Comparisons of δC values in leaves of aquatic macrophytes from different habitats in Britain and Finland; some implications for photosynthetic processes in aquatic plants.

The δC values of submerged aquatic plants from contrasting but relatively defined habitats, and the δC values of emergent, floating and submerged leaves of dimorphic aquatic plants, were measured. In many instances the δC values of dissolved inorganic carbon in the water were also measured. Plant δC values in the vicinity of-40 to-50‰ were found in rapidly flowing spring waters with carbonate δC values of-16 to-21‰, consistent with the notion that species such as Fontinalis antipyretica almost exclusively assimilate free CO via RuP carboxylase. Plant δC values in the vicinity of-10 to-15‰ in sluggish water with carbonate δC values of about-5‰ were observed, consistent with the notion that boundary layer diffusion and/or HCO uptake may determine the δC value of submerged aquatic plants in these circumstances. Comparisons of δC values of the same or related species growing in waters of similar carbonate δC value but different flow rates confirmed this view; more negative δC values were frequently associated with plants in fast moving water. In Britain, but not in Finland, the δC values of submerged leaves of dimorphic plants were almost invariably more negative than in aerial leaves. The δC value of carbonate from chalk streams and in acid springs indicate substantial inputs of respiratory CO, as opposed to atmospheric carbon. The contributions of these variations in δC of the carbon source, and of isotope fractionation in diffusion, to the δC value of submerged parts of dimorphic plants is discussed.