Introduction

Global and regional climates have already begun changing. The World Meteorological Organization (WMO) (2006) has pointed out that “since the start of the 20th century, the global average surface temperature has risen approximately 0.7°C. But this rise has not been continuous. Since 1976, the global average temperature has risen sharply, at 0.18°C per decade. In the northern and southern hemispheres, the period 1997–2006 averaged 0.53 and 0.27°C above the 1961–1990 mean, respectively.” The Intergovernmental Panel on Climate Change report (IPCC 2007) indicates that (1) there is high agreement and much evidence that, with current climate change mitigation policies and related sustainable development practices, global greenhouse gas (GHG) emissions will continue to grow over the next few decades; (2) continued GHG emissions at or above current rates would cause further warming and induce many changes in the global climate system during the twenty-first century that would very likely be larger than those observed during the twentieth century; (3) even if radiative forcing (e.g., GHG-driven longwave radiation) were to be stabilized, thermal expansion (i.e., expansion of seawater volume due to global warming) would continue for many centuries, due to the time required to transport heat into the deep ocean.

Meteorology is the driving force for lake internal heating, cooling, mixing and circulation, which in turn affect nutrient cycling, food-web characteristics and other important features of lake/reservoir limnology. Therefore, climate changes will affect the physical, chemical and biological attributes of lakes and reservoirs because of changes that include (1) the thermodynamic balance across the air–water interface, (2) the amount of wind-driven energy input to the system, and (3) the timing of stream delivery into a lake/reservoir. These processes can exert changes across the entire water column depth. These processes vary for different systems and their geographical location. Most existing water management and restoration plans, as well as water-supply and -drainage systems, are based upon historic climatic and hydrological records, and assume that the future will resemble the past. Although these systems may be sufficient to handle most changes in mean conditions associated with climate change over the next couple of decades or less, management problems are likely to arise if there is an increase in climate variability and the occurrence of extreme events due to climate change. The existing problems may be exacerbated due to such changes.

The broadening consensus about the inevitability of global warming and climate change is forcing both decision makers and researchers to evaluate the probable consequences for lakes and reservoirs. There is a growing concern that aquatic ecosystem function may also be affected. Lake Tahoe is an ultra-oligotrophic and sub-alpine lake renowned for its deep blue color and clarity. Due to concerns about progressive loss of clarity [i.e., 0.22 m per year, Tahoe Environmental Research Center (TERC), UC Davis], the lake has been the focus of major efforts by local, state and federal agencies and policy-makers to halt the worsening trends in clarity and trophic status. Records from the past 33 years (i.e., 1969–2002) also show that Lake Tahoe has become both warmer and more stable (Coats et al. 2006). With continued climate change, Lake Tahoe can be expected to continue to become more stable and, as a consequence, to experience water quality changes over time.

Meteorological predictions from a global climate model (GCM)—the national oceanic atmospheric administration (NOAA) geophysical fluids dynamic laboratory (GFDL) CM 2.1 model, downscaled to northern regional scales (Cayan et al. 2008)—were used to investigate the effects of climate change on Lake Tahoe. The changes in lake dynamics over time were examined using the lake clarity model (LCM). The LCM was developed to study the impacts of pollutant load on lake clarity as part of the science-based restoration plan, i.e. the Lake Tahoe Total Maximum Daily Load (Lake Tahoe TMDL). A TMDL is a water quality restoration plan required under the United States Federal Clean Water Act to ensure the achievement of water quality standards in impaired surface water bodies. Therefore, the objective of this study were (1) to examine the lake warming rate using LCM and GFDL-predicted meteorological variables, (2) to assess lake dynamics using possible climate change scenarios, and (3) to discuss possible effects on lake water quality and general lake management alternatives for restoration. Modeling of lake water quality changes under these changed climatic inputs is beyond the scope of the present contribution.

