What determines the diversity of life in the ocean?

Oceanic water contains substances that are essential for life. Living things are found in the ocean at any depth. They exist even at the bottom of the Mariana Trench – the deepest point in the World Ocean – at a depth of 11,000 meters, even where hot magma flows from the depths of the Earth through faults, even where there are high temperatures and tremendous pressure. It is safe to say that life in the ocean is all-pervading.
Life in the ocean is extremely diverse, since its conditions from the poles to the equator, from the surface of water masses to deep ones are very different. In terms of the diversity of plant and animal species, the ocean is comparable to land. The ocean is still full of secrets to this day. When exploring the depths of the sea, organisms unknown to science are found.
According to most scientists, the ocean is the cradle of life on Earth, since all life on our planet came out of the ocean. The development of life in it led to a change in the properties of water masses (salinity, gas content, etc.). For example, the appearance of green plants in the ocean led to an increase in the oxygen content of the water. Oxygen was released from the water into the atmosphere, changing its composition at the same time. The appearance of oxygen in the atmosphere has led to the possibility of the colonization of land by organisms originating from the oceans.

All inhabitants of the World Ocean, according to their habitat conditions, can be combined into 3 groups:
organisms that live on the surface of the ocean and in the water column and do not have active means of transportation;
organisms actively moving in the water column;
bottom-dwelling organisms.

Analysis of living organisms and their habitats suggests that the ocean is inhabited by organisms unevenly. Especially densely populated are coastal areas with depths of up to 200 meters, well illuminated and warmed up by the sun’s rays. On the continental shelf you can see forests and algae meadows – pastures for fish and other inhabitants of the ocean. Far from the coast, large algae are rare, as the sun’s rays hardly penetrate the water column. Plankton reigns here (Greek planktos – wandering). These are plants and animals that are unable to withstand currents that carry them over considerable distances. Most of these organisms are very small, many of them visible only under a microscope. Distinguish between phytoplankton and zooplankton. Phytoplankton are various algae developed in the upper, illuminated layer of water. Zooplankton inhabits the entire water column: these are small crustaceans, numerous protozoa (unicellular animals of microscopic size). Plankton is the staple food of most ocean dwellers. Naturally, the areas rich in them are also rich in fish. Baleen whales can also live here, in the diet of which plankton occupies the main place.
Benthos lives on the bottom of the sea or ocean (Greek benthos – deep). This is a collection of plant and animal organisms that live on the soil or in the soil of the seabed. Benthos includes brown and red algae, molluscs, crustaceans and others. Among them, shrimps, oysters, scallops, lobsters, and crabs are of great commercial importance. Bentos is an excellent food base for walruses, sea otters and some fish species.
The depths of the ocean are sparsely populated, but they are not lifeless. Of course, there are no plants there anymore, but in complete darkness, under great pressure, amazing fish swim in cold water: they have huge toothy mouths, luminous bodies, “lanterns” on their heads. Some of them are blind, others can see poorly in the dark. They feed on the remains of organisms falling from above, or eat each other. The water column is home to many bacteria that live in the deepest water masses. Thanks to their activity, dead organisms decompose, and the elements necessary for the nutrition of living beings are released.