Phytoplankton receive energy through photosynthesis. Photosynthesis is the process of making glucose using sunlight. Carbon dioxide, sunlight, and water combine, producing glucose sugars that the being can use for energy. . Zooplankton however feed off phytoplankton and decaying matter. They are known as detrivores. . Phytoplankton use photosynthesis to make food; zooplankton get energy from other phytoplankton and decay matter.