Nitrogen Load (Tributaries)
Nitrogen 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 nitrogen load 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 1989 through 2014. The yearly load for each stream is calculated as the sum of daily loads s for a given water year. The combined total nitrogen load represents an estimate of the total mass of nitrogen that is 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 is from the Lake Tahoe Interagency Monitoring Program (LTIMP).
Evaluation Map
The seven streams routinely monitored for total nitrogen 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
- Little or No Change
- 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
- This indicator measures the total nitrogen delivered to Lake Tahoe via seven monitored streams (reported as total nitrogen load). 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 (Lahontan and 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 near-shore, 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. 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. Nitrogen occurs naturally in the soils of the Lake Tahoe Region, but organic nitrogen primarily comes from the decomposition of plant material. Atmospheric deposition of automobile exhaust is considered a primary source of inorganic nitrogen to Lake Tahoe (Lahontan & NDEP, 2010a). Between 40 and 80 percent of the total organic nitrogen load is dissolved organic nitrogen (Coats and Goldman, 2001). If only a fraction of the dissolved organic nitrogen is biologically available, its importance as a nitrogen source for algal growth would outweigh that of dissolved inorganic nitrogen (equals nitrate plus nitrite plus ammonium). In a study of the bioavailability of dissolved organic nitrogen to bacteria and phytoplankton, Seitzinger et al. (2002) found that 30 to 45 percent of the dissolved organic nitrogen in stream water from a pine forest was biologically available (Seitzinger et al., 2002). The availability of dissolved organic nitrogen from urban runoff varied from 48 to 70 percent. The bioavailability of humic-associated nitrogen in a river draining coniferous forest (generally thought to be refractory) may be as high as 37 percent (Carlsson et al., 1999).
- 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
- No target established. Established numerical standards (targets) for dissolved and total nitrogen load have been identified for the pelagic and littoral zones of Lake Tahoe. However, the comparative basis for these standards (i.e., average loads from the 1973 to 1981 period) from the various sources identified above are not available; thus, no determination of status can be rendered, and status is unknown. The annual total nitrogen load is strongly influenced by the volume of total annual runoff, perhaps more than the other constituents (i.e., suspended sediment and total phosphorus). The combined total annual total nitrogen load for the seven monitored streams averaged (1989 to 2014) 38,556 kilograms/year, with a median value of 35,051 kilograms/year, and a range from 12,268 kilograms/year in 1994 to 89,030 kilograms/year in 1995. Note that 1995 had both the highest total annual total nitrogen load, and the highest total annual runoff in the data record (1989 to 2014).
Blackwood Creek and the Upper Truckee River continue to be the most important source of total nitrogen entering the lake. Blackwood Creek has the greatest yield per unit area. The Upper Truckee River had a lower total nitrogen yield per unit area, but its large size makes it the greatest contributor of total nitrogen load. Trout Creek basin has a much lower yield than the other monitored watersheds, due perhaps to the relatively gentle slope of the extensive flood plain in the lower part of its basin. - Little or no change. No significant trend for change in total nitrogen load over time was detected. Highly significant downward trends (P < 0.003) in nitrate (NO3-N) loads were found for all seven streams with Alley's Adjusted Variable Kendall Test (Alley, 1988; Helsel and Hirsch, 2002).
In order to test for trends in total nitrogen, we added the nitrate (NO3-N) loads and total Kjeldahl nitrogen loads for each of the seven streams by year for 1989 to 2014. With total Kjeldahl nitrogen included, only Third Creek showed a significant downward trend (a 51.1 percent reduction in load over 24 years; less than 0.002). And with total nitrogen loads and flows aggregated across watersheds, no time trend was detected (see Figure 1). The apparent lack of a trend in total Kjeldahl nitrogen masks the basin-wide downward trend in nitrate-N.
Highly significant downward trends (P < 0.003) in nitrate (NO3-N) loads were found for all seven streams with Alley's Adjusted Variable Kendall Test (Alley, 1988; Helsel and Hirsch, 2002). Table 2 shows the percent changes in regression-estimated nitrate (NO3-N) loads for given annual hydrologic conditions. For example, at a given total annual and maximum annual discharge in Blackwood Creek, the estimated total annual nitrate (NO3-N) load in 2014 was (on average) 55.8 percent less than it would have been for the same hydrologic conditions (had they occurred) in 1975. Note that the annual nitrate (NO3-N) loads were estimated with the period-weighted sample method, and the residuals for trend analysis were taken from the regression of total annual load versus total annual and maximum annual daily discharge.
Confidence Details
- Low. Where insufficient data exists to determine status, confidence in the status determination is low. The confidence in an estimate of total nitrogen annual load depends on the number of samples, and on the variance of the daily loads that are sampled to derive the annual load. (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 nitrogen. Currently the LTIMP stream sampling program collects about 25 to 30 samples per year at each station. With the method used here to estimate total nitrogen loads based on 28 samples per year, we can be 95 percent sure that the true load is within +/- 20 percent of the estimated load. Since much of the total nitrogen load is dissolved, continuous turbidity monitoring may not provide much additional confidence in the estimates, but this has yet to be determined.
- Low. The Kendall test reports the P value for the slope of a trend, that is, the probability that the slope is not different from zero. The trend test for total nitrogen had a p-value = 0.86, tau = -0.028.
- Low.
Additional Figures and Resources
No documents available.