Distribution of quagga mussel veligers, Dreissena bugensis, in the reservoirs of the Colorado River Aqueduct

Abstract

This study examined the distribution of quagga mussel veligers, Dreissena bugensis, in Copper Basin Reservoir and Lake Mathews, the 2 deepest reservoirs on the Colorado River Aqueduct system in California. Density stratification, driven by temperature differences, was a critical factor in controlling vertical distribution of veligers within the water column. Copper Basin Reservoir had the strongest stratification from May to August, with veliger concentrations higher than 80/L near the thermocline but much lower in the epilimnion and hypolimnion. Veliger concentrations were also elevated near the thermocline in Lake Mathews in late summer 2008 and spring 2009, although operation of a bubble aeration system later in 2009 eliminated the strong thermocline and may have influenced veliger abundance generally present there. Average chlorophyll a concentrations were low (<5 μg/L) in both reservoirs, although an increase in chlorophyll a concentration (to ∼ 10 μg/L) was observed within the spring thermocline in Lake Mathews; a chlorophyll a maximum near the thermocline was not present in Copper Basin Reservoir. These observations suggest that stratification, and not chlorophyll a concentration, determined the vertical distribution of veligers. The thermocline is a zone of increasing density that often concentrates particles. Statistically significant horizontal gradients in veliger concentrations were absent, suggesting that lateral currents mix and distribute veligers across these basins. © 2010 Copyright Taylor and Francis Group, LLC.