Long-term Increases in Oxygen Depletion in the Bottom Waters of Boulder Basin, Lake Mead, Nevada-Arizona, USA

Abstract

Long-term changes in the hypolimnetic volumetric oxygen demand (HVOD) of Boulder Basin, Lake Mead were determined from dissolved oxygen profiles collected from 1991 to 2007. HVOD is the rate at which oxygen in a deep layer in contact with the sediments is depleted during the period of thermal and/or chemical stratification. Generally, the rate at which oxygen is depleted is correlated to the amount of organic debris in the hypolimnion and sediments. The sediment oxygen demand reflects historical organic loading, while HVOD is a measure of productivity because of the organic particles settling from above. The lower hypolimnion in Boulder Basin remains relatively stable during the stratification period, enabling the calculation of HVOD in the near-bottom water layer. Small increases and/or decreases that occur in temperature and dissolved oxygen concentrations are detectable. Boulder Basin fully destratifies every other year on average, but mixes only partially in the spring (before May) of the remaining years. The HVOD rates after partial and complete destratification have been assessed separately for 1995-2005. The annual HVOD rate is generally lower the year after partial destratification than after complete destratification due to greater downward transport of oxygen into the hypolimnion. The HVOD of Boulder Basin is variable depending on loading of nutrients and water into the Basin. The rate dropped significantly following commencement of advanced wastewater treatment practices in 1994. The rates then increased 1996-2006 at a rate of approximately 0.75 mg DO/m 3 /day per year, or about 7% annually. During those years the inputs of nutrients steadily increased. Rates have been dropping from 2005 to present (2007) following further reduction of phosphorus input. A multiple regression analysis revealed that HVOD is significantly positive related to the total phosphorus concentration in Las Vegas Bay, but significantly negative to inflows of Colorado River water. That means HVOD was highest when reservoir water was nutrient-rich and flow rates were low. HVOD should be considered a major tool for monitoring trophic state changes in Boulder Basin.

Modeling water ages and thermal structure of Lake Mead under changing water levels

“…The relatively younger water at the lake surface compared to the older water at the lake bottom suggests that distinct vertical patterns exist in the lake's circulation processes. Additionally, it has been shown that in many years Boulder Basin does not completely destratify (LaBounty and Burns 2007). This vertical pattern could impact the lake's hydrodynamic process as well and may impact the distribution of the simulated water age.…”

“…Moreover, numerical models can provide a useful method to analyze the impact of different water availability and management scenarios. Recent research on Lake Mead has primarily focused on field measurements and lab experiments (e.g., LaBounty andBurns 2007, Steinberg et al 2009). As for the present state of numerical models of Lake Mead, most have been conducted to predict hydrodynamic processes and water quality to understand past and present limnological conditions (SCOPEIS 2006).…”

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

“…Lake Mead is mostly mesotrophic and monomictic (LaBounty and Burns 2005). LaBounty and Burns (2005Burns ( , 2007 defined three regions of Boulder Basin as the Inner, Middle, and Outer basins. The Inner Basin is characteristically more eutrophic, the Middle tends to be mesotrophic, while the Outer is meso-to oligotrophic.…”

Evaluating lake stratification and temporal trends by using near-continuous water-quality data from automated profiling systems for water years 2005-09, Lake Mead, Arizona and Nevada

“…They found that higher concentrations of total phosphorus in Boulder Basin increased the hypolimnion oxygen depletion. With enhanced removal of phosphorous from wastewater entering Las Vegas Wash commencing in 2002, hypolimnion oxygen depletion rates in 2005 and 2006 were lower than previous years (LaBounty and Burns, 2007). This trend toward lower hypolimnion oxygen depletion rates from reduced nutrient input to Boulder Basin is likely responsible for the trend of increasing DO concentrations in the hypolimnion at Sentinel Island.…”

“…At other times, DO is well below saturation because of demand from bacteria breaking down organic matter. LaBounty and Burns (2007) found that hypolimnetic oxygen depletion was strongly positively associated with the amount of organic matter and nutrients entering Boulder Basin. They found that higher concentrations of total phosphorus in Boulder Basin increased the hypolimnion oxygen depletion.…”