The objective of Arctic Bloom is to understand how light controls the diversity of phytoplankton blooms in the Arctic Ocean and the dominance of two major microalgal groups: the diatoms and the Prymnesiophytes (Phaeocystis sp.). Two complementary aspects will be studied: (1) the physiological status (growth, photosynthesis, etc.) of uni-algal cultures and co-cultures under different light climates in lab-controlled conditions, (2) the potential succession of diatoms and Prymnesiophytes using in situ mesocosms.