Abstract
The photosynthetic responses of two arctic intertidal seaweeds, Fucus spiralis L. and Laminaria longicruris DE LA PYLAIE were studied during the summers of 1977 to 1986 at the head of Frobisher Bay. Samples were incubated at different depths under natural sunlight and at ambient temperature with constant salinity and nutrients. Maximum photosynthetic rate of the upper intertidal alga, {iF. spiralis} was 0.98 mg c.g-1 dry wt. hour-1 and occurred at a light saturation of about 760 μEm-2s-l and a temperature of 2.9° C in the surface water. The rate by the lower intertidal alga, L. longicruris was 0.31 mg c.g-1 dry wt. hour-1 and reached a maximum at lower light saturation (250 µEm-2s-1) and lower temperature (0.9° C) in the deeper water (5 m). The photosynthetic capacity of F. spiralis was greater than that of L. longicruris enabling it to sustain more extreme conditions of light and temperature. These two parameters seem to be the principal factors that regulate their vertical distribution in the Arctic.