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

Abstract

Zooplankton density and community composition were examined before and after an experimental flood (22 March–7 April 1996) in the Colorado River, Grand Canyon, north-central Arizona, USA, and comparisons were made across sampling 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 (R2=0.79) but declined with the fourth power of distance downriver following the flood (R2=0.59). Zooplankton community similarity indices between mainchannel and backwater habitats ranged from 0.08 to 0.89 prior to the flood but increased significantly to 0.64–0.98 after the flood, indicating increased homogeneity between habitat types. The flood displaced resident (i.e. benthic, littoral and/or phytophyllic) taxa from backwater habitats and replaced them with limnetic organisms. Total zooplankton density was greater after the flood (4174/m3) than before (2733/m3), but the increase may have been an artifact of normal seasonal cycles in the Colorado River and/or the upstream reservoir (Lake Powell). Copyright © 2000 John Wiley & Sons, Ltd.