Persistence of Daphnia in the epilimnion of Lake Mead, Arizona–Nevada, during extreme drought and expansion of invasive quagga mussels (2000–2009)

Abstract

Lake Mead has experienced drought conditions since 2000 and the introduction of invasive quagga mussels since about 2004. Empirical evidence from the Great Lakes region suggests a potential for energy transfer to the benthos and away from planktivorous fish through competition between quagga mussels and filter-feeding, herbivorous zooplankton. We describe Daphnia dynamics from 2043 analyses performed on the 3 basins of subtropical Lake Mead during 2000–2009. Daphnia were generally most numerous in shallower areas strongly influenced by nutrient inputs and with decreased clarity and higher chlorophyll a (chl-a) concentrations. Daphnia populations increased in mid- to late winter and early spring prior to the spawning of gamefish populations and rapidly decreased with increasing predation pressure, water temperature and cyanobacteria populations. Concurrent with the reduction in Daphnia populations in late spring, quagga veliger abundance rapidly increased, declined in summer, and peaked again in the fall. Quagga veligers in Lake Mead were generally most numerous in open water areas with maximum depth, greater clarity and low chl-a. Although mean Daphnia abundance since the beginning of drought conditions and the establishment of quagga mussels is lower in Boulder Basin, the temporal and spatial dynamics of the zooplankton community remain comparable to previously described historic patterns in all 3 basins of Lake Mead.