Which of the following is a primary producer in surface water ecosystems?

Phytoplankton are considered primary producers in surface water ecosystems because they perform photosynthesis, converting sunlight into energy and organic matter. This process allows them to produce their own food, which serves as the foundation of the aquatic food web. Phytoplankton, being microscopic plant-like organisms, utilize nutrients from the water to grow and thrive, thereby supporting a range of organisms in the ecosystem, including herbivorous zooplankton and larger fish that feed on them.

In contrast, fish, bacteria, and insects play different roles in the ecosystem. Fish are consumers rather than producers, as they rely on other organisms for food. Bacteria primarily decompose organic matter, recycling nutrients back into the ecosystem, but they do not produce energy through photosynthesis. Insects, while they can be primary consumers or secondary consumers depending on their diets, do not generate their own energy like primary producers do. Thus, phytoplankton's role as a primary producer is essential for energy flow in surface water ecosystems.