Lake and Reservoir Management 23:69-82, 2007
© Copyright by the North American Lake Management Society 2007




          ...
LaBounty and Burns



by Matthews and Effler (2006). Because the AHOD is the sum
of the oxygen consumed under unit area of ...
Long-term Increases in Oxygen Depletion in the Bottom
                                         Waters of Boulder Basin, La...
LaBounty and Burns



Water samples were collected weekly for analysis of nutrients    time (days), and then these data we...
Long-term Increases in Oxygen Depletion in the Bottom
                                                                    ...
LaBounty and Burns



a fully destratified year (Type 1 year). But Boulder Basin                     2005 – 2006 (Type 2) D...
Long-term Increases in Oxygen Depletion in the Bottom
                                                                    ...
LaBounty and Burns



wastewater treatment remained the same, loading increased                                           ...
Long-term Increases in Oxygen Depletion in the Bottom
                                Waters of Boulder Basin, Lake Mead, ...
Table 2.-Estimate of downward transport of dissolved oxygen (DO) into the hypolimnion of Boulder Basin 1995-2005 by eddy d...
Long-term Increases in Oxygen Depletion in the Bottom Waters of Boulder…
Long-term Increases in Oxygen Depletion in the Bottom Waters of Boulder…
Long-term Increases in Oxygen Depletion in the Bottom Waters of Boulder…
Long-term Increases in Oxygen Depletion in the Bottom Waters of Boulder…
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Long-term Increases in Oxygen Depletion in the Bottom Waters of Boulder…

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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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Long-term Increases in Oxygen Depletion in the Bottom Waters of Boulder…

  1. 1. Lake and Reservoir Management 23:69-82, 2007 © Copyright by the North American Lake Management Society 2007 Long-term Increases in Oxygen Depletion in the Bottom Waters of Boulder Basin, Lake Mead, Nevada- Arizona, USA James F. LaBounty and Noel M. Burns* Southern Nevada Water Authority 1900 East Flamingo Road, Suite 255 Las Vegas, Nevada 89119 (702) 822-3357, jim.labounty@SNWA.com Lakes Consulting * 42 Seabreeze Rd. Devonport, New Zealand 0624 (09) 445-7561, lakescon@xtra.co.nz 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 an- nual 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. Key Words: applied limnology, dissolved oxygen, HVOD, hypolimnion, Lake Mead, phosphorus, reservoir ecology, temperature A fraction of the organic material produced by primary pro- 1979, Wetzel 2001, Beutel 2003). Development of the theory duction in the epilimnion of lakes settles into the hypolimnion of oxygen deficits as a measure of lake health has a long his- where it decomposes and causes a hypolimnetic oxygen tory since the fundamental work by Birge and Juday (1911), deficit. Changes in the hypolimnetic oxygen depletion rates Thienemann (1928), and Juday and Birge (1932). The relative over long periods can be indicative of overall changes of areal deficit (AHOD) was introduced by Strøm (1931) and productivity and lake trophic level (Lasenby 1975, Walker modified by Hutchinson (1938, 1957), and was recently used 69
  2. 2. LaBounty and Burns by Matthews and Effler (2006). Because the AHOD is the sum of the oxygen consumed under unit area of hypolimnion, it Virgin River Moapa River is sensitive to changes in hypolimnion thickness. In contrast, the volumetric oxygen deficit, the rate of disappearance of oxygen from unit volume of the hypolimnion, or hypolimnetic volumetric oxygen demand (HVOD), is less dependent on hypolimnion thickness. The HVOD is an indirect measure of epilimnetic primary production, provided no significant Overton Arm Grand Wash primary production is occurring in the hypolimnion (Burns Colorado River Iceberg Canyon et al. 2005). HVOD rates can also be modified by oxygen Boulder Canyon transported downward into the hypolimnion. Significant and “The Narrows” Las Vegas Wash variable amounts of DO transferred from the thermocline Virgin Canyon Virgin Basin into the hypolimnion must be determined if HVOD rates are Bo u ld e r Ba s i n Gregg Basin Temple Basin to be meaningful indicators of change in trophic level. The Pearce Ferry occurrence of these processes has been documented in Lake Hoover Dam (Black Canyon) Colorado River Erie (Rosa and Burns 1987, Burns et al. 2005). HVOD has Figure 1.-Location map of Lake Mead, Arizona-Nevada, with been used repeatedly to investigate changes in the health of an insert of a map of the Colorado River Watershed in the southwestern U.S. lakes and reservoirs (Burns 1995, Burns et al. 2005, Charlton 1980, Cornett and Rigler 1979, Hieskary and Wilson 2005, Lasenby 1975, Vincent et al. 1984). The primary focus of this investigation was to evaluate long- Virgin, Temple, and Gregg. Between these basins are 4 nar- term trends of HVOD, phosphorus concentrations and inflows row canyons: Black, Boulder, Virgin, and Iceberg (Fig. 1). in Boulder Basin, and determine the causes of variability. The most complete sets of annual data are from May 2000 to 2007. Retention time in the reservoir ranges between approximately A previous analysis of data from 2000 to 2004 was reported 1 to 3 years, depending on release and inflow patterns as in LaBounty and Burns (2005). The oxygen data collected well as reservoir volume at any one time. The Colorado