The (typically) sudden and strong bloom of phytoplankton in the spring in temperate and sub-polar oceans. In the winter, the ocean waters are mixed, i.e., the water is circulated from the bottom to the top of the ocean because the water is relatively cold (and thereby have the same density) throughout the water column. In the early spring, the upper water layers therefore have enough nutrients (circulated up from bottom waters) but phytoplankton are unable to thrive because they are circulated down to depths where there is not enough light for them to survive. However, as the ocean becomes warmer in the spring, the warm water will tend to stay at the top, stabilizing the water. At this time, the phytoplankton are kept in waters with enough lights and with abundant nutrients, and their population numbers explode. However, the phytoplankton use up the available nutrients during a relatively short time (a few weeks to a few months), and their numbers dwindle in summer.