Phytoplankton convert energy from sunlight into food for zooplankton. Zooplanton become food for larger animals. Phytoplankton need sunlight and nutrients to grow. In winter nutrients are abundant, but light is limited. In spring more light penetrates surface waters and phytoplankton grow rapidly. In the summer phytoplankton growth stops becauses the sun warms surface waters and the cooler, denser depths are not stirred up and there is no mixing of the layers that could replenish nutrient.