The response of Lake Tahoe to climate change G. B. Sahoo & S. G. Schladow & J. E. Reuter & R. Coats & M. Dettinger & J. Riverson & B. Wolfe & M. Costa-Cabral Received: 11 August 2011 / Accepted: 11 September 2012 /Published online: 11 October 2012 # Springer Science+Business Media Dordrecht 2012 Abstract Meteorology is the driving force for lake internal heating, cooling, mixing, and circulation. Thus continued global warming will affect the lake thermal properties, water level, internal nutrient loading, nutrient cycling, food-web characteristics, fish-habitat, aquatic ecosystem, and other important features of lake limnology. Using a 1-D numerical model—the Lake Clarity Model (LCM) —together with the down-scaled climatic data of the two emissions scenarios (B1 and A2) of the Geophysical Fluid Dynamics Laboratory (GFDL) Global Circulation Model, we found that Lake Tahoe will likely cease to mix to the bottom after about 2060 for A2 scenario, with an annual mixing depth of less than 200 m Climatic Change (2013) 116:71–95 DOI 10.1007/s10584-012-0600-8 This article is part of a Special Issue on Climate Change and Water Resources in the Sierra Nevada edited by Robert Coats, Iris Stewart, and Constance Millar. G. B. Sahoo (*) : S. G. Schladow : J. E. Reuter Tahoe Environmental Research Center, University of California Davis, One Shields Avenue, Davis, CA 95616, USA e-mail: [email protected]G. B. Sahoo : S. G. Schladow Department of Civil and Environmental Engineering, University of California Davis, One Shields Avenue, Davis, CA 95616, USA J. E. Reuter : R. Coats Department of Environmental Science and Policy, University of California Davis, One Shields Avenue, Davis, CA 95616, USA M. Dettinger US Geological Survey and Scripps Institute of Oceanography, La Jolla, CA 92093, USA J. Riverson Tetra Tech, Inc., 10306 Eaton Place, Suite 340, Fairfax, VA 22030,USA B. Wolfe Northwest Hydraulic Consultants, 870 Emerald Bay Road, Suite 308, South Lake Tahoe, CA 96150, USA M. Costa-Cabral Hydrology Futures, LLC, 4509 Interlake Avenue N #300, Seattle, WA 98103, USA
Transcript
The response of Lake Tahoe to climate change
G. B. Sahoo & S. G. Schladow & J. E. Reuter & R. Coats &M. Dettinger & J. Riverson & B. Wolfe & M. Costa-Cabral
Received: 11 August 2011 /Accepted: 11 September 2012 /Published online: 11 October 2012# Springer Science+Business Media Dordrecht 2012
Abstract Meteorology is the driving force for lake internal heating, cooling, mixing, andcirculation. Thus continued global warming will affect the lake thermal properties, waterlevel, internal nutrient loading, nutrient cycling, food-web characteristics, fish-habitat,aquatic ecosystem, and other important features of lake limnology. Using a 1-D numericalmodel—the Lake Clarity Model (LCM)—together with the down-scaled climatic data of thetwo emissions scenarios (B1 and A2) of the Geophysical Fluid Dynamics Laboratory(GFDL) Global Circulation Model, we found that Lake Tahoe will likely cease to mix tothe bottom after about 2060 for A2 scenario, with an annual mixing depth of less than 200 m
This article is part of a Special Issue on Climate Change and Water Resources in the Sierra Nevada edited byRobert Coats, Iris Stewart, and Constance Millar.
G. B. Sahoo (*) : S. G. Schladow : J. E. ReuterTahoe Environmental Research Center, University of California Davis, One Shields Avenue, Davis, CA95616, USAe-mail: [email protected]
G. B. Sahoo : S. G. SchladowDepartment of Civil and Environmental Engineering, University of California Davis,One Shields Avenue, Davis, CA 95616, USA
J. E. Reuter : R. CoatsDepartment of Environmental Science and Policy, University of California Davis, One Shields Avenue,Davis, CA 95616, USA
M. DettingerUS Geological Survey and Scripps Institute of Oceanography, La Jolla, CA 92093, USA
J. RiversonTetra Tech, Inc., 10306 Eaton Place, Suite 340, Fairfax, VA 22030, USA
B. WolfeNorthwest Hydraulic Consultants, 870 Emerald Bay Road, Suite 308, South Lake Tahoe, CA 96150,USA
M. Costa-CabralHydrology Futures, LLC, 4509 Interlake Avenue N #300, Seattle, WA 98103, USA
as the most common value. Deep mixing, which currently occurs on average every 3–4 years, will (under the GFDL B1 scenario) occur only four times during 2061 to 2098.When the lake fails to completely mix, the bottom waters are not replenished with dissolvedoxygen and eventually dissolved oxygen at these depths will be depleted to zero. When thisoccurs, soluble reactive phosphorus (SRP) and ammonium-nitrogen (both biostimulatory)are released from the deep sediments and contribute approximately 51 % and 14 % of thetotal SRP and dissolved inorganic nitrogen load, respectively. The lake model suggests thatclimate change will drive the lake surface level down below the natural rim after 2085 for theGFDL A2 but not the GFDL B1 scenario. The results indicate that continued climatechanges could pose serious threats to the characteristics of the Lake that are most highlyvalued. Future water quality planning must take these results into account.
