Increased CO2 in the water around Littorella uniflora raises the sediment O2 concentration

In the present study, we set up laboratory experiments with the isoetid plant Littorella uniflora to test whether higher water column CO2 concentrations affect (1) O2 concentrations in leaves and sediment, and (2) nutrient dynamics in the porewater. The experiments showed that at 175 μM CO2 in the water column, which is 10-fold atmospheric equilibrium, a higher biomass of L. uniflora developed (from 302 ± 51 to 390 ± 86 g dry wt m−2) and there were higher O2 concentrations in the leaves (from 120 up to 160% of air saturation) and in the porewater of the sediment (1.7-fold higher). The O2 concentration in particular increased in the upper 10–30 mm of the sediment at the elevated CO2 concentrations. The increase in biomass of L. uniflora mostly derived from a significant increase in the leaf biomass (from 97 ± 14 to 137 ± 21 g DW m−2), while root biomass at high CO2 (253 ± 82 g dry wt m−2) was similar to that of plants growing at low CO2 (205 ± 40 g dry wt m−2). This implied that the increase in O2 concentration in the sediment was a result of increased O2 production in the leaves rather than of increased root biomass. Porewater samples from three sediment depths (20, 70 and 120 mm) showed reduced NO3− concentrations at high CO2 concentration, especially at 20 mm depth layer (mean values for low CO2 ranged from 25 to 60 μM while they were below 25 μM at high CO2), presumably as a result of uptake by L. uniflora. NH4+ concentrations increased with depth (from nearly 0 up to 100 μM) but were not significantly related to the CO2 concentration. The PO43− concentrations were low (less than 0.5 μM) and similar at both low and high CO2 treatments at all three depths. The study showed that photosynthesis of L. uniflora is limited by CO2 at ambient concentrations and that higher CO2 concentrations result in higher O2 release to the sediment which is important for the cycling and retention of nutrients in sediments of oligotrophic softwater lakes.