TUNDRA STREAM MACROALGAE OF NORTH-AMERICA - COMPOSITION, DISTRIBUTIONAND PHYSIOLOGICAL ADAPTATIONS

Eighty-three infrageneric taxa of stream macroalgae have been reported from tundra regions of North America, composed of 32 cyanobacteria, 3 5 Chlorophyta, 10 Chrysophyta and 6 Rhodophyta. There are few if any e ndemics represented in this flora. The most widespread species are the cyanobacteria Rivularia minutula, Nostoc commune and Tolypothrix tenu is as well as asexual populations of the chlorophyte genus Zygnema. Th e relative contribution of cyanobacteria compared to that of the Chlor ophyta increases from the low to high arctic. Number of species per se gment ranges from 0 to 7, with a mean of 2.8, and varies little betwee n the low and high arctic. The percentage of stream bottom covered by macroalgae ranges from 0 to ca. 75%; mean cover values for low and hig h arctic streams are ca. 12 and 8%, respectively. Tundra macroalgae te nd to be more abundant and diverse in less rigorously flowing stream s ections. Most species tolerate prolonged freezing by forming resistant vegetative cells with thick walls, plentiful reserves and low molecul ar weight solutes to lower the freezing point. Many tundra stream macr oalgae also produce 'sunscreen' pigments to reduce exposure to damagin g radiation in the blue and ultraviolet regions. Nutrients tend to be low and phosphorus is often limiting in these systems. Arctic streams appear to differ from those of Antarctica in having potential grazers of macroalgae, such as the chironomid Diamesa, the mayfly Baetis and t he caddisfly Brachycentrus.