1 Introduction
Climate change at both regional and global scales is evident in the shifts of time-series trendsand patterns of long-term weather and hydrologic observations that include maximum andminimum air temperature, snow accumulation, snow to precipitation ratio, snow melt timing,and stream runoff (Cayan et al. 2008, 2009; Coats 2010; Dettinger and Cayan 1995; Hansenet al. 2006). Meteorology is the driving force for lake heating, cooling, mixing, andcirculation; thus, climate change affects features of physical limnology including, but notlimited to (1) the heat budget and thermodynamic balance across the air-water interface; (2)formation and stability of the thermocline; (3) the amount of wind-driven energy input to thesystem; (4) the water budget including evaporative loss: and (5) the timing of streamdelivery into a lake or reservoir. Several authors have evaluated the impact of climate changeon the thermal behavior of lakes (Austin and Colman 2008; Coats et al. 2006; Livingstone2003; Schneider et al. 2009).
Lake Tahoe (CA-NV, USA) is world renowned for its natural beauty and cobalt-bluecolor. Observed trends in air temperature, precipitation, percent of total annual precipitationfalling as snow, and snowmelt timing indicate that the Sierra Nevada region is warming(Sahoo et al. 2011; Schneider et al. 2009; Stewart et al. 2005), and that the Tahoe basin iswarming faster than the surrounding region (Coats 2010). Lake Tahoe is an ice free warm-monomictic lake with deep-mixing only in the winter. Lake Tahoe mixes completely to its500 m bottom on the average once every 3 to 4 years (Tahoe Environmental Research Center(TERC), University of California Davis 2008). A stable thermocline is established eachsummer at a depth of approximately 20 m. As the lake cools in the fall, the thermoclinetypically lowers and by October is at a depth of 32 m (Coats et al. 2006). It has beenhypothesized that deep-mixing could cease entirely if the warming trend continues (Coats etal. 2006). Since deep mixing supplies dissolved oxygen from surface to bottom, reducedmixing may result in evolution of anoxic condition near the deep-sediment interface.Existing water quality and quantity problems at Lake Tahoe include (1) declining Secchidepth transparency (2) increasing primary productivity rate (5 % per year), (3) pervasivethick growths of attached algae along parts of the once-pristine shoreline, (4) increasingvolume weighted mean temperature (0.013 °C per year), (5) increasing resistance to mixing,and (6) invasion of non-native species (Sahoo et al. 2011; Tahoe Environmental ResearchCenter (TERC), University of California Davis 2008). Continued climate change couldpotentially exacerbate all of these issues.
The Fourth Assessment Report of the Intergovernmental Panel on Climate Change(Intergovernmental Panel on Climate Change (IPCC) 2007) indicates that: (1) global green
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house gas (GHG) emissions will continue to grow over the next few decades and (2)continued GHG emissions at or above current rates would cause further warming and inducechanges in the global climate system during the 21st century. With this global situation as aback drop, the objective of this study was to use spatially downscaled meteorology output(air temperature, precipitation, wind speed, longwave radiation, and solar radiation) for theTahoe basin obtained with the Geophysical Fluid Dynamics Laboratory Model (GFDLCM2.1) (Delworth et al. 2006) applied to the A2 and B1 IPCC emission scenarios(Dettinger, this issue) to estimate lake response (thermal properties, lake water level, andlake water quality in terms of dissolved oxygen and nutrients) during the 21st Century. Theclimatic response to A2 green house gas emission scenario is based on assumptions of a veryheterogeneous world economy with increasing global population, regionally oriented eco-nomic development, and more fragmented and slower technological changes. The B1scenario is based on assumptions of a greener future with same global population that peaksin mid-century and declines thereafter, but with rapid changes in economic structurestowards information and service, with introduction of clean and resource-efficient technol-ogy and reduction in material intensity (IPCC 2007). In addition, implications for lakemanagement due to changes in thermal properties in the lake are discussed. Note that theclimatic scenarios represent a range of possible future, modeled situations. These are usefulfor exploring the potential implications of a changed climate.
