LaBounty, J.F. and Burns, N.M. 2007. Long-term increases in oxygen depletion in the bottom waters of Boulder Basin, Lake Mead, Nevada-Arizona, USA. Lake Reserv. Manage. 23:69-82. 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/m3/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.

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