Comparisons of δ¹³C values in leaves of aquatic macrophytes from different habitats in Britain and Finland; some implications for photosynthetic processes in aquatic plants

Summary

The δ¹³C values of submerged aquatic plants from contrasting but relatively defined habitats, and the δ¹³C values of emergent, floating and submerged leaves of dimorphic aquatic plants, were measured. In many instances the δ¹³C values of dissolved inorganic carbon in the water were also measured. Plant δ¹³C values in the vicinity of-40 to-50‰ were found in rapidly flowing spring waters with carbonate δ¹³C values of-16 to-21‰, consistent with the notion that species such as Fontinalis antipyretica almost exclusively assimilate free CO₂ via RuP₂ carboxylase. Plant δ¹³C values in the vicinity of-10 to-15‰ in sluggish water with carbonate δ¹³C values of about-5‰ were observed, consistent with the notion that boundary layer diffusion and/or HCO₃ ^- uptake may determine the δ¹³C value of submerged aquatic plants in these circumstances. Comparisons of δ¹³C values of the same or related species growing in waters of similar carbonate δ¹³C value but different flow rates confirmed this view; more negative δ¹³C values were frequently associated with plants in fast moving water. In Britain, but not in Finland, the δ¹³C values of submerged leaves of dimorphic plants were almost invariably more negative than in aerial leaves. The δ¹³C value of carbonate from chalk streams and in acid springs indicate substantial inputs of respiratory CO₂, as opposed to atmospheric carbon. The contributions of these variations in δ¹³C of the carbon source, and of isotope fractionation in diffusion, to the δ¹³C value of submerged parts of dimorphic plants is discussed.