Invasive submerged freshwater macrophytes are more plastic in their response to light intensity than to the availability of free CO₂ in air-equilibrated water

The future increase in the atmospheric CO₂ concentration is likely to affect the growth and performance of submerged freshwater macrophytes because of higher concentrations of free CO₂ in the water at air equilibrium. We measured the plastic responses to free CO₂ and light for several traits of four invasive aquatic plants (Elodea canadensis, Egeria densa, Hydrilla verticillata and Ceratophyllum demersum) that use bicarbonate. The plants were grown in standard culture media at either high (HiC, c. 50 μmol L^-1) or low concentrations of free CO₂ (LowC, c. 19 μmol L^-1) combined with high (HiL, c. 150 μmol m^-2 s^-1) or low (LowL, c. 21 μmol m^-2 s^-1) light intensity in a 2 × 2 factorial experiment. We compared the relative growth rate (RGR), several morphological traits, the photosynthetic response to light intensity and the bicarbonate uptake capacity of the four species. For every trait measured, we established plasticity indices for light and CO₂ availability. Light intensity had a greater effect than CO₂ concentration on all species. The RGR of all four species was higher at high light intensity, and photosynthetic light responses acclimated to low light with a lower light compensation point, and with higher concentrations of photosynthetic pigments and quantum yield. The bicarbonate uptake capacity was generally highest at the high light intensity and high concentrations of free CO₂. Plasticity indices for light intensity were consistently higher than for CO₂ availability in all four species. As the growth and photosynthesis of the four invasive bicarbonate users were only slightly affected by the CO₂ availability in air-equilibrated water, the future rise in atmospheric CO₂ is unlikely to exacerbate their invasive behaviour and may even reduce their competitiveness against species unable to use bicarbonate, especially at limiting light.