Reduce Total Annual Phosphorus Load
Phosphorus is a nutrient important to the growth and reproduction of plants and is considered a pollutant of concern in the Lake Tahoe Region. Nitrogen and phosphorus together support the growth of algae in Lake Tahoe and contribute to the decline in water transparency and adversely affect nearshore aesthetics. Landscape disturbances including impervious surfaces, residential and commercial development, wildfire, and the degradation of stream environment zones (SEZs), can contribute to sediment and nutrient inputs to the lake or its tributaries. Projects such as restoring SEZ and limiting fertilizer use in the Region seek to reduce phosphorous in Lake Tahoe's tributaries. Phosphorous reductions are monitored by partners through the Lake Tahoe Total Maximum Daily Load Program (TMDL).
Status
Phosphorus load reduction with the target shown in 2019 in light blue.
Evaluation Map
Lake Clarity Tracker Projects
2023 Evaluation
- Status
- At or Somewhat Better Than Target
- Trend
- Moderate Improvement
- Confidence
- Moderate
Applicable Standard
WQ35: Reduce total annual phosphorus load to achieve long-term pelagic water quality standards (WQ1 and WQ2) and littoral quality standards (WQ5 and WQ6).
Key Points
- Between 2020-2023, TMDL implementors (CalTrans, CLST, Douglas, El Dorado, NDOT, Placer, Washoe) collectively prevented 6550 pounds of phosphorus from reaching the lake, an average of 1638 pounds a year.
- The Winter of 2022 - 2023 was a historic year in terms of snowfall and low temperatures, which led to long and continuous periods of deep snow and cold temperatures that prevented full inspections necessary to award credits (2024 TMDL Performance Report).
- Urban land uses account for over 38 percent of total phosphorus loading to the lake (Lake Tahoe TMDL). The goal of the TMDL is nearly 100 feet by 2076, with an interim target of the achieve an interim target of 78 feet of clarity by 2031. Achieving the interim target requires reducing phosphorus by the load in 2026 by 17% from the TMDL baseline while achieving 100 ft of clarity will require a 35% reduction.
- The TRPA Code of Ordinances provides specific direction to ensure that activities and development in the Region are compatible with the Regional Plan and support the attainment and maintenance of the Region’s shared goals for restoration and environmental quality as expressed in the threshold standards.
- The Tahoe Science Advisory Council recommended that the discharge standards WQ19-WQ32 are not suitable for threshold standards, but should be retained in the TRPA Code (currently at Chapter 60) as management restrictions on discharge
About the Threshold
- Phosphorus is a nutrient important to the growth and reproduction of plants and is considered a pollutant of concern in the Lake Tahoe Region (Lahontan and NDEP, 2010b)
- Nutrient (nitrogen and phosphorus) inputs from anthropogenic sources are considered the primary driver of increasing PPr in temperate lakes (Conley et al., 2009). It is suspected that activities associated with urbanization and watershed disturbance influence Lake Tahoe’s PPr through the release of nutrients and subsequent transport in runoff, or through the atmospheric deposition of nutrients. The nutrient source analysis for the Lake Tahoe TMDL indicates that both urban and non-urban sources of nitrogen and phosphorus are important contributors of nutrients to Lake Tahoe (Lahontan and NDEP, 2010a; Sahoo et al., 2013). Meteorological conditions (e.g., wet vs. dry years) also affect PPr, due to changes in tributary loads of nutrients, and differences in the magnitude of physical processes within the Lake (e.g., deep lake mixing). However, the trend suggests these factors have not substantially influenced the overall trend. The source of nutrients that are driving the increase in PPr is currently unknown.
Delivering and Measuring Success
EIP Action Priorities
- Acquire Environmentally Sensitive Lands for Restoration and Protection
- Operate and Maintain Stormwater Infrastructure
- Reduce Stormwater Pollution From: Roads and Highways, Forest Roads, Public and Private Parcels
- Restore Priority Meadows, Wetlands, and Lake Tahoe Tributaries
EIP Indicators
- Acres of SEZ Restored or Enhanced
- Impervious Coverage Retired
- Miles of Street Sweeping
- Parcels with Stormwater BMPs
Lake Clarity Indicators
Monitoring Programs
Rationale Details
- At or Somewhat Better Than Target. WQ35) Reduce total annual phosphorus load to achieve long-term pelagic water quality standards (WQ1 and WQ2) and littoral quality standards (WQ5 and WQ6). The Lake Tahoe TMDL establishes a series of increasing pollutant load reduction targets necessary to restore the historic clarity of Lake Tahoe. During the four-year evaluation period, TMDL implementors exceeded load reduction targets by 8 percent, obtaining 11,319 credits relative to a target of 10,528. Implementors have exceeded annual load reduction targets in seven of the last eight years. In 2023, implementors fell just short, achieving 98 percent of the load reduction target. The estimated total phosphorus load reduction was 18.5%, higher than the 17% load reduction target for 2026 (2023 Findings & Program Recommendations Memo). The phosphorus load reduction necessary to achieve other water quality standards have not been defined, therefor status is assessed relative to the defined load reduction target for clarity.
- Moderate Improvement. Load reduction target attainment is verified through the Lake Clarity Crediting Program. Urban implementers document and report load reduction activities through the Lake Clarity Crediting Program which provides the standardized tools and protocols to consistently and transparently estimate, account, and report accomplishments. Lake Clarity Credit (credit) targets define load reduction milestones in California stormwater permits and Nevada interlocal agreements. Use the map and check boxes below to see the achievements relative to targets for each jurisdiction, state, and basinwide. To evaluate the effectiveness of management and track load changes over time, the crediting program modeling tools provide estimates on the average annual load that is verified through on-the-ground field condition assessment.
Confidence Details
- Moderate. The load reduction targets of the TMDL are designed to restore the long-term clarity of the lake. The TMDL crediting program is well-documented and science-based. Full details on implementation and crediting are available on the program dashboard: https://clarity.laketahoeinfo.org/Home/ProgramManagement
- Moderate. TMDL load reduction targets accelerate each year of implementation, building towards 17% phosphorus load reduction in 2026 from the 2004 baseline.
- Moderate.
Additional Figures and Resources
No photos available.
No documents available.