Phytoplankton are microscopic plants that grow in aquatic ecosystems. Most phytoplankton use photosynthesis to obtain energy and are found in the upper layer of water. They are extremely abundant in the ocean and are the foundation of the marine food web and even affect the Earth's climate. The distribution of phytoplankton in the world's oceans is governed by sunlight, water, and nutrient availability. Diatoms and dinoflagellates are the most common types of phytoplankton.