Grantee Research Project Results
1999 Progress Report: Watershed Response to Natural and Anthropogenic Stress: Lake Tahoe Nutrient Budget
EPA Grant Number: R825433C029Subproject: this is subproject number 029 , established and managed by the Center Director under grant R825433
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
Center: UC Davis Center for Children's Environmental Health and Disease Prevention
Center Director: Van de Water, Judith
Title: Watershed Response to Natural and Anthropogenic Stress: Lake Tahoe Nutrient Budget
Investigators: Reuter, John E. , Heyvaert , Alan C. , Jassby, Alan D. , Goldman, Charles R.
Institution: University of California - Davis
EPA Project Officer: Packard, Benjamin H
Project Period: October 1, 1996 through September 30, 2000
Project Period Covered by this Report: October 1, 1998 through September 30, 1999
RFA: Exploratory Environmental Research Centers (1992) RFA Text | Recipients Lists
Research Category: Center for Ecological Health Research , Targeted Research
Objective:
To develop methods for assessing lake response to anthropogenic stresses by analyzing sediment profiles and quantifying the sources and sinks of phosphorus.
Progress Summary:
Previous studies used sediment profiles to assess watershed and lake response to anthropogenic stress. With the recent discovery that phosphorus loading is a primary factor affecting algal growth in Lake Tahoe, it is now more important than ever that there is a P-nutrient budget which quantifies the critical sources and sinks of this nutrient. Five major sources of nutrients (including phosphorus and nitrogen) to Lake Tahoe have been identified: (1) direct wet and dry atmospheric deposition, (2) stream discharge, (3) overland runoff directly to lake, (4) groundwater and (5) shoreline erosion. The major losses include settling of material from the water column to the bottom and discharge to the Truckee River outflow. Preliminary estimates of P and N loading have been made.
Atmospheric Deposition. Using previously published data, in concert with isohyetal map for Lake Tahoe (which shows the spatial distribution of precipitation over the entire lake and watershed), loading values for N and P which fall directly on the lake surface were estimated. Nutrients deposited on the watershed that are subsequently transported to the lake are included in the calculations of stream discharge, direct runoff and groundwater loading. The data base used for calculations in this section included: Ward Valley Lake Level Station - 100 m from lakeshore at an elevation of approximately 1,895 m (1983-1992); Ward Valley Bench - 6.8 km west of lakeshore at an elevation of 2,200 m (1983-1992); anchored buoys at four lake stations, three forming an east-west transect from Ward Valley to Mid Lake and the remaining one offshore of South Lake Tahoe (1986-1992); and stations in Glenbrook, NV and Incline Village, NV which were operational only in Water Year 1982.
For bulk deposition (wet plus dry), the estimated rates for both nitrate and ammonium ranged from 250-450 µg/m2/day, depending on location. Typically, the open water portions of the lake were characterized by having concentrations at the lower end of this range. Estimates of bulk soluble reactive-P deposition ranged from approximately 15 µmol/m2/day along the South Shore to 55 µmol/m2/day near Glenbrook. For particulate-N, defined as PN=TN-nitrate-ammonium, bulk deposition ranged from 580 to 1,025 µg/m2/day; particulate-P ranged from 20 to 65 µg/m2/day, again depending on location.
Based on the distribution of deposition measurements and the pattern of precipitation denoted by isohyetal contours, the lake was divided into 8 regions. Within each region, the deposition rate was multiplied by the area to determine bulk deposition. The data below represent conditions during the period 1989-1993 and can be entered into the lake's overall nutrient budget:
Atmospheric Deposition | ||
(metric tons per year) | ||
Nitrogen | Phosphorus | |
Soluble | 107 | 5.6 |
Particulate | 128 | 6.8 |
TOTAL | 234 | 12.4 |
For the entire lake surface area, the contribution of P was an estimated 12.4 metric tons (MT; where 1 MT = 1000 kg or 2205 pounds). Direct N-loading to the lake surface was estimated at 234 MT. This accounts for 27% and 56% of the annual TP and TN budgets, respectively.