Description of Lake Tahoe

Lake Tahoe has long been known for its beauty and spectacular clarity. Millions of people visit Lake Tahoe every year. Lake Tahoe is a subalpine lake that was formed by a graben fault about 2 million years ago (Marjanovic 1989). Lake Tahoe is the 11th deepest lake in the world, with a maximum depth of approximately 500 m. The average elevation of Lake Tahoe is 1,897 m from mean sea level, with an average depth of 303 m. The total area of the Lake Tahoe basin is 1,310 km2, with 813 km2 in the surrounding watershed area. The total volume of the lake is 158 km3, with a hydraulic residence time of 650–700 years. Elevations in the Lake Tahoe Basin range from over 3,050 m at Mountain Rose down to 1,897 m at lake level. The basin consists of a total of 63 inflow tributaries and 1 outflow tributary. The Truckee River is the only outflow, and is an inland drainage, ending at Pyramid Lake, Nevada (Rowe et al. 2002).

Methods

To examine the impact of global warming and climate change on Lake Tahoe, the LCM was used as a tool to simulate lake dynamics. The LCM is a complex system of sub-models including thermodynamic, hydrodynamic, ecological, zooplankton, nutrient, particle, and optical sub-models, although for the present study only the thermodynamic and hydrodynamic modules were used. The conceptual design of the LCM is shown in Fig. 1 (Sahoo et al. 2007a). All the LCM sub-models operate inside the dashed line box in Fig. 1. The pollutant sources and amounts of inorganic particle loading from atmospheric deposition, groundwater, tributaries and various land uses (urban and non-urban) are shown above this box. Groundwater contributes only nitrogen and phosphorus for algal growth. The optical sub-model estimates Secchi depths based on scattering and absorption characteristics of particles, algae, colored dissolved organic matter (CDOM), and water itself.

Fig. 1
figure 1

Schematic representation of the lake clarity model (LCM). The dashed line box encloses all in-lake processes. N nitrogen, P phosphorous, Si silicon

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To run the LCM, a series of simulation years into the future (a 40-year period, i.e., 2000–2040) was established. This time period was selected for example purposes only to examine the effect of global warming and climate change on Lake Tahoe. The loading simulation program in C++ (LSPC) watershed model (Tetra Tech 2007) provided stream inputs (i.e., inflow) for the period 1994–2004. Therefore, the precipitation information (and associated LSPC loading results) for these 11 years (i.e., 1994–2004) was used to populate the LCM runs for the period 2000–2040. The precipitation distributions used for LCM modeling during 2000–2040 are shown in Fig. 2. The proposed precipitation frequency distribution for the period 2000–2040 in Fig. 2 is similar to that of past years (i.e., 1965–2005). Sahoo et al. (2007b) developed a subroutine to estimate stream temperature from air temperature and solar radiation.

Fig. 2
figure 2

Proposed annual total precipitation distribution for 1999–2040 for generation of the base case. The year used to supply input data for runoff and pollutant, and meteorological inputs, is indicated above each bar

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The meteorological inputs to the LCM for the establishment of baseline estimates for the years 2000–2040 were set to the same as proposed for precipitation years. The lake dynamics will continues for the next 40 years if there is no change in the current status. Thus, the values obtained using these values are referred to as the base case.

Previously published monthly and seasonal air temperature and precipitation change data (Cayan et al. 2008), as shown in Table 1, were used to estimate the net change per year. The total changes during a specific period were spread progressively over the entire period, i.e., the net temperature change in a year during the period 2005–2034 is ΔT = (1.5/30) × (2035 − year). Similarly the net change in precipitation was estimated using Table 1.

Table 1 Geophysical fluids dynamic laboratory (GFDL)-predicted temperature and precipitation changes during the twenty-first century for A2 emission scenario and northern California (Source: Cayan et al. 2008)
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GFDL CM2.1 model predictions indicate that shortwave radiation does not change over time. The stream water temperature was estimated using the air temperature and solar radiation values of the climate change scenarios. The change of flows and pollutant loads, and delivery timing of each stream into the lake due to climate change is the subject of another study. This study presents lake dynamics where temperature acts the dominant variable. Pollutant load values are kept the same for both with- and without-climate-change scenarios except stream water temperature. Detailed water quality studies are currently underway.

Data used

Required daily meteorological data for the LCM include solar short wave radiation (KJ m2 day−1), incoming longwave radiation (KJ m2 day−1), or a surrogate such as fraction of cloud cover and air temperature (°C), vapor pressure (mbar) or relative humidity (%), wind speed (m s−1 at 10 m above the water surface) and precipitation (mm, 24-h total). Data from 1994 and 2004 were collected at Tahoe City meteorological station SNOTEL gage located at 39.172°N latitude and 120.138°W longitude maintained by the United States Natural Resources Conservation Services. The hourly recorded data were then further averaged or integrated as necessary to obtain daily values.

