This parameter describe the volumetric mass concentration [in units of mg m-3] of phytoplankton (algal) pigment chlorophyll a in the model’s surface layer. Chlorophyll a in photoautotrophic phytoplankton is used to "harvest" sunlight energy and drive all cellular processes, especially the build-up of new biomass. In a photoacclimative model such as MAECS, cells adapt their chlorophyll to biomass ratio to ambient light concentration: when there is a lot of light (in summer and near the ocean surface), cells need less chlorophyll per biomass than in darker conditions, such as in spring and in deeper water layers.