Which of the following describes the importance of phytoplankton in aquatic ecosystems?

Phytoplankton play a critical role in aquatic ecosystems primarily as primary producers. They are microscopic organisms that undergo photosynthesis, converting sunlight, carbon dioxide, and nutrients into organic matter. This process not only supports their own growth but also serves as the foundation for the entire aquatic food web. Phytoplankton are consumed by various organisms, including zooplankton, small fish, and other marine fauna. In essence, they sustain higher trophic levels by providing a vital source of energy and nutrients.

While other options describe important aspects related to aquatic ecosystems—such as the filtering of harmful chemicals or the regulation of temperature—these roles are typically associated with different organisms or processes. Phytoplankton's unique function as primary producers underscores their significance in supporting life in both freshwater and marine environments.