The changes in the phytoplankton crop of the sea have been viewed from a physical stand-point. A coefficient has been developed which is related to the quantity of phytoplankton in the sea. The changes in the coefficient are related with the supply of nutrients and the availability of sunlight. The nutrient and optical conditions required for optimal occurrence of the coefficient have been derived. It is further inferred that the optimal occurrence of the coefficient takes place at relatively great depths for relatively clear waters. In case of turbid waters, the optimum is very close to the sea surface