during that time have now been combined with additional River contributes about 97% of the annual inflow to Lake data from 1991 to 2007 to calculate 16 annual HVOD rates. Mead; the Virgin and Muddy rivers and Las Vegas Wash Drastic changes in annual HVOD rates required arranging the provide the remainder. Inflow to Lake Mead is controlled rates into three groups. Rates from 1995 to 2006 are further by the amount of water released from Glen Canyon Dam. used to elucidate potential consequences when various sce- The minimum release from Glen Canyon dam is 10.2 × 109 narios of environmental conditions occur. Downward mixing m3 yr-1, the 10-year average is <18.5 × 109 m3 yr-1. Annual of oxygen into the hypolimnion was estimated for each of the inflow via Las Vegas Wash is currently about 1.9 × 108 m3, eleven stratified seasons from 1995 to 2005, and the effects about 1.5% of total inflow to Lake Mead. Discharge from on the observed HVOD rates determined. Hoover Dam is hypolimnetic and occurs from 2 intakes, one at elevation 272.8 m and the other at 318.5 m a.s.l. Annual discharge is approximately 9 × 109 m3. During the time of this Materials and Methods investigation the 2 intakes for the Southern Nevada Water System (SNWS) were located at 304.8 m and 320 m a.s.l Description of Study Area until mid-2004, when they were combined at 304.8 m a.s.l. Annual withdrawal through the SNWS in Boulder Basin is Lake Mead is a large mainstream Colorado River reservoir in currently about 5.5 × 108 m3. the Mohave Desert, Arizona-Nevada (Fig. 1). Its lower end is 15 km east of Las Vegas, Nevada. Lake Mead, formed in LaBounty and Horn (1997), LaBounty (2005), LaBounty and 1935 following construction of Hoover Dam, is the largest Burns (2005) and Holdren et al. (2006) describe the hydro- reservoir in the United States by volume (36.7 × 109 m3, 2.975 dynamics and relationships between various limnological × 107 ac-ft)1, and is second only to Lake Powell in terms of variables of Boulder Basin. As with other reservoirs, dam surface area (660 km2; Lara and Sanders 1970). At full pool operations exert a great influence on the water quality and (reservoir elevation 374 m a.s.l.), Lake Mead extends 106 ecology of the system (Thornton 1990). The hydrodynamics km from Black Canyon (Hoover Dam) to Pearce Ferry. Its of such large reservoirs are complex and require analysis of greatest width is 15 km, and the highly irregular shoreline is large data sets to understand them. Each basin within Lake 885 km in length. Lake Mead has 4 large sub-basins: Boulder, Mead is ecologically unique and therefore responds differ- ently to the inflow-outflow regime. Furthermore, the different sources of water entering Lake Mead, as in other reservoirs, 1 ac-ft = 3.259 × 105 gal 1 70
  3. 3. Long-term Increases in Oxygen Depletion in the Bottom Waters of Boulder Basin, Lake Mead, Nevada-Arizona, USA Pre- and Post 2000 sample locations Post 2000 sample locations T emper at u re ( ° C ) Las Vegas Wash 12 14 16 18 20 22 0 DO m g/L Epilimnion DO % /10 20 Temperature Las Flow Wash Veg low a sB Metalimnion rF 40 ay ve Depth (m) Ri o ad 60 lor Co 80 Classic Hypolimnion 100 120 2 3 4 5 6 7 8 9 DO mg/L -- DO %Sat/10 Hypolimnion used in this Hoover Dam study Figure 2.-Map of Boulder Basin, Lake Mead, Arizona-Nevada. The Figure 3.-Temperature and dissolved oxygen profiles from sampling locations for this investigation are depicted. sampling location CR346.4 on 20 October 2005. often retain their identity and influence for substantial dis- Profile data used for the reported analyses were collected by tances into the reservoir and do not necessarily mix com- 3 agencies: the City of Las Vegas, Bureau of Reclamation, pletely with the rest of the water column (Ford 1990). This and the Southern Nevada Water System. HVOD analysis spatial heterogeneity can lead to significant underestimates was performed using data from the hypolimnetic portion of of water retention time and uncertainties in the conveyance 8 deep-water sampling locations within the Colorado River and fate of materials transported into the reservoir. thalweg of Boulder Basin from the Narrows to Hoover Dam (Fig. 2). Data from 1991 through April 2000 are from 2 Boulder Basin (Fig. 2) is the most downstream basin and deep-water sampling sites, CR346.4 and CR342.25. These collects the combined flows from the reservoir’s 2 main locations were sampled twice monthly from March through arms. Additionally, it receives all drainage from the Las November and at least monthly from November through Vegas Valley via Las Vegas Wash into Las Vegas Bay. April. Data collected after May 2000 through early 2007 are Boulder Basin is 15-km wide from Boulder Canyon to from all stations shown in Fig. 2. Each location is sampled at Hoover Dam (Black Canyon), and the distance from the least once per month; many are sampled once or twice weekly confluence of Las Vegas Wash to Hoover Dam is approxi- by one or more of the participating agencies. mately 16 km. The historical Colorado River channel lies along the eastern side of the basin. Lake Mead is mostly Field and Analytical Methods, and Data mesotrophic and monomictic (LaBounty and Burns 2005). Analysis Boulder Basin destratifies completely about every other year. Stratification lasts for approximately 265 days from 1 May Each profile was sampled in a similar manner. Data were to 15 January during years when complete destratification collected from the following depths: surface-30 m every 1 m; occurs. During the stable summer stratification