Phytoplankton are the primary producers of the aquatic ecosystem, which means they are the base of the food chain. They convert the energy from the sun into organic matter through photosynthesis. As a result, they form the foundation of the entire aquatic food web.
Large fish, including many commercially important species, are higher up in the food chain and are often predators of smaller fish and zooplankton that eat phytoplankton. The phytoplankton serve as the primary food source for zooplankton, which in turn are eaten by small fish, and the small fish are eaten by larger fish.
Therefore, the growth of phytoplankton in shallower banks due to increased sunlight penetration can lead to an increase in the population of zooplankton and small fish, which in turn can support the growth of larger fish. This is why shallow banks that have higher levels of phytoplankton often have rich stocks of large fish.