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Annual Average Secchi Depth

Status and Trend

Interpretation and Commentary

Annual Average Secchi

Status: Somewhat Worse Than Interim Target
Trend: Moderate Decline
Confidence: Moderate

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  • Relevance - This indicator measures the annual average Secchi depth at the Lake Tahoe Index Station and provides a measure of Lake Tahoe transparency each year. Federal, state, regional, and local agencies have all adopted numerous regulations to protect Lake Tahoe’s renowned transparency. California has designated Lake Tahoe an Outstanding National Resource Water under the Federal Clean Water Act, and it considers aesthetic enjoyment of the Lake’s clarity a primary beneficial use. Similarly, Nevada has designated Lake Tahoe as a “water of extraordinary ecological or aesthetic value.”  The protection of Lake Tahoe’s transparency is also a key component of the Regional Plan and priority focus of the Environmental Improvement Program; restoring Lake Tahoe's transparency is considered an important socioeconomic value.
  • Adopted Standards  - California standard:Achieve an annual mean Secchi disk transparency of 29.7m (97.4ft).
  • Indicator - Each annual value is the mean of 20-25 individual measurements taken throughout the year at an established index station. Individual measurements are recorded in meters.
  • Status – Lake Tahoe is considered an “impaired” water body under the Federal Clean Water Act (Section 303d). Lake Tahoe has not met the California transparency standard of 29.7m since this standard was first adopted in the early 1970s. In 2011, the Secchi depth was 21m (68.9ft), an increase of 1.4 m (4.5ft) from the previous year. However, the reader is cautioned from placing too much importance to this year-over-year change. This amount of change between years is not extraordinary for the annual average Secchi depth. The status of Lake transparency is somewhat worse than the interim target because the 2011 value is only 12 percent less than the interim target of 23.8m (78 ft.).

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  • Trend Trend in overall exceedance rate was determined to be “moderate decline” based on combined data collected at Barton Beach and Airport sites (2008 to 2011). A simple linear regression test indicated that trend in exceedance rate calculated from airport monitoring effort showed an increasing trend of 0.59/day/year, however, this trend was not statistically significant (Airport - R2=0.638, P = 0.411). There was no trend in exceedance rate recorded at the Barton Beach site (R2=0.25, P=0.667). The trend in overall average exceedance rate (exceedance rate averaged over all monitoring sites between 2008 and 2011) indicated an increase in exceedance rate of 0.021/day/year. However this was not statistically significant (R2=0.352, P=0.406) probably due to small sample size (n=4 years) and inter-annual variation in exceedance rate.

Confidence

  • Status – There is high confidence in the status determination. Secchi depth measurements are used widely as a measure of water transparency in oceans and lakes; it is a highly reliable, relatively simple, and inexpensive measurement of lake transparency. It is among the oldest limnological devices and was first used by Italian Professor P.A. Secchi in the 1860s. Jassby et al. (1999)evaluated the general precision of the method used in Lake Tahoe, and estimated the average precision based on the two observers was  +0.027 m. A recent analysis of annual average Secchi depth readings (includes water conditions down to a depth of ~20 m in recent years) and the vertical extinction coefficient (a measure of the rate of light attenuation, measured with a sensor down to ~ 100 m), has shown these two measures of light penetration in Lake Tahoe to be well correlated over the entire period of record (TERC 2011b).
  • Long Term Trend – Confidence in the long-term trend between 1968 and 2011 is “moderate.” The long-term trend is estimated using a general additive model (GAM), which blends properties of generalized linear models and additive models. The purpose of a GAM is to maximize the quality of prediction of a dependent variable from various distributions, by estimating unspecific (non-parametric) functions. The intra-annual variability associated with each average annual estimate is expected as part of the normal ecosystem response due to year-to-year changes in precipitation, runoff, Lake mixing, and meteorology. There is a moderate level of confidence that the trend of improvement in annual average lake transparency observed since about 2000 will continue into the future. Continued monitoring is required to see how this apparent improvement progresses into the future.
  • Overall Confidence - The overall confidence in this indicator is “moderate” because there is high confidence in the condition status, and moderate confidence in the long-term trend.
  • Human and Environmental Drivers - Water transparency in Lake Tahoe is almost exclusively the result of particles blocking light penetration either by scattering or by absorption (Swift et al. 2006). Particles in Lake Tahoe are composed of both small, microscopic, free-floating algae (phytoplankton) and fine sediment that is transported to the lake with stream and stormwater runoff (Swift et al. 2006), or from atmospheric deposition (Lahontan and NDEP 2010). Excess nutrient (nitrogen and phosphorus) loading which stimulates algal growth also contributes to the loss of transparency (Lahontan and NDEP 2010). Drivers influencing the delivery of fine sediment and nutrients include urban development (including the transportation network and vehicle density), anthropogenic and natural disturbance in the undeveloped portions of the watershed, and local and regional climate (especially wind and precipitation).
  • Monitoring Approach – Transparency measurements are taken in Lake Tahoe using a 25-cm, all white Secchi disk. The disk is lowered into the water column from a boat to a depth at which it is no longer visible by the observer, and then raised slowly until visible again. The midpoint of these two depths is called the Secchi depth. Between 20 and 25 individual measurements are taken throughout each year to arrive at an estimate of the annual average Secchi disk depth. The measurements presented in this document are taken from the Index Station, where monitoring has occurred uninterrupted since mid-1967. Although this station appears close to the shoreline, it is >150m deep and is characteristic of open-water. Early studies by UC Davis show that this location is representative of the Lake’s deepwater condition (Goldman 1974).
  • Monitoring Partners – University of California at Davis (Tahoe Environmental Research Center) and Tahoe Regional Planning Agency.

Links

 
  • Monitoring Plan
  • Conceptual Model

Map

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Locations where UC Davis measures various Lake Tahoe condition indicators.Trend Charts

Trend Charts

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Annual average Secchi disk depth measurements recorded at the Lake Tahoe index station (1968 to 2012). Each annual value is the mean of 20-25 individual measurements taken throughout the year. The line of best fit was determined statistically using a general additive model (GAM). The 2011 measurement 21 m (68.9 ft) is somewhat worse than the interim target of 23.8 m (78 ft.). The long-term trend has shown a historically declining condition; however, over the last decade (2002-2011) the rate of decline in Lake transparency appears to have slowed relative to the trend prior to 2000. Data are from the UC Davis – Tahoe Environmental Research Center (TERC 2011a).

Additional Info

References

Additional Information

Last Updated on Monday, 11 March 2013 12:58