This study reports the change in weight and the net photosynthetic response of the aquatic lichen Peltigera hydrothyria across long-term incubations of water temperatures from 5 - 30°C, from periods of hours to months. At a temperature of 5°C there was little change in weight or net photosynthetic response for periods as long as 400 days. At a temperature of 11°C, a decline in net photosynthetic response was apparent after 150 days; at 15°C, after 120 days; at 18°C, in 30 days. At 21°C net photosynthesis ceased within 15 days; at 26°C, within six days; and at 30°C, within three days. Although some decline in weight also occurred, the magnitude was not as great as that observed for net photosynthesis. The results helps support the observation that this species is restricted to cold, temperate streams