In a pond ecosystem, removal of phytoplankton would most directly affect ____________.

The correct option is Primary consumers like zooplankton.

In a pond ecosystem, phytoplankton (microscopic algae and plants) serve as the primary producers. They perform photosynthesis to convert solar energy into chemical energy, forming the foundation of the aquatic food web.

Primary consumers, such as zooplankton, feed directly on these producers. If phytoplankton are removed from the ecosystem, the primary consumers lose their immediate food and energy source, leading to a direct and rapid decline in their population.

Other components of the ecosystem are affected less directly: secondary consumers (like fish-eating birds) are affected indirectly as the disruption moves up the food chain, decomposers rely on dead organic matter which decreases over time, and abiotic components are non-living chemical and physical factors that do not depend on phytoplankton for their existence.