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Biosphere as Place
Ocean Life Zones

  Abyssopelagic Life Zone


Ocean Life Zones

Ocean biomes dominate the surface of the earth. They cover 71 percent of Earth's surface. Ocean biomes are called Life Zones. Ocean biomes at the bottom of the sea are called Benthic Biomes. The oceans are very deep. Life Zones are defined by their depth from the surface, so most ocean life zones are layered like a cake according to how much light they receive and how deep they are. Near coasts, intertidal and estaury zones are the interfaces between land and water--ecotones.

As it gets deeper, there is less and less light, and at about 150-200 meters twilight turns into eternal darkness.

Life in the ocean that lives below the sunlight penetration lives in a three dimensional blackness. They can move in any dimension, but have few clues about where because there are zero landmarks, or better, signs to tell them where they are. It is hard for gravity-bound beings like us to imagine free floating all the time. No autotrophs (self-feeders) live here--there is no light to photosynthesize. All organisms here must eat other organic beings. So they hunt, or they wait for something to fall down from the life-rich surface layers. Hunting is very hard, for there are few others here. Many organisms that are here in the 3-D dark are filter feeders such as salps and siphonophores which depend on the 'rain' of small dead organics from above.

With increasing depth, there are fewer animals, both in numbers and diversity. As depth increases, pressure from the weight of the water column increases enormously.

Relative Depth Life Zone
surface to 100-150 m.

Plankton (surface waters) euphotic zone (lighted zone)

surface and land interface

Intertidal (littoral)

surface and river interface


less than 200 meters

Continental Shelf

150 to 1,000 meters

Mesopelagic (twilight top, dark bottom)

1,000 to 3,000 meters

Bathypelagic (deep, cold and dark)

3,000 to 7,000 meters


Plankton Life Zone

Plankton means floating life that moves around by wind and currents.The entire surface of the world-ocean is covered with a thin drifting film of life which is mostly invisible to our eyes.  This film is as much as 150 meters deep--it is  thin only compared to the size of the earth.

The plankton zone is extremely productive because it is bathed in sunlight. In clear waters light reaches down 150 meters. In murky waters light is less.

Almost all ocean food webs begin in the plankton. This is where solar radiation is converted into chemical energy. Phytoplankton are the ocean's primary producers .

 Plankton is made of two main kinds of life.

Phytoplankton is photosynthetic, the green life. They are self-feeders, or autotrophs. Some greens are microscopic algae such as diatoms. Other greens are cyanobacteria, the kind of life that long ago invented photosynthesis (using sun energy to make food energy) and, as a by-product, created our oxygen atmosphere. Phytoplankton depends on nutrients available only in relatively shallow water, such as the continental shelves, where upwellings bring nutrients to the surface waters and cause great planktonic 'blooms'. In open oceans, the average depth is two miles. Nutrients are deep, so they are not available to phytoplankton. Where nutrients from dead organisms collect on a deep bottom, life is scarce; but there are upwellings at sea mounts that become "islands" of life in the virtual pelagic desert .

Filaments of cyanobacteria, oldest photosynthesizer to give Earth oxygen.
Micromonas, a tiny protist of polar waters, one of the smallest eucaryote cells
A dinoflagellate protist, risking only its flagellum--the rest is inside its shell.
Ceratium, a stunning dinoflagellate protist.
assorted diatoms x 40, a bounty of forms: circles. stars,rectangles, filaments, spicules. All protists

Ostreococcus tauri, a protist, the smallest known eurcaryote, with one chloroplast and one flagellum. photo courtesy of Hervé Moreau, Laboratoire Arago


Zooplankton can't feed themselves. They are heterotrophs, they feed on other life , mainly the phytoplankton. Zooplankton are mostly protozoans such as forams, which we can't normally see. Vast numbers of multicellular lives (eucaryotes) such as copepods and krill are visible to our eyes. These tiny lives are joined in their drifting by large quantities of animal spawn, eggs, and just-born organisms for whom the surface layers are the cradle.


An ostracod, ripe with eggs, enlarged. Image courtesy of James Morin krill, zooplankton,
cold water food, enlarged, NOAA
copepod with eggs, zooplankton, a speck to the naked eye. photo by NOAA.

Krill is a vital food source for many animals, especially baleen whales. Krill thrive in arctic and antarctic waters, which are warming. Krill are dying.

