Phosphorus Load (Surface Runoff)
This indicator has been retired and is no longer reported. This standard was retired on May 23, 2018, as part of the Threshold Update Initiative: Reorganization and Non‐Policy Technical Corrections. See the Governing Board staff report at https://www.trpa.gov/wp-content/uploads/documents/archive/May-23-2018-Governing-Board-Packet-Final.pdf for more information. The content on this page is archived and available for evaluations prior to May 2018, but will not be updated or maintained for future evaluations.
Phosphorus Load (Surface Runoff) is include in the Threshold Dashboard. Threshold Indicators are evaluated against Threshold Standards every 4 years. Thresholds are environmental goals and standards for the Lake Tahoe Basin that indirectly define the capacity of the Region to accommodate additional land development.
Status
Evaluation Map
Map shows all stormwater monitoring locations during water years 2014 and 2015. Source: Tahoe Resource Conservation District
2015 Evaluation
- Status
- Insufficient Data to Determine Status or No Target Established
- Trend
- Insufficient Data to Determine Trend
- Confidence
- Low
Applicable Standard
Reduce total annual nutrient and suspended sediment loads as necessary to achieve loading thresholds for tributaries and littoral and pelagic Lake Tahoe.
Key Points
No Key Points
About the Threshold
- This indicator measures the total phosphorous load delivered to Lake Tahoe and its’ tributaries via direct surface runoff. Nitrogen and phosphorous nutrients are important to the growth and reproduction of plants, and they are considered a pollutant of concern in the Lake Tahoe Basin (Lahontan & NDEP, 2010a). Nitrogen and phosphorus together support the growth of algae in Lake Tahoe (Lahontan & NDEP, 2010a). Free-floating algae (i.e., phytoplankton) occur throughout Lake Tahoe and contribute to the decline in water transparency by absorbing light for photosynthesis. Attached algae (i.e., 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 are considered undesirable. Phosphorus occurs naturally in the soils of the Lake Tahoe Basin, and is delivered to surface waters and Lake Tahoe through soil erosion and subsequent transport in streams and storm water, atmospheric deposition, and fertilizer runoff (Lahontan & NDEP, 2010a). The Lake Tahoe Total Maximum Daily Load (TMDL) estimated that a 46 percent reduction (from 2004 loading estimate) in total phosphorus load from urban upland sources over 65 years would be required to achieve the pelagic clarity standard.
- Landscape modification (e.g. impervious cover such as roads or residential and commercial development or logging) influences the volume of runoff, erosion rates, and the ability of the watershed to retain sediment and nutrients. Sediment and nutrient load in stormwater runoff is influenced by the type, magnitude, and location of landscape modifications and the extent to which practices to mitigate potential impacts (e.g. water quality BMPs) are in place. A variety of natural factors also influence the load of sediment and nutrients in stormwater including climate, weather, landscape topography, and vegetation. The TMDL estimated that 26 percent of total phosphorus load originates from urban upland sources.
Rationale Details
- Insufficient data to determine status. Data reported in this assessment is load as measured at specific catchments and no overall estimate of load was available at this time. Load reduction estimates and condition assessment commitments are documented in the credit accounting platform of the TMDL. A more robust picture of load in stormwater will be available in March 2017 after credit declaration and associated verification associated with the first TMDL milestone is complete. In sites that monitored in both the 2014 and 2015 water years total phosphorous load was 24.5 kilograms in 2014 and 13.7 kilograms in 2015. Three additional sites were added in water year 2015 and total monitored load was 33.2 kilograms in water year 2015. The two years are not comparable because three additional sites were added in 2015 and total surface volume was much greater in 2014.
- Insufficient data to determine trend. The first Region wide load reduction estimates are expected in March 2017. These estimates will serve as the basis for evaluating trend in future evaluations.
Confidence Details
- Low. Where insufficient data exists to determine status, confidence in the status determination is low. There is moderate confidence in the data because it is collected using widely recognized, standardized national protocols (see monitoring approach) with quality assurance/quality control procedures. Only a small proportion of the outflows are sampled and not all runoff events are sampled. Regional estimates of overall load and load reduction are not available at this time.
- Low. No trend assessment was performed because both the nature and limited duration of the data preclude trend assessment. v
- Low.
Additional Figures and Resources
No photos available.
No documents available.