period the 30-60 m every 2 m; 60-100 m every 5 m; 100-5 m from the epilimnion, metalimnion and hypolimnion are about 15, 20, bottom every 10 m; and from 5 m-bottom at least every 1 m and up to 120 m thick respectively, depending on the depth (Fig. 3). Each profile includes temperature and DO. Profile of the reservoir and particular location. The hypolimnion data were collected using profiling sondes manufactured by is the thickest and most stable layer, with an average water Hydrolab Corporation (1995-early 2007) and Eureka Corpo- temperature of 12°C. The deepest portion of Boulder Basin ration (mid-2005-early 2007). Repeatability of DO measure- does not completely destratify each year, leaving a remnant ments is within 0.2% and instrument accuracy = 0.1 mg/L for portion of the hypolimnion >20 m above the bottom in about readings <8 mg/L, 0.2 for readings >8 mg/L. Air calibration 40% of the years (LaBounty and Burns 2005, Holdren et al. is electronic, utilizing the barometric reading at the beginning 2006). Ecological dynamics until early 2007 are described of the survey. Quality control was accomplished by annual in this paper. Significant populations of the quagga mussel comparison of up to 11 probes from participating data col- (Dreissena bugensis) were discovered throughout Boulder lection groups at the same time and place. Basin in January 2007. As populations of this invasive species explode, the ecological dynamics of Boulder Basin could be drastically altered. 71
  4. 4. LaBounty and Burns Water samples were collected weekly for analysis of nutrients time (days), and then these data were fitted to a straight line including total phosphorus (TP). Samples were collected us- by linear regression analysis. This annual rate of DO loss ing a Van Dorn sampler at all depths except from the surface, (HVOD) is expressed in mg/m3/day. Regressions were cal- where grab samples were collected. Two samples were taken culated at the observed water temperatures and then corrected within the epilimnion when it was ≤15 m thick; 3-4 samples to a standard water temperature of 12°C for all years. This within the thermocline, including one at peak conductivity; method was chosen because the HVOD rate is presumed to and up to 4 samples within the hypolimnion. Analyses for TP double for every 10°C rise in temperature, as rates do in many and orthophosphate were by the USBR laboratories in Den- biochemical reactions. Standardization is necessary for the ver, Colorado and Boulder City, Nevada, the Clark County comparison of data from different years because they have Water Reclamation District Water Quality Laboratory, Las different average temperatures. The temperature of 12°C was Vegas, Nevada, and Montgomery Watson Harza Laborato- chosen as the standard temperature because it was close to ries, Monrovia, California. Standard Method SM4500P-E, the average temperature observed for all years. The annual colorimetric analysis was used for most analyses. Detection HVOD rates were plotted separately for 1995-2006, and limits were 10 µg P/L for TP. All values that were nondetect- linear regressions were obtained for each plot for 2 types of able were halved for calculations of averages. years: Type 1 year follows complete destratification; Type 2 follows partial destratification (as described in the follow- Processing of data, statistical analysis and development ing section). The slopes of the regression lines gave the rate of graphical presentations were completed using Excel, of change of HVOD in Boulder Basin in mg DO/m3/day PowerPoint (www.microsoft.com), and LakeWatch (www. per year. The strength of relationships between HVOD and earthsoft.com; www.lakewatch.net). other reservoir characteristics was determined by simple and multiple regression analysis. Hypolimnetic Volumetric Oxygen Depletion (HVOD) Calculations Results The HVOD rates for Boulder Basin were determined using Vertical Mixing the HVOD module in the LakeWatch software. To determine While temperature and DO of the epilimnetic layer are sea- the most adequate method, HVOD analysis of the bottom sonally variable, within the hypolimnion they change slowly layer was attempted using 3 methods. In the first, the hypo- throughout the stratified period. During fall the epilimnion limnion layer was selected from the normal lower inflexion thickens due to seasonal cooling. From May to early October point on the temperature/depth profile. In the second and third the epilimnion layer of Boulder Basin is generally 11-13 methods, the bottom layer depths were selected as being 5 m m. In early October cooling begins to thicken this layer. and 20 m, respectively, above the bottom. The first method For example, on 20 October 2005 the epilimnion was 25 m was rejected because the hypolimnion water mass under (Fig. 3). Despite cooling temperatures in fall 2005, neither analysis changes as the selected hypolimnion depth deepens the water temperature nor the DO profiles indicated strong as the season progresses. The bottom 5-m layer did not yield vertical mixing of the hypolimnion. A typical metalimnetic enough observations to give stable values. Consequently, DO minimum was prominent, while the concentrations within the bottom 20-m layer was chosen for analysis because the hypolimnion steadily decreased with depth until there it gave stable values from an unchanging water mass and was a sudden drop of >2 mg/L within 5 m of the bottom. contained water with the lowest DO concentrations in the Below 90 m, temperature cools with depth into the