The word pelagic is derived from the Greek work pélagos, meaning Open Ocean. It is the name for oceanic water not in direct contact with a shore or the bottom.
All in all, it is by far the largest aquatic biome terms of volume, but in comparison to many of the other biomes it is a desert.
The pelagic zone is further divided up into vertical sub-zones as seen in the image below. This biome vertically joins with the Deep Sea biome once the illuminated surface layers are passed. For more information on the deeper pelagic waters, please also visit our DEEP SEA [1] biome page.
Seen from space we can see that Earth is truly a water planet. About 71% of Earth's surface is water, and the average depth of the oceans is just under 4,000m (about 13,000ft.) Life on the planet have a few basic requirements to survive. We need energy of some kind, and to most animals that mean that they will have to eat. And to get food, an animal has to be where there IS food, or be able to go where there is food. The same goes with reproduction. Many marine organisms reproduce sexually and need to find a mate to reproduce. Most other biomes described here are in close proximity to land of some kind, which usually helps in both these cases but the pelagic zone is simply defined as waters that are not directly connected to land in any direction, neither horizontally nor vertically. So organisms living in the pelagic zone are definitely faced by some problems to survive - go where there is food and locate a partner to reproduce. In the aquatic world, the clear blue pelagic waters for that reason means somewhat of a water-desert. The biomass out here is much lower than many coastal waters per unit volume, but there is still a lot of organisms that live here.
A lot of the marine fish we eat come from pelagic fisheries. Some commercially important species of fish that are fished are Pacific mackerel, jack mackerel, Pacific sardine and Blue-fin tuna. Unfortunately toady, many fish stocks have been over-exploited and some species, such as many shark species, even face extinction due to over fishing. On top of this many pelagic animals that are not targeted by the fishing boats, such as dolphins and turtles, sometimes also get affected by negative fishing methods.
For more information on the deep zones, including video go to our DEEP SEA [1] biome page.
The epipelagic zone stretches from the surface down to the depth where photosynthesis can no longer occur because of the limited light. In general the epipelagic zone is considered to be between the surface down to about 200m, although since light is absorbed quickly with depth, only a small percentage of the sunlight ever reaches this far down.
Since sunlight is needed for photosynthesis, nearly all primary production of the ocean occurs here. In fact, a great percentage of the oxygen in the atmosphere comes from the primary production out in the open oceans! As a result of this, the epipelagic zone is also where most pelagic animals are found. And if you were animals here, they are often big. Tunas, sharks and large marine mammals such as whales and dophins travel in these waters. We also find planktonic jellyfish and comb jellies.
The photosynthetic organisms here are dominated by phytolankton, diatoms and dinoflagellates that have evolved specialized features to stay in the surface waters and not sink such as air bubbles or small droplets of special oils. Some also have spines that increase their surface area and slows down sink rate.
Clear, well lit open water is also a dangerous place to be in for many organisms when such large predators are around. For this reason many small animals only come up to the epipelagic zone at night and spend the sunlit hours deeper.
A camoflage coloration found on many animals that live in the open ocean is counter-coloration: Light colored undersides and darker backs. Seen from above, a dark back blends in better with the darkness of the depth, but a light colored belly will blend in better with the bright surface when seen from below.
General depth range of the Epipelagic Zone: 0 - 200m
In the mesopelagic zone there is no oonger enough light for photosynthesis, although some light still penetrates down here and provides enough light for hunting if you have good eyes. Many animals do vertikal migrations down to the mesopleagic to hide during the day. Then when the darkness of the night comes, it is safer again to migrate closer to the surface where there is generally more available food.
Many creatures that live in this zone are also transparent, which a good camoflague in this zone where there is barely enough light to see. Some animals here have also developed larger eyes to make best use of the limited light.
Another problem is that at these depths light is not the only thing that is decreased, oxygen concentrations are also very limited. So organisms that live down here have to be able to survive low oxygen levels as well.
Squids, nautilus shells and swordfish are a few species that can be found down here.
General depth range of the Mesopelagic Zone:200 - 1000m
The bathypelagic zone is defined as the zone that goes down and passed the continental slope. The pressure down here is great and only organisms with special adaptations to surive such pressures can live this deep. For example, the swimbladder that we see in many fish at the surface is missing in fish down here. The food source here is limited to the debris of dead material that sinks like snow from the above zones so conserving energy by being still is common. Some fish attracts prey by going fishing... Anglerfish have a small glowing bioluminiscent rod that is attached to its head. Other fish get attracted to the light and becomes a meal for the anglerfish.
The water temperature stays fairly constant down here between about 2-4 degrees C, (about 35-39 deg. F).
General depth range of the bathypelagic Zone: 1000 - 4000m
This zone is usually where the continental slope levels off. More than 30% of the bottom of the ocean is said to be situated here.
General depth range of the Abyssopelagic Zone: 4000 - 6000m
Some parts of the ocean floor has deep trenches that can reach several kilometers deeper than the surrounding ocean floor. Thess zones, which only cover less than 2% of the ocean bottom, are referred to as the hadopelagic zones.
A lot of these trenches are still unexplored and only a few species have so far been observed here. Not many organisms down here would ever survive being brought up to the surface because of the incredible pressure and temperature difference on the way. Very little detritus falls this far down so food is thought to be very limited to these organisms.
General depth of the trenches in the Hadopelagic Zone: 6000 - 11000m The deepest part of the ocean is The Challenger Deep in the Mariana Trench at approximately 11,021 meters (36,160 feet). (Ref. NODC Frequently Asked Questions [2] )
Find out more about pelagic fisheries by the Pelagic Fisheries Research Programme [3].
Do you want to make sure the fish you are eating isn't over-fished? Check out FishWatch [4] to find a list of good choices.
Links:
[1] http://www.untamedscience.com/biology/world-biomes/deep-sea-biome
[2] http://www.nodc.noaa.gov/General/NODC-oceanfaq.html
[3] http://www.soest.hawaii.edu/pfrp/
[4] http://www.nmfs.noaa.gov/fishwatch/