Response capacity to CO2 depletion rather than temperature and light effects explain the growth success of three alien Hydrocharitaceae compared with native Myriophyllum triphyllum in New Zealand

• We studied the impact of light, temperature and carbon on four submerged aquatic plant species.
• The growth of H. verticillata, E. densa and L. major was affected by temperature.
• The growth of Myriophyllum triphyllum was mainly affected by the CO2 availability.
• Response to CO2 depletion is a major factor for plant success in dense macrophyte stands.

Light, temperature and the availability of carbon and nutrients are major factors affecting aquatic plant growth. A three factorial experimental design in a large outdoor mesocosm experiment was used to evaluate the effects of light, temperature and carbon on the growth parameters of RGR (relative growth rate), LDMC (leaf dry matter content) and root:shoot ratio as well as pigment characteristics of invasive Hydrilla verticillata, Egeria densa and Lagarosiphon major and a native New Zealand milfoil, Myriophyllum triphyllum. The RGR of H. verticillata, E. densa and L. major was mainly affected by temperature, while RGR in M. triphyllum was predominantly affected by carbon. In H. verticillata light had a significant effect on the LDMC, whereas the LDMC of the other species was predominantly affected by carbon.The pigment contents of all species were significantly higher in the low light treatments, and for H. verticillata, E. densa and M. triphyllum a significant effect of carbon was found. The Chla:b ratio tended to increase with decreasing temperature (except for M. triphyllum) and decreasing light availability (except for E. densa). Carbon only had a significant effect on Chla:b ratio of M. triphyllum. Generally, the growth and pigments of the three Hydrocharitaceae were mostly affected by temperature and light, while in M. triphyllum a strong effect of carbon was found. This could in part explain the relative success of the three exotic species within New Zealand waters, where M. triphyllum has been displaced by dense populations of these species in numerous waters.