The ocean is divided into two main aquatic life zones: the pelagic and the benthic zones. Both of these life zones can sustain life. The vertical dimension of an aquatic system is known as the water column. The physical characteristics comprising a water column include sunlight, temperature, pressure, mineral nutrients and oxygen. The distribution of these elements varies considerably, and the adaptations that marine animals go through determine which kinds of organisms will survive in specific zones of the water column.
The salt water life zones are found in the oceans. Oceans comprise approximately 71% of the Earth’s surface, and are inhabited by more than 240,000 known species of marine animals and plants; however, more than two million marine species may exist in the unexplored depths of the ocean. Oceanographers divide the ocean into distinct zones depending on biological and physical conditions. The distribution of nutrients and oxygen varies considerably, and the variation of these characteristics determines the type of species thriving in a particular ocean zone.
Factors that limit life at different depths in aquatic life zones
The pelagic zone (open-ocean zone) is the illuminated zone in the surface of the sea. In this zone, there is sufficient light to support photosynthesis. Nearly all primary producers are found in the pelagic zone, including algae seaweed and phytoplankton. As a consequence, animal diversity is varied. Animals found in this zone include the shark, dolphin, tuna and many more marine species and mammals. Biodiversity decreases significantly in the deeper zones. Below the pelagic zone, water temperature decreases, water pressure increases, food sources are rare and light becomes dimmer until it finally disappears.
Surface temperatures may vary from warm tropical water at 30 °C (86 °F) to an ice-covered surface in the poles. Oxygen in the surface of the ocean is usually rich in oxygen, and deep ocean water has a high saturation rate of oxygen. The mesopelagic zone at around 500 m (1640 ft.) is not as rich in oxygen. This creates a problem for mid-water species which have managed to survive by implementing behavioral and biochemical adaptations. When organisms die, they sink to the bottom of the ocean where decomposers release their minerals. Ocean currents, climate and geography force deep water to the surface, causing primary production increases with high levels of mineral nutrients.
Pressure increases with depth. Every 10 m (33 ft.) down into the ocean, one atmosphere of pressure is added. For instance, at 100 m (329 ft.) a person would experience 1atm of air pressure plus 10 atm of water pressure; totaling 11 atm. Animals in the ocean perform vertical migrations every day with no harmful effects. To cope with these extreme pressures, deep ocean fish are small with a gelatinous body and minimal skeletal structure. These creatures also lack excess cavities, such as swim bladders and lungs which would collapse under great pressure.
In the photic zone at 200 m (656 ft.), sunlight penetrates enough to support the growth of phytoplankton; however, in the mesopelagic zone lying in between 200-1000 m (656-3289 ft.), light penetration is minimal, thus, preventing the process of photosynthesis. Animals living in this zone include the sword fish, wolffish, squid and some species of mollusks. Many animals thriving in this zone have also developed bioluminescence. Some creatures in this zone tend to migrate to the epipelagic zone during the night to feed.
In the bathypelagic zone situated deep down in the ocean at 4000 m (13,100 ft.), sunlight does not penetrate. Most animals living in this zone feed from the nutrients found in the bodies of dead organisms falling from the zones above. Animals inhabiting this zone include the giant squid, smaller squids and the dumbo octopus. Bioluminescent organisms, such as the lantern fish live in this zone, as well. Sperm whales make deep dives to this zone in search of the giant squid on which they feed. Occasionally, the dead bodies of large animals, including dead sharks or whales descend to this zone.
The abyssopelagic zone lies at 4,000 m (13,100 ft.) just above the ocean floor. Not much marine life is found in this zone due to the cold temperatures, extreme water pressures and lack of light. Among the few species living in this zone are various species of squid, swimming cucumber, basket star, and the sea pig. Marine arthropods, such as the sea spider are found, as well. As a result of the absence of light, many marine animals have developed adaptations, including transparency and the lack of eyes.
The hadopelagic zone comprises the bottom of the ocean and the deepest ocean trenches. Only 2% of this zone has been explored by humans. Darkness, water pressure, cold temperature and food all contribute to limit life in this zone; however, there is biological life thriving around hydrothermal vents. According to marinebio.org, there are a number of factors limiting life along the ocean water column, including pressure, current, temperature, oxygen and nutrients, as well as potential predators.