hypo- water column. These values could thus show how low the limnion by approximately 0.1°C per 20 m. This temperature oxygen concentrations became at the end of each stratified gradient is maintained from May to December indicating a season. The average DO content for the bottom layer was very stable hypolimnion. At the same time the bottom 20 calculated for each sampling station on each sampling date. m water temperatures increased by about 0.3°C throughout In the remainder of this study, the term hypolimnion refers the stratified season, indicating a small, steady downward to the 20-m layer above the bottom (Fig. 3). transfer of heat (Fig. 4). HVOD rates were determined according to the stratification There was a typical annual pattern of temperature and DO conditions prevailing each year. The numbers of days the within the bottom 20 m of the hypolimnion (Fig. 4) indicating reservoir remained stratified each year was determined by 4 phases of vertical mixing: examining water temperature and DO from the initiation of stratification (usually 1 May) to the point of its interruption Phase 1. During the first 220 days of stratification (normally (usually 15 January, or 265 days later). The stratification 1 May to about 6 December) the average water temperature period lasted from 190 to 280 days. The annual oxygen deple- of the bottom 20 m of the hypolimnion gradually but steadily tion rate was calculated by plotting the average hypolimnetic warmed from 11.9°C to 12.2°C, or 0.3°C/220 days. At the DO concentration sampled at each station as a function of 72
  5. 5. Long-term Increases in Oxygen Depletion in the Bottom Waters of Boulder Basin, Lake Mead, Nevada-Arizona, USA same time the average DO concentration steadily declined Dissolved Oxygen (mg/L) Temp ºC 2 9 12.4 from 5.2 mg/L to 2.1 mg/L, or -3.1 mg/L/220 days. DO mg/L 8 Temperature ºC 12.3 7 4 1 Phase 2. From day 220 to day 300 (about 6 December to 24 6 12.2 3 5 February), warming of the bottom 20 m of the hypolimnion 12.1 4 accelerated, and partial vertical mixing occurred as indicated 3 12 by the reoxygenation from 2.1 to 8.5 mg/L in the early part 2 0 40 80 120 160 2 00 240 28 0 3 20 36 0 of Phase 3. Water temperature increased nearly 0.3°C dur- Ma y 1 April 30 Dec 6 Feb 24 Number of Days from May 1 ing this phase (12.2°C to about 12.5°C). The surface water temperature at the end of February 2006 was 13.2°C, so when 1 Slow warming – Weak vertical mixing with slight downward heat and DO transfer the surface water mixed downward there was relatively little 2 Faster warming – Partial mixing; Some DO transfer increase in temperature because the surface water tempera- 3 Intrusion of Colorado River water; Reoxygenation to 80% saturation ture was close to that of the deep water, while at the same 4 time there was considerable downward transfer of DO by Stratification strengthens-Oxygen uptake recommences vertical mixing. However, complete vertical mixing was not Figure 4.-Average daily water temperature and dissolved oxygen achieved because a bottom water temperature of 13.2°C was (DO) concentrations from the bottom 20 m of the hypolimnion, 1 May 2005 to 30 April 2006. not obtained. Complete vertical mixing on this date proceeded only to a depth of 46 m. Phase 3. After day 300 (about 24 February) intrusion of low De st ra tif i ca t ion temperature, low conductance, higher DO Colorado River Some reoxygenation water from Boulder Canyon was observed, lowering the water temperature suddenly from 12.4 to 12.1°C. Phase 4. After 1 May (day 1) the cycle began as stratifica- Dissolved Oxygen (mg/L) DO mg/L tion strengthens. Oxygen uptake in the bottom 20 m of the T e m p er a tu r e ° C hypolimnion occurred. 9 12.4 Temperature °C 8 7 12 6 11.6 Fully Destratified (Type 1) Versus Partially 5 4 11.2 Destratified Years (Type 2) 3 1 00 200 300 100 200 300 Ma y 1 May 1 Jan 1 M ay 1 Jan 1 Lake Mead is a monomictic lake with turnover (destratifica- 2002 2 0 03 20 0 4 tion) occurring generally from 1 March-1 May (LaBounty Figure 5.-Average daily temperature and dissolved oxygen (DO) and Burns 2005). Since stratification is usually complete by concentration in the bottom 20 m of the hypolimnion of a partially 1 May, the “lake year” for every period of 365-days is de- destratified year (Type 2) followed by a fully stratified year (Type fined as extending from 1 May to 30 April, and thus spans 2 1), 1 May 2002 to 1 May 2004. calendar years. Lake years are named from the second of the 2 years spanned. The Type 1 group are those years follow- Average Monthly Temperature ing the complete destratification of the reservoir (lake years 1997, 1999, 2001, 2002, 2004, and 2006). The Type 2 group 12.5 Temperature (° C) includes those years following a year when the reservoir did 12 not fully destratify (lake years 1995, 1996, 1998, 2000, 2003 11.5 and 2005). The pair of years plotted in Fig. 5 (May 2002 through April 2004) are Type 1 and 2, respectively. The 11 Type 2 group of years have higher hypolimnion temperatures 10.5 2007 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 and usually lower DO concentrations than the Type 1 years Average Monthly Dissolved Oxygen Dissolved Oxygen (mg/L) 12 (Figs. 6 and 7). 10 8 Study of numerous profiles of temperature, DO and specific 6 conductivity show that Boulder Basin has normal vertical 4 temperature and DO structure for most of the year, with the Following Partial Following Full Destratification Destratified 2 thermocline deepening after mid-November. In some years Y e ar 2 ty p e Y e ar 1 t y p e 0 2007 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 the downward mixing progresses only to a depth of 70 m to give a partially destratified year (Type 2 year). In other Figure 6.-Average daily water temperature and dissolved oxygen (DO) concentration of the bottom 20 m of Boulder Basin, 1995- years, downward mixing proceeds to the lake bottom to give 2007. 73