2 Methodology
2.1 Lake clarity model
The Lake Clarity Model (LCM) (Sahoo et al. 2010) is the customized model based on theUC Davis - Dynamic Lake Model with Water Quality (DLM-WQ) (Chung et al. 2009;Fleenor 2001; Hamilton and Schladow 1997; Heald et al. 2005; Perez-Losada 2001). Thehydrodynamic component of the model is based on the original DYRESM (Imberger et al.1978). Fleenor (2001) added river plunging algorithms into the hydrodynamic module. Theprimary hydrodynamic model is one-dimensional (1-D) and is based on a horizontally mixedLagrangian layers approach (Hamilton and Schladow 1997); however, the stream inflowsand mixing due to stream turbulence are two-dimensional (2-D). All the ecological modulesare incorporated into the 1-D hydrodynamic model (Sahoo et al. 2010). The hydrodynamicmodel simulates stratification, mixing, the transport of all pollutant in the vertical direction,and determines the stream plunging depths. Lake Tahoe has 63 tributary inflows. Theecological modules simulate transformation processes associated with algal photosynthesis(Sahoo et al. 2010). External flows and pollutants (nutrients and fine sediment particles) intothe lake are from atmospheric deposition, streams and intervening zones (both urban andnon-urban), groundwater and shoreline erosion. Sahoo et al. (2010) calibrated and validatedthe LCM using estimated (1) stream flows and associated pollutant loads (2) atmosphericpollutant loads, (3) shoreline erosion, (4) groundwater flow and pollutant loads and (5)5 years (i.e. year 2000 to 2004) of in-lake data. Model validation demonstrated the ability ofthe LCM to capture the seasonal temperature and DO patterns.
It is evident in Fig. 1 that (1) DO concentrations continuously decrease in absence of deepmixing and (2) the lake becomes homogenized because of the winter mixing (see March2007 winter mixing in Fig. 1). DO concentration declines at the sediment surface (450 mbelow the lake water surface) at the rate of approximately 0.1 mg/L per month as a result ofthe biological and chemical processes that typically create water column biological oxygen
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demand (BOD) and chemical oxygen demand (COD), and sediment oxygen demand (SOD).At this rate DO concentration at the lake bottom would be reduced to zero in approximately6 years in complete absence of deep mixing.
2.2 Model assumptions and LCM modification
2.2.1 Sediment release rates
As part of this study, the treatment of sediment nutrient release in LCM was modified toaccount for deep-water column anoxia (Sahoo et al. 2010), a condition not currently knownto exist in Lake Tahoe. Nutrients are released from the sediments when anoxia occurs at thesediment-water interface (Wetzel 2001). Due to the expected increase in lake stability (i.e.reduced mixing) under future climate conditions (Winder et al. 2008); a reduction in oxygentransfer to the sediments was expected.
Sahoo and Schladow (2008) using just the hydrodynamic model of LCM demon-strated that deep lake mixing can be reduced because of lake warming. However theydid not show how this would affect dissolved oxygen, possible impacts of nutrientrelease from anoxic sediments, and the magnitude of this additional nitrogen (N) andphosphorus (P) source relative to the complete nutrient input budget. The presentstudy calculated DO concentrations in the lake at each modeled depth layer. Thesediment nutrient release rates (Table 1) were assigned based on experimental resultsusing intact deep sediment cores and water specifically from Lake Tahoe water(Beutel 2000, 2006). In the study, it was assumed that rates of N and P release fromthe sediment remained uniform over the modeled period.
Fig. 1 Dissolved oxygen concentrations based on SEABIRD profiles taken at approximately monthlyintervals at the mid-lake station. The open circles at the top of the figure indicate the profiling dates. Verticalresolution is approximately 0.5 m
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2.2.2 Lake water level
Water level is estimated based on the following water balance equation:
DWt ¼ DWt�1 þ St þ GWt þ Rt � Et � Ot � Ovt
Where,
DWt Water level at current time step tDWt-1 Water level at previous time step t-1St Stream inflow contribution between time steps t-1 and t, expressed
as an equivalent height of water at the surface.GWt Groundwater inflow contribution between time steps t-1 and t, expressed as an
equivalent height of water at the surface. Groundwater inflow rate is from Trask(2007). The daily value of groundwater is assumed to be the same for all years.