In-lake vertical profiles of temperature, chlorophyll a (Chl a), dissolved oxygen (DO), biological oxygen demand (BOD), soluble reactive phosphorous (SRP), particulate organic phosphorus (POP), dissolved organic phosphorus (DOP), nitrate (NO3) and nitrite (NO2), ammonia (NH4), particulate organic nitrogen (PON), dissolved organic nitrogen (DON), and concentrations of seven classes of particles collected at the mid-lake station (i.e., in the deeper part of the lake—460 m deep) were used for calibration and validation purposes and as initial profile for lake simulation. Flows and loads from all streams estimated using LSPC++ (Tetra Tech 2007) were used in this study. The values of groundwater discharge and nutrient loading to Lake Tahoe reported by the United States Army Cores of Engineers USACOE (2003) were used in this study. The values of nutrient and inorganic particulate matter estimated by Adams and Minor (2001) were used as shoreline erosion load. The California Air Resources Board (CARB 2006) conducted the Lake Tahoe atmospheric deposition study (LTADS) to quantify atmospheric deposition from nitrogen, phosphorus, and particulate matter loading into Lake Tahoe. Estimates of wet deposition came from UC Davis–TERC monitoring. Phosphorus deposition was also estimated by the UC Davis DELTA Group (Cahill 2006; Gertler et al. 2006) and the UC Davis–TERC (Hackley et al. 2004, 2005). Nitrogen deposition was estimated by the Desert Research Institute (DRI, Reno, NV), and UC Davis–TERC. Deposition of particulate matter was performed by CARB (2006). The nutrients and particulate matter loads from the atmosphere, groundwater discharge, and shoreline erosion were kept the same for all the simulation years because these are the best estimates available at present.

Results and discussion

Lake Tahoe: current status

During late spring, summer, and early fall (June–October), Lake Tahoe becomes physically stratified into three identifiable layers—the epilimnion, metalimnion, and hypolimnion (Fig. 3). The epilimnion is the upper, warm layer, and is typically well mixed. Below the epilimnion is the metalimnion or thermocline region—a layer of water in which the temperature declines rapidly with depth. The hypolimnion is the bottom layer of colder water, isolated from the epilimnion by the metalimnion. The change in temperature (thus density) at the metalimnion acts as a physical barrier that prevents mixing of the upper and lower layers for several months during the summer.

Fig. 3
figure 3

Measured in-lake temperature profiles during the year 2000

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The depth of mixing depends on the exposure of the lake to wind. As the weather cools during winter (January–March), the epilimnion cools too, reducing the density difference between it and the hypolimnion (Fig. 3). As time passes, winds mix the lake to greater depths, and the thermocline gradually deepens. As the atmosphere cools, the surface water continues to cool, leaving it unstable. This cold water plunges in a turbulent plume, mixing with the water beneath. When surface and bottom waters approach the same temperature and density, winds can mix the entire lake; the lake is said to “turn over” (see Fig. 3 and March 07 line). Mixing distributes oxygen throughout the lake. Lakes that do not mix, or that have high oxygen demand, may have low oxygen levels in the hypolimnion. The cold water in the hypolimnion (bottom) can hold more oxygen than warmer water in the epilimnion (top). But if the lake produces an overabundance of algae, which fall into the hypolimnion to decay, oxygen becomes depleted. The steep temperature gradient of the metalimnion prevents any surface water with dissolved atmospheric oxygen from reaching the bottom waters.

Lake Tahoe’s DO level never falls below 6 mg/l although there is seasonal variation of DO during the year (Fig. 4b). The surface DO during summer is low because the oxygen gas holding capacity of water decreases at higher temperatures and vice versa (Fig. 4a). In oligotrophic lakes like Lake Tahoe, low algal biomass allows deeper light penetration and less decomposition. Algae are able to grow relatively deeper (nearly 20–50 m below surface) in the water column (Fig. 4c) and less oxygen is consumed by decomposition because the mortality rate is low (Sahoo et al. 2007b). The DO concentrations may therefore increase with depth below the thermocline where colder water is carrying higher DO leftover from winter mixing (recall that oxygen is more soluble in colder water). In extremely deep, unproductive lakes such as Lake Tahoe, DO persists at high concentrations, near 100% saturation, throughout the water column in all years. DO concentration is high at a depth of 50 m below the surface during summer because algae release oxygen during photosynthesis.

