Clarity - Vertical Extinction Coefficient (VEC)
This indicator has been retired and is no longer reported.
The vertical extinction coefficient (VEC) measures optical properties of Lake Tahoe similar to the Secchi depth, however VEC measures beyond the depth at which the Secchi Disc is visible, providing information on biological properties of the lake down to the bottom of the photic zone (approximately 50 to 60 meters). Light extinction results from one or more of the following factors: colored dissolved organic material, organic and inorganic particles and the water molecules themselves. The decline in VEC is a result of additions of fine sediment particles and growth of phytoplankton (algae). Drivers influencing the delivery of fine sediment and nutrients include urban development, anthropogenic and natural disturbance in the undeveloped portions of the watershed, and local and regional climate. Many programs throughout the Tahoe Basin are aimed at improving VEC such as stormwater reduction and stream restoration projects. VEC has been measured at Lake Tahoe by UC Davis since 1968.
Status
Annual average vertical extinction coefficient (VEC) recorded at the Lake Tahoe Index Station. No data was collected between 1977 and 1982 or in 2012.
Evaluation Map
Locations of long-term monitoring sites for VEC and other water quality constituents. The Index Station is located along the west shore.
2015 Evaluation
- Status
- At or Somewhat Better Than Target
- Trend
- Little or No Change
- Confidence
- Moderate
Applicable Standard
No Adopted Standard - State standard
Key Points
No Key Points
About the Threshold
- Both the vertical extinction coefficient (VEC) and the Secchi depth measure optical properties of Lake Tahoe. State standards define VEC as measure of clarity and Secchi depth as measure of transparency. VEC is related to Secchi depth, but the measure extends beyond the depth at which the Secchi Disc is visible, providing information on biological properties of the lake down to the bottom of the photic zone (approximately 50 to 60 meters). Light extinction results from one or more of the following factors: colored dissolved organic material, organic and inorganic particles and the water molecules themselves (Swift et al., 2006). The depth of one percent light transmission is typically considered the vertical extent of the photic zone. VEC can be interpreted as the amount of light extinguished as it passes through each meter of water. The mean annual VEC since 2000 has ranged from 0.07 to 0.09 m-1 meaning that seven to nine percent of the light is extinguished per meter of depth. VECs in some mesotrophic lakes can reach as high as 2.0 m-1 and in eutrophic lakes can approach 4.0 m-1.
- Water clarity in Lake Tahoe is largely controlled by particles blocking light penetration either by scattering or by absorption. Particles in Lake Tahoe are composed of both microscopic, free-floating algae (phytoplankton) and fine sediment. Algal growth is stimulated by excess nutrient (nitrogen and phosphorus) loading from stream and stormwater runoff and atmospheric deposition(Lahontan and NDEP, 2010a). Fine sediment loading is also from stream and stormwater runoff and atmospheric deposition. 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). The below average stream inflows and stormwater runoff due to the continuing drought, are substantial contributing factors in the recent improvement of lake transparency.
Rationale Details
- At or somewhat better than target. The annual average in 2014 was a Vertical Extinction Coefficient of 0.075, which means the standard is in attainment. However, 12 of 27 VEC measures in 2014 were above the state standard of 0.08.
- Little or no change. Since around year 2000, VEC measurements have been better than predicted by the long-term trend of linear decline observed since 1969. Statistical analysis supports the observation that the decline in VEC has slowed since 2000, and the overall trend is now better represented by a curve (see figure above), rather than a straight line. The line of best fit to describe the long-term trend was determined statistically using a polynomial regression.
Confidence Details
- Moderate. The annual average in 2014 was a Vertical Extinction Coefficient of 0.075, which means the standard is current in attainment. However, 12 of 27 VEC measures in 2014 were above the state standard of 0.08 per meter.
- High. According to the standard methodology of this report, the trend is statistically significant R² = 0.7672, P < 0.01.
- Moderate. Overall confidence takes the lower of the two confidence determinations.
Additional Figures and Resources
No photos available.
No documents available.