We measure turbidity, light attenuation and chlorophyll fluorescence.

Turbidity

Lake Tahoe shoreline

Turbidity is a measure of how water scatters light. Water with a lot of small particles scatters lots of light and has a cloudy, or murky, appearance. The murkier the water appears the higher the turbidity. Turbidity is determined by shining a controlled light source into a sample of water. The mount of light that is scattered in a direction that is 90 degrees away from the incoming light is proportional to the turbidity. Turbidity is different measurement than water clarit that is determined by measuring the greatest depth that a Secchi disk (which is sort of a white disk a foot in diameter) can be observed from the surface. In the shallow depths of the near shore zone, a Secchi disk will remain visible when it hits the bottom of the lake.

Clarity measurements have several advantages over turbidity measurements. Clarity is directly related to what a person sees, it is easy to explain and understand, it is sensitive to conditions over a range of depths, and there is a long measurement record for Lake Tahoe. The Tahoe Research Group at the University of Davis has been measuring clarity at several locations in the middle of the lake since the 1960s.

Turbidity measurements have several advantages over clarity measurements. Unlike clarity measurements, turbidity measurements can be made from a moving boat or by unattended instruments. This makes it practical to produce maps of turbidity or to remotely collect turbidity data at a fixed location more often than clarity measurements can be made. Unlike clarity measurements, turbidity measurements can be made in water a few inches deep, so it works well in the near shore zone. This makes it possible to consider the influence of localized on-shore activates on lake water quality.

Our turbidity measurements are made at a single water depth, so they do not say much about conditions at other depths unless multiple measurements are made at different depths at the same location. In our studies we operate on a moving boat, continuously pumping water from a water depth of 1-meter through our onboard instruments. A GPS and computer system records and displays the value of the turbidity, water temperature, and the boat location in real-time.

Clarity and turbidity measurements complement each other. Clarity is a great indication of the overall health of the lake and is well understood by the public. Turbidity is great for spatially detailed investigations that can be used to determine which portions of the lakeshore are contributing undesirable material to the lake.

Light Attenuation

Light attenuation measures how much light is attenuated, or reduced, when light travels through water. Light is attenuated throubh both the absorption and scattering of light. Absorption occurs when particles and dissolved material in the water absorb light. Scattering occurs when particles in water scatter light in a direction that is different from the incoming light. The amount of light that is scattered and the angle at which it is scattered is different for different colors of light and different particle sizes and composition.

Chlorophyll Fluorescence

We are also occasionally measuring chlorophyll fluorescence which is proportional to the concentration of chlorophyll in the water. This measurement is made by illuminating a water sample with one wavelength of light and measuring how much fluorescence there is at a second wavelength. The light wavelengths are selected so that the measurement is sensitive to the concentration of chlorophyll in the water sample. The chlorophyll measurements can also be made from a moving boat and provide an excellent indication of the algae concentrations.