Temporal and altitudinal variations in the attached algae of mountain streams in Colorado

Attached algae were examined from eight sites in Central Colorado streams ( five sites on St. Vrain Creek and three on the Snake River) between June an d November, 1995. The sites ranged in elevation from 1600 m to 3500 m a.s.l .(plains zone to alpine zone). Seasonal variation in algal communities were qualified in forms of species composition and abundance (cell counts and b iovolumes). The Ochiai coefficient of similarity was used to estimate degre e of similarity between communities at different times, on different stream s, and at different locations on the same stream. The composition of commun ities across streams for alpine and foothills zones was very close, but mon tane communities differed greatly across streams (similarity <0.1). Tempora l variability of attached algae was almost absent in the alpine zone, but w as high in other zones. The composition of the alpine community in August d eveloped downstream with a temporal shift: October in the montane and footh ills zones, November in the plains zone.