Fig. 4
figure 4

Measured in-lake temperature, dissolved oxygen (DO), and chlorophyll a (Chl a) concentration at different depths

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The in-lake profile for the year 2000 is presented herein as an example to show some important features of lake dynamics such as the occurrence of a deep chlorophyll maximum (DCM) during summer, full or deep mixing events during winter, and high DO concentration year round throughout the lake. Note that the mixing depth, and hence lake dynamics, varies every year. However, with continued climate change, this pattern may change over time.

Lake status under climate change

The LCM was calibrated and validated using measured data from 2000 and 2001–2002, respectively. The measured in-lake profiles of temperature, DO, Chl a, nutrients, and particle concentrations of the first month of 2000 and 2001 were taken as the initial profile for calibration and validation, respectively. Although the model runs in 1-h time steps, the boundary conditions of stream inflow, outflow, and meteorology are average/total daily values. The simulated time series–depth profiles temperature values were close to measured values (Sahoo et al. 2007a). This indicates that the LCM simulates lake dynamics well. In addition to temperature, the LCM was calibrated and validated for phytoplankton concentration, nutrient concentrations, particle concentration, and Secchi depths. Sensitivity analysis was carried out for the modeling parameters. The calibrated values of modeling parameters were found to be well within the ranges reported in the literature and the coefficient of variation (average value/standard deviation) of Secchi depth for a combination of a ±25% change of modeling values of three important parameters (coagulation rate, chlorophyll growth rate, and chlorophyll light scattering rate) was only 11%. The reader is referred to Sahoo et al. (2007b) for detailed results of calibration and validation. The calibrated LCM was used in this study to estimate effect of climate change on lake dynamics.

The LCM was run for 40 years using the data with and without climate change scenarios as described in Methods. There is no change in temperature and precipitation during the period 2000–2004, because actual measured data were used for these years. The results of daily volume–average lake water temperature for 2005–2040 presented in Fig. 5 show that the lake is warming up at 0.005°C per year, which is lower than the estimate of Coats et al. (2006) (i.e., 0.015°C per year). This indicates that other meteorological variables, such as longwave radiation and wind speed, may have an influence on lake warming. Using data from the 5th generation of Mesoscale Model (MM5) of the National Center for Atmospheric Research (NCAR), Coats et al. (2006) showed that the downward longwave radiation trend was upward during the period 1970–2000, while the trend of shortwave radiation did not change during this time.

Fig. 5
figure 5

One-year running average volume-average lake temperatures using geophysical fluids dynamic laboratory (GFDL) A2 scenario temperature change data reported by Cayan et al. (2008). Thin black line Volume average temperature, thick black line linear trend

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Meteorological data for the GFDL CM2.1 model were downloaded from the website http://www-pcmdi.llnl.gov/ipcc/about_ipcc.php. There is no monthly longwave radiation estimation available for the twenty-first century A2 scenario. Thus, the monthly net longwave radiation for A1 scenario and for Lake Tahoe (i.e., GFDL cell latitude 38.43°–40.45°N and longitude 117.5°–120.0°W) was estimated using the upwelling and downwelling longwave radiation data of the period 2001–2100. Figure 6 shows the upward trends for net longwave radiation. The slopes of the upward trend vary in the range 0.03–0.11 W m−2 during October–March (Fig. 6). The values of slopes for other months are less than 0.03 W m−2. The important point to mention here is that deep mixing occurs only during winter. Figure 6 shows the increasing trends of net longwave radiation during winter. Thus, in addition to the change in air temperature and precipitation, the longwave radiation should be increased so that the warming rate approaches 0.015°C per year as estimated by Coats et al. (2006).

