How to Use a Secchi Disk to Measure Turbidity in a Water Body

In order to understand the physical, biological and chemical characteristics in a body of water, such as a lake, it is critical to know how much light is penetrating through the water column or how far the light is traveling. For this reason, Scientists often measure turbidity, or how clear a fluid is.

One simple, cost-effective method of measuring turbidity is the use of a Secchi Depth Disk.

These instructions are designed to teach scientists, limnologists or those interested in light values in a body of water, on how to use a Secchi Depth Disk.

A secchi disk is a round flat disk with contrasting colors, typically 20-30 cm in diameter. It was developed in the late 1800's by one of the Pope's scientific advisors, Father Pietro Secchi. It is now a widely used method of Limnologists, Ecologists, Biologists and more to measure turbidity in water, specifically lakes and freshwater ecosystems. Turbidity is a measure of how clear a body of water is. Turbidity or “cloudiness” in water can be caused by a multitude of factors, such as suspended particles, microorganisms or dead organic matter floating through the water column.

Step 1: Preparation

Obtain a secchi disk or make your own following these Instructions

Ensure the disk has contrasting colors and an attached cable or rope with labeled measurements, half meter increments are best when measuring in small and large body lakes.

In order to avoid experimental error avoid sampling up to 3 days after a large rain event. This is due to the fact that precipitation impacts light and particle scattering in the water column. Turbidity and Light readings are most accurate on relatively clear days.

Step 2: Position Yourself

Position Yourself on the shady side of the boat or surface where you are taking a reading from. Remove any eyewear, as reflections from glasses can impair the accuracy of your reading.

Step 3: Lower the Disk

Slowly lower the disk into the water, using one hand to hold the disk steadily and the other hand to feed yourself slack rope. Lower disk until you can no longer see the contrast in colors on the disk. After you can no longer see the disk, slightly raise the disk to see it again, then use a clothes pin or hold the rope/cable where it enters the water, and record how far you have to lower the disk to not see it anymore.

Step 4: Repeat Measurement

Because many people have different eyesight capabilities, and the Sun is constantly being covered by clouds, its important to do multiple trials to ensure the validity of your measurement. When working in teams, it may be helpful to have everyone on the team take their own Turbidity Measurement using the Secchi Disk and then take an average of all the measurements.

When taking measurements try to keep the following variables constant, or keep them in mind when analyzing your data:

• Time of day and Season
• Weather conditions (i.e cloud cover)
• Person taking measurement
• Latitude and Altitude
• Atmospheric Transperency (i.e haze, smoke, particles)

These above factors strongly affect the light intensity and quality in a body of water.

Step 5: Use Your Measurements to Make Light Assumptions

Using your secchi depth reading you can approximate a light extinction coefficient (η) using the following equation:

η=1.7/Zsd

where.... Zsd= Secchi depth (m)

η= Light Extinction Coefficient

The light extinction or light attenuation coefficient tells you how easily a material can be penetrated by light or how far that light can travel through the surface. η can then be used in the following negative exponential equation to estimate the value of light at depth and value of light at the surface of the fluid.

lz=loe^-ηz

where...lz= Light at depth z

lo= Light at the surface

e= natural logarithm

η= Light Extinction Coefficient

z= depth

Light Attenuation in Water occurs due to the processes of absorption and scattering. Knowledge of how far light can travel through water can give insight on the particles present in the water, what can live there and the amount of solar energy penetrating through the water column. Light is very important to all organisms, especially in an aquatic food web. Primary producers such as phytoplankton require light as solar energy to make food through the process of photosynthesis. Therefore, the whole food web in an aquatic ecosystem relies on light energy to function properly as an ecosystem.

Step 6: Learn More

For centuries, Scientists and non-scientists across the globe have used Secchi Disks to obtain critical information regarding light and transparency in bodies of water. Today, organizations of volunteers have created an event called Secchi Dip-in, in which people worldwide take turbidity measurements and submit their data to their website. This incredible movement has promoted awareness of the importance of water quality across the globe. Use these instructions to take your Secchi measurement and join the movement today!