EFFECTS OF ATMOSPHERIC EXPOSURE ON CHLOROPHYLL-A, BIOMASS AND PRODUCTIVITY OF THE EPILITHON OF A TAILWATER RIVER

Field experiments were conducted to determine the effects of atmospher ic exposure on the chlorophyll a content, biomass and gross primary pr oductivity (GPP) of littoral epilithon in the Colorado River below Gle n Canyon Dam, Arizona. The chlorophyll a content of the epilithon was much more sensitive to exposure than the biomass. The epilithon was ra pidly bleached during summer daytime exposures, but algal filaments re mained attached for several weeks after reinundation. The percentage o f initial chlorophyll a remaining after one day of exposure was not di fferent from the percentage remaining after two days of exposure. Howe ver, significant reductions in chlorophyll a content were detected for daytime exposures as short as six hours. Overall, there were close in verse relationships (r(2) greater than or equal to 0.73) between the t ime exposed or cumulative solar radiation (400-700 nm) and the percent age of initial chlorophyll a remaining after reinundation. The GPP of Cladophora glomerata-dominated epilithon from the permanently inundate d channel was 10 times higher than the GPP of epilithon from the zone of daily water level fluctuation. Experimental atmospheric exposure of the epilithon from each zone reduced the GPP, but not the assimilatio n ratio (GPP per unit of chlorophyll a) of the epilithon. The Glen Can yon epilithon has low resistance to exposure disturbances, and recolon ization is slow under hydropower peaking how regimes. Cladophora glome rata has an important structural role in Glen Canyon, the disruption o f which is likely to precipitate effects at higher trophic levels.