What are phytoplankton, and why are they important in aquatic ecosystems?

Phytoplankton are microscopic plants that inhabit the upper layers of oceans, rivers, and lakes. They play a crucial role in aquatic ecosystems as the primary producers, meaning they are capable of photosynthesis, converting sunlight and carbon dioxide into energy and organic matter. This process not only produces oxygen, which is vital for aquatic life, but also forms the foundational base of the aquatic food web.

Phytoplankton are consumed by zooplankton, which in turn are eaten by larger fish and other marine organisms. Thus, they are essential for transferring energy throughout the ecosystem and supporting a diverse range of aquatic life. Their abundance and health can directly influence the productivity and overall health of marine and freshwater ecosystems.

In contrast, the other options do not accurately describe the role of phytoplankton. Predators of aquatic pests do not contribute to the food web in the same foundational way that phytoplankton do. Large mammals living in water, while an important part of some ecosystems, do not provide the same base of the food chain. Fungi contribute to decomposition, but they do not function as primary producers like phytoplankton.