A planktivore is an aquatic organism that feeds on planktonic food, including zooplankton and phytoplankton. Phytoplankton are usually photosynthetic one-celled plant organisms. These organisms are usually found near the surface of the water due to their need for light energy for their photosynthetic processes. Phytoplankton provide most of the oxygen that is in the water and provide a large amount of food for other organisms in the water column. Zooplankton, in contrast, are heterotrophic plankton, animals which ingest nutrients rather than producing it via chemical reactions. Some taxa are planktonic for only part of their life cycle.