Rt Direct precipitation on the lake between time steps t-1 and t, expressed as anequivalent height of water at the surface. Isoheytal map of Lake Tahoe (LahontanRegional Water Quality Control Board (Lahontan) and Nevada Division ofEnvironmental Protection (NDEP) 2010a; Simon et al. 2003) shows thatprecipitation on the lake varies nearly 50 % from the shore to the middle of thelake, so the estimated precipitation is reduced by 35 %. This estimate wasderived from a best fit for comparing the daily estimated lake water elevation tothose of the historical measured records during calibration and validation.
Et Evaporation contribution between time steps t-1 and t, expressed as anequivalent height of water at the surface.
Ot Outflow contribution between time steps t-1 and t, expressed as an equivalentheight of water at the surface. Outflow was estimated based on the regressionequations (see Table 2).
Ovt Overflow contribution between time steps t-1 and t, expressed as an equivalentheight of water at the surface. This applies if the water level goes above themaximum legal limit for Lake Tahoe (1898.63 m Bureau of Reclamation Datum,or 1899.86 m NAVD) and water is spilled to the Truckee River.
The regression equations for outflow (O) were developed based on lake water depthabove the lake’s natural rim (D). The sixth order regression equations were developed toprovide the highest R2 value and match the modeled outflow as close to the measuredoutflow as possible. The U.S. Geologic Survey (USGS) measures lake level at Tahoe City(site number: USGS 10337000 LAKE TAHOE ATAHOE CITY CA). When the lake levelfalls below the natural rim, there is no outflow to the Truckee River. Although data areavailable since 1950, recent data 2000 to 2009 data were used in the analysis because recentdata reflects the updated gate operation at Tahoe City. While a regression was developed
Table 1 Sediment oxygen demand (SOD) and nutrient release rate of soluble reactive phosphorus (SRP),nitrate (NO3) and ammonium (NH4) in oxic and anoxic phases (Source: Beutel 2000, 2006)
between lake level and outflow for the purpose of this study, in reality the outflow rate isgoverned by operating rules determined by the Federal Water Master, based on negotiateddownstream water needs (Truckee River Operating Agreement (TROA) http://www.troa.net/).These rules change over time, and are based not only on conditions at Lake Tahoe but alsoconditions of lakes, reservoirs and rivers in the Truckee River Basin (such as Lake Tahoe (andits semi-enclosed embayment Emerald Bay), Donner Lake, Independence Lake, StampedeReservoir, Boca Reservoir and Pyramid Lake). Thus, the developed regression is used forpredicting future release rates, however, it is recognized that the estimated release rates usingregression equations may deviate from future actual rates.
3 Data inputs
3.1 Meteorological data input
The data used to support the two emissions scenarios (B1 and A2) of the GFDL GCM werebased on the downscaled meteorological projections (Dettinger, this issue). The details ofusing the output of only one GCM (i.e. GFDL) for the lake and watershed models wereexplained in (Dettinger, this issue). Briefly, the archives containing GCMs’ future projectionfor the IPCC Assessment (2007) contain outputs of temperature and precipitation than otheroutputs of surface variables like radiative fluxes, winds, and humidities. For these othervariables, archives often are limited to certain period or monthly statistics. Because theseother variables have significant influence on lake warming and dynamics (Sahoo et al.2011), and we were able to obtain output of all variables for the 21st century from GFDL weused only GFDL outputs. The uncertainties in downscaled GFDL climatic data are discussedin the (Dettinger, this issue). Briefly, the correlation of estimated and observed dailytemperature and precipitation anomaly are above 0.9 and 0.7 for Lake Tahoe. Similarly,the downscaled downward longwave fluxes, surface-wind speeds, and downward solarradiation are very well correlated when aggregated to monthly time scales (correlation>0.95, >0.9, and >0.8, respectively), giving confidence in the downscaled projections foruses. Multiple regression equations were developed to correct biases in downscaled histor-ical data. This was performed using measured data from the Tahoe basin (1989 to 1998) anddownscaled historical data (GFDL A2 and B1) over the same time period with algorithmspublished by Woods et al. (2002, 2004) (see the details in our report http://www.fs.fed.us/psw/partnerships/tahoescience/bmp_climate_change.shtml). The bias corrected climate data wereused in the watershed model for generation of flows and pollutant loads. The same climate dataand generated flows were used in the lake model. There are 36 grid points for the downscaled airtemperature (maximum and minimum) and precipitation data; and 81 grid points for thedownscaled shortwave radiation and wind speed on and around the lake. LCM (Sahoo et al.2010), being a 1-D model, requires meteorological information at a single representative gridpoint over the lake. That point was chosen to be the grid point which is close to the center of thelake. In addition to downscaled precipitation, air temperature, shortwave radiation, and windspeed, LCM requires longwave radiation and vapor pressure data. Regression equations betweenair temperature and dew point were developed using the South Lake Tahoe Airport meteoro-logical station data from 1989 to 2004. The longwave radiation was estimated using algorithmsdescribed in Tennessee Valley Authority (TVA) (1972), with downscaled air temperature andestimated cloud fractions data. Vapor pressure was estimated using dew point temperature.
