Chlorophyll-a
The primary photosynthetic pigment in all algae and cyanobacteria (blue-green algae), and the most direct real-time proxy for algal biomass in a water body.
Chlorophyll-a is present in every photosynthetic organism in freshwater and marine systems, from diatoms to cyanobacteria, regardless of color. Because its concentration scales with the amount of living algal cells in the water column, it is the most useful indicator of standing algal biomass and the trophic state of a lake, reservoir, or coastal water. In-situ fluorescence sensors excite chlorophyll-a with blue light (~430 nm excitation) and read the red fluorescence (~685 nm) it emits, giving a continuous, non-destructive estimate without lab extraction. Chlorophyll is present even in non green algae: red algae, brown algae, golden algae, all contain chlorophyll and their presence can be monitored with chlorophyll.