The ocean is the world’s largest aquatic biome, and it consists of zones which describe whether there is light to stimulate plant growth which provided nutrients and carbon/oxygen exchange; whether there is depth that increases the pressure and coldness; and whether there is interface with land, either above the surface or in the form of silt, soil and outcroppings beneath the water.
The world’s oceans are considered to be a connected, continuous body of water, with various currents, separation of continents, and locations on the planet to distinguish them. The Atlantic Ocean, for example, separates the Americas from the Eurasian continent, and extends from the cold North through the tropics and equator, and to the Antarctic.
In terms of light, there is daylight or euphotic zone. Then there is the twilight or disphotic zone. Finally, midnight is the aphotic zone, where no sunlight penetrates. In terms of depth there are the intertidal, pelagic, benthic, abyssal, and hadal zones. The hadal zones are the deep trenches of the ocean.
In terms of distance from the land masses, there is the intertidal and the pelagic zone, or open ocean which is farther away from any land.
The open ocean is thus in the pelagic and aphotic zones. The open ocen is generally cold, but is at the mercy of the warm and cold ocean currents that move through the water. The pelagic zone supports surface seaweed, plankton, fish, and mammals such as dolphins and whales.
The start of most of the open ocean food chains are the phytoplankton, algae and other plants which thrive through photosynthesis. Plankton are a widely diverse group of microscopic life forms that feed themselves through sunlight transactions. They are divided into two categories: photosynthetic autotrophs, or phytoplankton and zooplankton.
Phytoplankton are microscopic plants that live in the surface of the water, where the light penetrates most thoroughly. These life forms are single celled and may or may not have a nucleus to the cells. These are truly the start of the open ocean food chain.
Zooplankton are hetertrophic organisms. They must feed on other organisms, such as phytoplankton or organic waste. They can be single celled or very large. Crustaceans, jellyfish, mollusks are zooplankton.
Bacterioplankton are diverse bacterial forms of life that can be either autotrophic or heterotrophic. These are the scavengers and recyclers of the open ocean, as they contribute to the oxygen/nitrogen/carbon and other geochemical cycles.
An excellent discussion of the open oceans zones, invertebrates, fish and mammals is here. The most well known invertebrates in the open ocean are the whales, sharks and dolphins which can feed on plankton, fish and other mammals.
UC Berkeley, Ocean Biome
University Corporation For Academic Research (UCAR)