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Home Water Streams and Wetlands

Total Phosphorus Concentration

Status and Trend

Interpretation and Commentary

Status: Considerably Worse Than Target
Trend: Moderate Improvement
Confidence: Moderate

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Map


The ten streams routinely monitored for total phosphorus concentrations include five streams in Nevada and five streams in California: Ninety percent of the cumulative total inflow from the ten monitored streams is from the five California streams, and ten percent is from the five Nevada streams. The sub-watersheds where stream monitoring occurs are colored in the figure above.
  • Relevance - Phosphorus is a nutrient important to the growth and reproduction of plants, and it is considered a pollutant of concern in the Lake Tahoe Basin (Lahontan 1995). Nitrogen and phosphorus together support the growth of algae in Lake Tahoe (TERC 2011a). Free-floating algae (i.e., phytoplankton) occur throughout Lake Tahoe and contribute to the decline in water transparency by absorbing light for photosynthesis and scattering light. Attached algae (i.e., periphyton) coat rocks in the near shore, adversely impacting near shore aesthetics. From an ecological perspective, algae are a dominant component of the aquatic food web, providing an important source of energy and nutrients that support other organisms in the food web (e.g., zooplankton and herbivorous fish). Under the Federal Clean Water Act, and in California under the Porter-Cologne Act, each State develops a set of water quality standards designed to protect the beneficial uses of a waterbody. In the Tahoe Basin, Lahontan Regional Water Quality Control Board (CA), Nevada Division of Environmental Protection (NV), and TRPA have the individual authority to determine if standards have been violated.
  • Adopted Standards  - TRPA: Attain applicable State standards for concentrations of dissolved phosphorus; Nevada: The yearly average TP concentration cannot exceed 0.05 mg/L in all tributaries to Lake Tahoe located in Nevada (NAC [no date]); California: The yearly average concentration cannot exceed 0.015 mg/Lin the Upper Truckee River, and Trout, General, Blackwood, and Ward creeks (Lahontan 1995).
  • Indicator - The Indicator isaverage yearly total phosphorus (TP) concentration measured over a water year (Oct. 1-Sept. 30). Average TP concentration is based on atotal of 20-35 individual samples currently collected each water year from each of the ten monitored streams, although the number of individual samples collected each water year has varied during the period of record from three to 138. The average yearly TP concentration values presented here differ from the flow-weighted concentrations in that the former value is the straight arithmetic mean of the measured values within a given water year, regardless of inflow. All average TP concentrations are reported in milligrams per liter (mg/L).
  • Status – In the table below, scores forannual averageTP concentration status, trend, and confidence were assigned for each of the ten regularly monitored streams in the Lake Tahoe Basin. Assigned scores for individual streams for California and Nevada and overall, are based on: 1) percent to target calculations, 2) standard exceedance rate (see also Appendix WQ-1), 3) visual inspection of graphed data record, and 4) the aggregation methods described in the Methodology Chapter of this report.

    The annual average TP concentrations for water year 2010 for the five Nevada streams ranged from 0.030 mg/L (Logan House Creek; 40 percent of standard) to 0.080 mg/L (Glenbrook Creek, 60 percent greater than the standard); and from 0.029 mg/L (General Creek, 93 percent greater than the standard) to 0.072 mg/L (Blackwood Creek, 480 percent greater than the standard) for the five California streams. Ninety percent of all inflow delivered to Lake Tahoe from the ten monitored streams comes from the five California streams; thus, the total contribution of TP from California streams was substantially greater than from the Nevada streams. Overall, the current status of tributary TP concentration was determined to be “considerably worse than target.”

    Stream Status Trend Confidence
    Nevada Streams
    Third -2 4 0
    Incline -4 4 0
    Glenbrook -4 -4 0
    Logan House 4 0 0
    Edgewood 2 4 0
    Nevada Summary -0.8 3.2 0
    California Streams
    Trout -4 0 0
    Upper Truckee -4 0 0
    General -4 0 0
    Blackwood -4 0 0
    Ward -4 0 0
    California Summary -4 0 0
    Overall Summary -2.4 1.6 0
    Overall Qualitative Assessment Considerably Than Target Moderate Improvement Moderate
       

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  • Trend Overall, it was determined the trend in TP concentration exhibited “moderate improvement” based on the change in standard exceedance rate between the 1990-1999 and the 2000-2010 periods (i.e., fewer exceedances of adopted standards were observed in the most recent period). Between water years 1981 and 2010, the average TP concentrations for the five Nevada streams ranged from 0.013 mg/L (26 percent of the standard) to 0.315 mg/L (630 percent greater than the standard). The yearly average TP concentrations for the five California streams ranged from 0.019 mg/L (27 percent greater than the standard) to 0.218 mg/L (1,453 percent greater than the standard) over the same period (see also Appendix WQ-1). At no time during the monitoring record have California streams attained the TP concentration standard.