  6. 6. LaBounty and Burns a fully destratified year (Type 1 year). But Boulder Basin 2005 – 2006 (Type 2) DO Depletion Rate 12.2 has a unique mixing pattern in one respect. In a normal lake, Temperature C DO (mg/L) 5 the temperature at the bottom of the lake at the end of the isothermal period is the lowest temperature observed in the 12.1 4 water column that year. However, in Boulder Basin, when 3 12 the surface water begins to warm after the isothermal period, DO Depletion 0 the hypolimnion from a depth of about 95 m can continue Regression 0 30 60 90 120 150 180 210 T em p er atu r e Nov 15 M ay 1 Jul 6 Sept 10 to get colder. This is attributed to a large pool of cool water Regression 2004 – 2005 (Type 1) DO Depletion Rate from the Colorado River slowly moving into Boulder Basin 11.8 8 11.7 Temperature C into the bottom of the deepest part of the Basin and usually DO (mg/L) 7 11.6 results in the deeper hypolimnion having a temperature 11.5 6 of ≤12°C. The process of an intrusion of Colorado River 11.4 5 water into Boulder Basin takes 2 months to arrive from the 11.3 11.2 4 Colorado River inflow (Dr. Imad Hannoun, Flow Science, 0 30 60 90 120 150 180 210 240 Harrisburg, VA., pers. comm.). The intrusion (interflow) Jan 21 May 1 Jul 6 Sept 10 Nov 15 of Colorado River water forming the deeper hypolimnion Figure 7.-Dissolved oxygen depletion (HVOD) rate (regression is usually complete by 1 May and sets up a Type 1 year. = -0.01375 mg DO/L/day; p < 0.0001) and temperature increase During the remainder of the year, until the end of Novem- (regression = +0.00112°C/day; p < 0.0001) for (top panel) the first 210 days of 2005 (beginning 1 May 2005), and (bottom panel) ber when the Colorado River interflow is found within the the first 265 days 2004 (beginning 1 May 2004) dissolved oxygen upper portion of the hypolimnion or in the metalimnion, depletion (HVOD) rate (regression = -0.01471 mg DO/L/day; p < the temperature in the bottom hypolimnion layer increases 0.0001) and temperature increase (regression = +0.00112°C/day; slowly as the DO concentrations decrease (Fig. 4 and 5). A p < 0.0001). Type 1 year is usually followed by a Type 2 year with some downward mixing and/or some Colorado River water being added to the deep hypolimnion between February and May, menced with a Colorado River intrusion of 11.1°C and 980 with DO concentration increasing but with little change in µS/cm water on 12 April 2004. temperature (Fig. 6). At the end of the Type 2 year the hy- polimnion temperature is usually above 12°C enabling full Average monthly temperatures of the bottom 20 m of the vertical mixing (Holdren et al. 2006), illustrated by the data hypolimnion ranged between 11 and 12.6°C from 1995 to on a biennial period (Fig 5). early 2007 (Fig. 6). Holdren et al. (2006) stated that mixing occurs in Boulder Basin when early spring hypolimnetic The water in Boulder Basin has a higher conductance than temperatures are above 12°C; mixing does not occur below the water from the Colorado River because of evaporation 12°C. When destratification is not complete between strati- and the higher conductivity water from Las Vegas Wash and fied periods (black arrows, Fig. 6), the water temperature the Virgin and Muddy Rivers. The first year (1 May 2002-30 continues to rise until destratification does occur (red arrows, April 2003) of a pair of years (Fig. 5) follows a complete Fig. 6). Full mixing that results in bottom waters with the destratification during February-April 2002. At Station same temperatures as the surface waters occurs only after CR346.4 on 12 February 2002, the water was isothermal at the temperature difference between the surface and bottom 11.9°C with a conductance of 935 µS/cm, but by March the waters is <1°C. Once destratification occurs, its onset is sud- Colorado River intrusion water had changed the tempera- den and lasts 30-60 days. The pattern for DO concentrations ture to 11.1°C with a conductance of 919 µS/cm to set up a reflects that of temperature. In years when destratification was Type 1 year. The second year (1 May 2003-30 April 2004) not complete (black arrows), DO concentrations remained followed partial destratification. Stratification weakened generally below 6 mg/L. Upon destratification, however, between January and April 2003 but remained present. It reoxygenation to saturation near 80-85% was achieved (red was weakest on 4 March 2003 with a surface temperature arrows). A similar pattern, but the reverse of that of tem- of 12.8°C and a bottom temperature of 11.6°C. Average perature, existed for average DO concentration from 1 May bottom water temperature began at about 11.1°C on 1 May 2002 to 30 April 2004 (Fig. 5). Average DO concentrations 2002 and was 11.8°C on 30 April 2003, a rise of 0.7°C in were 8.2 mg/L on 1 May 2002 and 5.1 mg/L 265 days later 365 days. From 1 May 2003 to about 1 February 2003 (275 in mid-March 2003, a loss of 3.1 mg/L DO in the bottom 20 days) the temperature rose from 11.8°C to about 12.4°C, or m of the hypolimnion. DO depletion continued after some 0.6°C. The temperature rose at a rate 12.1% greater in the oxygenation in late April and early May 2003. The average second year. After 1 February 2004 (within the second year) DO concentration was 5.8 mg/L in May 2003 and ended at destratification, or complete mixing, occurred with the water 3.6 mg/L in mid-January 2004, about 245 days later, a loss column isothermal at 12.2°C and conductance between 998 and 1000 µS/cm on 13 February 2004. A Type 1 year com- 74
