The annual cycle of phytoplankton production varies in different parts of the North Shelf ecosystem. The northern parts of the ecosystem, including the Gulf of Maine and Georges Bank areas, tend to have a spring bloom and usually, but not always, a well developed fall bloom. These two areas can be divided into five seasonal time periods: late winter, spring, summer, fall, and early winter. The five-period cycle is not present in the southern areas of the ecosystem. A spring bloom usually develops in the Southern New England area, but a fall bloom does not consistently develop. This area is divided into four time periods: winter, spring, summer, and fall. The Mid Atlantic Bight can best be characterized with three seasonal time periods: winter, summer, and fall. The 2006 annual cycles for these four regions, and historical data on the chlorophyll concentration during each time period is provided below. Chlorophyll concentration is characterized in two ways:

Seasonal Chlorophyll Concentration: is the mean chlorophyll concentration for the seasonal time period in a region. This is analogous to intensity.

Integrated Seasonal Chlorophyll Concentration: is the mean chlorophyll concentration for a seasonal time period times the length of the time period. This is analogous to magnitude. In some years, the time period start or stop day was not obvious from the annual data, in these cases the climatological time period was used instead.