EFFECTS OF SUSPENDED SEDIMENT AND DESICCATION ON THE BENTHIC TAILWATER COMMUNITY IN THE COLORADO RIVER, USA

We demonstrated that differences in habitat requirements by C. glomera ta and Oscillatoria have a profound bottom-up influence on the foodweb in the tailwaters below Glen Canyon Dam in the Colorado River through Grand Canyon National Park, USA. We examined the effects of suspended sediment and desiccation on the colonization sequence of Cladophora g lomerata and Oscillatoria spp. and the consequent effects on macroinve rtebrates in each algal community in a series of reciprocal transplant s in the regulated Colorado River, AZ. Our experiments showed that C. glomerata grows best in continuously submerged, clear-water, stable ha bitats, whereas Oscillatoria forms dense mat-like matrices of trichome s and sand in varial zones and habitats with high suspended sediments typical of many southwestern USA streams. Varial zone conditions have a stronger influence on community structure than habitats with high su spended sediments. Recruitment by chironomid larvae was less dependent on C. glomerata and less affected by suspended sediment and periodic desiccation than Gammarus lacustris. We estimated the energy from macr oinvertebrate biomass associated with tufts of C. glomerata to be an o rder of magnitude higher than that in Oscillatoria matrices. Therefore , loss of C. glomerata and replacement of habitat more suitable for Os cillatoria as a result of regulated flows indirectly reduces potential energy flow in the Colorado River foodweb.