  7. 7. Long-term Increases in Oxygen Depletion in the Bottom Waters of Boulder Basin, Lake Mead, Nevada-Arizona, USA of 2.2 mg/L. The depletion rate for DO was 23.2% greater Table 1.-Annual dissolved oxygen depletion rates for 16 years, 1991–2006, Boulder Basin, Lake Mead, Nevada-Arizona. during the first year than the second. DO on Ending Annual Lake Type May 1 DO Rate2 Annual HVOD Rates Year1 of Year (mg/L) (mg/L) (mg/m3/day) The flow into Las Vegas Bay of Boulder Basin consists 1991 1 8.8 5.6 18.1 mostly of treated wastewater from the Las Vegas Valley. 1992 1 8.4 4.9 17.2 Since 1994, several improvements to waste water treatment 1993 1 9.0 5.1 15.1 (i.e., tertiary treatment of phosphorus and nitrogen) were 1994 1 8.8 5.1 17.4 made, affecting Las Vegas Bay. As a result of this and other 1995 2 5.2 3.6 8.1 1996 2 5.5 4.1 7.5 external factors (such as above vs. below average inflow 1997 1 8.4 6.3 9.3 to Lake Mead), certain patterns form individual groups of 1998 2 8.2 6 9.6 years, each having one or more major events that cause them 1999 1 8.1 5.7 10.7 to be considered a separate group no matter which years are 2000 2 7.1 4.8 11 included in other groups. Accordingly, annual depletion rates 2001 1 8.2 5.2 11.8 for lake years 1991-2006 are in 3 distinct groups, each with 2002 1 8.2 5.1 14.1 a major event related to changes in treatment of nutrients 2003 2 5.8 3.6 11.7 released into the Las Vegas Wash and thus into Las Vegas 2004 1 7.8 4.2 14.7 Bay (Table 1; Fig. 8). 2005 2 5.1 2.7 13.7 2006 1 8.8 4.8 11.6 Group 1, 1991-1996, includes the initiation of advanced Average 1 8.5 5.2 14.0 wastewater treatment in 1994. The early HVOD rates were Std Dev 1 ±0.4 ±0.5 ±2.9 the highest of the 15 years (15.1-18.1 mg/m3/day)2. Group Average 2 6.2 4.1 10.3 2, 1996-2004, begins with the lowest annual rates (7.5-8.1 Std Dev 2 ±1.2 ±1.1 ±2.3 mg/m3/day), and ends with an annual rate of 14.7 mg/m3/day. May of named year to April of the next year 1 This group includes the commencement of tertiary treatment of nutrients in discharged wastewater as well as a period of The HVOD at standard temperature (12°C) regression presented in this 2 table abnormally high inflow to Lake Mead (1995-1999). While average outflow from Glen Canyon Reservoir was always <335 m3/day in 1991-1994 and 2000-2006, average outflow in 1995-1999 was >400 m3/day (peaking at about 600 m3/day in 1997). There was ever-increasing organic loading during Enhanced this time period due to the population surge in the watershed Advanced W int e r Treatment Treatment Treatment f or P leading to steadily increasing flows of treated wastewater Average Glen Canyon Begins Begins Begins Outflow > 400 m3/day via Las Vegas Wash. Group 3, 2005-2007, shows declining annual rates (14.7-11.6 mg/m3/day). Advanced treatment HVOD Regression of phosphorus in the discharged wastewater effluent is not 18 HVOD at Std Temp Regression DO Depletion Rate (mg/m3/day) officially regulated from October through March. However, 16 beginning in winter 2002 treatment of phosphorus in the 14 Group 2 wastewater outflows from all facilities occurred much more Group 1 12 frequently from October to April. Additionally, in 2005 10 enhanced treatment for phosphorus occurred that further Group 3 reduced the total load of organics to Boulder Basin. All 8 these efforts reduced the current TP load by 97-98% from 1991 1993 1995 1997 1 99 9 2001 2003 2005 levels from the 1970s and 1980s (Dr. Doug Drury, Clark County Water Reclamation District, Las Vegas, Nev., pers. Figure 8.-Rate of HVOD change in the bottom 20 m of the comm.). hypolimnion of Boulder Basin, 1991-2006. Group 1 (1991-1996): high HVOD rates early (during period of high organic loading), dramatically lower rates later (following implementation of Annual HVOD Rate for Years of Type 1 and 2 advanced wastewater treatment technology); Group 2 (1996- 2004): progressively increasing HVOD rates due to rapid Data for 1995-2005 were selected for analysis and clas- population expansion within the watershed leading to continuous sification into Type 1 and 2 years, because even though the increase in organic loading; Group 3 (2004-2006): beginning of a trend of decreasing HVOD rates due to decreased organic loading. HVOD rates at standard temperature of 12°C 2 75
  8. 8. LaBounty and Burns wastewater treatment remained the same, loading increased Type 1: Years following complete destratification DO Depletion Rate (mg/m3/day) with population surges. Dissolved oxygen depletion pro- 14 gresses at a steady rate, but there is a measurable difference 12 between Type 1 and Type 2 years (Fig. 7). Disruption of the 10 upward temperature trend and the downward DO depletion HVOD Regression trend happens sometime after day 180 (Fig. 4, a temporal 8 HVOD at Std Temp Regression extension of the top panel of Fig. 7). In 2005, the pattern 1997 1998 1999 2 0 00 2 001 20 02 20 03 2 00 4 began falling apart after day 210 (6 December 2005). After Type 2: Years following partial destratification DO Depletion Rate (mg/m3/day) complete destratification, the stable period lasted at least 265 14 days to 21 January. 