    Patterns and trends in the streams from each State are evaluated separately, since the TP numeric standard for Nevada (0.05 mg/L) is 3.3 times larger than the California standard (0.015 mg/L). In Nevada, average TP concentrations in Logan House Creek were below the 0.05 mg/L standard, and in Edgewood Creek were near (within 0.02 mg/L) or below the 0.05 mg/L standard. Annual average Glenbrook Creek TP concentrations were at or above the standard more frequently than Edgewood, but the yearly average concentrations hovered close to 0.05 mg/L. The time series for these three streams exhibit low but relatively stable annual average TP concentrations over the period of record. Annual average TP concentrations in Incline Creek and Third Creek frequently exceeded the standard during the period 1989-1999. However, the number of exceedances has declined since 2001. The annual average TP concentrations have been near (within 0.02 mg/L) or below the standard since 2001, with the exception of values measured in Incline Creek in 2003 and 2006, and Third Creek in 2002. Overall, data from four of the five Nevada streams showed little or no trend in annual average TP concentration (see figure above). Third Creek shows an improving trend, possibly due to the dissipation of sediment from a 1986 avalanche and the rerouting of the Rosewood Creek tributary in the early 2000s. In California, Trout Creek, the Upper Truckee River, and General Creek show low but relatively stable average TP concentrations that exceed the 0.015 mg/L standard by minimal or modest amounts in all years. Blackwood and Ward creeks exceeded the standard by the greatest amounts, and the magnitudes of exceedance from the standard were higher in wetter years. Overall, the data from four of the five California streams showed moderate declines (i.e., improvements) in annual average TP concentrations. General Creek shows no trend in annual average TP concentrations (see figure above).
Confidence
  • Status – There is high confidence in the reliability of the TP concentration data as the data collection consistently followed national field monitoring protocols established by the U.S. Geological Survey for stream monitoring (USGS variously dated; Rowe et al. 2002). All field and laboratory data are subject to extensive quality assurance requirements (USGS 2006). Currently, a total of 20-35 individual samples are collected each water year from each of the ten monitored streams. This sampling frequency is considered sufficient to characterize different inflow conditions observed during the water year. The sampling frequency has varied over the period of record. The stream monitoring program focuses on both event-based conditions (large runoff events associated with rainfall and snowmelt) and baseline conditions (low inflow during summer when precipitation is negligible). The analytical methods for measuring nutrients have been developed and customized for use in aquatic systems where concentrations can be extremely low (Goldman et al. 2009). Thus, there is high confidence in the status determination.
  • Trend – Confidence in the trend determination is considered "low". Although there is high confidence in the data, assessments and interpretations of trend are all based on visual inspection (i.e., qualitative).
  • Overall – Overall confidence in the status and trend determination is moderate given the high confidence in status and the low confidence in trend.
  • Human and Environmental Drivers - Within the Tahoe Basin, all the tributaries deliver sediment and nutrients to a single downstream water body, Lake Tahoe. Lake Tahoe has 63 individual tributaries and associated watersheds, each with their own drainage area, slope, geology, and land-use characteristics. Furthermore, variability in the amount, timing, and type of precipitation strongly influences runoff patterns. A substantial rain shadow exists across the Lake from west to east, where precipitation can be twice as high on the West Shore relative to the East Shore. Both new and legacy disturbances to the landscape can affect the volume of runoff, erosion rates, and the ability of the watershed to retain nutrients. Landscape disturbances including but not exclusive to impervious road and parking lot surfaces, residential and commercial development, wildfire, and the degradation of stream environment zones, can contribute to sediment and nutrient inputs to the Lake or its tributaries. Weather variations and long-term climate change are considered to be among the most important environmental drivers of tributary runoff.
  • Monitoring Approach – The LTIMP stream monitoring program was first developed in 1979 to provide a Basin-wide evaluation of sediment and nutrient input from tributaries to Lake Tahoe, and to support research efforts that aim to understand the drivers affecting the transparency of Lake Tahoe. Ten streams have been monitored since the early 1990s; five in California (Upper Truckee River and Trout, General, Blackwood, and Ward Creeks) and five in Nevada (Third, Incline, Glenbrook, Logan House, and Edgewood Creeks). Six of these streams have been monitored since water years 1980 or 1981. A few of the ten streams have had multiple monitoring stations along the tributary and all have primary monitoring stations at or near the point of stream discharge to Lake Tahoe. Currently, a total of 20-35 individual samples are collected each water year from each of the monitoring stations. The 10primary stations allow for the evaluation of the cumulative conditions within the watershed and represent approximately 50 percent of the yearly tributary inflow into Lake Tahoe (Lahontan 1995). U.S. Geological Survey gauging stations are located at each of the monitoring stations, where inflow (discharge) measurements are collected and continuous inflow is calculated. Other water quality-related constituents monitored include water and air temperature, pH, specific conductance, and dissolved oxygen.
  • Monitoring Partners – U.S. Geological Survey (Nevada and California Water Science Centers), University of California at Davis (Tahoe Environmental Research Center), Tahoe Regional Planning Agency, and U.S. Forest Service (Lake Tahoe Basin Management Unit).

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Trend Charts

Chart data not available at this time
These plots show how average yearly total phosphorus (TP) concentration, determined from samples collected during a water year (Oct. 1-Sept. 30), compare to the water quality standard in place for each of ten regularly monitored streams in the Lake Tahoe Basin. The individual bars represent the average TP concentration, based on atotal of 3-138 individual samples collected, depending on the water year from each of the ten monitored streams.The horizontal red line represents the Numeric Standard of 0.05 milligrams per liter (mg/L) for streams in Nevada (graphs on left side, above), and the Numeric Standard of 0.015 mg/L for streams in California (graphs on right side, above). Stream monitoring data used to evaluate TP concentrations are from the sampling locations closest to where the tributaries discharge to Lake Tahoe. Data are from the Lake Tahoe Interagency Monitoring Program (LTIMP).

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References

Additional Information

Last Updated on Tuesday, 13 November 2012 07:16