Nutrient limitation in a southwestern desert reservoir: Eutrophication of Las Vegas Bay, Lake Mead, Nevada

Algal bioassay tests were conducted with Selenastrum capricornutum and natural algae on inner Las Vegas Bay, Lake Mead, Nevada, from December 1992 through September 1993, to identify any nutrient limitation in an area of the reservoir that has experienced problems associated with severe nutrient enrichment. Three areas were sampled based on a gradient of water quality conditions that existed in Las Vegas Bay (LVB). Disodium ethylenedinitrilotetraacetate (EDTA) significantly stimulated algal growth compared to non-EDTA treatment. Algal bioassays indicated that phosphorus (P) was the primary limiting nutrient at all stations for most of the test dates. Chi a response with EDTA + phosphorus (EDTA+P) was significantly greater (p<0.05) than the control response. These tests suggest that continued P enrichment of lower LVB may well result in increased algal productivity. INTRODUCTION Lake Mead, Nevada, was formed in 1935 by impoundment of the Colorado River by Hoover Dam. Lake Mead has the largest surface area of any reservoir in the northern hemisphere and is used for flood control, irrigation, domestic and industrial water supply, power generation, navigation, recreation, fish and wildlife conservation, and sediment control (Hoffman and Jonez 1973). The trophic status of lower Colorado river reservoirs ranges from oligotrophic to mesotrophic (Paulson and Baker 1984). Nutrient concentrations are low and phosphorus is often limiting. Nutrient fertilization experiments have been conducted to increase plankton productivity with the goal of improving the fisheries in sections of Lake Mead (Vaux and Paulson 1989). Much of Lake Mead is considered oligotrophic (Axlef et al. 1987), although in the early 1970s eutrophication was thought to be in progress (Staker et al. 1974). U.S. Department of the Interior (1970) reported that Las Vegas Bay (LVB) was the most eutrophic section of Lake Mead. Today, LVB has been classified from hypereutrophic to oligotrophic based on chlorophyll and nutrient concentrations (LaBounty 1994). Phosphorus deficiency in Lake Mead began to develop in 1963 when Glen Canyon Dam was constructed 460 km upstream. As the reservoir slowly filled, it substantially reduced the phosphorus supply in the Colorado River inflow to Lake Mead (Paulson and Baker 1980, Paulson et al. 1980, Evans and Paulson 1983). By contrast, lower sections of Las Vegas Bay, which lie within Boulder Basin have had severe problems with excess nutrient enrichment from point and nonpoint source pollution. Summer surface chl a concentrations often exceed 80 ug/L (LaBounty 1994).