The ocean is the largest marine biome. It is a continuous body of saltwater relatively uniform in chemical composition mineral salts and decomposed biological matter.
The ocean is divided into a few zones depending on the distance reached by light in the water. Each zone has a different group of species adapted to the particular biotic and abiotic conditions of that zone. The intertidal zone, that is, the area between high and low tides, is the marine region closest to land. In general, most people consider this part of the ocean to be a sandy beach. In some cases, the intertidal zone is actually a sandy beach, but it can also be rocky or muddy.
The organisms are exposed to the air and the sun at low tide and are submerged most of the time, especially at high tide. Therefore, creatures that live in the intertidal zone are adapted to be long periods of drought. The exoskeletons of coastal crustaceans (such as the shore crab Carcinus maenas) are robust and protect them from desiccation (drought) and wave damage.
Another consequence of pounding waves is that fewer algae and plants settle in the water.
The neritic zone extends from the intertidal zone down to a depth of about 200 m (or 650, ft) at the edge of the continental shelf. Because light can penetrate so deeply, photosynthesis can take place. The water here contains silt and is well oxygenated, has low pressure, and stable temperature. Phytoplankton and floating sargassum (a species of free-swimming algae) provide a habitat for some forms of marine life found in the neritic zone.
Zooplankton, protists, small fish and shrimp are found in the neritic zone and form the basis of the food chain for most of the world’s fisheries.
Beyond the neritic zone lies the area of the open ocean known as the oceanic zone. In the ocean zone, there are thermal stratifications where warmer water and colder water mix due to ocean currents. Abundant plankton serves as the basis of the food chain for big animals like whales and dolphins. Nutrients are scarce and it is a relatively less productive part of the marine biome. When photosynthetic organisms and protists and the animals that feed on them die, their bodies fall to the bottom of the ocean where they remain. Most of the organisms in the aphotic zone are sea cucumbers (Phylum Echinodermata) and other organisms that survive on nutrients contained in the carcasses of organisms in the photic zone.
The deepest part of the ocean is the abyssal zone, which is found at a depth of 4000 m or more. The abyssal zone is very cold and has very high pressure, high oxygen levels, and low nutrient levels. This area has different invertebrates and fish, but there are no plants in the chasm area due to a lack of light.
Fissures in the Earth’s crust, called Hydrothermal Vent, are found primarily in the abyssal zone. Around these springs, chemosynthetic bacteria use the emitted hydrogen sulfide and other minerals as an energy source and serve as the base of the food chain found in ‘s abyssal zone.
Below the water is the benthic zone, which consists of sand, silt, and dead organisms. This is a nutrient-rich part of the ocean. Due to this high nutrient content, there are a wide variety of sponges, sea anemones, sea worms, starfish, fish, and bacteria.
Coral reefs are characterized by high biodiversity and structures created by invertebrates living in warm, shallow waters within the ocean’s photic zone. They are mostly found within 30 degrees north and south of the equator. The Great Barrier Reef is a well-known reef system that lies several kilometers off the northeast coast of Australia.
Coral organisms (members of the phylum Cnidaria) are colonies of saltwater polyps that secrete a skeleton of calcium carbonate. These calcium-rich skeletons slowly accumulate to form the submarine reef have an interrelationship with photosynthetic unicellular algae. The relationship provides corals with most of the nutrients and energy they need.
The waters in which these corals live are nutrient-poor and without this reciprocity, it would not be possible for large corals to grow. Some corals that live in deeper, cooler waters don’t have a mutual relationship with algae; These corals gain energy and nutrients by using stinging cells in their tentacles to capture prey.
More than 4,000 species of fish are estimated to inhabit coral reefs. These fish may feed on coral, other invertebrates, or algae, and are associated with coral.
Global Coral Reef Decline
It takes a long time to build a coral reef. The animals that form coral reefs have evolved over millions of years and continue to slowly deposit the calcium carbonate that forms their distinctive oceanic home. Bathing in warm tropical waters, coral animals and their symbiotic algae partners evolved to survive in the upper limit of ocean water temperature.
Together, climate change and human activities pose a dual threat to the long-term survival of the world’s coral reefs. As global warming increases sea temperatures due to emissions from fossil fuels, coral reefs suffer. Excessive heat causes reefs to expel their symbiotic food-producing algae that give rise to a phenomenon known as bleaching. When bleaching occurs, reefs lose much of their distinctive color as algae and coral die if the loss of symbiotic zooxanthellae is prolonged.
Rising levels of atmospheric carbon dioxide threaten corals in other ways; As CO2 dissolves to % in ocean water, it lowers the pH and increases the acidity of the ocean. As acidity increases, it disrupts the calcification that normally occurs when coral animals build their homes from calcium carbonate. When a coral reef begins to die, biodiversity decreases as animals lose food and shelter.
Coral reefs are also economically important tourist destinations, hence the decline of coral reefs poses a serious threat to the coastal economy. Population growth has also damaged corals in other ways. As the human population on the coast has increased, so has the runoff of sediment and agricultural chemicals, rendering some of the once clear tropical waters murky.
Although a 1–2 °C increase in global temperatures (a conservative scientific forecast) over the next decades may not seem large, it is of great importance for this biome. When changes happen quickly, species can become extinct before evolution leads to new adaptations. Many scientists believe that global warming with its rapid (relative to evolutionary time) and the inexorable rise in temperature, is tipping the scales beyond the point where many of the world’s coral reefs can recover.
Place where the ocean meets freshwater. Estuaries are biomes found where a freshwater source, such as a rivER meets the ocean. Thus, both fresh water and salt water are in the same neighborhood. Dilluted salt wate is called brackish.
Estuaries form sanctuaries where many of the young hatchlings of crustaceans, mollusks and fish begin their life. Salinity is a very important factor affecting organisms and the adaptations of organisms in estuaries. The salinity of estuaries varies and is based on the flow rate of their freshwater sources.
Once or twice a day, high tides bring saltwater into the estuary. Equally, frequent low tides reverse the flow of saltwater. The rapid and short-term fluctuations in salinity due to the mixing of freshwater and saltwater are a difficult physiological challenge for plants and animals that inhabit estuarine rivers.
Many plant species in the estuary are halophytes: plants that can tolerate saline conditions. In some halophytes, filters in the roots remove salt from the water the plant ingests. Other plants can pump oxygen to their roots. Animals such as mussels and clams (Phylum Mollusca) have evolved behavioral adaptations and expend large amounts of energy to function in this rapidly changing environment.
When these animals are exposed to low salinity, they stop eating, close their shells, and switch from aerobic respiration (using gills) to anaerobic respiration (a process that does not require oxygen). When the tide returns to the estuary, the salinity and oxygen content of the water increases, and these animals open their shells, begin feeding, and return to aerobic respiration.