Lake Mead prefertilization study: Preliminary nutrient enhancement studies in Lake Mead

Studies conducted by the University of Nevada-Las Vegas (UNLV), the Nevada Department of Wildlife (NDOW), the Arizona Game and Fish Department (AGFD), The Nevada Division of Environmental Protection (NDEP), and the United States Bureau of Reclamation (USBR) have identified decreased algal production as a major factor involved in the decline of the Lake Mead sport fishery. Phosphorus-laden silt particles in the Colorado River have been sedimenting out in Lake Powell since the completion of Glen Canyon Dam 286 miles upstream in 1963. This sharp decrease in phosphorus loading to Lake Mead (\u3e5000 tons per year) has resulted in decreased biomass and growth at all levels of the food chain. Phosphorus loading to the lower basin (Boulder Basin) has decreased even further since 1981 when Clark County and the City of Las Vegas began removing phosphorus from wastewaters discharged into Las Vegas Bay. Most of Lake Mead is now oligotrophic according to almost all of the trophic status indices which have been developed. Only the inner and middle regions of Las Vegas Bay (treated wastewater influent), the Overton Arm upstream of Fish Island (Muddy and Virgin River discharges), and the Iceberg Canyon/Grand Wash area (Colorado River influence) have been found to have phosphorus levels sufficient to sustain relatively higher productivity. Phytoplankton production becomes tightly regulated by the supply of phosphorus during most of the growing season. Zooplankton graze on planktonic algae, and threadfin shad feed primarily on these zooplankton and phytoplankton. Since game fish feed primarily on either zooplankton or shad at different stages of their life cycle, it is clear how a nutrient limitation of phytoplankton growth can cascade up the food chain. The declines in the sport fisheries, particularly largemouth bass, striped bass and trout, began in the early 1960\u27s and have become much more dramatic since the mid to late 1970\u27s as evidenced by declines in total yields of largemouth bass and trout, and striped bass condition factors, and increased angler effort. It is likely that as fish were increasingly stressed by food shortages, conditions were worsened by indirect factors such as lack of suitable cover for littoral populations and by fish predation. The only way to restore the previous fertility of the lake water is to add nutrients. Large-scale fertilization programs in British Columbia and Alaska have been very successful at stimulating the productivity of all levels of the food chain, ultimately producing more salmon. Nevada and Arizona completed an intensive study of the black bass (largemouth bass) fishery in Lake Mead in 1982 which was funded by the U.S. Bureau of Reclamation (USBR). A major recommendation was that pilot fertilizations be conducted as a demonstration project for future large-scale nutrient enrichment. UNLV subsequently submitted a proposal to the Bureau of Reclamation in November 1984 to artificially fertilize about 30,000 acres in the Overton Arm and about 11,000 acres in Gregg Basin. Since that time a technical advisory panel (the Lake Mead Nutrient Enhancement Technical Committee) comprised of representatives from UNLV, NDOW, AGFD, USBR, NDEP, the National Park Service (NPS), and the U.S. Fish and Wildlife Service (USFWS), was formed to review the original proposal and develop a feasible plan for implementing an experimental program of large-scale fertilization. Subsequently, the USBR funded the present study for 16 months (May 1986-August 1987) to conduct laboratory and pilot-scale field experiments designed to evaluate the potential for successfully stimulating algal growth on a large scale in Lake Mead using artificial fertilization