• Lemanca mamillosa;
  • Rhodophyta;
  • nutrition;
  • growth;
  • unstirred layers;
  • carbon;
  • nitrogen;
  • phosphorous

Abstract. Field measurements of the growth rate of the red freshwater macroalga Lemanca mamillosa Kutz, in the Dighty Burn, together with measurements of water velocity, [CO2], [NO3], [NH3+ NH4+] and [phosphate], have been made between February and July. This period covers the growth of the erect gametophyte and later of the carposporophyte inside the gametophyte. Hydrodynamic studies in the laboratory on benzoic acid models of the gametophyte suggest an average in situ unstirred layer some 12 μm thick. For growth of the gametophyte, this estimated boundary layer thickness, together with the measured inorganic C transport pathway within the plant, suggest that growth is not significantly restricted by CO2 transport from the bulk phase to the plastids. δ13C measurements on source CO2 and on plant organic C bear this out. Habitat choice (low temperatures: CO2 enrichment from ground-water input: rapid water flow), plant morphology and anatomy (turbulence-generating ‘knobbles’ on the nodes; plastids close to the outside of the plant), and plant biochemistry (high CO2 affinity of the RUBISCO carboxylase; quite high carbonic anhydrase activity) are responsible for this lack of limitation by inorganic C transport in the growing gametophyte which lacks HCO3 transport and a CO2 concentrating mechanism. Transport through the boundary layer does not significantly restrict acquisition by the plant of N (probably as NH4+, despite the preponderance of NO3 in the environment) or of P (as orthophosphate) in the field. The membrane transporters, which have high substrate affinities (K½'s about 2 mmol NH4+ m-3 and < 2 mmol inorganic phosphate m−3), probably impose the major limitation. The development of the carposporophyte later in the season, and an increase in the thickness of the cortex of the gametophyte, result in an increased (less negative) δ13C, suggesting a significant diffusion limitation to CO2 transport. This conclusion is reinforced by consideration of the opposing effect on Δδ13 C of the decreased demand for products of phosphoenolpyruvate carboxylase activity as the N/C ratio decreases late in the growing season.