Kent Turner

Characteristics of Lake Mead, Arizona–Nevada

Abstract Lake Mead was formed in the 1930s by the construction of Hoover Dam on the Colorado River. Hoover Dam was one of the first multipurpose dams constructed by the federal government. Lake Mead is the largest reservoir in the United States by volume (3.5479 × 1010 m3 at full pool) and is second only to Lake Powell in surface area (637.05 km2). Water from Lake Mead is used for municipal and industrial purposes and irrigation water by approximately 25,000,000 people. The Colorado River provides approximately 97% of the inflow, with the remainder coming primarily from Las Vegas Wash and the Virgin and Muddy rivers. Flow in Las Vegas Wash more than doubled over the past 30 years as a result of the rapid population growth in Las Vegas. Lake surface elevations have dropped by about 40 m since 1999 as a result of extended drought and increasing water demands brought about by population growth. This elevation change impacted water quality and recreation and is just one of many management challenges facing the lake. The discovery of adult quagga mussels (Dreissena rostriformis bugensis) in Lake Mead in February 2007 created infrastructure management problems and raised concerns about lake biology and water quality. The impacts of this invasion are still under investigation. This manuscript provides information on the history of Lake Mead, as well as descriptions of lake and watershed characteristics, hydrology, water quality, fisheries, and recreation that lake managers can use to guide future assessment and management of Lake Mead.

Potential ecological consequences of invasion of the quagga mussel (Dreissena bugensis) into Lake Mead, Nevada–Arizona

Abstract The recent invasion of the quagga mussel (Dreissena bugensis) in Lake Mead, Nevada–Arizona, USA has the potential to alter biological relationships in this western reservoir. We evaluated the potential impacts by examining several measurements in the Boulder Basin of Lake Mead after the introduction of quagga mussel (2007–2008). Analysis of variance did not reveal any basin-wide changes in chlorophyll a concentrations or water clarity (Secchi disk depth). Although significantly lower chlorophyll a concentrations were found in the outer basin, this reduction was likely related to the decline of dissolved phosphorus concentrations. The abundance of cladocerans, copepods or rotifers has not changed since 2007. Overall, the results suggest that there are no statistically significant changes to many of the standard water quality indices routinely measured in the Boulder Basin of Lake Mead; however, given the complexity of biological, chemical and physical processes driving this ecosystem, the long-term impacts of quagga mussels remain uncertain. This manuscript identifies impacts known to be altered by quagga and zebra mussels in other systems and aims to help lake managers develop experimental and monitoring programs that will accurately address the impacts of quagga mussels.

Early invasion population structure of quagga mussel and associated benthic invertebrate community composition on soft sediment in a large reservoir

Abstract In 2007 an invasive dreissenid mussel species, Dreissena bugensis (quagga mussel), was discovered in Lake Mead reservoir (AZ–NV). Within 2 years, adult populations have spread throughout the lake and are not only colonizing hard substrates, but also establishing in soft sediments at depths ranging from 1 to >100 m. Dreissena bugensis size class and population density distribution differs between basins; cluster analysis revealed 5 adult cohorts within Boulder Basin and Overton Arm but low densities and low cohort survival in the Las Vegas Basin. Regression analysis suggests depth and temperature are not primary controllers of D. bugensis density in Lake Mead, indicating other factors such as sediment type, food availability or other resource competition may be important. Monthly veliger tows showed at least 2 major spawning events per year, with continuous presence of veligers in the water column. Adult mussels have been found in spawn or post-spawn condition in soft sediments in shallow to deep waters (>80 m) indicating the potential for reproduction at multiple depths. Comparisons to a 1986 benthic survey suggest there have been shifts in nondreissenid macroinvertebrate composition; however, it is unclear if this is due to D. bugensis presence. Current distribution of nondreissenid macroinvertebrates is heterogeneous in all 3 basins, and their biodiversity decreased when D. bugensis density was 2500/m2 or greater.