= Solution
<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 \b[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.<Chlorophyll maps by NASA Earth Observatory>[Chlorophyll alters sunlight absorption and reflection, enabling mapping of near-surface phytoplankton distributions; pigment is used as an index rather than a direct carbon measurement.]{ref}
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