Stream Loading. A total of 63 streams drain into Lake Tahoe. These streams are characterized by different levels of urban development and disturbance. The Lake Tahoe Interagency Monitoring Program (LTIMP) has been sampling up to 32 sites in 14 streams since 1980. LTIMP is a cooperative program including both state and federal partners, and is operationally managed by the U.S. Geological Survey (USGS), The UC Davis - Tahoe Research Group and the Tahoe Regional Planning Agency (TRPA). Because of variation in watershed characteristics around the Basin and significant 'rain shadow' effects along the west-to-east direction across the lake, no single location is representative of all watersheds. Cumulative flow from these monitored streams comprises 50-55% of the total discharge from all tributaries. Each stream is monitored on 40-60 dates each year. N and P loading calculations were made using this data base.
Using data from the early 1980s to the early 1990s the stream loads for N and P has been calculated as part of two separate studies (Marjanovic 1989; Jassby et al. 1994). The results for annual N-loading were 81.1 MT and 55.2 MT for the beginning and end of this period, respectively. Comparable loading values for total-P were 12.5 MT, and 11.2 MT. Differences between these periods reflect the variation in precipitation and runoff. The USGS, in a cooperative study with the TRPA also provided a very preliminary estimate of nutrient loading to Lake Tahoe during the period 1990-1993 (Thodal 1997). In this latter study, annual nutrient loads associated with streamflow was estimated by multiplying the mean annual volume of surface-water runoff by the mean annual nutrient concentration. Using this simple approach, loading from runoff was reported as 70 MT for total-N and 20 MT for total-P (Thodal 1997). A very early estimate of streamflow nutrient load by Dugan and McGauhey (1974) estimated 120 MT of total-N and 9.2 MT for total-P. Despite these data being much older and perhaps done using different methodologies for nutrient analysis, they are similar to the other estimates. Taking the mean of these values, which represent different time periods and consequently different precipitation conditions, loading estimates of 81.6 ± 27.7 (mean ± standard deviation) MT and 13.3 ± 4.7 MT were obtained for total-N and total-P, respectively. These accounted for 20% and 29% of the N and P budgets.
Soluble reactive-P accounted for 18% of total-P for the ten primary LTIMP streams sampled between 1989-1993. This is exactly the mean SRP:TP ratio found for the Upper Truckee River (South Lake Tahoe site) during the period 1981-1997.
Direct Runoff. The Tahoe basin has 52 intervening zones which drain directly into the lake without first entering the streams. These intervening zones are generally found between individual watersheds and as such are distributed around the entire lake. These zones range in size from 0.1 km2 to 10.5 km2. The range for covered or otherwise disturbed land within these intervening zones ranges from 0 to 63%. The overall ratio of disturbed to total area is 27%, with runoff from the intervening zones accounting for 10% of the entire drainage.
Calculations of loading from direct runoff requires quantification of flow and concentration. Flow from each of the intervening areas was calculated by Marjanovic (1989). Data on N and P concentrations in direct runoff are less available than concentrations from other sources since this type of study has not received priority funding in the Basin. However, based on previous studies, concentration data for the purpose of this preliminary nutrient budget were estimated. Clearly, a significant amount of new monitoring that focuses on urban runoff, which is not monitored at the LTIMP stream sites, is still needed.
For the purpose of calculation, an intervening area was considered urban if 25% or more of its area was classified as covered or disturbed. Concentrations representative of urban and 'non-urban' conditions were taken from the field studies cited above and used in the quantification of loads. Values represent mean ± standard deviation and are expressed as mg/L.
