Utilization of sediment CO2 by selected North American Isoetids
The photosynthetic uptake of root-zone CO2 was determined for Eriocaulon septangulare, Gratiola aurea, Isoetes macrospora, Littorella uniflora var. americana and Lobelia dortmanna as part of a study of the photosynthetic carbon economy of submerged aquatic isoetids. The pH and dissolved inorganic carbon (DIC) of the sediment interstitial water in four Wisconsin lakes reflected the water column character, where the DIC increased with depth in the sediment to concentrations five to ten times those of the water column. Sediment free CO2 concentrations were 5–50 times those in the water column and were similar at all sites (about 05–1.0mM CO2 in the root-zone). In ‘pH-drift’ studies these plants were unable to take up HCO2-. Laboratory determinations of the carbon uptake from the root and shoot-zones were made for all five species. These experiments showed that CO2 in the root-zone accounted for 65–95 per cent of external carbon uptake for the five species. For G. aurea and E. septangulare, root-zone CO2 was \textgreater 85 per cent of carbon uptake.