Carbon acquisition and carbon dynamics by aquatic isoetids

Isoetids are small, submerged vascular plants that often dominate the vegetation of soft-water lakes. The isoetids have short stiff leaves and large roots that might make up more than 50% of plant biomass. Leaves and roots have a system of internal gas spaces that allow efficient intra-plant gas transport and are the morphological basis for the remarkable use of sediment CO(2) in photosynthesis by isoetids. This ability allows the plants to exploit the rich CO(2) pool in the sediment of soft-water lakes and thereby partly ameliorate the low CO(2) availability in the bulk water. The transport Of CO(2) from roots to leaves is by diffusion in the lacunae. Diffusion is relatively slow over longer distances and appears to set an upper limit to the leaf-length of isoetids. Quantitatively, the use of sediment CO(2) accounts for most of the inorganic carbon assimilated by isoetids and only at very high CO(2) concentrations in the bulk water significant amounts are taken up through the leaf surface. Physiologically some of the isoetids are unique among submerged macrophytes by featuring the crassulacean acid metabolism (CAM).,CAM in isoetids is considered a carbon conserving mechanism and is an ecological advantage in typical isoetid habitats. CAM allows the plants to take up CO(2) from the water at night when competition from other plants is eliminated and, since daytime CO(2) uptake is unaffected by CAM, the time-slot during which CAM-isoetids can assimilate inorganic carbon is expanded compared to non-CAM species. The unique morphological and physiological traits and adaptations of isoetids allow them to prosper under the conditions prevailing in soft-water lakes. Still, growth is restrained by low resource availability and by an inherent low capacity for growth. Isoetids grow throughout the year with higher rates in summer and lower in winter. Annual leaf turnover rates are low and correspond to a leaf lifetime as long as 700 days in some species. A close relationship between production and mortality of leaves results in a relatively constant biomass over the year with only slightly higher values in summer than winter. However, the small stature and low weight of isoetid plants only allow moderate standing biomass per ground area despite high plant densities and accordingly the low leaf turnover rates result in low annual primary production in isoetid populations. (C) 2002 Elsevier Science B.V. All rights reserved.