XVI International Botanical Congess

Carbon uptake by submersed plants is potentially limited by CO2 availability. Since CO2 concentrations are typically higher in sediments than in the water column, a strategy for sediment CO2 uptake would be advantageous. The network of lacunae in macrophytes provides a pathway for sediment CO2. We report very high concentrations of CO2 in leaf lacunae of Vallisneria that do not vary with light/dark cycles, (in contrast to diurnal changes in internal O2 concentrations). The gradient of CO2 concentration from leaf base to tip is similar to that of CH4 and opposite to the O2 gradient, suggesting influx of CO2 and CH4 from sediments. Plants grown in sediments with contrasting delta-13C values have leaf and lacunar gas delta-13C values that reflect sediment pore-water values, rather than water column values, indicating that the majority of carbon fixed is sediment-derived.