Development of a phosphorus budget for Lake Mead

Abstract

Phosphorus is the growth-limiting nutrient for phytoplankton in Lake Mead. Multiple agencies have developed an extensive phosphorus dataset with low detection limit since 2007 by obtaining monthly or biweekly samples at 19 monitoring stations throughout the lake and at all the major inflows. Approximately 2000 phosphorus grab samples were collected and analyzed as part of this effort. We developed a phosphorus budget for Lake Mead during 2007–2008 using these measured phosphorus data and evaluated its accuracy using the water and bromide budget developed in this study. Based on the phosphorus budget, total phosphorus (TP) loading from the Colorado River dominated external TP loading to Lake Mead, accounting for 98% of total external loading. The orthophosphate (Ortho-P) portion that is soluble and bioavailable accounted for <2% of the TP loading from the Colorado River, indicating that a significant portion of the Colorado River TP loading was in particulate form. Our analysis showed that 88% of this TP loading settled within 8 km of the river confluence. Calculated annual retention coefficients for Ortho-P indicated that Lake Mead retained about three-fourths of the influent Ortho-P; thus, Lake Mead is a significant phosphorus sink for the Colorado River, limiting the transport of phosphorus to downstream reaches or attached systems. In Boulder Basin, the most downstream basin where major drinking water intakes are located, Las Vegas Wash was the largest Ortho-P contributor during 2007–2008, validating the recent management efforts to reduce phosphorus loading in the Las Vegas Wash.

Key words:

• budget
• Lake Mead
• loading
• mass balance
• phosphorus