Learn More: Bay Water Clarity

The relative abundance of seagrasses in coastal waters is a key indicator of coastal and estuarine ecosystem health. Like other plants, seagrasses produce food using photosynthesis, but they require significantly more sunlight than terrestrial vegetation. The amount of light that is available to them for photosynthesis is dependent on water clarity, as is their potential range; where there is good water clarity, they can grow in deeper water. The suspended sediments, organics, and microorganisms present in seawater affect its transparency, and therefore the amount of sunlight that can reach the leaves of seagrasses. As light passes through water, its intensity decreases, and this reduction over a given distance is known as light attenuation. The main contributors to light attenuation in the water column are turbidity, phytoplankton, and dissolved organic matter. Light attenuation can also occur at the surface of the leaf due to the growth of algae/epiphytes.

Water that is not perfectly clear may be referred to as having "color." 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.

Light attenuation: Its low cost and convenience have made the Secchi disk one of the most widely used tools for measuring water clarity. A Secchi disk (photo, right) 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 depth, which is reported in feet or meters.

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 be measured accurately only 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.

Light availability: Not all wavelengths of light are equally usable for photosynthesis by seagrasses and other submerged aquatic vegetation. Generally, wavelengths in the range of 400-700 nanometers (nm) can be used. Special optical sensors have been developed to measure this photosynthetically active radiation (PAR) in surface water. PAR is normally quantified as μmol photons/m²/second, which is a measure of the photosynthetic photon flux density (PPFD), i.e., how many photons in the selected wavelengths pass through the water in a finite period of time.

Total suspended solids: These 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 water depth happens to be eight feet, then our calculation tells us that light is probably reaching the bottom.

Light falloff in water can be expressed as a light attenuation coefficient, k. Secchi depth can be related to k by the following equation:

k = 1.7/(Secchi depth)

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.

• Characterization of the Underwater Light Environment and Its Relevance to Seagrass Recovery and Sustainability in Tampa Bay, Florida
• Colored Dissolved Organic Matter (CDOM) Workshop summary
• Colored dissolved organic matter in Tampa Bay, Florida
• How to Make a Secchi Disk