Carbon dioxide and submersed macrophytes in lakes: linking functional ecology to community composition

Evaluating plant community response to atmospheric CO 2 rise is critical to predicting ecosystem level change. Freshwater lakes offer a model system for examining CO 2 effects as submersed macrophyte species differ greatly in their growth responses to CO 2 enrichment, and free CO 2 concentrations among these habitats show a wide range of natural, spatial variation. We determined free CO 2 concentrations in the water column and sediment porewater in littoral zones with pH < 6.0 in Adirondack Mountain (New York, USA) lakes, and derived a community CO 2 responsiveness index (CCRI) based on quantitative sampling of 15 submersed macrophyte communities coupled with greenhouse‐derived growth responses to CO 2 enrichment of constituent species to test two hypotheses: (1) CCRI, which is higher for communities dominated by species with greater growth responses to CO 2 enrichment, is positively correlated to free [CO 2 ] in the water column, and (2) in natural communities, the percent of sediment CO 2 ‐using species, which are relatively unresponsive to CO 2 enrichment, is negatively correlated to free [CO 2 ]. A significant positive correlation ( P  = 0.003) between our physiologically based CCRI and the concentration of free CO 2 in the water column supported our primary hypothesis that sites with higher levels of free CO 2 are dominated by species with greater growth responses to CO 2 enrichment. Our CCRI is also highly significantly correlated ( P  < 0.001) to the first axis scores for the same vegetation data from polar ordination. Finally, the relative importance of species that use sediment CO 2 as a photosynthetic carbon source is significantly negatively correlated ( P  = 0.029) with the concentration of free CO 2 in the water column. Our results indicate that natural variations in CO 2 levels are important determinants of submersed macrophyte community composition. Further, we demonstrate the utility of a physiologically‐based index of community composition, our CCRI, as an ecologically valid measure of community response to CO 2 .