12 Dissolved oxygen concentrations at the beginning and end 10 HVOD Regression of annual stratification were different for the two categories 8 HVOD at Std Temp of years (Table 1). Each Type 1 year started and finished the Regression stratified season with higher DO concentrations than Type 1 99 5 19 9 6 19 9 7 1 9 98 19 99 2 000 20 01 2002 2 00 3 20 04 2 0 05 2 years. The HVOD rates for the Type 1 group (14.0 ± 2.9 Figure 9.-Rate of HVOD change. Top panel: Type 1 Category mg/m3/day) are higher than those of Type 2 (10.3 ± 2.3 years; years that followed complete destratification (regressions: HVOD = +0.803 mg/m3/day per year, p < 0.01; HVOD at 12°C mg/m3/day). The annual DO depletion rates for 1995-2005 = +0.906 mg/m3/day per year, p < 0.05). Bottom panel: Type were plotted separately for Type 1 and Type 2 years (Fig. 2 category years; years that followed a short period of partial 9). Regressions were calculated and corrected to HVOD destratification (regressions: HVOD = +0.581 mg/m3/day per year, at 12.0°C and illustrate increasing trends since 1995 for p < 0.01; HVOD at 12°C = +0.586 mg/m3/day per year, p < 0.01). both types of years. The annual rates, corrected to 12.0°C, increased by 0.91 mg/m3/day per year for the Type 1 years Average Daily Outflow, Glen Canyon Dam and 0.59 mg/m3/day per year for the Type 2 years. Each of these trends is statistically significant (p < 0.01). 650 600 HVOD Change Based on Type 1 and Type 2 1 9 95 , 96 , 9 7 , 9 8 , & 9 9 550 Flows > average, Years reservoir rose 11 m 500 m3/day The mixing period of larger lakes is usually annual if the 450 lake is monomictic. Finding a biennial mixing period was 400 unexpected, but even more unexpected was the discovery 350 of different HVOD rates for the different types of years. A 300 number of scenarios were investigated to determine the cause for the different HVOD rates. The first possibility was that 250 1991 1993 1995 1997 1999 2001 2003 2005 2007 different mixing regimes resulted in different organic carbon Figure 10.-Average daily outflow from Glen Canyon Dam (Lake concentrations in the hypolimnia of the 2 different types of Powell), 1991-2006. Outflow from Glen Canyon Dam travels years, thus causing different depletion rates; this was found through the Grand Canyon into Lake Mead unimpeded. Circled not to be the case, however; organic carbon concentrations values are years when average flow from Glen Canyon Dam was were similar in the pairs of years (Type 1 years 2.94 ± 0.31 >average. mgTOC/m3; Type 2 years 3.00 ± 0.30 mgTOC/m3). Varying inflow rates and water temperature from Lake Pow- (Fig. 6; 1997, 1999) and 3 Type 2 years (Fig. 6; 1995, 1996, ell (Glen Canyon Dam outflow, Fig. 10) were investigated 1998), which means the distribution of year types was similar as possible causes of the different HVOD rates. The inflow to the low flow period. from Lake Powell via Boulder Canyon dominates the water mass in Boulder Basin, and the volume and temperature of The possibility of variable downward transport of oxygen was this inflow is quite variable. Both the volume/inflow rate next investigated. The full and partially destratified years tend and water temperature depend on the outflow from Glen to occur in pairs (Fig. 6). However, as Holdren et al. (2006) Canyon Dam (Lake Powell). In general but not always, the pointed out, and as we have shown, winter destratification temperature of Lake Powell outflow depends on the depth of was not completed for the 3-year period 1994-1996. Inflow Lake Powell. At lower lake levels, outflow temperatures are of colder water into the hypolimnion as an interflow from higher because water is removed from a shallower layer. The the Colorado River occurred in those 3 years but also in the magnitude of the flow did not affect the type of year because other years when destratification occurred. The resistance in the 1995-1999 high flow period there were 2 Type 1 years 76
  9. 9. Long-term Increases in Oxygen Depletion in the Bottom Waters of Boulder Basin, Lake Mead, Nevada-Arizona, USA to vertical destratification is strongest in the first year of a a small rise in temperature of the water column. The transfer pair because the water temperature of the hypolimnion is 1-2 of DO into the bottom 20 m of Boulder Basin by eddy dif- degrees cooler than in the second year of the pair. When the fusion was estimated by an equation successfully developed water temperature rises during the second year of the pair, and applied in Lake Erie (Rosa and Burns 1987): resistance to full vertical mixing weakens and full vertical ∆O = (Ot - Oh)/(Tt - Th) × ∆T eq. (1) mixing can occur. where ∆O = increase in DO concentrations if DO was con- From 1995 to 2005, the average HVOD rate for Type 1 served; ∆T = increase rate in hypolimnion temperature; Ot = years was 2.0 mg/m3/day higher than Type 2 years (Table 2). average thermocline DO concentration for stratified period; Further, the rate of increase in the HVOD rate was less for Oh = average hypolimnion DO concentration for stratified Type 2 years. We investigated the cause of the differences by period; Tt = average thermocline temperature for stratified determining whether there is a difference in the downward period; and Th = average hypolimnion temperature for strati- transfer of heat and DO between the two types of years. fied period. The temperature increase rates in the bottom 20 Downward transfer of heat occurs by conduction from m are small but can explain substantial transfer of DO into warmer, upper waters to cooler, deeper waters. Similarly, the hypolimnion (Table 2). oxygen can move downward