Phytoplankton are the base of the marine food web and are crucial players in the Earth's carbon cycle. The large phytoplankton (red, yellow) have fastest growth rates and dominate where there are lots of nutrients to nourish them at high latitudes and near the equator where nutrient-rich water upwells from the deep ocean. On the other hand, the small phytoplankton (green, blue) are more competitive where nutrients are very low. Read more about this model here: https://sos.noaa.gov/datasets/phyto