In the beginning there are Phytoplankton.(Algae with chlorophyll)
they provide nourishment for both Zooplankton(organisms drifting in the water)and energy for the biological pump(an important source of Carbon,basically a compost heap on the ocean floor, required for life in general)and Krill(shrimp like crustaceans,and small fry).
Phytoplankton multiply in cold waters as do their predators the Zooplankton and Krill.These make up the largest part of the diet of fish around the globe.krill generally do a diurnal vertical migration, providing food for predators near the surface at night and in deeper waters during the day.
So both herbivores and Carnivores are catered for.