Lake Mead is an impoundment of the Colorado River along the borders of Arizona and Nevada. While the primary source of water to the reservoir is the Colorado River, there are additional inputs from the combined Virgin and Muddy Rivers from the North and the Las Vegas Wash from the east. These three inflows establish distinct gradients in the reservoir with regard to conditions influencing the phytoplankton community. The Colorado River generally enters Lake Mead as a cold, turbid inflow as it leaves Grand Canyon National Park. This results in a seasonal shift in this gradient where the inflow plunges to the bottom for part of each year and periods of low light penetration when the water mixes with the surface waters. The Virgin and Muddy Rivers bring water to Lake Mead from Southern Utah, a generally rural agricultural area with some small to midsize towns. This produces a gradient in the lake influenced by the relatively low nutrients, higher salinity, and low turbidity. The Las Vegas Wash carries a combination of natural groundwater, urban runoff, storm flows, and treated wastewater to the lake from the Las Vegas Valley. This produces a third gradient in Lake Mead of higher nutrient loading and moderate salinities. Basic water quality data, nutrient and chlorophyll a concentrations, and phytoplankton community composition data have been collected along all three gradients at weekly to monthly intervals for over 10 years.
Results/Conclusions
In general all three gradients yield the anticipated pattern of algal biomass predicted by reservoir limnology; low concentrations in the inflowing my Rivers leading to elevated concentrations further into the reservoir as retention time increases and finally low concentrations further from the inflow. Comparing across the gradients the role of advertise nutrient loading is particularly obvious between the Las Vegas Wash and the Virgin and Muddy Rivers. The higher nutrient loading by the Las Vegas Wash produces higher chlorophyll a concentrations despite increases in wastewater nutrient removal. In the Colorado River gradient the seasonal shift in inflow water column position has had impacts, with intermediate concentrations appearing during the underflow period and lower concentrations during the mixed periods. Despite these changes in overall phytoplankton biomass measures of species diversity have been reasonably stable over time and across the three environmental gradients. Overall this low variability in diversity suggests that the relatively oligotrophic conditions found in the lake limit the phytoplankton community diversity and biomass