carbon compensation points expressed as total DIC, free CO2, or the atmospheric CO2 concentration in equilibrium with the corresponding free CO2 level are strongly pH‐dependent and do not differ substantially between M. spicatum and V. americana.