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.