The one-year running average of the daily meteorological data from the downscalingexercise, over the 21st Century, along with the best fit trend lines are plotted for shortwave
Fig. 2 a USGS-recorded and LCM-estimated lake water surface b LSPC-estimated and USGS-recordedstream flow of the 10 LTIMP streams, and c estimated flow percentage change to the mean of LSPC andUSGS flows
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radiation, longwave radiation, air temperature, wind speed, and annual precipitation. Wefound that shortwave radiation remains largely unchanged while air temperature is expectedto increase approximately 4.5 °C and 2.0 °C, and longwave radiation will increase approx-imately 10 % and 5 % for the A2 and B1 scenario, respectively. The wind speed showed adecline on the order of 7–10 %. Note that these types of trends help to determine thestatistics of future climate; however, extreme weather conditions over periods of days maychange lake mixing and subsequent lake ecology without significantly altering the meteo-rologic long-term trend.
3.2 Stream inflow and pollutant loads
Streamflow, including tributary flow and direct runoff to the lake via intervening zones, andassociated pollutant loads through year 2100 were provided by the load simulation programin C++ (LSPC) watershed model (Riverson et al., this issue) forced by the same downscaledmeteorological data sets. Concentrations of fine sediment particles are estimated from theLSPC-derived stream flow based on algorithms described in Lahontan Regional WaterQuality Control Board (Lahontan) and Nevada Division of Environmental Protection(NDEP) (2010a). The stream temperatures are estimated based on the algorithms describedin Sahoo et al. (2009). Groundwater pollutant loads are based on the estimates of USACE(United States Army Corps of Engineers), Sacramento District (2003). However, the actualgroundwater flux was based on the estimates of Trask (2007). Estimates of atmosphericdeposition and shoreline erosion reported in Lahontan Regional Water Quality ControlBoard (Lahontan) and Nevada Division of Environmental Protection (NDEP) (2010a) areused in this study. Inputs from atmospheric deposition, groundwater and shoreline erosionwere assumed to be the same for all years (Sahoo et al. 2010) because of the lack ofadequate, long-term loading data from these sources.
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These assumptions imply that the loads over the next 100 years will bear the samerelationship to the meteorology and stream flows as they have in the past. For example, wedid not assume the success of the Tahoe Maximum Daily Load (TMDL) program for waterquality restoration, nor take account of possible future land use changes.
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3.3 Lake data
Lake data are required to provide initial conditions for the LCM model runs. Verticalprofiles of temperature, chlorophyll-a, DO, biological oxygen demand (BOD), solublereactive phosphorous (SRP), particulate organic phosphorus (POP), dissolved organicphosphorus (DOP), nitrate (NO3
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+), particulateorganic nitrogen (PON), dissolved organic nitrogen (DON), and concentrations ofseven classes of sediment particles (0.5–1.0, 1.0–2.0, 2.0–4.0, 4.0–8.0, 8.0–16.0,16.0–32.0, and 32.0–63.0 μm) are collected at two lake stations by UC Davis TahoeEnvironmental Research Center (TERC). Data from the mid-lake station in the deeperpart of the lake (460 m depth) were used to provide the initial conditions. Down-scaled meteorological data are available starting from January 1, 2001, however, thelake profile monitoring data used to define the initial conditions was first taken onJanuary 3, 2001.
The elevation of a spillway constructed at the lake outlet is approximately 1,899 mBureau of Reclamation Datum. Water level above 1,899 m is discharged to theTruckee River. Bottom elevation of lake is approximately 1,400 m Bureau of Recla-mation Datum. The elevation of each stream before it enters the lake was estimatedfrom GIS DEM and used along with stream and lake water temperature to estimatethe plunging depth of the stream discharge.
