Phytoplankton effects on optics
Photosynthetic organisms contain chlorophyll a, which selectively absorb light in most wavebands except the green to green- yellow, leaving a color like one evident in this photograph. The Secchi disk contrasts the green of the water, showing the extent of the bloom in the Rhode River, a tributary of the Chesapeake Bay.
The effect of phytoplankton on light attenuation is actually twofold. The cell wall of the phytoplankton acts as organic particulate matter, absorbing and scattering light. Additionally, the photosynthetic pigments with the phytoplankton, chlorophyll a, strongly absorbs light, differently than CDOM.
The graph depicts three typical absorbance spectra for various types of phytoplankton. As you can see, absorption is greatest in the blue region of the spectrum, with a significant peak in the orange to red region of the spectrum. Absorption is the lowest in the green region, hence the green color of the water.
Human activities can increase phytoplankton concentration to ecologically detrimental levels, by increasing nutrient loadings to a water body. High concentrations of phytoplankton can decrease light penetration, sometimes with catastrophic consequences to seagrass communities and other marine life.