Phytoplankton are critical in marine food webs as they form the base of the oceanic food chain. These microscopic organisms perform photosynthesis, converting sunlight into energy and producing a significant portion of the planet's oxygen. They are a primary food source for various marine organisms, including zooplankton and small fish, thus supporting larger fish species and marine mammals. The health of phytoplankton populations is essential for maintaining oceanic ecosystems, impacting biodiversity and overall marine productivity.