There are also animals in the plankton zone that can swim on their own, rather than drift. Many are young fish. Plankton swimmers are called nekton.

Larval codfish just hatched, nekton

bubble nudibranch lives under surface film, nekton
lobed sailor jelly,nekton

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Intertidal Biomes

Intertidal Biomes are sometimes submerged and sometimes in air, "between tides." This is where ocean meets land. Another name for it is the Littoral Zone The intertidal biome is the tidepool region of rocky shores, mudflats, salt marshes, and in warm latitudes, saltwater mangrove thickets. These habitats are nurseries for many kinds of life, places where young animals Like all places where biomes come together, intertidal zones are rich with life.

giant kelp, California

blue sea star
Stilts make a living with long probing beaks. Photo credit: Los Angeles Times
Predators such as cormorants and seals hunt the breakers.
This sea lion colony off the Oregon coast uses tide pools as rich foraging grounds.
Fiddler crab from the mangroves mangrove thicket

Rocky Shores, Sandy Shores


On rocky shores, rocks provide many niches. Each irregularity, each crack, however small, is colonized by larval animals and by macroalgae. Rocks are often covered with red coralline algaes, which tend to be rough-surfaced. These bumpy surfaces are colonized by (or recruit, in science parlance) larval molluks, which then graze on green algae which can overlay and eventually kill the coralline algae that first colonized the rock.

Sandy shores also offer multiple habitats, especially for meiofauna. Meiofauna are minute but not microscopic animals that live in tiny niches, such as those between grains of sand. They include nematode worms, copepods and many others.

Sands are made of fairly uniform-sized grains. Spaces between sand grains are interstices; animals who live there are called interstitial animals. The larger the sand grains, the larger the space available, so larger interstices mean larger animals.  Sands also support burrowing macrofauna, such as clams, snails, crabs, and worms.

Gravel is looser than sand; it’s made of small, water-smoothed stones and pebbles of different sizes. Only larger animals can dig in gravel, but if you’re tiny, you can just slip between the pebbles. Unlike fine sands or muds, gravels can offer flow. Flow means that small suspension feeders who filter food from water passing by can colonize the upper layers of gravel or attach to the tallest stone on the gravel surface.

Estuary Biomes



Estuaries are a special kind of Intertidal Biome where ocean meets a river instead of shore. When the tide comes in salt water is pushed upstream. The result is brackish water when fresh and salt waters are mixed. But what often happens in the absence of turbulence is that a layer of river water (fresh) floats on top of the salt water, which is heavier. Estuaries are often long and form bays where life flourishes. Like other places where life zones merge, estuaries are filled with life from microscopic protists to mollusks to shrimp to fish. All these make favored nest sites for wading birds. Many juvenile animals find a place to grow in estuaries. Human impact on estuaries is large and is growing. We like to locate cities where rivers flow into the sea; as a result, sewage and runoff, especially of heavy metals, are destroying life in estuaries.

A snowy egret fishes in the Moro Bay estuary
marbled godwits hope for a meal in seaweed wrack at Morro Bay estuary
estuary cordgrass
saltmarsh estuary, Britain
A New Zealand saltmarsh estuary

An estuary in Wales,
photo © Dave Newbould

Salt marsh habitats often accompany estuaries. Spartina cordgrasses dominate salt marshes. These marshes are tidal habitats, flushed daily by the tides, and are hugely productive rearing grounds for many fish and invertebrates. Many birds nest and feed in saltmarshes. A rich benthos of salt-tolerant invertebrates flourishes in the muds, many molluscs such as clams and mussels, crustaceans like blue and fiddler crabs, and a host of smaller creatures that are crucial to the lives of fish and the many larval marine animals that are born in the marshes and later ride the tides to the sea.





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Shelf Shallows Life Zone

The continents all have "shelves" of rock that extend out into the ocean up to 200 miles. By shallows we mean up to 200 meters deep. The life zone created at the edges of each landmass on Earth is enormous and filled with life. Solar light and heat penetrate deep into such shallows. Rivers outflow into the shelf zone, bringing mineral riches in solution and organic remains in suspension. Seasonal upwellings bring nutrients up to the plankton as waters warm and ocean currents follow the coasts. Such biomes as kelp forests and coral reefs provide a great flowering of shelf-sea organisms.

