Clearly there are many ways by which a surface deficit of water can occur, forcing upwelling! While this is a process that is particularly important to coastal systems and the people who live and depend on the ocean's coastal resources, upwelling can also be a significant factor in the open ocean. Satellites are particularly useful for understanding upwelling in these areas, since they occur where people aren't normally around to notice and monitor them!

Similarly, the level to which upwelling will impact a system is in part dependent on other factors necessary for phytoplankton growth. For example, phytoplankton inhabiting waters which exhibit higher temperatures are often able to respond far more quickly to increased nutrient levels than are phytoplankton located in colder waters. This is due to the fact that the biochemical reactions that make up an organism's metabolism will occur at a faster rate when in warmer conditions until reaching the maximum possible, as famously documented by Eppley in his study relating photosynthesis rate to water temperature:

Light also has an impact, since phytoplankton need light to grow.