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Home Water Streams and Wetlands

Total Phosphorus Load

Status and Trend

Interpretation and Commentary

Status: No Target Established
Trend: Litle or No Change
Confidence: Moderate

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The ten streams routinely monitored for phosphorus load includes five streams in Nevada and five streams in California: Ninety percent of the cumulative total inflow from the 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 - This indicator measures how much phosphorus is delivered to Lake Tahoe via monitored streams (measured as phosphorus load). 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 and NDEP 2010). 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 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). Soluble reactive phosphorus approximates the amount of orthophosphate that is directly available for use by plants, whereas total phosphorus includes all forms of phosphorus that are directly and indirectly available to plants. Phosphorus occurs naturally in the soils of the Lake Tahoe Basin, and is delivered to surface waters and Lake Tahoe through soil erosion and fertilizer runoff (Lahontan and NDEP 2010).
  • Adopted Standards  - 1)Tributaries:reduce total yearly nutrient and suspended sediment load to achieve loading thresholds for littoral and pelagic Lake Tahoe; 2) Littoral and Pelagic Lake Tahoe related standards for nutrient and sediment including: a) reduce dissolved inorganic nitrogen (N) loading from all sources by 25 percent of the 1973-81 annual average; b) reduce dissolved inorganic nitrogen loads from surface runoff by approximately 50 percent, from groundwater approximately 30 percent, and from atmospheric sources approximately 20 percent of the 1973-81 annual average; and c) reduce the loading of dissolved phosphorus, iron, and other algal nutrients from all sources, as required, to achieve ambient standards for primary productivity and transparency.
  • Indicator - The yearly load for each stream is calculated as the sum of daily loads for a given water year. Currently, a total of 20-35 individual samples are collected each water year from each of the ten streams. The combined yearly load represents an estimate of the total mass of the constituent transported by ten streams to Lake Tahoe during a single water year. Indicators measured include total phosphorus load (expressed as kilograms per year; kg/yr) and soluble reactive phosphorus (expressed as kilograms as phosphorus per year; kg as P/yr).
  • Status – The combined yearly load for total phosphorus (TP) was 10,424 kg in water year (WY) 2010. The soluble reactive phosphorus (SRP) load for WY 2010 was 1,292 kg as P. The total yearly inflow for all ten streams was 176,000,000 (or 1,760 x 105) cubic meters in WY 2010. The WY 2010 load estimates are similar to the combined yearly load estimates for WY 2000, which had a similar level of total yearly inflow. TP and SRP loads were largely driven by the yearly loads from the Upper Truckee River, and Trout, Blackwood, and Ward creeks. Together these streams contributed 85 percent and 80 percent of the average yearly TP and SRP loads, respectively, from all ten streams (Appendix WQ-2). There is no well-defined Numerical Standard (numeric target) for TP or SRP loads, so no status determination was provided.

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  • Trends Tributary phosphorus loads are a function of the constituent concentration and amount of inflow. Combined yearly tributary loads of total phosphorus (TP) and soluble reactive phosphorus (SRP) are strongly influenced by inflow, and in the Lake Tahoe Basin there can be considerable year-to-year variation in combined total yearly inflow from the ten monitored streams. Tributary flows depend on the amount and timing of precipitation (snow and rain) that falls in each watershed. Factors such as air temperature and snowmelt, rain-on-snow events, and rain versus snow all affect the timing of inflow within the water year. Subsurface infiltration and stormwater discharge from developed areas also affect tributary flows; thus, estimates of phosphorus load presented here represent some combination of non-urban and urban sources.

    From water years 1993 to 2010, the total yearly inflow ranged from 772 to 3990 x 105 cubic meters. During the 18-year period, total yearly inflow was highest in water years 1995 through 1998, and 2006, and was above 3000 x 105 cubic meters in those water years. Inflow was extremely high in water year 1997 when a huge rain-on-snow event caused massive flooding, especially on the West Shore. These types of large hydrologic events have the capacity to transport significant amounts of material to the Lake as they are intensive and erosive. For the period 1999-2010, total yearly inflow was moderate to low, ranging from 827 to 2770 x 105 cubic meters, except for the large total yearly inflow in water year 2006––the result of a large New Year’s rain-on-snow event. Water Years 1994, 2001, 2002, 2004, and 2007 to 2009, were characterized by low total yearly inflow (less than 1500 x 105 cubic meters) from the 10 monitored streams.