Urban | Non-urban | |
Total Suspended Solids (TSS) | 238 ± 234 | 45 ± 28 |
Total Kjeldahl Nitrogen (TKN) | 1.01 ± 0.62 | 0.33 ± 0.11 |
Nitrate | 0.03 ± 0.02 | 0.03 ± 0.03 |
Total Phosphorus (TP) | 0.43 ± 0.13 | 0.07 ± 0.03 |
Soluble-reactive-P | 0.14 ± 0.05 | 0.02 ±< 0.01 |
N-loading was calculated at 41.8 MT or 10% of the total-N budget, while P-loading was 15.5 MT or 34% of the total-P budget. Runoff values calculated by Marjanovic (1989) for each intervening zone were used in concert with the measured concentration values. The percent contribution of SRP to TP for direct runoff was estimated at 32% based on the concentration levels. The observation regarding the high contribution of P-loading from direct runoff is particularly important since a significant portion of the urbanization at Tahoe is found in the intervening zones. It provides project planners with the understanding that control and treatment of urban/direct runoff to the lake is critical and should be a high priority.
Groundwater. Quantitative estimates documenting the contribution of groundwater discharge to the lake's nutrient budget is limited. The most comprehensive, basin-wide effort to date comes from Thodal (1997) as part of a hydrogeology study of the Tahoe basin. Data on the results of a groundwater quality monitoring study done in 1990-1992 are presented. By multiplying mean nutrient concentrations from their groundwater survey (N = 1.0 mg/L; P = 0.074 mg/L) and estimates of total annual groundwater discharge to the lake (5.15 x 107 m3), Thodal calculated 'rounded estimates' of 60 MT for N-loading and a 4 MT for P-loading. This accounted for 14% of the TN budget and 9% of the TP budget. It was assumed that nitrogen and phosphorus loading was in the dissolved form.
Shoreline Erosion. The process of shoreline erosion and its quantitative importance to the nutrient and sediment budgets of Lake Tahoe have received very little attention. However, the importance of shoreline erosion has been highlighted in recent years as a result of high lake levels and strong, sustained winds that altered some of the west shoreline by many feet. We present here a very preliminary estimate of the 'order-of-magnitude' contribution by this process. We emphasize that additional studies must be performed before any action is based on these rough numbers.
Quantification of the contribution of shoreline erosion to the nutrient budget of Lake Tahoe requires two components; first, the concentrations of nitrogen and phosphorus associated with shorezone soils and second, the amount of material lost to the lake. In response to the former need, we conducted a pilot study to determine a first-order estimate of N and P concentrations in backshore and shorezone sediment samples. This work was performed between July and September 1998 at a total of six locations.
The amount of shoreline eroded each year has not been quantified nor to the best of our knowledge even roughly estimated. With this understanding, we attempted a very rough estimate. Again, we can not emphasize too strongly that our estimate represents a guess at this time, but none-the-less may form the basis for a more comprehensive estimate. This estimate is needed for a number of reasons including both refinement of the nutrient budget and extent of shorezone sediment loss. That being said, if we assume that 55% of the Lake's 113 km shoreline is subject to erosion and that on average, a cross-section with an area of 5 cm x 3 cm (0.0015) to 8 cm x 4 cm (0.0032 m2) is lost from each kilometer of erodable shoreline, then on the order of 100-200 m3 of material may be lost to the lake in a year. Over a 10-year period this would amount to 1,000-2,000 m3 of shoreline sediment. Using a sediment density of 2.5 g/cm3 this amounts to 2,500-5,000 MT of material during that 10-year period. Further, it is assumed that once every 10 years, a very large erosion event results in an equivalent amount of erosion during a single year. Under these circumstances during this hypothetical 11-year period there would be [e.g. 2500 MT + 2500 MT]/11 years or approximately 450-900 MT of shoreline material eroded per year.
Using the field concentrations measured in the summer of 1998 (see above) the average ratio of TP:TSS was estimated at 0.0007. Concentration of total-P per unit wet sediment in a single sample ranged from a high of 0.0013 to a low of 0.00003 g TP per g sediment. The mean values for TP per g of wet weight sediment at the sampling sites were close to within a factor of 2, ranging from 0.00041 to 0.00098. The exception to this was Pope Beach which was very low in TP, 0.000068 g TP per g of wet weight sediment. Applying a mean TP concentration of 0.00068, a total-P load of 0.3-0.6 MT is calculated.