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Fig. 5 Simulated annual average soluble reactive phosphorus release from the sediments for a GFDLA2 andb GFDLB1 scenario
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4 Results and discussion
4.1 Calibration and validation
Sahoo et al. (2010) illustrates the calibration of LCM. The watershed model LSPCwas calibrated and validated for 1991 to 2008. Detailed lake data (nutrients, finesediments of seven bins (0.5–1 μm, 1–2 μm, 2–4 μm, 4–8 μm, 8–16 μm, 16–32 μm,32 to <63 μm), algae, dissolved oxygen, temperature) are available since 1999. Thelake data needed for the model was not available until 1999 even though the streamloading data was available from 1999 forward. Thus, in this study the lake water levelwas calibrated and validated using measured weather and lake level records for10 years (1999 to 2008). Figure 2a demonstrates the overall ability of LCM toestimate the lake water level. Figure 2a shows that water level closely follows thatof USGS-recorded water level except during 2003 and 2005. Note that years 2005and 2006 were characterized by high precipitation and LSPC overestimated stream-flow by approximately 20 % to 35 % relative to the mean of LSPC and USGS 2003to 2006 (Fig. 2b and c). Note that the relative percent difference (0(LSPC flow –USGS flow)/mean of LSPC and USGS flow) on annual stream runoff during 1991 to1998 for 10 LTIMP streams are only −12.0 %,–5.1 %, 7.1 %,–17.4 %, 3.0 %, 4.8 %,14.5 %, and 4.7 %, respectively (Fig. 2c). Since the LSPC with this set up generatedstreamflows of the 63 streams using downscaled meteorological data (Dettinger, this issue) for
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Fig. 6 Simulated annual average NH4-N release from the sediments for a GFDLA2 and b GFDLB1 scenario
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the period 2001 to 2099, the LSPC model values are not changed in the estimation of the lakewater elevation.
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4.2 Lake stratification and mixing
Lake stratification and mixing are strongly influenced by the meteorological conditions. Typi-cally in the summer, lakes undergo thermal stratification that stabilizes the water column into an
Fig. 8 Close view of the bottom 45 m (450 m to 495 m) simulated soluble reactive phosphorus release for aGFDLA2 b GFDLB1 scenario. X-axis values represent the beginning of the year
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upper, warmer and less dense epilimnion and a deeper, cooler and denser hypolimni-on. These two zones do not readily mix, and the strength of the thermocline boundarybetween these two layers intensifies with increased warming of the epilimnetic waters.In winter, the opposite occurs when the lake cools and the thermocline deepens.When surface and bottom density differences are reduced to zero, the lake can mixcompletely from top to bottom, a process termed turnover. At Lake Tahoe completeturnover typically occurs every 3–4 years on average (Tahoe Environmental ResearchCenter (TERC), University of California Davis 2008).
Lake mixing is important as it redistributes dissolved and particulate material. Forexample, nutrients such as nitrate, which typically accumulates in the hypolimnion throughthe summer, are reintroduced to the epilimnion when the lake mixes in the winter. Similarly,dissolved oxygen, which is introduced across the air-water interface, is redistributedthroughout the lake when deep mixing occurs.
The maximum annual mixing depths for the period 2001 to 2098 are shown in Fig. 3, whichillustrates that mixing to the bottomwill largely cease after 2060 for the GFDLA2 scenario. Forthe GFDL B1 scenario, the LCM predicted deep mixing to occur only four times during theperiod 2061 to 2098. For either of these emission scenarios, this would represent a verysignificant change relative to historic and current conditions. There are many implications forlake ecology based on a reduction in mixing of this magnitude (see below). The results alsoindicate that deep mixing events persist for shorter periods of time than they have in the past.
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Climatic Change (2013) 116:71–95 85
4.3 Implication of mixing effect on DO and nutrients
Modeled DO concentration at the deep sediment-water column interface for both emissionscenarios reached zero in approximately 6 to 7 years in the absence of deep mixing (Fig. 4)as surface water oxygen could not be transferred into deeper waters as a result of a persistentresistance to mixing. Ammonium and SRP have been shown to be released from Lake Tahoesediment under these conditions (Beutel 2000, 2006). These forms of biologically availableN and P will continue to be released from the sediment at the assumed rate (Table 1) whileDO concentration is less than 0.01 mg/L (Figs. 5 and 6). It is clear from Figs. 7 and 8 that theNH4
+−N and SRP released from the sediment at the deepest part of the lake are confined in
the bottom waters because of density stratification. Due to the absence of light at that depth,the released nutrients do not contribute to photosynthesis. That will only happen when thereleased nutrients are eventually mixed to the photic zone during the period of mixing andupwelling. The transport of nutrients by vertical eddy diffusion is significant in lakeenvironments (Robarts and Ward 1978; Salonen et al. 1984).