    The combined yearly TP loads for the ten monitored streams during water years 1993-2010 ranged from 2,180 kg in 2001 to 42,900 kg in 1997. Generally, combined yearly TP loads varied directly with total yearly inflow. Water year 1997 is a notable exception, which is thought to be due to increased erosion during an extreme rain-on-snow event. The pattern of combined yearly TP loads during water years 1993-2010 was similar to suspended sediment (SS) loads (see also Appendix WQ-2). This is expected since phosphorus is attached to soil sediment and transported along with SS.

    The evaluation of trend in total phosphorus loads from tributaries to Lake Tahoe is complicated by large inter-annual variability in precipitation and inflow, and the response of individual streams. It would be erroneous to make conclusions based on apparent short-term trends in combined yearly loads, given the known variability in yearly inflow. Yearly flow-weighted concentrations (FWC) of total phosphorus were calculated to investigate long-term pollutant loading trends in the presence of variable inflows (Appendix WQ-3). Further, a five-year moving average of sediment FWC was constructed to help reveal long-term trends. The combined yearly FWC for TP followed a similar pattern to suspended sediment. Phosphorus adsorbs to soils and the two are typically transported together. Even though there are periods over the 1993-2010 record where the five-year moving average shows signs of either improvement or degradation, there is no observable overall trend.

    The combined yearly SRP loads for the ten monitored streams during water years 1993-2010 ranged from 411 kg as P in 2001 to 2,880 kg as P in 1995. The year-to-year variations in combined yearly SRP loads generally tracked the combined yearly TP loads. The average ratio of SRP to TP combined yearly load was 13 percent, and the ratio ranged from 6-19 percent throughout the period of record. Water Years 1997 and 2006 had the lowest SRP to TP ratios of 6-9 percent, respectively. It is hypothesized that the time needed for the phosphorus attached to suspended sediment to detach and reach equilibrium with the stream water, was longer than the time of travel during the rapid inflow that occurred during the huge rain-on-snow events in those years. The five-year moving average of SRP showed a decline between 1997 and 2005, an increase from 2006 to 2010, but showed no observable overall trend (Appendix WQ-3).

    A determination of little or no change in the overall trend of phosphorus load is based on no observable trend in the FWC of TP.

  • Status – There is high confidence in the reliability of the data used to calculate yearly phosphorus loads 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 has been developed and customized for use in aquatic systems where concentrations can be extremely low (Goldman et al. 2009).
  • Trend – There is high confidence in the reliability of the data used to calculate yearly flow-weighted concentrations as the data collection followed national and consistent field monitoring protocols established by the U.S. Geological Survey for stream monitoring (USGS variously dated; Rowe et al. 2002). However, visual inspections of the five-year moving average was used to estimate the long-term trend in TP and SRP loads, yielding a “low” confidence in trend assessment.
  • Overall – Because there is high confidence in status and low confidence in trend, the overall confidence was determined to be “moderate.”
  • 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, excess fertilizer application, wildfire, and 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 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 and NDEP 2010). Reporting of combined yearly loads begins in water year 1993 because that was when the 10th stream was included in the monitoring program. 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.



Trend Charts

Chart data not available at this time
Combined yearly total phosphorus loads (provided as soluble reactive phosphorus and other phosphorus) and total yearly inflow for tenstreams routinely monitoring in the Lake Tahoe Basin. Data are displayed for each water year (Oct. 1 - Sept. 30) from 1993 through 2010. The yearly load for each stream is calculated as the sum of daily loads for a given water year. The combined total phosphorus load (soluble reactive phosphorus plus other phosphorus) represents an estimate of the total mass of phosphorus transported by ten streams to Lake Tahoe during a single water year. The dashed line is the combined total inflow from the ten streams for each water year, which represents approximately 50 percent of the inflow from all 63 tributaries to Lake Tahoe (Lahontan and NDEP 2010). Stream monitoring data used to calculate loads 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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Additional Information

Last Updated on Tuesday, 13 November 2012 07:14