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A widespread photosynthetic pigment with prominent blue and red absorption bands. Its concentration is a useful index of phytoplankton biomass, but pigment per unit carbon varies with light acclimation and physiological condition.

Many open-ocean chlorophyll retrieval algorithms infer chlorophyll a from the blue-to-green remote-sensing reflectance ratio. Increasing pigment generally depresses blue reflectance relative to green. In coastal waters, however, coloured dissolved organic matter and non-algal particles can independently absorb blue light, as in the sketches, and therefore mimic an increase in pigment. Their concentrations need not covary with phytoplankton biomass: river runoff and sediment resuspension can change them separately.

Moreover, the satellite measures emerging radiance, not absorption directly. Schematically,where $b_b$ is the backscattering coefficient; this is an explanatory approximation, not a complete retrieval formula. Suspended particles change both absorption and backscattering, and different mixtures can produce similar band ratios. Shallow-bottom reflection, land-adjacency effects and more difficult atmospheric correction introduce further coastal errors. Consequently a simple blue/green ratio can overestimate chlorophyll a when excess CDOM removes blue light, although sediment scattering and other effects mean the bias is not universally positive. Multiband or hyperspectral methods that separately constrain pigment, dissolved absorption and particles, supported by local observations, are preferable in optically complex water.^[ref]

Chlorophyll a occurs in nearly all oxygen-producing phytoplankton groups and is central to their photosynthesis. Its concentration can be measured reproducibly by pigment extraction or calibrated optical methods, and its absorption affects ocean colour, making it observable over large areas. It is therefore a practical common index of photosynthetic standing stock across diverse communities, unlike a pigment limited to one group.

It is a proxy, not a fixed conversion to phytoplankton biomass or primary production. The carbon-to-chlorophyll ratio varies with species, light acclimation, nutrient supply and physiological condition; low-light cells often contain more pigment per unit carbon. Dead pigment and methodological differences can also affect interpretation, and nonphotosynthetic plankton are not represented. A pigment increase need not mean the same proportional increase in living carbon biomass.^[ref]