Zooplankton density and community composition following an experimental flood in the Colorado River, Grand Canyon, Arizona

Zooplankton density and community composition were examined before and afte r an experimental flood (22 March-7 April 1996) in the Colorado River, Gran d Canyon, north-central Arizona, USA, and comparisons were made across samp ling locations and habitat types. The flood primarily affected longitudinal distribution and community composition of zooplankton. Prior to the flood, zooplankton density declined linearly below Lee's Ferry (R-2 = 0.79) but d eclined with the fourth power of distance downriver following the flood (R- 2 = 0.59). Zooplankton community similarity indices between mainchannel and backwater habitats ranged from 0.08 to 0.89 prior to the flood but increas ed significantly to 0.64-0.98 after the flood, indicating increased homogen eity between habitat types. The flood displaced resident (i.e. benthic, lit toral and/or phytophyllic) taxa from backwater habitats and replaced them w ith limnetic organisms. Total zooplankton density was greater after the flo od (4174/m(3)) than before (2733/m(3)), but the increase may have been an a rtifact of normal seasonal cycles in the Colorado River and/or the upstream reservoir (Lake Powell). Copyright (C) 2000 John Wiley & Sons, Ltd.