A community of submerged aquatic CAM plants in Lake Kalgaard, Denmark

Abstract

The CO 2 exchange pattern of leaves of dominant Isoetes lacustris L., Littorella uniflora (L.) Aschers. and Lobelia dortmanna L. from oligotrophic, carbon-poor Lake Kalgaard, Denmark was examined by gas-exchange experiments and by following the diurnal acidity rhythm. Both variables suggest Crassulacean Acid Metabolism (CAM) in Littorella and Isoetes. During the dark, both species had a continuous, but declining CO 2 uptake. The maximum uptake in Littorella was 28 μmol CO 2 g −1 DW h −1 or about 36% of the light CO 2 assimilation rate. In Isoetes, corresponding figures were 19 μmol CO 2 g −1 DW h −1 and 22%. A peak of 49.5 μEq g −1 FW titratable acidity ( Littorella) and 41.9 μEq g −1 FW ( Isoetes) was found at the end of the dark period, reflecting an increase of 11.6 μEq g −1 FW (9 h) −1 ( Littorella) and 16.3 μEq g −1 FW (9 h) −1 ( Isoetes). These results agreed with the cumulative CO 2 assimilation during the dark. The assimilation of CO 2 in the dark is important, apparently, for growth of Littorella and Isoetes in Lake Kalgaard. Lobelia, which exhibits little dark CO 2 assimilation, is also the least abundant species, at less than 1% of the total cover.