Plankton are the unicellular organisms that make up the bottom of the marine food chain. Plankton are split into two different categories: phytoplankton, plant-like organisms that photosynthesize, and zooplankton, animal-like organisms. They are considered drifters which means that their movement is dictated by the ocean current and wind that affects the surface of the ocean. Most plankton are very small and microscopes are necessary to see them in detail. You can categorize plankton into three groups by size too: microplankton, macroplankton, and nannoplankton.(2) The reason they are so small is because they are so abundant and would otherwise take up too much space. Also, the smaller size helps keep the plankton from sinking which is especially helpful for the phytoplankton that need the sunlight for energy. Some plankton have also developed spikes and flagellum that help prevent them from sinking quickly.(1)
Sources:
1. "Plankton." Plankton. Accessed January 01, 2017. http://www.whoi.edu/science/B/people/kamaral/plankton.html.
2. The Editors of Encyclopædia Britannica. "Plankton." Encyclopædia Britannica. February 21, 2013. Accessed January 01, 2017. https://www.britannica.com/science/plankton.