As discussed elsewhere in this web site (eutrophication and trophic states), good water clarity does not mean that one water body is "better" than another. Clear water and murky water look equally attractive depending on your view of what the "management objectives" for the lake should be.

Serious freshwater anglers who have fished a variety of lakes may realize that the best fishing isn't always in "gin-clear" water. Lakes that are considered more productive, with quite a few pounds of fish averaged per acre, are normally fairly green with greater amounts of algae. They have a more productive food web, starting with free-floating algae (phytoplankton), which reduces visibility.

Water clarity is merely a measure of how far you can see into the water; that is, its visible depth.

In Florida, about 12% of our lakes have a visible depth of greater than 12 feet (oligotrophic). Some especially clear lakes in north Florida have visible depths of 30 feet and more.

About 31% of our lakes have a visible depth of between 8 and 12 feet (mesotrophic).

About 41% of our lakes have a visible depth of between 3 and 8 feet (eutrophic).

And about 16% of our lakes have a visible depth of less than 3 feet (hypereutrophic). Some especially murky lakes have a visible depth of only a few inches.

These depths correlate to the different trophic states of Florida lakes. Learn more about trophic states and eutrophication on another page of this web site.

There is a humorous tale (humorous at least among fisheries people) of two young fishery graduate students who were sent to Lake Apopka a few years ago to sample fish by pulling a trawl net (similar to a shrimp net) behind their boat. At the time, the lake was completely murky and considered unsuitable for recreation, even fishing. The two budding biologists set out the net, opened two beers, and leisurely trawled up and down the lake, without a set time or distance. When they did stop, the net was so full of fish, it couldn't be pulled back in the boat. There were hundreds of pounds of fish in that one net. It had to be cut and the fish released!

Lake Apopka does have its problems, and has earned the reputation of one of Florida's murkiest lakes, but the fish remain.

WATER CLARITY FACTORS
While clarity in some Florida lakes has been altered by storms or recent human activity, others have not. Many of our lakes were murky or tannin-stained long before Columbus sailed, back when human impact on Florida waters were rather negligible by today's standards.

Most often, a water body's water clarity is the result of:

Suspended sediments that have been stirred up from the bottom by strong winds and currents.

The underlying soils of a lake that may have high phosphorus content that helps algae grow, thus making the water cloudy.

Surrounding geology also helps determine a lake's clarity. The lake itself may have sandy nutrient-poor soil, but the surrounding watershed may be high in nutrients, which wash into the lake and cause algae to grow. See the geology page of this web site.

Water color may make a lake darker and less clear. See the water color page of this web site.

Human impacts certainly contribute to the nutrient load in a lake and therefore to its algal load and water clarity. See the human impacts page of this web site.

See the lake trophic state page of this web site to learn about eutrophication and water clarity.

Measuring Water Clarity with a Secchi disk
The most common tool for determining lake clarity is a Secchi disk, a 20 centimeter (8-inch) black and white plastic disk that is lowered into the water at the end of a string or cord. A small weight is often located underneath the disk so that it can sink quickly. The attached string is usually has each foot marked off in increments of inches or centimeters, depending on your preferred unit of measurement. Note: Some monitoring groups are now using disks that are solid white in color.

As the disk is lowered into the water, the individual holding the device is instructed to “mark” the point at which the disk disappears from view by pinching the line with their fingers at the water's surface (a clothespin also works well). Once the disk and string are pulled from the water, the distance between the disk itself and the pinched string is tabulated and recorded. This measurement is known as a Secchi depth.

It's a simple technique first used in 1865 by Pietro Secchi in Italy (scientific advisor to the Pope), and it's still widely used today.

Measuring a lake's clarity on regular occasions, along with monthly water sampling can provide valuable information about a lake’s biological productivity.