Secchi transparency of Boulder Basin, Lake Mead, Arizona-Nevada: 1990–2007

Abstract

Over 5,500 individual Secchi readings were taken from Boulder Basin of Lake Mead, Nevada-Arizona, between July 1990 and December 2007. Annual and seasonal patterns are plainly displayed by these data. Variations in Secchi depth measurements are both spatial and temporal. Peak water transparency >15 m generally occurs in April and March and at sampling locations along the thalweg of the Colorado River channel in the Outer portion of Boulder Basin. Transparency steadily decreases toward the tributary inflow into Las Vegas Bay. Transparency in open water of Las Vegas Bay ranges from <1 m during the growing season to 5–7 m in winter. These differences are generally driven by varying algal production which decreases in a cline from Las Vegas Bay to the Outer Basin. Entrainment of small sediment particles from the urban inflow also influences water transparency. Steady lowering of the reservoir during the 2000 to 2007 drought exacerbated the influence of turbidity due to entrainment of former lake bottom sediments into the epilimnion. However, the strongest statistical relationships with decreasing Secchi transparency are with chlorophyll a (chl-a) and total phosphorus (TP). The patterns and relationships presented establish the nature of Boulder Basin's water clarity to 2008 that will be useful in light of imminent and future alterations of the aquatic ecosystem by both anthropogenic and nonanthropogenic factors. Perhaps the largest change will occur due to the recent introduction of the quagga mussel (Dresseina rostriformis bugensis). After discovery in January 2007, the mussel population has been increasing rapidly. The data sets in this presentation form the basis of conditions in Boulder Basin prior to the introduction of quagga mussels.

Key words:

• chlorophyll a
• lake management
• Lake Mead
• phosphorus
• quagga mussel
• reservoirs
• Secchi
• water clarity
• water transparency