G. Chris Holdren

Chemical and physical characteristics of the Salton Sea, California

A 1-year sampling program was conducted to assess current chemical and physical conditions in the Salton Sea. Analyses included general physical conditions and a suite of water quality parameters, including nutrients, trophic state variables, major cations and anions, trace metals and organic compounds. Samples were collected from three locations in the main body of the lake and from the three major tributaries. Nutrient concentrations in the Salton Sea are high and lead to frequent algal blooms, which in turn contribute to low dissolved oxygen concentrations. The tributaries consist primarily of agricultural return flows with high nutrient levels. Concentrations of trace metals and organic compounds do not appear to be of major concern. Two geochemical models, PHRQPITZ and PHREEQC, were used to evaluate potential chemical reactions limiting the solubility of selected water quality variables. Modeling indicated that the Salton Sea is supersaturated with respect to calcite, gypsum, and other minerals. Precipitation of these minerals may serve as a sink for phosphorus and limit the rate of salt accumulation in the Salton Sea.

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.

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.

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.

Evaluation of potential impacts of perchlorate in the Colorado River on the Salton Sea, California

Ammonium perchlorate, a component of rocket fuel, entered Lake Mead through drainage and shallow groundwater in the Las Vegas Valley, Nevada, and is now found in the lower Colorado River from Lake Mead to the international boundary with Mexico. Perchlorate is a threat to human health through reduction of thyroid hormone production. Perchlorate has been found in water throughout the lower Colorado system and in crops in the California’s Imperial Valley, as well as in several other states, but it has not previously been included in investigations of the Salton Sea. Because perchlorate behaves conservatively in the Colorado River, it was postulated that it could be accumulating at high levels along with other salts in the Salton Sea. Results show that perchlorate is not accumulating in the Sea, although it is present in tributaries to the Sea at levels similar to those found in the Colorado River. Bacterial reduction of perchlorate is the most likely explanation for the observed results.