The upper part of the sea, specifically the photic or sunlit zone, is home to a diverse range of organisms. Plankton (phytoplankton and zooplankton) are microscopic or small organisms that drift in the water column and form the base of the food web in this zone (phytoplankton are primary producers). Nekton are actively swimming organisms like fish, marine mammals, and squid that inhabit and feed within this zone. Benthos refers to organisms that live on or within the seafloor, which is typically below the upper parts of the ocean.