Phytoplankton are the primary source of food in coastal ecosystems, forming the base of the marine food web.

Phytoplankton, microscopic plant-like organisms, are the primary producers in coastal ecosystems. They use sunlight to photosynthesize, converting solar energy into chemical energy, which is then consumed by herbivores like zooplankton. These, in turn, are eaten by larger organisms such as fish, shellfish, and marine mammals. Seaweed, while also a source of food for some marine life, does not form the foundational energy source in the same way as phytoplankton. Shellfish and fish are secondary consumers in the food web.