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We recognize that future development due to population growth and socio-economic changes could increase the fine sediment and phosphorus loads from storm-water runoff. Included in the recent Total Maximum Daily Load (TMDL) program forLake Tahoe (adopted in 2011), is a strategic plan to implement best managementpractices and water quality improvement projects to reduce future pollutant loads to a levelbelow the existing the existing condition and offset the impacts of future development. Evenunder a scenario of 10 % above full build-out, fine sediment loading was estimated to onlyincrease by 2 % relative to all sources (Coats et al. 2010; Lahontan Regional WaterQuality Control Board (Lahontan) and Nevada Division of Environmental Protection(NDEP) 2010a).
The annual sediment release of SRP and DIN (as nitrate, nitrite and ammonium) for theA2 scenario at the end of 21st Century are compared to other sources of the current N and Ploading budget (Lahontan Regional Water Quality Control Board (Lahontan) and NevadaDivision of Environmental Protection (NDEP) 2010a, b) in Fig. 9. When the hypolimnion isanoxic and nutrients are released from the sediment, the lake internal SRP load contributesapproximately 51 % of the total load. Although atmospheric deposited DIN is highest among
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Climatic Change (2013) 116:71–95 87
all sources (66 %), sediment derived DIN contributes approximately 14 % to the DIN pool.Clearly, internal nutrient load due to climate change can be significant to the lake nutrientbudget.
Fig. 12 Daily lake water temperature for a GFDL A2 scenario and b GFDL B1 scenario. X-axis valuesrepresent the beginning of the year
88 Climatic Change (2013) 116:71–95
4.4 Timing and delivery of the streams
The depth of insertion of each stream into Lake Tahoe is a complex process governed by thedensity (primarily water temperature) of each stream, the stratification of the lake, thestreamflow, and the geometry of the streambed and alluvial fan. A stream inflow thatplunges into the hypolimnion of the lake has different ecological consequences than whenit is inserted closer to the water surface. The seasonal pattern of Secchi depth water claritywill be affected by the depth at which fine sediment is delivered to the water column. Theinsertion depth of the Upper Truckee River is shown in Figs. 10 and 11 for GFDL A2 andGFDL B1 scenarios, respectively.
Figures 10b and 11b show a much more finely resolved (4 years), temporal view of thedaily insertion depth during the longer modeling period of record; while Figs. 10a and 11ashow the daily insertion depth for GFDL A2 and GFDL B1 scenarios, respectively for the100-year model output. The river plunges deeply most of the time during January to March(Figs. 10b and 11b); however, discharge and loads are delivered to the photic zone(approximately 0 to 50 m) during rest of the year. Due to climate change, the lake waterwill warm for the GFDLA2 scenario (Fig. 12). The lake epilimnetic temperature normallyexperiences significantly seasonal warming and cooling (red during summer and blueduring winter in Fig. 12). This pattern will continue; however, the simulation shows aprogressive deepening of higher temperature contour lines (Fig. 12a). The deeper partof the lake (>100 m) becomes warmer after 2080 for GFDL A2 while the warmingeffect on the hypolimnion is less for the GFDL B1. Because stream temperature wasestimated from air temperature and shortwave radiation (Sahoo et al. 2009), streamwater temperature also increases. Since hydraulic residence time of the lake is verylong (650–700 years), lake water temperature in the upper 100 m is critical for riverinsertion, since river water entrains lake water first and plunges depending on thedensity of the mixed water and stratified lake. Temperature contours in Fig. 12illustrate that the lake is strongly stratified after 2080 for GFDL A2 scenario. Theinsertion depth of Upper Truckee River is below a depth 200 m for the period 2034–
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Climatic Change (2013) 116:71–95 89
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Climatic Change (2013) 116:71–95 91
2076 except 2056 and 2061, and thus out of the photic zone. By contrast, for the caseof GFDL B1 scenario more of the winter discharge occurs in the photic zone during2077 to 2090. Stream discharge in the photic zone should have a more immediateeffect in stimulating algae growth and the associated loss of lake transparency.
