Inorganic C-sources for Lemanea, Cladophora and Ranunculus in a fast-flowing stream: Measurements of gas exchange and of carbon isotope ratio and their ecological implications.

CO2-and O2-exchange characteristics and δ13C values have been measured in a rhodophycean haptophyte (Lemanea mamillosa), a chlorophycean haptophyte (Cladophora glomerata) and a magnoliophyte rhizophyte (Ranunculus sp.) from a 5 m stretch of the Dichty Burn near Dundee. Light-and CO2-saturated rates of photosynthesis are greatest on a dry weight basis for Cladophora and lowest for Lemanea; the order is reversed on a surface area basis. The CO2 concentration at pH 6.5 at which photosynthesis is half-saturated is 25-40 μM, with Lemanea rather lower than Cladophora or Ranunculus; these half-saturation values are similar to the free CO2 concentration in the Burn water. Lemanea cannot use HCO 3- in photosynthesis, while Cladophora and Ranunculus can. Despite being within a factor or two of saturation with free CO2 in terms of the bulk water concentration, the growth habit of Cladophora and, particularly, Ranunculus means that the high water velocity in the Burn does not necessarily prevent C depletion effects around the plants, thus providing a possible role for HCO 3- use by these plants. Lemanea lives in the fastest-growing parts of the Burn, and its growth habit insures that it is exposed to this high water velocity, thus minimising CO2 depletion during photosynthesis despite the low surface/volume ratio for this plant. δ13C measurements on the inorganic C in the Burn water are consistent with at least part of its excess (above air-equilibrium) inorganic C levels coming from heterotrophic activity. Lemanea has the most negative δ13C value of the three plants, consistent with CO2 use and small diffusion resistances. Ranunculus has the least negative δ13C value, consistent with some CO2 depletion and/or HCO 3- use in situ related to a high diffusion resistance in a rhizophyte which does not have to obtain all of its N and P from the bulk water but can obtain some from the sediments. Cladophora is intermediate, suggesting some CO2 depletion and/or HCO 3- use in this densely growing haptophyte.