Growth limitation of submerged aquatic macrophytes by inorganic carbon

Abstract

SUMMARY1. This study determined the effects of CO2 and HCO3‐ enrichment on in situ growth of two submerged macrophytes, Elodea canadensis and Callitriche cophocarpa, in two Danish lakes: Lake Hampen and Lake Væng. Lake Hampen is an oligotrophic low‐alkaline lake (0.4 meq −1) and Lake Væng is mesotrophic with an alkalinity of 1.1 meq 1‐−1. In Lake Hampen experiments were carried out throughout the growth season, whereas experiments in Lake Væng were restricted to late summer. The CO2 and HCO3‐enrichment procedures used increased the concentration of free‐CO2 by 500–1000 μM and the concentration of HCO3‐ by about 80 μM.2. The concentration of free‐CO2 in Lake Hampen was about five times atmospheric equilibrium concentration (55 μM) in early summer declining to virtually zero at the end of summer.3. Under ambient conditions Callitriche, which is restricted to CO2 use, was unable to grow and survive in both lakes. In contrast, Elodea, which has the potential to use HCO3‐ in photosynthesis, grew at rates varying from 0.046 to 0.080 day−1 over the season.4. Under CO2 enrichment the growth rate of Callitriche varied from 0.089 to 0.124 day−1 and for Elodea from 0.076 to 0.117 day−1 over the season. Enrichment with HCO3‐affected Elodea only and only to a limited extent. This may be a result of insufficient increase in [HCO3‐] upon enrichment or to a limited capacity of the plants to take up HCO3‐.5. The substantial stimulation of in situ growth of Elodea and Callitriche by enhanced concentrations of free‐CO2 shows that inorganic carbon is an important determinant of growth of submerged macrophytes and that inorganic carbon limitation of in situ growth may be a common phenomenon in nature, even in lakes with an alkalinity as high a 1 meq 1‐−1. Inorganic carbon, however, is only one of many parameters important for growth, and the growth rates of Elodea at both ambient and high free‐CO2 were closely coupled to day length and photon irradiance, indicating that light had an ultimate control on growth.