Ocean Zonation: Biogeography Series (Part 6)

Part 6, in which we visit the Zonation of Life in the Oceans.

While it is a well-known fact, it can never be stressed enough how essential water is to life on this planet. Water has some very unique properties that are it a very special compound. It is the “universal solvent”, able to make available minerals more useful to organisms. Its density changes with temperature, and it has a high boiling point and low freezing point. This allows for a large range of habitable temperatures. Water also has a high specific heat, meaning a lot of energy is required to heat or cool water and so for its form to change. This helps to maintain a homogenous environment.

98% of the water on Earth is found in the oceans, and the oceans make up about 70% of the planet. While this leaves only a small percentage of water available for drinking, it provides an enormous area for biodiversity to flourish. Three main factors affect the distribution of life in the oceans: light, temperature, and oxygen availability. The amount of life available below the ocean’s surface depends on the depth of the water column as well as the turbidity of the water. Temperate and coastal waters tend to exhibit high turbidity while tropical waters and open ocean exhibit mostly low turbidity. Scientists have organized the ocean into particular zones with their own unique characteristics. The boundaries of these zones are readily ignored by the organisms confined to them, but the zones divide the ocean into more or less specific types of environments. Each layer is distinguished by the depth of the water column, the amount of sunlight is receives, and the degree of pressure found there.

Epipelagic Zone This includes the intertidal zone where the ocean and land meet. Communities are constantly changing due to the ebb and flow of the tide and crashing of waves. The biota includes worms, clams, crustaceans, and shorebirds. The bottom of the intertidal zone exhibits more biodiversity, as it is exposed only during the lowest tides. The epipelagic zone is also called the photic zone because it is the part of the ocean which receives enough light for photosynthesis to occur. The temperature here is greater than 10°C. Most of the primary production in the ocean occurs here and has the highest concentration of flora and fauna. This zone extends to a depth of 200 meters.

Mesopelagic Zone Some light enters the mesopelagic zone, but not enough for photosynthesis. The temperature is less than 10°C. At about 500 meters the water becomes depleted of oxygen. Animals here have more efficient gills or they minimize their amount of movement. Swordfish, cuttlefish, and squid live in this zone. Some organisms will travel up to the epipelagic zone to feed.

Bathypelagic Zone After about 1000 meters we enter the bathypelagic zone, where the temperature drops to less than 4°C. It is totally dark in this zone except for bioluminescent organisms. Most animals survive on the detritus that falls from the zones above, but some species are predatory. This zone is home to such organisms as the giant squid and dumbo octopus.

Abyssal Zone At 4000 meters, very few creatures are able to survive in the cold and tremendous pressure found in the abyssal zone. This water is around 3° or 4°C. There is a lot of oxygen here but there is little nutritional content. Mid-ocean ridges and hydrothermal vents are found here. Chemosynthetic bacteria, invertebrates, and some fishes inhabit this zone.

Benthic Zone This zone is the lowest level of the ocean. The benthic region actually begins at the shore line and extends downward, following the continental shelf. The organisms in this zone generally have a very close relationship with the ocean floor substrate. The ocean floor is not flat; it is carved with submarine ridges and dee ocean trenches, forming the halal zone.


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