Pelagic zone

The pelagic zone is the vast, open water column of the ocean. It is defined as the area away from the shore where marine life can swim freely in any direction. This zone is not the seafloor, which is known as the benthic zone. Instead, the pelagic zone is the water itself, stretching from the surface down to the very bottom. It covers about 71 percent of the Earth's surface. The mean depth of the ocean is 3,688 meters. However, the maximum depth can reach 10,924 meters.

Conditions in the water column change significantly as you descend. As depth increases, the amount of light and the temperature both decrease. At the same time, the water pressure increases. Other factors like salinity, oxygen, and micronutrients such as iron, magnesium, and calcium also change. These changes create different environments. Scientists divide the water column into up to five vertical layers. This stratification is similar to how the Earth's atmosphere is divided into layers.

The first layer is the epipelagic zone, also called the sunlight zone. This is the only region with enough light for photosynthesis. Photosynthesis is the process where plants use sunlight to make food. Because of this light, nearly all primary production occurs here. About 90 percent of all marine life is concentrated in this top layer. It includes plankton, jellyfish, tuna, whales, sharks, and dolphins. This zone sits just above the continental shelf.

Below the sunlight zone is the mesopelagic zone, or the twilight zone. This layer is located right under the continental shelf. It contains only a trace amount of sunlight. The pressure here ranges from about 20 to 100 atm. A thermocline, which is a layer where temperature changes quickly, acts as the boundary from the warmer top zone. Many creatures here, such as squid and swordfish, have evolved bioluminescence. Bioluminescence is the ability to produce light through chemical reactions in the body.

As you go deeper, you enter the bathypelagic zone, known as the midnight zone. This environment is pitch black with no trace of sunlight. The temperature and salinity in this zone remain stable. Since no plants can live here, many creatures survive on detritus. Detritus is organic matter that falls from upper zones, often called "marine snow." Some animals, like the marine hatchetfish, survive by preying on other inhabitants. The anglerfish is a famous example of a creature that uses bioluminescence in this dark space.

The next layer is the abyssopelagic zone, or the abyssal zone. This zone is near the ocean floor, where tectonic plate movements form volcanoes and mountains. It is characterized by cold temperatures, high pressure, and complete darkness. Only a few specialized creatures live here. These include species of squid, sea spiders, and echinoderms like the sea pig. Many animals at these depths are transparent or have no eyes at all.

The deepest region is the hadopelagic zone, named after Hades. This zone is found in narrow ocean trenches and troughs. These depths can reach more than 10,000 meters. The deepest trenches stretch to 10,924 meters, while average trenches are 5 to 10 kilometers deep. The atmospheric pressure here can reach 1,100 atm. Interestingly, temperature can actually increase here due to adiabatic heating. Very few creatures live in this extreme environment, including some mollusks and amphipods.

The pelagic ecosystem is built upon phytoplankton. These tiny organisms manufacture food through photosynthesis in the sunlit zones. Interestingly, marine biomass pyramids are often inverted at the base. This means the biomass of consumers, like krill and forage fish, is larger than the biomass of primary producers. This happens because phytoplankton grow and reproduce very rapidly. They also sequester about 2 billion tons of carbon dioxide each year. This process helps the ocean act as a sink for carbon dioxide.

Whales play a vital role in moving nutrients through the water. This process is known as the whale pump. Whales feed on krill in deeper levels and return to the surface to breathe. When they defecate, they release liquid rich in nitrogen and iron. This liquid stays at the surface where phytoplankton can consume it. In the Gulf of Maine, the whale pump provides more nitrogen than rivers do.