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1) Laboratory experiments were carried out on the photosynthetic physiology of two algal communities and two species of aquatic moss from lakes on Signy Island (Antarctica). (2) Net rate of oxygen production by algal `felts’ was measured at very low irradiance (up to 1.5 W m-2) at 2⚬ C. A community based on the blue-green algae Tolypothrix and Plectonema (Sombre Lake) had a light compensation point of 0.17 W m-2, with maximum net rate of oxygen production per unit ash-free dry weight of c. 0.6 μg mg-1 h-1, and a high and variable rate of oxygen uptake in the dark (respiration), mean value 0.41 μg mg-1 mg-1. A second community, in which Phormidium spp. predominated (Changing Lake), had a compensation point of 0.09 W m-2, but a lower maximum net rate of oxygen production (c. 0.2 μg mg-1 h-1) and a low respiration rate (mean value 0.09 μg mg-1 h-1). (3) Two species of aquatic moss, Calliergon sarmentosum and Drepanocladus sp., had similar respiration rate per unit ash-free dry weight at normal lake temperature (up to 5⚬C)–c. 0.3 μg mg-1 h-1. Respiration rate increased linearly with temperature between 1.2 and 30⚬C, but more rapidly for Calliergon than for Drepanocladus. The light compensation point at 2⚬C of the two mosses differed markedly. Drepanocladus had a compensation point similar to the algal communities investigated (0.11 W m-2), but Calliergon, which generally occurs in shallower water, had a higher compensation point (0.64 W m-2). Increase in temperature in the range of irradiance used in the experiments (maximum 2.4 W m-2) caused the compensation point to shift to higher irradiance. The effect was more pronounced in Calliergon.