- The largest of all the ecosystems, oceans are very large bodies of water that dominate the Earth’s surface. Like ponds and lakes, the ocean regions are separated into separate zones: intertidal, pelagic, abyssal, and benthic. All four zones have a great diversity of species. Some say that the ocean contains the richest diversity of species even though it contains fewer species than there are on land.
- The intertidal zone is where the ocean meets the land - sometimes it is submerged and at other times exposed, as waves and tides come in and out. Because of this, the communities are constantly changing. On rocky coasts, the zone is stratified vertically. Where only the highest tides reach, there are only a few species of algae and mollusks.
- In those areas usually submerged during high tide, there is a more diverse array of algae and small animals, such as herbivorous snails, crabs, sea stars, and small fishes. At the bottom of the intertidal zone, which is only exposed during the lowest tides, many invertebrates, fishes, and seaweed can be found. The intertidal zone on sandier shores is not as stratified as in the rocky areas. Waves keep mud and sand constantly moving, thus very few algae and plants can establish themselves-the fauna include worms, clams, predatory crustaceans, crabs, and shorebirds.
- The pelagic zone (0 to 200 m) includes those waters further from the land, basically the open ocean. The pelagic zone is generally cold though it is hard to give a general temperature range since, just like ponds and lakes; there is thermal stratification with a constant mixing of warm and cold ocean currents. The flora in the pelagic zone includes surface seaweeds. The fauna include many species of fish and some mammals, such as whales and dolphins. Many feed on the abundant plankton.
- The benthic zone (200 to 1500 m) is the area below the pelagic zone, but does not include the very deepest parts of the ocean. The bottom of the zone consists of sand, slit, and/or dead organisms. Here temperature decreases as depth increases toward the abyssal z one, since light cannot penetrate through the deeper water. Flora are represented primarily by seaweed while the fauna, since it is very nutrient-rich, include all sorts of bacteria, fungi, sponges, sea anemones, worms, sea stars, and fishes.
- The deep ocean is the abyssal zone (> 1550 mt). The water in this region is very cold (around 3° C), highly pressured, high in oxygen content, but low in nutritional content. The abyssal zone supports many species of invertebrates and fishes. Mid-ocean ridges (spreading zones between tectonic plates), often with hydrothermal vents, are found in the abyssal zones along the ocean floors. Chemosynthetic bacteria thrive near these vents because of the large amounts of hydrogen sulfide and other minerals they emit. These bacteria are thus the start of the food web as they are eaten by invertebrates and fishes.
- Ecological classification of marine biota: The organisms living in different zones of the sea are classified ecologically. They are:
- Plankton: Organisms which float or drift in the surface layers of the sea and are passively moved about by winds and currents are known as plankton. It includes protests, plants and animals and most of these are microscopic to minute in size. The plankton constitutes a major source of food for the fishes.
- Nekton: These are animals which move about freely by their own efforts, in open sea. Eg. Fishes, turtles, sea snakes, seals, whales, dolphins etc.
- Neuston:These are organisms which swim of rest on the surface of the sea. Eg. Flying fishes, dolphin fishes etc.
- Benthos:These forms inhabit the bottom of the seas and are called benthic forms
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