Limiting factors in aquatic life zones are sunlight, water temperature, oxygen, and nutrients. Depending on the amount of sunlight that filters through the water, the varying temperature of the water, and the amount of oxygen in the water; sea life will live or die according to the amount needed for a particular species to survive.
There are two major types of aquatic life zones that support sea life, which are the saltwater and seawater life zones. Aquatic life zones are equivalent to biomes that are located in the water whether it is a pond, lake, river or ocean.
Aquatic zones are divided into layers – lower (abysmal zone), middle (bathyal zone), and upper layers (euphotic zone). Each of these layers or zones will have different levels of sunlight for photosynthesis, different temperatures, different levels of oxygen and nutrients.
Sunlight
Sunlight can only filter through the water at about 30 meters or 100 feet below the water’s surface. Therefore only the upper layer of the aquatic life zone will support photosynthesis. Sea life that depends on photosynthesis such as algae can only survive at this level.
Temperature
The water temperature falls with increasing depth of the water and therefore the water would be coldest in the lower layer. This layer is between 1500 meters and 10,000 meters and it is not only colder but darker without sunlight.
The middle layer is known as the twilight layer and is between 200 and 1500 meters below the surface of the water.
The upper layer is the photosynthesis layer and between zero and 200 meters below the water surface.
Aquatic organisms, due to the water’s inability to readily change temperature, have not evolved with the ability to withstand differing water temperatures. Therefore, small water changes can have a drastic effect on water life.
Oxygen
The amount of oxygen at different ocean depths can vary widely according to temperature; producers that add oxygen; and consumers and aerobic decomposers that remove oxygen.
The producers are the photosynthetic organisms such as algae and plankton. Aerobic decomposers are usually bacteria. Consumers are nonphotosynthetic herbivores that feed on phytoplankton.
Fish and other aquatic organisms will die without the appropriate level of oxygen such as levels below three to five parts per million (ppm). Decreasing levels of oxygen are due to the lack of photosynthesis caused by the decreasing filtering of sunlight in the middle and lower depths.
Nutrients
Nutrients are plentiful in shallow waters of rivers, ponds, streams, and the coastal areas of the ocean. Here, the aquatic life that survives on nutrients or the producers is also plentiful.
The open ocean will often be in short supply of nutrients such as nitrates, phosphates, and iron; but in upwellings, nutrients from the bottom of the ocean may come to the surface for use by producers.
However, there are also pollution nutrients in water that contribute to the death of certain aquatic creatures and aquatic life.
Conclusion
The ocean depths are varying according to sunlight, temperature, oxygen, and nutrients. Therefore different depths of the ocean support different types of aquatic life. Knowledge of ocean depth factors are important to oceanographers and scientists with regard to supporting sea life and working to help create sustainable ocean systems.
Additional reference:
Miller, G. Tyler Jr., Living in the Environment, tenth edition, Wadsworth Publishing Company, 1998.