4.5 Lake level
Figure 13 shows water level of the lake for both the GFDL A2 and GFDL B1 scenarios.Note that outflows are estimated based on the lake level above its natural rim and theregression analysis developed using 2000 to 2009 lake level and discharge data. Outflow iszero when the lake level falls below the natural rim. The lake level dips down below thenatural rim when evaporation rate is higher than sum total of stream inflows, groundwatercontributions and on-lake precipitation over the lake. As long as the lake level is below therim, the effects of annual evaporation and inflow are cumulative, and cannot be influencedby gate operation.
It is clear in Fig. 12a that that modeled lake temperature is predicted to signifi-cantly warm in the last 20 years of the 21st century for the GFDL A2 scenario. Thisis due in large part to the air temperature and longwave radiation increasing at ahigher rate for GFDL A2 case (nearly at double rate) than those of the GFDL B1case. As a result, lake evaporation is higher (Fig. 14). Figures 14 and 15 also indicatethat precipitation over the lake during 2075 to 2095 is lower for the GFDL A2 casethan for GFDL B1 so the stream inflow is lower. Due to the combination of thesefactors, the lake surface level dips below the natural rim after 2086 for the GFDL A2but not the GFDL B1 scenario.
5 Conclusions
The meteorologic and geographic conditions in the Tahoe basin combine to create avulnerable ecosystem. Temperatures in the Basin are increasing faster than in the surround-ing region and even under historic and current conditions the lake only mixes completely tothe bottom on the average of once in 3–4 years. Processes such as climate change that warmthe surface waters will increase the resistance to deep mixing. The most significant impactsof a future, modeled climate change at Lake Tahoe are as follows:
& Under the GFDL A2 emissions scenario, the Lake Clarity Model suggests that by themiddle of the 21st Century (after about 2060) Lake Tahoe will cease to mix to the bottom,with an annual maximummixing depth of only less than 200 m as the most common value.A similar, albeit not as severe outcome is seen for the GFDL B1 emissions scenario. As thesurface water heats, the resulting density difference between the warmer surface water andthe colder deeper water will be too strong for the wind energy to overcome. Indeed, thischange in density can already be seen in the measured historic data.
& When the lake fails to mix completely, the bottom waters are not replenished withoxygen and eventually dissolved oxygen at these depths will be depleted to zero. Whenthis occurs both soluble reactive phosphorus and ammonium-nitrogen (both biostimula-tory) are released from the deep sediments resulting in an increase in nutrient loadingrelative to that under the lake’s current deep mixing regime. The model shows this as anew and significant source of nutrients, heretofore not seen in Lake Tahoe. The modelindicates that under the GFDL A2 scenario, dissolved oxygen at the lake bottom could
92 Climatic Change (2013) 116:71–95
reach a sustained level of zero by about 2075. At the depths below 200 m,oxygen concentrations could be at levels inhospitable to native salmonids (<6 mg/L) even earlier. The model also suggests that intermittent periods of anoxiain the deepest waters could occur within the next 20 years. Under the GFDL B1scenario, deep-water anoxia will also occur, albeit not as sustained as seen in theGFDL A2 scenario; this results from the observation that while complete mixingwill be less frequent than historically observed, it will occur. Sensitivity analysison the 21st century modeled daily wind speed indicated that a 10 to 15 % increasewould be needed to maintain the historic deep mixing frequency (once in 3 to4 years) in the late 20th century.
& Based on published results for soluble phosphorus (SRP) and ammonium release fromanoxic Lake Tahoe sediments, the annual loading of SRP under sustained conditions oflake stratification (no deep mixing) and anoxic sediments would be twice the currentload from all other sources for GFDL A2 scenario. Loading of ammonium under theseconditions would increase the amount of biological available nitrogen that enters the lakeby 14 %. This effect on the nutrient loading budgets of Lake Tahoe, in addition to thepredicted loss of functional habitat for certain species under anoxic conditions, couldhave a dramatic and long-lasting impact on the food web and trophic status of the Lake.
& Should the nutrients released from the bottom sediments periodically mix or otherwisebecome entrained into the upper waters we expect that the impact on algal growth withinthe photic zone should be significant, with an attendant impact of lake food webdynamics and trophic status of both the pelagic and littoral regions of the lake. Thesenutrients, particularly phosphorus, will be available to drive algal growth. Reducing theload of external nutrients entering the lake in the coming decades may be the only possiblemitigation measure to reduce the impact of climate change on lake clarity and trophic status.
& The lake model suggests that climate change will drive the lake surface level downbelow the natural rim after 2086 for the GFDL A2 but not the GFDL B1 scenario. Theresults indicate that continued climate changes could pose serious threats to the charac-teristics of the Lake that are most highly valued. Future water quality planning must takethese results into account.
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