Fig. 6
figure 6

Monthly net longwave radiation (W m−2) for the period 2001–2100 using predicted values of GFDL CM2.1 climate model and A1 scenario. Data points GFDL CM2.1 model predicted values, line linear trend, LW longwave radiation, y year, R 2 coefficient of determination

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In addition to the change in air temperature and precipitation, the 10% progressive change in longwave radiation over the period 2005–2040 shows that the lake warming rate is approximately 0.015°C per year (see Fig. 7a), which is close to the value estimated by Coats et al. (2006). To cancel out the seasonal variations, 1-year running averages of daily values are presented in Fig. 7. Lake stability—the amount of work needed for a water column to overcome thermal stratification, and hence vertical density differences in order to completely mix—increases as shown in Fig. 7b. Schmidt stability is the amount of work needed to be done by the wind for a water column to overcome thermal stratification, and hence vertical density differences, in order to mix completely. Estimation of Schmidt stability requires surface area, area at each depth measured, the depth to the center of gravity for a stratified lake, total lake volume, and the density at each depth measured. Thus, lake stability increases and mixing reduces for increasing total and Schmidt work. Deep mixing has reduced as the weather continues to become warmer, as shown in Fig. 8. Each bar in Fig. 8 shows the maximum mixing depth occurring in that year. In the 40-year simulation period, the lake is expected to mix completely ten times if there is no change in climate (i.e., no base case). Figure 8 indicates that deep mixing will cease after 2019 with continued global warming and climate change. Note that the results presented herein are based on the assumptions considered in this study. Lake warming and mixing pattern will change if the assumptions made regarding the rate and magnitude of meteorological variables (air temperature, longwave radiation, wind speed etc.) change. Wind speed, which was not considered in this exercise, can change the mixing pattern. The reader should not take the findings presented herein as predictions and/or estimations, but rather should take the message that lake dynamics will change with continued climate change. The results presented are preliminary and detailed further study is required to verify these findings.

Fig. 7
figure 7

One-year running average (a) volume average lake temperature (thin black line) and linear trend (thick black line), and (b) average total work and Schmidt stability (dotted line) in Lake Tahoe using the GFDL A2 scenario temperature change data reported by Cayan et al. (2008) and a 10% progressive increase in longwave radiation. Thick black lines linear trends

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Fig. 8
figure 8

Maximum mixing depth in a year for the following cases: no climate change and with climate change scenario using GFDL A2 scenario temperature change data reported by Cayan et al. (2008) and a 10% progressive increase in longwave radiation

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Possible adverse impacts on ecology due to climate change in lakes

The changes in Lake Tahoe’s thermal structure reported herein are consistent with the findings of other researchers and other lakes around the world (Table 2). Analyzing in-lake measured data from 1969–2002, Coats et al. (2006) pointed out that (1) the volume average rate of warming in Lake Tahoe amounted to 0.015°C year−1, and (2) lake warming was associated with increased thermal stability and resistance to mixing. The findings of this study using the twenty-first century GFDL A2 scenario for Lake Tahoe are identical to the findings of Coats et al. (2006).

Table 2 Warming trends in lakes around the world
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O’Reilly et al. (2003) reported that, along with increased temperatures, wind velocities in the Lake Tanganyika watershed have declined by 30% since the late 1970s. The combined effect increased lake stability and reduced mixing. They also reported that, because of reduced mixing, the loading of internal nutrients was reduced resulting in declining productivity. This reduction in internal loading and primary production allowed expansion of the anoxic water mass in Lake Tanganyika. O’Reilly et al. (2003) indicated that algal abundance declined 20% over the 80-year period for which data exists. This decline is a direct result of the reduction in lake circulation. Based on earlier studies on other lakes (Nixon 1988), a 20% decline in primary productivity would lead to a 30% reduction in fish stocks in addition to any possible effects of over-fishing. Reduced mixing and lake warming has also been reported in Lake Malawi (Verburg et al. 2003).

Austin and Colman (2007) reported that (1) the surface warming rate is about 0.11 ± 0.06°C year−1, and (2) average wind speeds at the open water surface increased on the order of 0.05 ms−1 year−1 during the period 1979–2006 in Lake Superior. They hypothesized that the increased wind speeds might have been caused by destabilization of the atmospheric boundary layer due to a decreased air density gradient (since the surface air temperature is becoming progressively warmer). Analyzing data for the period 1979–2006, they showed similar trends of surface water temperature, air temperature, and average wind speeds for Lake Michigan, Lake Huron, and Lake Erie. Applying a one-dimensional temperature model to Lake Michigan, McCormick (1990) showed that climate warming decreases summer thermocline depth increase resistance to mixing, and could even lead to the evolution of a permanent deep-water thermocline.

