All phytoplankton cells contain chlorophyll

Primary production is the formation of organic carbon from carbon dioxide, using light as a source of energy. All phytoplankton cells contain chlorophyll to harvest the light energy. Therefore, chlorophyll might be used to estimate the biomass of phytoplankton. However, the chlorophyll content per cell depends on the available light and the traits of a specific phytoplankton type. The chlorophyll content is therefore dynamically modeled for each phytoplankton type.

In the North Sea the growth of phytoplankton (= primary production) starts in March in the South where there is more light and the water temperature is higher. In April the growth starts in the northern part of the North Sea. In spring diatoms have the highest production and determine to a large extent phytoplankton biomass and hence the chlorophyll concentration.