by molecular diffusion to the The temperature increase rate is similar for both Type 1 and deeper waters containing less oxygen, 2 independent mecha- Type 2 (Table 3), but because of the greater Ot - Oh gradi- nisms. The conduction of heat through water is calculated by ent in Type 2 years, eddy diffusion is able to transfer more using the thermal conductivity coefficient of water (0.0014 oxygen downward than in Type 1 years. The extra oxygen cal/cm/sec) and the vertical temperature gradient, which at added to the hypolimnion in Type 2 years significantly lowers the depth of 20 m above the bottom is approximately 0.6 × the observed HVOD rate in those years. Also, the true (as 10-4°C/cm in Boulder Basin. This produces a warming in the adjusted) oxygen uptake rates are significantly higher than 20-m thick hypolimnion of approximately 0.0017°C from 8 the observed HVOD rates. The rates of HVOD change are May-28 December of this particular year. In 2000, the bot- 0.59 mg/m3/day per year for Type 2 years and 0.91 mg/m3/day tom waters at the central sampling site (CR346.4) warmed per year for Type 1 years. The increased downward diffusion by 0.3°C during this period. Thus the downward transfer of also explains why Type 2 HVOD rates are increasing more heat by conduction is of little importance in the warming of slowly than Type 1 rates. As hypolimnetic DO concentrations the hypolimnion. become lower, the DO concentration gradient between the The molecular diffusion of oxygen through the surface of the hypolimnion and the water above it becomes greater; there- hypolimnion can be estimated by using the diffusion coef- fore, eddy diffusion transports more oxygen downward. ficient of oxygen in water of 2 × 10-9 m2/sec (Pilson 1998) and Factors other than hypolimnetic temperature prevent or the average DO concentration gradient at the hypolimnion enhance complete mixing after the second year of a pair of boundary of 0.20 g/m4. This yields a downward flux of DO years: winter climatic factors such as ambient air tempera- of 3.5 × 10-3 mg/m2/day, which would increase the concen- ture, wind speed and duration; and as discussed, erratic high tration of DO in a 20-m thick hypolimnion by 0.17 × 10-3 and low flow patterns to and from Lake Mead (Holdren et mg/m3/day. This is much smaller than the observed HVOD al. 2006). These factors altered the pattern of full biennial rates ranging from 8.1 to 14.7 mg/m3/day. Molecular diffu- restratification, caused complete mixing in 2 subsequent years sion of oxygen would be of little importance in explaining in 2000 and 2001 (Fig. 4), and prevented complete mixing the difference of 2.0 mg/m3/day between the Type 1 and for 3 other years (1994-1996). Type 2 HVOD rates. During the stratified season, heat and DO mostly move Causes of Change in HVOD downward in lakes and reservoirs by a vertical mixing process known as vertical eddy diffusion, a process found to be sig- The period of relatively low HVOD rates from 1995 to 2000 nificant in Onondaga Lake, New York (Mathews and Effler occurred just after the commencement of advanced treatment 2006). In eddy diffusion, heat and DO are moved downward of the wastewater entering the Las Vegas Wash (Fig. 8), in the same water mass; thus, the increase of temperature which indicates that the supply of phosphorus to Boulder in the hypolimnion is an indication of the quantity of DO Basin probably has an effect on the HVOD rates. However, transferred downward in the water. However, the increase this period of low HVOD rates also coincides partially (1995- in DO concentrations will not be observed if oxygen is being 1999) with the period of high flows from the Glen Canyon consumed via the degradation of organic matter at a greater Dam, which also tend to lower the HVOD rates (Fig. 10 and rate than its downward displacement. If the eddy diffusion 11). Thus both the Glen Canyon Dam outflow and TP con- is not extensive and happens to a similar degree from the centrations in Las Vegas Bay were investigated as affecting thermocline downward, it is not easily discernable except for 77
  10. 10. Table 2.-Estimate of downward transport of dissolved oxygen (DO) into the hypolimnion of Boulder Basin 1995-2005 by eddy diffusion. Av Av Av Av Temp Downward Thermo Hypo Thermo Hypo Tt - Th increase Downward HVOD HVOD DO flux as Year Type DO DO Ot - Oh Temp Temp Temp rate DO flux Obs. Adj. % of HVOD and Year Adj. (g/m3) (g/m3) (g/m3) (°C) (°C) (°C) (°C/day) (g/m3/day) (g/m3/day) (g/m3/day) Type 1 – Years After Full Destratification LaBounty and Burns 1997 7.39 7.03 0.36 12.091 12.012 0.079 0.0009 0.004 0.009 0.013 29 1999 7.40 7.03 0.36 12.062 11.987 0.075 0.0011 0.005 0.011 0.016 33 2001 7.13 6.81 0.32 11.876 11.827 0.049 0.0012 0.008 0.012 0.020 39 2002 7.25 6.69 0.55 11.405 11.328 0.077 0.0015 0.011 0.014 0.025 43 2004 6.76 6.09 0.67 11.463 11.372 0.091 0.0018 0.014 0.015 0.028 48 Averages 7.18 6.73 0.45 11.779 11.705 0.074 0.0013 0.008 0.012 0.020 40 Type 2 - Years After Partial Destratification 1995 5.21 4.74 0.47 12.003 11.894 0.109 0.0017 0.007 0.008 0.015 47 1996 5.62 4.87 0.75 12.653 12.545 0.108 0.0011 0.007 0.007 0.014 52 1998 7.53 7.09 0.44 12.252 12.220 0.032 0.0016 0.022 0.010 0.032 70 2000 7.01 6.28 0.73 12.390 12.341 0.049 0.0013 0.019 0.011 0.030 63 2003 5.71 4.73 0.99 12.132 12.007 0.125 0.0015 0.012 0.012 0.024 50 2005 5.31 3.95 1.37 12.202 12.079 0.123 0.0016 0.017 0.014 0.031 56 Averages 6.07 5.27 0.79 12.272 12.181 0.091 0.0014 0.014 0.010 0.024 58 78

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