From the 1950s to the 1990s, a high warming rate (~0.024°C year−1) in the uppermost 20 m (i.e., epi/metalimnion) of Lake Zurich combined with lower warming rates (0.013°C year−1) below 20 m (i.e., in the hypolimnion) resulted in a 20% increase in thermal stability and a consequent extension of the stratification period by 2–3 weeks (Livingstone 2003). In several Swiss lakes, including Lake Zurich, a series of three consecutive warm winters (1987/1988–1989/1990, associated with an extremely positive phase of the North Atlantic Oscillation) with persistent stratification has been shown to have been responsible for the extremely low deep-water oxygen concentration (Livingstone 1997).

Coats et al. (2006) hypothesized that (1) decreased mixing helps retain small particle in the epilimnion, where they have maximum adverse impact on lake clarity; (2) reduced mixing will shut down the flow of oxygen to the bottom of the lake, and continued influx of increasing pollutants may cause hypoxia at the sediment surface in deep water triggering soluble phosphorus; (3) observed water quality in a large lake like Tahoe may response slowly in the short-term, although responses might be significant in decadal time; (4) the increased stability and decreased thermocline depth may affect the feeding behavior and population structure of zooplankton. It can also be hypothesized that decreasing transparency in the lake may decrease solar radiation penetration into the deep lake. The DCM may not occur and the lake may be strongly stratified in future.

Reviews of past literature and the current study show that there are positive upward trends in lake warming, stability, resistance to mixing and longer summer stratification periods. If the warming trend continues, lakes will be permanently stratified, resulting in low DO concentrations in the hypolimnion. The dynamics of many other important elements in lake ecosystems, such as phosphorous, nitrogen, sulfur, silica, and iron, are dominated by microbial activity, which is controlled primarily by DO concentration (redox potential), temperature, pH, and various concentrations of these elements. In general, increasing temperature accelerates reaction rates. Decreased redox potential caused by low DO availability affects many equilibrium reactions. The phosphorous (P) cycle is sensitive to DO concentration at the sediment–water interface. Even a short extreme anoxic period near the bottom can induce P mobilization from the sediment to the water body, and change the lake permanently to eutrophic. It is very difficult and expensive to reverse this development. Typically, if surface erosion from the catchment is increased, P input is also increased. The nitrogen (N) cycle is connected to the atmosphere, both through N fixation by blue-green algae and bacteria (source), and through denitrification (sink). With increasing temperature, the solubility of gases decreases and processes such as denitrification and nitrogen fixation are accelerated. Such changes will lead to many water quality problems in lakes.

Possible restoration managements

Total phosphorus (TP), total nitrogen (TN), Chl a, Secchi depth and, sometimes, suspended solids (SS) are the key variables used to evaluate the restoration of lakes (Søndergaard et al. 2007). To achieve lake restoration, the natural processes that allow a lake to assimilate the nutrient load it receives must be restored. However, it is hard to stop/reverse the effect of climate change on lakes since continued GHG emissions at or above current rates would cause further warming. Thus, efforts should be taken to reduce the load entering the lake by managing lakes and their watersheds.

Lake restoration is very broad and complex topic, and is very case specific. Some general alternatives that are not specific to any particular case are discussed herein. Specific strategies to address a lake’s nutrient enrichment problems must focus on activities in the watershed and, if needed, in-lake restoration techniques. Lake management approaches fall into two categories, the “quick-fix” and long-term management. The quick-fix offers a short-term solution such as the application of algaecides to kill unwanted algae. This approach treats the biological symptoms of a lake's problem but does not address the underlying causes of symptoms.

Varis and Somlyódy (1996) proposed a variety of alternatives for lake water management (see Table 3), which are in wide use globally. External nutrient sources such as fertilizer use, pet wastes, stormwater runoff, septic system effluents, waterfowl, agriculture, and even rainfall can contribute nutrients to a lake. Lake management removes or modifies as many of these nutrient sources as possible, especially those sources shown to be contributing the greatest nutrient load to the water body. In-lake restoration techniques are necessary and they should be followed by, or occur simultaneously with, appropriate long-term management actions to control sediments, nutrients, and toxic inputs. A successful lake restoration program should strive to manage both external and internal nutrient sources. Many of them, particularly the internal action alternatives, are best applied, and in some cases can only be applied, to small lakes or on a local scale.

