Which organisms serve as primary consumers in aquatic ecosystems?

Primary consumers in aquatic ecosystems are organisms that feed primarily on primary producers, which are typically plants or phytoplankton. In this context, zooplankton are the correct answer because they are small, often microscopic organisms that graze on phytoplankton and other organic material in the water. This feeding relationship places them in the role of primary consumers, as they convert the energy captured by primary producers into a form that can be utilized by higher trophic levels, such as fish.

Phytoplankton, while crucial to the aquatic food web as primary producers, do not consume other organisms, and zebras, being terrestrial herbivores, are not part of aquatic ecosystems. Fish typically occupy higher trophic levels, as they are often secondary consumers that feed on zooplankton or other smaller organisms. Thus, zooplankton are indeed the organisms that serve as primary consumers in aquatic environments.