EPIPHYTIC DIATOMS ON CLADOPHORA GLOMERATA IN THE COLORADO RIVER, ARIZONA: LONGITUDINAL AND VERTICAL DISTRIBUTION IN A REGULATED RIVER

Flagstaff, AZ 86011 ABSTRACT-Seventy-eight taxa of diatoms were identified from the epiphytic community of Cla- dophoraglomerata in the Colorado River through Glen and Grand Canyons. Achnanthes affinis, Cocconeis pediculus, Diatoma vulgare, and Rhoicosphenia curvata made up 80% of the diatom epiphyton community within 15 km of Glen Canyon Dam, Arizona. However, these taxa made up only 33% of the periphyton community 354 km downstream. Mean total diatom density associated with Cladophora glomerata was significantly reduced at each sequential station downstream. Diatom density decreased and community composition of diatoms changed with increasing water depth in the river channel. Cell densities of D. vulgare, C. pediculus, and R. curvata decreased markedly at Lees Ferry following a 3-month period of fluctuating flow during October through December 1985. Several studies on the algal communities in the Colorado River through Glen and Grand Can- yons, Arizona, were conducted prior to the com- pletion of Glen Canyon Dam near Page, Arizona, in 1963. Flowers (1959) observed 52 taxa of al- gae, including 29 taxa of diatoms in Glen Canyon, and Woodbury et al. (1959) were the first to report Cladophora glomerata (L.) Kiitz., now the dominant filamentous alga in the mainstream of the Canyon system (pers. obser.). Williams and Scott (1962) and Weber (1966, pre-dam data) listed the dominant species of diatoms at a water quality surveillance station near Page, Arizona. A few algal surveys have been conducted in the Colorado River since the completion of Glen Can- yon Dam in 1963. Crayton and Sommerfeld (1978) examined the planktonic algal species in the main river and concluded that many of the planktonic species were attached forms that had been dislodged by the current. S. W. Carothers and C. O. Minckley (in litt.) listed total numbers of epilithic diatom species for several major trib- utaries to the Canyon system. Czarnecki et al. (1976) studied the periphyton in the seeps and at the mouths of tributaries in the Grand Canyon and reported that C. glomerata was the dominant, attached green alga in the Canyon system, es- pecially at the mouths of major tributaries. Czar- necki and Blinn (1978) reported a total of 235 taxa of diatoms from the Grand Canyon system. Their study showed that Diatoma vulgare, Coc- coneis placentula, and Rhoicosphenia curvata were the dominant taxa of diatoms. Peterson (1987)