This study examines the response of two species of Potamogeton (Family: Potamogetonaceae), with differing canopy architectures, to an artificial light gradient. Potamogeton ochreatus Raoul and P. tricarinatus F. Meull. and A. Bennett were grown in water with an attenuation coefficient of 8.8 m-1 at various depths (10-81 cm) to give initial instantaneous irradiances between 0.4 and 460 µmol m-2 s-1. The average daily water column irradiances Iave between the planting depth and the water surface, over 15 daylight hours, ranged from 3.8 to 18.4 mol m-2. After about 80 days all P. tricarinatus plantings, except those at 81 cm, formed dense surface canopies which could access atmospheric CO2 and had a maximum relative growth rate (70 ± 4 mg g-1 per day) and net assimilation rates (0.1-0.9 mg cm-2 day-1) significantly above those of P. ochreatus (57 ± 3 mg g-1 day-1 and , 0.1-0.5 mg cm-2 day-1, respectively). P. ochreatus, which had a more diffuse and fully submersed habit, had a lower specific absorption coefficient (0.1 m-2 g-1) and average daily light compensation point (37 µmol m-2 s-1) than P. tricarinatus (0.9-1.2 m-2 g-1 and 57 µmol m-2 s-1, respectively), but had a relative growth rate of approximately 25 mg g-1 per day even at an initial instantaneous irradiance of 0.4 µmol m-2 s-1. In addition, P. ochreatus allocated about 80% of its biomass to leaves and stems irrespective of the light climate, whereas only small P. tricarinatus plants preferentially allocated biomass above ground. As energy levels increased, P. tricarinatus allocated a greater proportion of biomass to tissues capturing the limiting resource, light. As the light climate became more favourable, P. tricarinatus allocated more biomass to the rhizome. However, when compared to a wider range of submerged macrophytes, the two species optimised their respective growth rates by reacting to varying Iave in a similar way. Both responded to lower than optimal Iave by increasing photosynthetic area and to above optimal values of Iave by decreasing photosynthetic area. © 2001 Elsevier Science B.V.