The main plants and animals that float near the surface of an aquatic environment are called plankton. Plankton are divided into two main categories: phytoplankton (plant plankton) and zooplankton (animal plankton). As the lowest two levels of the food web, phytoplankton and zooplankton provide other animals with energy. Phytoplankton produces energy through photosynthesis; zooplankton receive that energy when they eat phytoplankton.
Phytoplankton, the microscopic plants that float near the water's surface, are the main source of energy in most aquatic food chains. Larger phytoplankton are classified as either diatoms or dinoflagellates, whereas smaller ones are classified as nanoplankton or picoplankton. Diatoms have a pillbox shape, and they become smaller and smaller with each generation. Dinoflagellates use flagella, or long whip-like structures, to move through their environments. Large amounts of dinoflagellates can cause a red tide, which can be lethal to other organisms.
Zooplankton are the microscopic animals that float near the surface of the water. Zooplankton are usually very weak at swimming, so they just float along with the water currents. Although they float near the surface at night to feed on phytoplankton, they often retreat a bit deeper into the water during the day to hide themselves from predators. There are several categories of zooplankton. For example, all zooplankton can be defined as either holoplankton or meroplankton. Holoplankton, such as diatoms, krill and copepods, remain microscopic for their entire lives, whereas merplankton, such as the larvae of fish, snails and sea stars, grow larger and eventually lose the term zooplankton. They can also be classified by size, with some zooplankton reaching less than 2 micrometers and others exceeding 200 milimeters, or about eight inches.
Phytoplankton, Zooplankton, and the Food Chain
In an aquatic food web, phytoplankton are arguably the most essential organisms. As the plant forms that supply food to other aquatic organisms, they are the source of energy for almost the entire food web. They also regulate carbon levels in the atmosphere. Because zooplankton eat phytoplankton, they are also essential links in the food web of many aquatic environments. Larger animals feed on the zooplankton, who get their energy from the phytoplankton. In this way, the entire food web depends on the organisms who live near the surface of the water.