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

April 23, 2007

Abstract

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.

Full PDF follows.

Read more

Lake Erie Trends in Temperature, Secchi Depth, and Dissolved Oxygen Rates in the Central Basin of Lake Erie, 1983-2002

April 23, 2002

Abstract: We examined temperature trends in a 20-year set of monitoring records collected at multiple deep-water stations in the central basin of Lake Erie. Data collected were statistically corrected (“deseasonalized”) to remove biases resulting from irregular sampling intervals within years. Depthintegrated summer temperature has increased by an average (±SE) of 0.037 ± 0.01°C per year. An observed reduction of Secchi depth (SD) by 7 ± 3 cm/y seems to be unrelated to variation in either total phosphorus (TP) or chlorophyll a concentrations. Midsummer midbasin SD values varied widely between 4 and 10 m, possibly depending on whether phytoplankton were concentrated in the epilimnion (giving shallow SD), or whether phytoplankton had settled out of the epilimnion into the lower layers, giving deeper SD values. Hypolimnetic volume-corrected oxygen depletion (HVOD) rates have also been highly variable, ranging from 2.68 to 4.72 mg/L/mo. These rates are sensitive to production of oxygen in the thermocline and hypolimnion by photosynthetically active phytoplankton that have settled from the epilimnion. The HVOD rate in any year was correlated with the previous year’s TP loading into Lake Erie. Since TP loading trends largely reflect the consequences of improving water treatment through the 1980s and increasing contributions from tributary run-off sediments through the 1990s, there is little direct evidence to suggest that the appearance of dreissenids has directly influenced hypolimnial oxygen depletion rates in the central basin. The observation that central-basin HVOD tracked the reductions in TP loadings through the 1980s may be the first affirmation that central basin hypolimnetic oxygen dynamics can be regulated by phosphorus inputs. This implies that TP loads must continue to be regulated if we wish to minimize oxygen depletion rates as a strategy to reduce the frequency of episodic central basin anoxia.

Full PDF follows.
Read more


EarthSoft Currents Week 30 2017 Video Newsletter

EnviroInsite Series: Part 4 - Srip Logs - Jul 2017