Phytoplankton play a crucial role in the oceanic food web as they are primary producers that convert sunlight into biomass through photosynthesis. They form the base of the aquatic food chain, providing essential nutrients and energy for a variety of marine organisms, including zooplankton, fish, and larger predators. Phytoplankton also contribute significantly to global oxygen production and carbon dioxide uptake, influencing the Earth's climate system. Changes in phytoplankton populations can therefore have cascading effects throughout the marine ecosystem, affecting biodiversity and fisheries health.