Charlotte Harbor NEP Water Atlas


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Water Clarity

What does this mean?

Water clarity is an obvious feature of surface waters and is an important indicator of waterbody health. It can also affect designated uses for a waterbody, such as swimming and other recreational activities. Erosion and pollution can severely affect the transparency of water measured by turbidity and total suspended solids. Often, however, the quality of water is judged—mistakenly—based on the visual appearance of the lake or stream. While waterbodies with low water clarity may be undesirable for contact recreation, that does not necessarily mean they are poorly suited for other activities such as fishing. Furthermore, streams with low water clarity are not necessarily "unhealthy" or polluted with toxic substances. Even crystal clear water can contain toxic substances. For more information read the Learn Mores "Trophic State Index" (TSI, pertains to lakes and estuaries), "Water Quality Index" (WQI, pertains to streams) and "Impaired Waters" the Water Quality pages of the Water Atlas.

Some waterbodies are referred to as "black waters" and are tea-colored or coffee-colored. But do not let color deceive you; many of these waterbodies are among the healthiest in the country. The color is mostly the result of humic acids (for example: tannins and lignins) that leach from living plants and dead plant matter that are either in the water or in low-lying areas (for example: marshes, and swamps) surrounding the waterbody.

Note: It is important to understand the difference between true color, which is caused by substances that are dissolved in water, and apparent color, which is caused by substances that are suspended in water, such as plankton, sediment, and leaf fragments. Apparent color is temporary, and often seasonally-dependent and flow-dependent, where true color is often a prevalent feature of the waterbody year-round.


How are the data collected? (Methods)

As mentioned previously, there are two methods commonly employed for measuring water clarity/transparency in streams, turbidity and total suspended solids; and three methods commonly employed for this determination in lakes: turbidity, total suspended solids and Secchi disk depth.

Total Suspended Solids: This measurement is determined according to procedures detailed in Standard Methods for Analysis of Water and Wastes (2540-D). Briefly, this procedure entails filtration of a water sample, and then drying and weighing the filter. The difference in filter weight before and after drying provides an accurate way to quantify the amount of material suspended in the water column at the time the sample was taken.

Turbidity measurements are made either in-situ (in the water) or in a laboratory. In either case, the methods use an optical sensor system set to transmit specific bandwidths of light and to receive light as it is scattered (reflected). The more particles suspended in the water, the more light is reflected from them, and the more 'turbid' or cloudy the water. Turbidity is expressed in Nephelometric Turbidity Units (NTU), and can only be measured accurately with a true nephelometer; that is, one measuring light reflected at exactly 90 degrees to the light source and using the specified bandwidth of light as the source. Many devices are called "turbidimeters" but few are true nephelometers.

Secchi disk depth remains the most commonly used method for water clarity/transparency measurement in lakes. A Secchi disk is a simple and inexpensive device used by both citizens and scientists for measuring water clarity. The device generally consists of a white disk 20-centimeters in diameter, painted with alternating black and white quadrants (some are reflective single color), with a light chain or non-stretching rope attached through the center. The chain or rope is graduated in increments of feet or meters. A small weight is attached beneath the disk so that it will sink quickly and the line will remain taut while measurements are being made. The disk is lowered below the surface until it just disappears from view; that depth is referred to as the Secchi disk depth.


Calculations

Total suspended solids (TSS) are reported in either grams/liter or milligrams/liter. A liter of water (or an exact fraction thereof) is filtered and dried. The dry weight and the original filter weight (measured in milligrams or grams) are subtracted from the wet weight (filter, including filtrate before drying), and the result is TSS in g/l or mg/l.

Turbidity, as measured by a nephelometer, is calculated by the system and typically includes temperature compensation and, depending on the system, compensation may also be made for true color.

Once a Secchi depth measurement has been attained, it can be used to estimate the depth at which light can penetrate into the water column, which is referred to as the compensation point or photic depth, and provides an approximate maximum depth to which light can penetrate. To make this approximation, one may multiply the Secchi depth by two. For example, if a Secchi depth is five feet, then we can multiply the Secchi depth by two to estimate the depth of light penetration:

2 X 5' (Secchi depth) = photic depth of 10 feet.

Note: Using the example above, if the lake happens to be eight feet deep, then our calculation tells us that light is probably reaching the bottom.


Caveats and Limitations

Water clarity can fluctuate quite a bit over the course of a year or over a period of years. This fluctuation can result from natural phenomena, such as flooding, drought, seasonal winds and temperatures, species introduction, pollution and dredging. There are many documented cases where water clarity has decreased, but there are also documented cases where it has increased. This is why long-term monitoring is so important to understanding the dynamics of waterbodies. By monitoring water clarity over a long period of time, one is able to see if the clarity is declining or increasing and if these changes are temporary or showing a trend in either direction.


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