Phosphorus Load (Tributaries)
Phosphorus Load (Tributaries) 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
Combined yearly total phosphorus load (Tonnes/yr), and total yearly inflow for seven streams currently monitoring in the Lake Tahoe Basin. Data are displayed for each water year (October 1 to September 30) from 1981 through 2014. The yearly load for each stream is calculated as the sum of daily loads for a given water year. The combined total phosphorus load represents an estimate of the total mass of phosphorus transported by seven streams to Lake Tahoe during a single water year. The solid line is the combined total yearly inflow from the same streams, having a total combined watershed area of 350.51 square kilometers. Data are from the Lake Tahoe Interagency Monitoring Program (LTIMP).
Evaluation Map
The seven streams routinely monitored for total phosphorous load includes two streams in Nevada: (1) Third Creek, and (2) Incline Creek; and five streams in California: (3) Trout Creek, (4) Upper Truckee River, (5) General Creek, (6) Blackwood Creek, and (7) Ward Creek.
2015 Evaluation
- Status
- Insufficient Data to Determine Status or No Target Established
- Trend
- Moderate Improvement
- Confidence
- Moderate
Applicable Standard
Reduce total annual nutrient and suspended sediment load to achieve loading thresholds for littoral and pelagic Lake Tahoe.
Key Points
No Key Points
About the Threshold
- 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, 2010a). 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. Nitrogen and phosphorus together support the growth of algae in Lake Tahoe (Lahontan and NDEP, 2010a). Free-floating algae (i.e., phytoplankton) occur naturally throughout Lake Tahoe and contribute to the decline in water transparency by absorbing light for photosynthesis and by scattering light. 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. 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 and NDEP, 2010a). This indicator measures how much phosphorus is delivered to Lake Tahoe via seven routinely monitored streams (measured as total phosphorus load).
- All the tributaries within the Tahoe Basin deliver sediment and nutrients to a single downstream water body: Lake Tahoe. The Tahoe Basin has 63 individual tributaries and associated watersheds, each with their own drainage area, slope, geology, and land-use characteristics resulting in high variability throughout the Region. Furthermore, variability in the amount, timing, and type 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. 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 its 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.
Rationale Details
- Insufficient date to determine status. There is no clearly established numerical target for total phosphorus load for any of the standards identified above; thus, no determination of status can be determined. The combined average total phosphorus load (1981 to 2014) for the seven monitored streams was 9,709 kilograms per year, ranging from 1,823 (2001) to 32,983 (1982) kilograms per year, with a median value of 6,660 kilograms per year. Individually, the seven monitored tributaries (1981 to 2014) varied considerably in their total phosphorus yield per unit area and loads (see Table 1). Blackwood Creek had the highest yield per unit area, although the Upper Truckee River, with the largest watershed area, contributed a greater total phosphorus load to Lake Tahoe. The steep and highly-developed watershed of Third Creek had a disproportionately high total phosphorus yield, but the small size of its contributing area resulted in a low average total phosphorus load. The Trout Creek basin was the smallest contributor of total phosphorus yield per unit area, but its large watershed area made it the third largest contributor of total phosphorus load to Lake Tahoe.
- Trend – Because the inter-annual variability in total phosphorus load is driven largely by the variability in annual runoff, it is virtually impossible to recognize long-term trends in load without first removing the effect of hydrology. The annual total phosphorus load for each of the seven streams was regressed against the associated total and maximum annual daily discharge, and tested for trends in the residuals. The downward trends were significant (P< 0.0035) for Blackwood, General, Third, and Ward creeks, and the Upper Truckee River. However, all of the streams except Blackwood Creek showed a leveling off or even an upturn in total phosphorus load beginning about water year 2000.
The basin-wide trend in total phosphorus load was analyzed by regressing the total annual total phosphorus load (summed over the seven stations) against the sum of the annual total and annual maximum daily discharge. The downward trend shown in Figure 1 is significant at P< 0.00025. As with the trends for four of the streams, the total trend shows a leveling off after about water year 2000, due to some high positive residual values after 2005, and low values between 2001 and 2004.
Percent changes in regression-estimated total phosphorus loads for given annual hydrologic conditions are presented in table 2. For example, at a given total annual and maximum annual discharge in Ward Creek, the estimated total annual total phosphorus load in 2014 was (on average) 42.4 percent less than it would have been for the same hydrologic conditions (had they occurred) in 1973. Trends are based on day-time samples only. Streams without a significant trend are not shown.
The reason for the 20-year downward trend and subsequent leveling off is unclear. It is hypothesized that the 20-year downward trend is due to long-term recovery from the 19th century clear-cut logging and mid-20th century land development. The record-low residuals of 2001 to 2004 may be an effect of the flushing of channel sediment (Simon et al., 2003). The cause of the high positive residual values after 2005 is unknown.
Confidence Details
- Low. Where insufficient data exists to determine status, confidence in the status determination is low. The confidence in an estimate of total annual total phosphorus load depends on the number of samples and the variance of the daily loads that are sampled to derive the annual load. Coats and Lewis (Coats and Lewis, 2014b, 2014a) presented tables that can be used to estimate the confidence intervals for average conditions in Tahoe Basin streams, for total phosphorus. Currently the LTIMP collects about 20 to 30 discrete water samples per year at each station. With the method used here to estimate total phosphorus loads based on 25 samples per year, we can be 90 percent sure that the true load is within +/- 20 percent of the estimated load
- Moderate. The downward trend shown in Figure 1 is significant at P< 0.00025. However, the total trend shows a leveling off after about water year 2000, due to some high positive residual values after 2005, and low values between 2001 and 2004.
- Low. Overall confidence takes the lower of the two confidence determinations.
Additional Figures and Resources
No documents available.