This magnificent kelp forest is off the coast of New Zealand.
Photo credit Ian Skipworth

orca pod in Puget Sound
coral reef
Sea otter basking in the kelp forest
A harbor seal imitates a pool toy

King penguins breeding at South Georgia Island
hunt the ocean shelf most of the year.
Photo © Owen Caddy

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Mesopelagic Life Zone

Mesopelagic simply means middle ocean. These ocean midwaters are sometimes called the Twilight Zone, because at its top (150 meters or so) there is still a little light. At the bottom of this zone (1,000 meters) it is entirely and permanently dark.
We know relatively little about the midwaters. We know more about the moon!

The mesopelagic is inhabited by fish and by a strange array of animal forms. Some are siphonophores, gelatinous creatures which have no brain, no heart, many stomachs and no skeleton, but they are very efficient predators. Some siphonophores are longer than a blue whale. At midwater depths life is relatively scarce.


a luminescent jelly
photo© NOAA

a siphonophore
small jellies pulse their bells
through the sea
a large jelly swims down,
trailing tentacles
a huge jelly
a siphonophore
photo © Monterey Bay Aquarium

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Bathypelagic Life Zone

Bathypelagic simply means deep ocean. It names permanently dark waters where there is some life, but not a lot. Animals in these depth eats organic remains that sink from above. They also eat each other./ Predators down this deep can't afford to lose any prey when they attack, so they have adapted to have huge mouths and long teeth. Here's a gulper eel. Notice the eyes.

Many creatures of the deep have also found a way to use light to attract prey. This light is called bioluminescence. It is created by a kind of bacteria that live in symbiosis with the animals. Most animals that use this light have developed muscles that act like shutters so they can turn the light on and off.

an anglerfish with a lighted bait and an overbite

Bathypelagic fermale anglerfish, tiny male attached on top; once a male finds a female and attaches, their blood vessels connect, and he lives as a parasite from then on, conveniently filled with sperm.
Photo credit David Paul and Mark Norman

Anglerfish Lasiognathus amphirhamphus pietschi photo courtesy T. W. Pietsch, U. Washington
This rattail fish sdoesn't chew

The snipe eel is incredibly long

The whip-barbel is longer than the snipe-eel
viperfish has transparent fangs

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Abyssopelagic Life Zone

This is the deepest water of all. It is ocean trenches that reach down 7,000 meters. But life is there. In the depths where we would expect no life, here and there along tectonic plate lines we find ocean vents called black smokers. These places are home to an ecosystem only recently discovered. Here, life is based on chemosynthesis instead of photo-synthesis. Bacteria transform dissolved sulfur into food that supports giant tubeworms and other life forms. There are also some deep-sea worms of beautiful form, and some sea cucumber relatives called seapigs that live on the rain of organic debris that rains down from the surface far above.

a black smoker seafloor vent
giant tubeworms
Microscopic forams from Challenger Deep at 36,000 feet
Photo courtesy JAMSTEC
a seafloor worm of beauty
seapig, a kind of sea cucumber

a basket star at enormous depth



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Explore Further in Biosphere

Biosphere: Introduction
Biosphere as Place: Introduction
Biosphere as Ocean: Life Zones
Biosphere as Ocean Floor: Benthic Biomes One
Biosphere as Ocean Floor: Benthic Biomes Two
Biosphere on Land: Terrestrial Biomes
Biosphere on Land: Anthropogenic Biomes
Biosphere as Process: Introduction
Biosphere Process: Floating Continents, Tectonic Plates
Biosphere Process: Photosynthesis
Biosphere Process: Life Helps Make Earth's Crust
Biosphere Process:
Rock Cycle--Marriage of Water and Rock
Biosphere Process: Marriage of Wind and Water
Biosphere Process: Gas Exchange
Biosphere as An Expression of Spirit
The Ecological Function of Art
The Earth Goddess
The Tree of Life
The Green Man
Earth Art
Biosphere as Community
Biosphere Microcosm: Bacteria and Archaea
The Procaryote Domain
Biosphere Microcosm: Germs
Biosphere Community: The Eucaryote Domain
Biosphere Community: Protists 1: Algae
  Biosphere Community: Protists 2: Protozoa
Biosphere Community: Plants: What's New?
Biosphere Community: Plant Diversity--Major Groups
Biosphere Community: Plant Defense
Biosphere Community: Plant Pollination
Biosphere Community: Plant Seed Dispersal
Biosphere Community: Kingdom Animals
Biosphere Community: Kingdom Fungi
Biosphere Community: Six Great Extinctions
Return to Ecology Index





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