Phaeocystis has a high light sensitivity 

The algae Phaeocystis appears after the diatom bloom when diatom growth becomes limited by silica only. If the other macro-nutrients (N and P) are still available there is a fair chance that Phaeocystis, which can reach very high concentrations by colony formation being too large for grazing by meso-zooplankton and which moreover protects the cells against virus infection, will bloom. This, in combination with the fact that Phaeocystis has a high light sensitivity, gives it a competitive advantage which is sufficient for this phytoplankton group to frequently reach high concentrations, especially in turbid coastal regions.