Principle

Visibility through the water column is a direct function of water clarity or, inversely, its attenuation properties. Light attenuation in water is caused by turbidity (suspended particulates that absorb and refract light), color (dissolved compounds that absorb light), and water's own absorbtive properties. Light attenuation in water follows a negative exponential relationship, as described by the following equation:

where Id is light flux to depth d, I0 is light flux at depth 0 (or some depth shallower than depth d), e is the base of the natural log, k is the light extinction coefficient (per meter), and d is the depth interval in meters. The visibility of a Secchi disk is one way to measure the light attenuation properties of a water column, and is surprisingly accurate for estimating k when compared to measurements made by more sophisticated light meters.

Method

Lower a standard (30 cm diameter) Secchi disk with a metered rope into the water until the disk is just visible. The depth from the water surface to the disk itself is called the Secchi depth. Secchi depth can be related to the light attenuation coefficient, k, by the following equation:

One can use an estimate of k derived from Secchi depth to estimate percent light flux to any given depth or, if actual light flux is known, to calculate actual light flux to any other depth using equation (1).

If a light meter is available with which to measure actual light fluxes at different depths, one can take measurements of I0 and Id and estimate k by rearranging equation (1) as follows:

For measurement of Secchi depth in very clear water (where k values might get as low as 0.06 or so), a larger Secchi disk with a blue rope work better than a standard rig.

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