Diatoms: These are microscopic, single-celled algae that are primary producers in aquatic ecosystems. They use photosynthesis to convert sunlight, water, and carbon dioxide into organic matter (food). Crustaceans: These are small aquatic animals like shrimp, crabs, and krill. They are primary consumers in the food chain and feed on diatoms and other phytoplankton (microscopic plants). Herrings: These are small schooling fish that are secondary consumers in the food chain. They feed on crustaceans and other zooplankton (microscopic animals). So, the energy flow starts with diatoms (producers) being eaten by crustaceans (consumers), which are then eaten by herrings (higher-level consumers).