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

Evaluating plant community response to atmospheric CO₂ rise is critical to predicting ecosystem level change. Freshwater lakes offer a model system for examining CO₂ effects as submersed macrophyte species differ greatly in their growth responses to CO₂ enrichment, and free CO₂ concentrations among these habitats show a wide range of natural, spatial variation. We determined free CO₂ 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 CO₂ responsiveness index (CCRI) based on quantitative sampling of 15 submersed macrophyte communities coupled with greenhouse-derived growth responses to CO₂ enrichment of constituent species to test two hypotheses: (1) CCRI, which is higher for communities dominated by species with greater growth responses to CO₂ enrichment, is positively correlated to free [CO₂ ] in the water column, and (2) in natural communities, the percent of sediment CO₂ -using species, which are relatively unresponsive to CO₂ enrichment, is negatively correlated to free [CO₂ ]. A significant positive correlation (P = 0.003) between our physiologically based CCRI and the concentration of free CO₂ in the water column supported our primary hypothesis that sites with higher levels of free CO₂ are dominated by species with greater growth responses to CO₂ 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 CO₂ as a photosynthetic carbon source is significantly negatively correlated (P = 0.029) with the concentration of free CO₂ in the water column. Our results indicate that natural variations in CO₂ 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 CO₂ .