Which of the following is considered phytoplankton?

Phytoplankton refers to microscopic plants that float in the water and form a vital component of the aquatic food web. These tiny, often unicellular organisms are capable of photosynthesis and contribute significantly to the primary production in aquatic ecosystems. They serve as a fundamental food source for a variety of marine and freshwater organisms, including zooplankton, small fish, and other aquatic animals.

In contrast, large aquatic plants that are easily visible would not fit the definition of phytoplankton, as they are typically classified as macrophytes or larger vegetative structures. Similarly, filamentous algae that are macroscopic do not qualify as phytoplankton, since they can be seen without a microscope and are often classified as a type of macroalgae. Lastly, organisms found on the surface of water plants are categorized differently, such as epiphytes or other surface-dwelling organisms, which do not share the defining characteristics of phytoplankton. This clarification reinforces why the option highlighting microscopic plants is the correct choice.