Table 3 General alternatives for lake water quality management (source: Varis and Somlyódy 1996)
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Anaerobic digestion of lake sediments is a much slower process than aerobic digestion. Whereas aerobic digestion can result in the control or reduction of organic sediment levels, anaerobic digestion almost always allows organic sediment levels to increase. During anaerobic digestion, bacterial enzymes and lack of oxygen make the nutrients in the bottom sediments soluble. The nutrients then return to the water column and are available to support new weed and algal growth. Anaerobic conditions at the lake bottom have a damaging effect on the food chain that supports fish populations as well as reducing or eliminating fish habitat, ultimately resulting in reductions in fish quality, size and quantity. Hypolimnetic aeration/oxygenation, which can be achieved by pure oxygen injection, or air injection (see McGinnis et al. 2004; Sahoo and Luketina 2006; Singleton et al. 2007), is an effective means of improving DO concentration in the water column. In the specific case of Lake Tahoe, however, injection of air or oxygen would be difficult due to the compression needs in overcoming the lake’s great depth.

The current Lake Tahoe TMDL (1) quantifies the source and amount of fine sediment and nutrient loading from a variety of activities and land-uses within the major categories of urban watershed, forest upland, atmospheric deposition, stream channel/shoreline erosion and groundwater; (2) uses the customized LCM to link pollutant reduction loading to lake response; and (3) develops the framework for an implementation plan to achieve an annual average Secchi depth of 29.7 m, as required by existing water quality standards (Roberts and Reuter 2007). This is a positive response for lake restoration. Sewage disposal to Lake Tahoe was halted in 1972. In the case of occurrence of low DO concentration in the hypolimnion, the possibility of installation of an aeration/oxygenation system should be investigated.

Conclusions and suggestions

The GFDL CM2.1 model shows that northern California air temperature is expected to increase by approximately 1.5 and 4.5°C at the end of 2034 and 2099, respectively, relative to 1961–1990. The model also indicates that net longwave radiation will increase by 3–11% during fall to winter at Lake Tahoe. The combination of both these factors will warm Lake Tahoe at a rate of approximately 0.015°C per year. Lake warming is associated with increased thermal stability at a rate of approximately 0.39 KJ m−2 per year. A review of the literature shows that lakes and reservoirs around the World are warming up because of global warming and climate change. Rising air temperatures account for only part of the recent warming of lakes. Other climate variables, such as changes in longwave radiation and wind speed, are also causes of lake warming. Lake warming has led to increased lake stability and resistance to deep mixing. It was shown that, at current rates of climate change, Lake Tahoe will stop deep mixing after 2019. However, this change depends on the future rate and magnitude of climate change. Reduced mixing in the lake may have significant adverse impacts on lake ecosystems. The possible scenarios are summarized as follows.

  1. 1.

    Reduced mixing in the lake helps retain fine particles in the epilimnion and may decrease penetration of solar radiation into the deep lake. This may contribute to a permanent stratification state.

  2. 2.

    Reduced mixing may limit the flow of DO to the bottom of the lake. Thus, continued influx of increasing pollutants may cause hypoxia at the sediment surface in deep water, triggering the release of soluble phosphorus.

  3. 3.

    Existing lake water quality problems may be exacerbated due to continuous lake water warming. Increasing temperature accelerates reaction rates. Decreased redox potential caused by low DO availability affects many equilibrium reactions. Such changes result in changes in lake dynamics in terms of nutrient cycling and primary productivity.

  4. 4.

    Future rates of lake warming depend on the rate and magnitude of climate change as well as on the success of on-going efforts to reduce the anthropogenic flux of nutrients to the lake.

The results shown here are not predictions and/or estimations. Rather, they are possible future scenarios. The results and findings would change if the rate and magnitude of the meteorological variables differ from the assumptions made in this exercise. Since climate change is inevitable, current lake management strategies should be integrated with new approaches and methodologies that can handle issues on water quality problems due to climate change. Research and development on lakes should focus on methodologies and approaches that can also handle extreme uncertainty.