Phytoplankton are microscopic plants that form the foundation of the marine food web. They convert sunlight into energy through photosynthesis, producing oxygen and serving as food for a variety of marine organisms. When phytoplankton populations decline, it disrupts the entire food chain. Zooplankton, which rely on phytoplankton for nourishment, will have less food available, leading to their decline. This, in turn, affects the species that feed on zooplankton, such as small fish, which can lead to a decrease in larger predators like tuna and sharks. The reduction of phytoplankton can also affect nutrient cycling and the overall health of marine ecosystems, highlighting the critical role that these organisms play in oceanic environments.