Historical patterns of phytoplankton productivity in Lake Mead

Lake Mead was impounded in 1935 by the construction of Hoover Dam. The Colorado River was unregulated prior to then and therefore was subjected to extreme variations in flows and suspended sediment loads. Hoover Dam stabilized flows and reduced suspended sediment loads downstream, but Lake Mead still received silt-laden inflows from the upper Colorado River Basin. The Colorado River contributed 97% of the suspended sediment inputs to Lake Mead, and up to 140 x 1O6 metric tons (t) entered the reservoir in years of high runoff. Most of the sediments were deposited in the river channel and formed an extensive delta in upper Lake Mead. However, sediments were also transported into the Virgin Basin and Overton Arm by the overflow that occurred during spring runoff. The limnology of Lake Mead is thought to have been strongly influenced by this turbid overflow until Glen Canyon Dam was constructed 450 km upstream in 1963. The construction of Glen Canyon Dam and formation of Lake Powell drastically altered the characteristics of the Colorado River inflow to Lake Mead. The operation of Glen Canyon Dam stabilized flows, reduced river temperatures and cut the suspended sediment loads by 70-80%. Nitrate loads decreased initially during 1963 and 1964, then increased through 1970, but have since decreased again to a lower steady state. Phosphorus loads were decreased due to reductions in suspended sediment inputs. Lake Powell now retains 70% of the dissolved phosphorus and 96% of the total phosphorus inputs that once flowed into Lake Mead. The Colorado River still provides 85% of the inorganic nitrogen to Lake Mead, but Las Vegas Wash now contributes 60% of the phosphorus inputs. Wastewater discharges from Las Vegas Wash into Las Vegas Bay increased steadily during the post-Lake Powell period. The morphometry and hydrodynamics of Lake Mead are such that the Las Vegas Wash inflow is confined to the Lower Basin where historically it has elevated phytoplankton productivity. However, high phosphorus loading and productivity have resulted in decreases in nitrate concentrations, and the Las Vegas Bay and parts of Boulder Basin have become nitrogen limited since 1972. A unique situation has therefore developed in Lake Mead in that the Upper Basin has become more phosphorus limited and the Lower Basin more nitrogen limited since the formation of Lake Powell. Paulson and Baker theorized that these changes in nutrient loading and limitation must also have been accompanied by decreases in reservoir-wide productivity. There is some evidence for this hypothesis in apparent improvements in water quality of Las Vegas Bay since 1968. Chlorophyll-a concentrations in the inner Las Vegas Bay have decreased considerably since the first measurements were made in 1968 and during the period of the Lake Mead Monitoring Program. Improvements in water quality of the bay have confounded efforts to establish water quality standards on effluent discharges and are contrary to predictions made in the early 1970s that water quality would continue to degrade with increased phosphorus loading. The decline in the largemouth bass fishery documented by the Nevada Department of Wildlife could also be a symptom of lower productivity in Lake Mead. In this paper, the hypothesis that algal productivity has declined in Lake Mead as a result of impoundment of Lake Powell is evaluated. The chemical status of six stations in the Upper and Lower Basins of Lake Mead is analyzed and current and past rates of organic carbon and phosphorus sedimentation are calculated. The relationship between algal productivity and accretion of organic carbon in sediment is determined, and this is used to construct a historical record of algal productivity for Lake Mead