The Secchi depth is a way to measure water transparency or clarity. It is determined by lowering a Secchi disk into the water and noting the depth at which it is no longer visible.
Secchi disk
The marine Secchi disc is a plain white, ø30 cm disc mounted on a pole, or a rope.
The freshwater Secchi disc is ø20 cm and features black quadrants
(courtesy of USGS)
In general, if the value of Secchi depth is low, it means the water is turbid and has high levels of suspended particles, which scatter and absorb the LiDAR signals. This reduces the ability to penetrate to the bottom. High turbidity can be caused by sediment, organic matter, or phytoplankton, all of which can obstruct the LiDAR’s laser pulses and impair the accuracy of depth measurements.
Measurement Recommendation
To ensure accurate and consistent Secchi depth measurements, conduct the measurement in the shade to avoid direct sunlight, which can create glare on the water surface and affect visibility. Try to have calm water conditions, as waves and ripples can distort the visibility of the Secchi disk.
Then, follow these steps:
- Get away from the shore on a boat, a paddle board, …
- Lower the Secchi disk slowly and steadily into the water.
- Note the depth at which the disk disappears. This is your Disappearance Depth
- After recording the disappearance depth, raise the disk slowly until it reappears.
- Note the depth at which the disk reappears. This is your Reappearance Depth
- Average the two recorded depths (disappearance and reappearance) to determine the Secchi depth.
Shortcomings
Despite its simplicity and wide use, the Secchi depth method has several shortcomings:- Subjectivity: The measurement relies on human observation, which can introduce variability due to differences in individual eyesight and judgment. What one person perceives as the disappearance point can differ from another’s perception.
- Light Conditions: Variability in ambient light conditions, such as overcast skies versus bright sunlight, can affect the visibility of the Secchi disk, leading to inconsistent measurements.
- Water Movement: Even slight waves or currents can impact the visibility of the disk, making it difficult to obtain a precise measurement.
- Turbidity Composition: The method does not differentiate between different types of particles in the water. For example, the presence of algae, silt, or organic matter can all reduce visibility but might have different implications for laser signal absorption.
- Depth Limitation: In very clear water, the Secchi disk may remain visible at greater depths than practical to measure accurately, limiting its usefulness in such conditions.
- Limited Scope: The Secchi depth provides a single point measurement and does not offer information on vertical profiles of water clarity or variations in turbidity at different depths.