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Water Clarity and Secchi Disk

Secchi disk measurements were originated in 1865 by Fr. Pietro Angelo Secchi, an astrophysicist who was requested to measure the transparency of the Mediterranean. Secchi was the scientific advisor to the Pope. The basic procedure remains in common use today as a simple means to determine water clarity (and assumed quality). We use an 8" diameter metal disk, with black and white quadrants which is lowered into the water on a line marked in meters. Clarity is measured as the depth at which the disk can last be seen.

FOLKS volunteers have been taking Secchi disk readings of water clarity, quarterly, since 1994. From these readings, we have found a significant correlation between rainfall and clarity depth. The reason is that the primary clarity problem in this lake today is the fine clay sediment washed downstream, and from the shoreline. Aerial videos confirm the primary source is from upstream, though new construction can be a significant contributor when not protected.

Two other events contributed to the results seen here:
In late '95 the lake levels were dropped about 6 1/2 feet in an attempt to control newly discovered Hydrilla, a fast growing noxious weed which has devastated some Florida lakes. While the lake was down, the normally occurring sediment accumulated on the lowered shoreline. Following the treatment, when the lake was brought back to normal levels, the accumulated sediment was agitated and went into suspension before finally settling to the bottom.

In 1998, the El Nino rains had a direct impact on the amount of sediment flowing into the lake, showing up as poor clarity readings.