Nitrogen is a nutrient important to the growth and reproduction of plants, and it is considered a pollutant of concern in the Lake Tahoe Region. Nitrogen and phosphorus together support the growth of algae in Lake Tahoe. Free-floating algae occur throughout Lake Tahoe and contribute to the decline in water transparency and attached algae coat rocks in the nearshore, adversely affecting nearshore aesthetics. Landscape disturbances including, but not limited 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 their effects on stream hydrology (particularly the extremes of droughts and floods), and long-term climate change are considered among the most important environmental drivers of tributary runoff. This indicator measures the average concentration of total nitrogen for each water year in the seven streams in the Tahoe Region routinely monitored by USGS.

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Photo Credit: David Smith - USGS

Status

Annual average total nitrogen concentrations for regularly monitored streams for the 2011-2023 water years.

Evaluation Map

Water Quality Monitoring Locations

2023 Evaluation

Status
Insufficient Data to Determine Status or No Target Established
Trend
Moderate Improvement
Confidence
High
View Evaluation

Applicable Standard

WQ15: Attain applicable state standards for concentrations of dissolved inorganic nitrogen.

Key Points

  • Domagalski et al., 2021 found that flow-normalized nitrate concentrations trended down at all sites, and that total Kjeldahl nitrogen showed decreasing trends in load and concentration at most sites. The study found that after an initial decline in ammonia concentrations, concentrations have been increasing since about 2000. Nitrogen concentrations declined by almost half when comparing the first decade on record (1981-1991) to the 2007-2017 period.
  • Reducing nitrogen loading is critical because excess loading of nitrogen supports the growth of algae which is a driver of clarity decline.
  • The Lake Tahoe TMDL lists nitrogen as a pollutant of concern. 
  • There has been a small reduction in overall nitrogen concentrations in monitored streams since monitoring became more widespread in 1988.
  • Real-time data on stream conditions is available via the USGS Hydromapper.

About the Threshold

Nitrogen is a nutrient important to the growth and reproduction of plants, and it is considered a pollutant of concern in the Lake Tahoe Basin. Nitrogen and phosphorus together support the growth of algae in Lake Tahoe. Free-floating algae (phytoplankton) occur throughout Lake Tahoe and contribute to the decline in water transparency by absorbing light for photosynthesis. Attached algae (periphyton) coat rocks in the nearshore, adversely affecting nearshore 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). However, persistently high levels of algae in Lake Tahoe would be considered undesirable. Nitrate and nitrite are inorganic forms of nitrogen that are directly available for use by plants, whereas total nitrogen includes all organic and inorganic forms of nitrogen that are directly and indirectly available to plants. Sources of nitrate in stream water include atmospheric deposition, urban runoff, and decomposition of organic matter, especially from alder, which is an important nitrogen-fixer.
All the tributaries within the Tahoe Basin deliver sediment and nutrients to a single downstream waterbody: Lake Tahoe. The Tahoe Basin has 63 individual tributaries and associated watersheds, each with its own drainage area, slope, geology, and land-use characteristics resulting in high variability throughout the Region. Variability in the amount, timing, and form of precipitation strongly influences runoff patterns. A substantial rain shadow exists across the basin from west to east; precipitation can be twice as high on the west shore relative to the east shore of Lake Tahoe. Both new and legacy disturbances to the landscape can affect the volume of runoff, erosion rates, and the ability of the watershed to retain sediment and nutrients. Landscape disturbances including, but not limited 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 their effects on stream hydrology (particularly the extremes of droughts and floods), and long-term climate change are considered among the most important environmental drivers of tributary runoff.

Delivering and Measuring Success

EIP Action Priorities

EIP Indicators

Lake Clarity Indicators

Monitoring Programs

  • Lake Tahoe Interagency Monitoring Program Stream Monitoring

    USGS led monitoring program developed in 1979 to assess sediment and nutrient input from tributaries.

  • Streams

    Surface Water Ambient Monitoring Program bioassessment protocol, Benthic Macroinvertebrate (BMI) composition and physical stream habitat to assess overall stream health.

Rationale Details

Insufficient Data to Determine Status or No Target Established. Neither California nor Nevada has adopted standards for dissolved inorganic nitrogen, so no status determination is possible relative to the standard which reads, “attain applicable state standards for concentrations of dissolved inorganic nitrogen.”
Moderate Improvement. Recent research (Domagalski et al., 2021) found that flow-normalized nitrate concentrations trended down at all sites, and that total Kjeldahl nitrogen showed decreasing trends in load and concentration at most sites. The observed declines were consistent with the findings of an earlier analysis by (Coats, R. 2016)

Confidence Details

High. The USGS led Lake Tahoe Interagency Monitoring Program (LTIMP) was started in 1979 to help understand why clarity was declining in Lake Tahoe. Currently, LTIMP stream monitoring is a partnership between USGS, U.C. Davis, the Tahoe Regional Planning Agency, Lahontan Regional Water-Quality Control Board, and the California Tahoe Conservancy.

Streamflow and water-quality samples are being collected at pertinent Lake Tahoe tributaries to estimate nutrient and sediment loads to the lake and assess trends in stream water quality. Streamflow and stage are measured at 9 gages in 8 basins within the Lake Tahoe watershed and water quality is monitored at 7 sites in 7 basins. Turbidity is monitored continuously at 5 of the water-quality sites.

Water-quality samples are collected during routine visits, storm events, and snow-melt runoff. Samples collected are analyzed for nitrogen, phosphorus, suspended sediment, and fine sediment at the laboratory, as well as for temperature, specific conductance, pH, dissolved oxygen, and turbidity in the stream. Additional details on the LTIMP program are available on the USGS program website: https://webapps.usgs.gov/tahoe/ltimp-sites/ltimp

The standardized stream monitoring protocols and length of record provide high confidence in the data.
High. A 2021 paper (Domagalski et al., 2021) found that flow-normalized nitrate concentrations trended down at all sites, and that total Kjeldahl nitrogen showed decreasing trends in load and concentration at most sites. The observed declines were consistent with the findings of an earlier analysis by (Coats, R. 2016)
High. See description of status in confidence.

Additional Figures and Resources

No photos available.


No documents available.