It provides an integrated measure of transmission and refraction with light absorbed by living particles, plankton and bacteria, and non-living colloidal sediment and the suspended detritus of dead organisms. In turbid waters, or those clouded by plankton blooms, Secchi visibility depths can be just a few centimeters. Over the past 4 centuries, water column visibility in the Chesapeake has been gradually reduced as agricultural erosion, suspended sediments and nutrient concentrations resulting from industrial pollution and human population has increased. The Bay's once-extensive underwater meadows helped to shape their own environment. Broad expanses of grasses quiet small wavelets while the blades themselves reduce currents, which allows suspended sediments to settle out.
Rhizomes bind underwater soils, inhibiting erosion.
The plants and rhizomes also take up tremendous amounts of nutrients from the sediments and release oxygen to the water. As the integrity of these beds erodes beyond some point, the deleterious effect of poor water quality on grass beds accelerates to become a self-fulfilling prophecy. Most researchers, and today's Baywide monitoring program use electronic meters that measure PAR, the photosynthetically active portion of the spectrum. But most programs continue parallel use of the Secchi disk because of the utility it has in making comparisons with the long and worldwide historical record. The Secchi disk requires no batteries and only a few simple precautions to provide repeatable measures, even by amateur users.