Phytoplankton are incredibly important to the aquatic food chain. They utilize nutrients in the water along with carbon dioxide to reproduce, and in the process, create essential fatty acids (EFAs) critical to the health and development of most marine organisms and people too (Omega 3 fatty acids-fish oils).
The phytoplankton utilize nitrates and phosphates, iron and other trace elements to produce proteins and essential fatty acids. The phytoplankton in turn are consumed by filter feeders and aquatic bugs like copePODS which pass these essential fatty acids on to fish and other predators.