SAV is considered to be an indicator of water clarity and nutrient levels and these factors were considered when attempting to find correlation between water quality and SAV survival. Vegetation losses are attributed to the synergistic effects of the factors affecting light penetration through the water column and water quality. Light reduction with depth is the major factor in the decrease in photosynthetic rates. Algae and macrophytes use between 4 to 9 quanta of light energy per molecule of carbon dioxide reduced. Absorption of light is specific for different chlorophylls.
Light energy penetrating the water is dispersed and absorbed.
This is called absorption. The diminution of light energy with depth by light scattering and absorption is called light attenuation. The light intensity or irradiance depends on the light intensity at the surface, light attenuation coefficient, and depth, with which it decreases exponentially. Light availability is a function of a complex interaction of factors which are directly or indirectly related to water quality such as total suspended solids, amount of phytoplankton, levels of nitrogen and phosphorus. In Chesapeake Bay a large scale decline of SAV occurred in the late 1960s and early 1970s.