What term is used to describe plankton consisting of algae found in bodies of water?

Phytoplankton refers specifically to the microscopic algae found floating in the upper layers of water bodies. These organisms are essential components of aquatic ecosystems because they perform photosynthesis, providing oxygen and serving as the primary food source for a variety of aquatic organisms, including zooplankton and larger fish. The term "phytoplankton" is derived from the Greek words "phyto," meaning plant, and "plankton," meaning drift or wanderer.

In contrast, other terms in the options relate to different types of aquatic life. Zooplankton are small, often microscopic animals that feed on phytoplankton. Planktonic bacteria refer to bacteria that are also suspended in water but are not classified as algae, and macroalgae refers to larger, multicellular algae, such as seaweeds, which are not classified under the term plankton since they are typically attached to substrates rather than floating freely. Therefore, the term that correctly describes the specific group of plankton comprising algae is phytoplankton.