Sediment total-N concentrations measured by the Tahoe Research Group in 1998 ranged from 0.00005 to 0.003 g TN per g sediment with a mean of 0.0011 g TN per g sediment. TN between locations was more variable than for TP with a range of 0.00025 to 0.00284 g TN per g of wet weight sediment. Again, the Pope Beach samples were much lower at 0.000084 g TN per g of wet weight sediment. Applying this to the calculation of total-N entering the lake via shoreline erosion an estimate of 0.5-1.0 MT. Again, it must be noted that these calculations depend on estimates of actual erosion shoreline erosion rates, which are extremely rough in this discussion.
Summary of inputs. The summary values presented below represents an initial estimate at quantifying the nutrient sources to Lake Tahoe. Depending on the amount and form of precipitation, individual Water Years will differ. Efforts are underway to provide estimates of both interannual and measurement variation to these values.
Our estimates suggest that approximately 17 MT or about one-third of the TP load is in the form of soluble-P and immediately available for biological uptake. Values of this magnitude are not common in the scientific literature.
The results at this time clearly suggest the importance of direct runoff from urban areas and highlight the need for additional study in this area. As restoration projects are being targeted and adaptive management proceeds, it will be very helpful to have more detailed data on the specific sources of nutrients within each of the major categories discussed above. Restoration should give priority to those areas which make the greatest contribution to the nutrient loading budget.
INPUTS | Nitrogen (MT) | Phosphorus (MT) | |
Total | Total | Soluble | |
Atmospheric deposition | 233.9 (56%) | 12.4 (27%) | 5.6 |
Stream Loading | 81.6 (20%) | 13.3 (29%) | 2.4 |
Direct Runoff | 41.8 (10%) | 15.5 (34%) | 5.0 |
Groundwater | 60 (14%) | 4 (9%) | 4 |
Shoreline Erosion | 0.75 (<1%) | 0.45 (1%) | No Data |
Total | 418.1 | 45.7 | 17.0 |
Losses. By analyzing material contained in a vertical series of sediment traps deployed in Lake Tahoe, we have found that nutrient sedimentation losses to the bottom of Lake Tahoe are 401.7 MT for total nitrogen and 52.8 MT for total phosphorus. These measured values agree remarkably well with the independent loading estimates given above. Loss of P and N via the lake's only outflow is minimal. This close agreement give us increased confidence that the loading rates are representative.
Future Activities:
A number of critical studies are needed to continue this line of research. These include (1) a more detailed investigation on the specific sources of P & N, i.e. where does the material in each of the major categories originate, (2) an increased understanding of the supply of biologically available-P and the biogeochemical processes which regulate its formation, and (3) what is the capacity of restoration projects and best management practices to reduce P-loading.
Supplemental Keywords:
RFA, Scientific Discipline, INTERNATIONAL COOPERATION, Water, ECOSYSTEMS, Ecosystem Protection/Environmental Exposure & Risk, Water & Watershed, Aquatic Ecosystems & Estuarine Research, Restoration, Aquatic Ecosystem, Environmental Microbiology, Terrestrial Ecosystems, Biochemistry, Ecology and Ecosystems, Aquatic Ecosystem Restoration, Watersheds, contaminant exposure, anthropogenic processes, biodiversity, watershed management, nutrients, restoration strategies, hydrology, wetland restoration, integrated watershed model, aquatic ecosystems, environmental stress, watershed sustainablility, ecosystem stress, ecology assessment models, Lake Tahoe, ecological impact, aquatic habitat protection , land use, ecological researchProgress and Final Reports:
Original AbstractMain Center Abstract and Reports:
R825433 UC Davis Center for Children's Environmental Health and Disease Prevention Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
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R825433C005 Fish Developmental Toxicity/Recruitment
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R825433C025 Regional Movement of Toxics
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R825433C035 Border Rivers Watershed
R825433C036 Toxicity Studies
R825433C037 Watershed Assessment
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R825433C041 Inorganic Analysis
R825433C042 Immunoassay and Serum Markers
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R825433C044 Molecular, Cellular and Animal Biomarkers of Exposure and Effect
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R825433C058 Time Series Analysis and Modeling Ecological Risk
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