A broad range of aquatic plants has been examined by means of three experimental techniques for the purpose of describing their abilities to use the different forms and concentrations of inorganic carbon which may be presented to them. Experiments were carried out on macrophyte shoots or algal suspensions in a specially designed apparatus under uniform conditions of light (560 ?Einsteins m-2 s-1) and temperature (20°C) while measuring the rate of oxygen production, and the rate of carbon uptake 3S determined indirectly by pH measurements. Kinetic experiments showed that macrophytes had more diffusive resistance to CO2 uptake than microalgae; that microalgae had a high affinity for CO2 and HCO3 whereas the macrophytes, including previously established bicarbonate users, had high affinities for CO2 only. The ratio of [HCO3] to [CO2] needed to produce the same rate of photosynthesis ranged from less than 1 in two of the microalgae to more than 100 in two macrophytes, implying a gradation in capability for bicarbonate use. Results of experiments at constant with varied pH showed that, over the range 7.4 to 9.0, net photosynthesis of the macrophytes was primarily dependent upon [CO2] in a given CT concentration but that of Anabaena cylinrica and Chlorella emersonii was not; their rates only became pH-dependent above pH 9.5. Bicarbonate use by macrophytes above pH 9 became apparent if the [CT] was high enough. The question of bicarbonate use by any specie depends upon the concentration of bicarbonate in the bathing solution, that species' affinity for bicarbonate and its bicarbonate compensation point. Using the pH-drift technique, a method is described for distinguishing CO2 and HCO3 compensation points. With this method, CO2 compensation points were determined as 0.9 muM for Cosmarium botrytis, 4.5 muM for Elodea Canadensis, 5.5 muM for Eurhynchium rusciforme, and 8 muM for Fontinalis antipyretica; HCO3 compensation points ranged from 0.05 mM in Cosmariuni to, respectively, 0.9, 0.95, and 3.3 mM in the macrophytes. From evidence from these three types of experiment, bicarbonate users and non-users do not exist as such but there is a graded capacity to use bicarbonate in which strong and weak users can be distinguished. Their poor affinity for bicarbonate means that most macrophytes can barely photosynthesise with this source at natural alkalinities when pH rises above 9. Macrophytes are largely CO2 dependent and, since CO2 is rarely saturating, they become pH- dependent. The photosynthetic activities of the plants in a lake, particularly those with a high affinity for bicarbonate can reduce the CT concentration below the air-equilibrium level. The resulting rise in pH, and at times precipitation of marl, causes a sever reduction in the free CO2 concentration which the weak users of bicarbonate rely on. It is suggested that competition for carbon arising from variations in alkalinity and from free CO2 depletion constitute an important factor in determining the composition of the plant community in lakes, particularly the macrophytes, and these plants can serve as indicators of the long-term carbon status of a lake.