Covering about 70 per cent of the earth’s surface ocean contains many communities.
The life extends to all depths of the ocean, although life is much denser around the margins of continents and islands. However, all the oceans are interconnected.
But temperature, salinity, and depths are the chief barrier to free movements of marine organisms. The sea is in continuous circulation because air temperature differences between equator and poles set up strong winds which, together with rotation of the earth, create definite currents.
The sea is dominated by waves of many kinds and by tides. The sea is salty, and salinity and temperature represent the two of the important limiting factors in the sea. The character of the radiation, as well as its intensity varies with depths. Besides the above important features of the sea which are of major ecological interest and greatly influence the marine community, the sea is the most fiercely competitive environment on the earth. For this reason marine organisms lead “eat-or-be-eaten” mode of existence.
Contents
Zonation of Ocean:
Marine habitats are separated into several divisions, the two largest of which are the neritic and the oceanic zones. (Fig. 4.14)
Neritic Zone:
The neritic zone (“near shore”) extends from the high tide mark to the edge of the continental shelf. Thus, it is the shallow water zone on the continental shelf. The biotic communities of the neritic zone consist of the following groups.
1. Producers:
The dominant producers in the neritic zone are passively drifting or floating algae, called phytoplankton, although in some locations the algae attached to the bottom also become important as producers. Most of the algal types included in the phytoplankton are microscopic. The most abundant are the phytoplankton diatoms and dinoflagellates (Fig. 4.15). Other marine algae include many types of variously pigmented forms, e.g., green, brown or red algae. These three groups show a depth distribution in the order named (with red algae deepest). Neritic phytoplankton also undergoes a seasonal density cycle. 
Surrounded on all sides by raw materials and bathed in sunlight, the passively drifting phytoplankton community inhabits a highly favourable, chemically rather stable environment. The death-rate resulting from animal feeding is high, but rapid reproduction sufficiently offsets it. Physical and chemical changes do not affect on algal cell too greatly. In winter, the temperature of surface waters may fell below the freezing point, but the salts of the ocean prevent actual freezing. Cold merely reduces the rate of metabolic processes and algal life continues at a slower pace.
2. Consumers:
The animal consumers belong to the following three types:
(i) Zooplankton:
Living side by side with the photosynthetic phytoplankton in the open waters are the small non-photosynthetic, free swimming, or floating forms. Several different phyla are represented in the zooplankton. Organisms which remain for their entire life cycle in the plankton are called holoplankton, or permanent plankton (Fig. 4.16).
The mature forms of protozoans (green flagellaters, rhizopod forminifera, actinopod radiolaria, and ciliate tintinnids), coelenterates (Pleurobranchia and many siphonophores), certain flat worms (Sagitta), segmented worms (Tomopteris), molluscs (pteropods), arthropods (copepods, calanus), and tunicates (Oikopleura) are most important permanent components of the zooplankton. Other forms are in larvel stages and form a considerable portion of the zooplankton. They are called meroplankton, or temporary plankton (Fig. 4.17) and grow into large organisms that will later become part of either the nekton or the benthos.
(ii) Nekton:
Nekton (Greek for “swimming”) comprises the large free-swimming forms that are capable of changing places at will (Fig. 4.18). Nektons are mostly fish such as sharks, flying fish, herrings, mackerels, as well as many others including numerous varieties of small species. The warm temperate or tropical waters of shallow seas and coastal waters have numerous varieties of coral reef fishes such as butterfly fish (Chaetodon) damsel fish (Abudefduf), parrot fish (Leptoscams).
However, the coastal and shallow sea fauna of temperate regions contains less number of species than that of warm waters. The sea also contains estuarine euryhaline fishes such as mullets, sciaenids and Bombay duck. Intertidal fishes such as pipe fish (Siphostoma), cling fish {Heteroclinus), butter fish (Pholis) and mudskipper (Periophthalmus) also from an interesting group. Nekton also include mammals (seals, porpoises, dolphins, and whales), certain arthropods (larger crustacea), molluscs (squids), and marine birds (penguins, pelicans). 
(iii) Benthos:
Benthos (Greek for “depth”) consists of crawling, creeping (crabs, lobsters, certain copepods, amphipods, other crustaceans, many protozoan’s, snails, echinoderms, some bivalves, and some crustaceans), and sessile organisms (sponges, barnacles, mussels, oysters, corals, hydroids, bryozoans, and some worms) along the sides and the bottom of the ocean basin. The sessile and creeping forms are often grouped as epifauna and the burrowing forms as infauna. The epifauna refers to the organisms living on the surface, either attached or moving freely on the surface, infauna refers to the organisms that dig into the substrate or construct tubes or burrows.
(iv) Decomposers:
The decomposers of the neritic zone are largely bacteria. According to Zobell (1963), the density of bacteria in sea water ranges from less than one per litre in the open ocean to a maximum of 10 per ml inshore.
Oceanic Zone:
The region of the open sea beyond the continental shelf is designated as the oceanic zone. The oceanic zone is divided vertically into three or sometimes four parts which are as follows:
(а) Euphotic Zone:
The euphotic zone (i.e., the “producing” region) consists of the waters into which sunlight penetrates, and extends down about 180 m below die surface. The producers of the euphotic zone are the phytoplankton; and consumers the zooplankton and nekton.
(b) Bathyal Zone:
The bathyal zone extends from the bottom of the euphotic zone to a depth of about 1800 m. It does not receive light.
(c) Abyssal Zone:
The abyssal zone includes all the sea below the bathyal. The area of the ocean ”deeps”, or the abyssal region, may lie anywhere from 1800 m to 4500 m. Like the bathyal zone, abyssal zone also receives no light, and they both form the aphotic zone.
(d) Hadal Zone:
The term hadal zone is used to designate the perpetually cold and dark supreme depths of the oceanic trenches. These lower regions have been explored very little, although we know they contain life in the form of bizarre fish and arthropods that have successfully adapted to the great pressures of the lightless habitat.