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

1. 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. 50molL(-1)) or low concentrations of free CO2 (LowC, c. 19molL(-1)) combined with high (HiL, c. 150molm(-2)s(-1)) or low (LowL, c. 21molm(-2)s(-1)) light intensity in a 2x2 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.