Water And Oceanography

A Guide to Ocean Habitats



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Oceans are a vast, mysterious place for many people, a watery part of earth that represents recreation by boat or beach.  Like land masses, however, oceans teem with life and have many different habitats to support the millions of creatures that live within the shallows and depths.  Though some of these habitats have been well-studied and are well-known, others are as mysterious as the surface of the moon.  The following is a basic guide to ocean habitats.

Beaches are one of the most well-known of the ocean habitats.  Though many people picture beaches as flat, white, sandy places to sunbathe, they actually have distinctive characteristics.  A beach’s topography includes offshore bars – what many people refer to as sand bars or shoals, which help protect beaches from erosion; the foreshore, otherwise known as the part of the beach that rises from the water; and a berm further back onshore.  Sand dunes are one type of berm.  Beaches are home to a variety of creatures.  Microscopic animals such as bacteria and diatoms live within the sand.  Turtles, fish, shore birds, and aquatic mammals also call beaches home.

Coasts are another well-known ocean habitat.  Beaches are a kind of coast, but coasts include any kind of shoreline such as rocky coasts and river deltas.  Coasts are divided into two kinds, depending on how they are formed.  Primary coasts are created by non-oceanic processes, such as river deltas and lava flows.  Primary coasts can further be divided into submergent and emergent coasts. 

A submergent coastline is one created by sea level rise whereas an emergent coast results from the land rising from tectonic activity.  Secondary coasts are created by oceanic processes, such as the emergence of barrier islands or coral reefs.  Examples of secondary coasts are mud flats, sea cliffs, and salt marshes.  Coasts are home to many of the same creatures that inhabit beaches – birds, aquatic mammals, turtles and microscopic animals.

Another popular and well-known ocean habitat is the coral reef.  These habitats are found primarily in warm tropical waters, particularly in the Caribbean Sea, Indian Ocean, and South Pacific Ocean.  Coral reefs are living habitats.  The reef is built over hundreds of thousands of years by tiny polyps that live within a limestone cup.  As polyps die, the cups are left behind and other polyps build on top of those cups.

Over time this building creates huge coral reef communities.  The variety of coral found on a reef – brain coral, branch coral, fans – shows the wide variety of polyps that create a coral reef.  Besides polyps, a coral reef is home to algae which helps keep the coral and sand in place.  Fish are also an integral member of a reef community, as are shrimp, crabs, sponges and sea grass.

Estuaries form one of the most crucial of ocean habitats.  An estuary can be a coastal area where fresh and salt water meet, a salt marsh or a mangrove swamp.  Estuaries are formed in a number of ways, some of the most common being river deltas, fjords, and bays.  Fish, crabs, snails and mollusks all call estuaries home.  For some, such as ocean fish, the estuary is an important part of their life cycle as these habitats are where eggs hatch and young fish grow before heading out to more open and unprotected waters.

Sea grass beds are another kind of habitat found in estuaries and calm bays.  Sea grass prefers to grow in warm shallow water along sandy bottom where sunlight easily reaches the grass blades.  Sea grass beds provide shelter, food, and nursery for a variety of sea life.  Diatoms and algae grow directly on the grass blades.  Fish, snails, scallops and horseshoe crabs live with the grass beds. 

Like sea grass, kelp forests are made of plants that grow from the bottom of the ocean and are home to a large array of sea life.  Kelp forests grow in water 18 to 90 feet deep along rocky coastlines.  Kelp is anchored to the ocean bottom by a root-like structure called a holdfast.  The plant grows towards the ocean’s surface.  Most species of kelp have many blades, each with a gas bladder that keeps the plant upright.  Along the ocean’s bottom, kelp is home to sea urchins, eels, sea cucumbers and fish.  The kelp blades also provide shelter and food to many fish species as well as sea otters.

Thus far, all of the ocean habitats discussed have been found along coastlines.  These habitats tend to be the most productive because of their relatively shallow waters where photosynthesis occurs, produces oxygen, and the nutrients needed for life are concentrated.  Compared to these habitats the open ocean is a desert of life.  It does, however, have a number of important habitats.  In general, the open ocean is called the pelagic zone.  It is characterized by not having any contact with land or the ocean bottom.

In the pelagic zone, the most productive habitat is the epipelagic zone, or the ocean’s surface and down to about 650 feet.  The primary inhabitants of this area are diatoms, dinoflagellates, phytoplankton, jelly fish, and ocean-going animals such as tuna, sharks, whales, and dolphins.  At night the epipelagic zone becomes more popular as creatures from the deeper zones migrate vertically to find food in the darkness of night.

Below the epipelagic zone, from 650 feet to about 3,300 feet, is the mesopelagic zone.  Also known as the twilight zone, this area receives little sunlight, certainly not enough for photosynthesis but just enough to hunt.  Creatures that live in this zone are often transparent making them more “invisible” in the water.  They also often have large eyes for better vision to hunt with.  Swordfish, nautilus, and squids are common inhabitants of the mesopelagic zone.

Deeper still, the bathypelagic zone starts around 3,300 feet and descends to around 13,000 feet, the average depth of the Earth’s oceans.  No sunlight penetrates this area.  In fact, the only light ever seen this deep comes from bioluminescence created by this zone’s inhabitants.  Oxygen is scarce this deep as well so fish and other creatures that live here move as little as possible.  Food comes detritus that slowly falls down from the upper zones, such as from dead animals and plants. 

One of the most well-known fish of the bathypelagic zone is the anglerfish.  This fish uses a unique way of hunting.  It has a small bioluminescent rod attached to its head that attracts prey to the anglerfish eliminating the need to hunt down its food.  Other animals found down here are the lanternfish, hatchetfish, giant squid, and dumbo octopus.  Sperm whales also dive down into this zone to hunt giant squid.

One of the most unique and bizarre of ocean habitats is hydrothermal vents.  First discovered in open ocean near the Galapagos Islands, at a depth of 8,000 feet, hydrothermal vents possess a community of organisms that thrive from the warm water that flows from chimneys in the ocean floor.  Hydrothermal vents are found where two ocean plates pull apart and lava seeps into the ocean.  The communities of organisms that survive in these places thrive on the chemicals that seep into the ocean, such as hydrogen sulfide and methane.  Animals that live around hydrothermal vents include chemosynthetic bacteria, and specialized species of fish, crabs, shrimp and tubeworms.

The last of the pelagic zones is sometimes divided into two zones – abyssopelagic (from 13,000 feet to the ocean floor) and hadopelagic (below 19,685 feet).  Characteristics of both of these zones are nearly identical so some scientists call anything below 19,685 feet hadopelagic while others consider only the ocean trenches to be hadopelagic.  Few creatures can survive the cold temperatures and extreme pressures of this area, though there are a few that survive and thrive here.  Marine arthropods such as the sea spider, as well as the swimming cucumber, basket star, and sea pig live at this incredible depths.  Creatures here are often eyeless and transparent.

The world’s oceans may be vast, mysterious and largely unexplored, but they are home to millions of plants and animals that need various conditions in order to survive.  The many different ocean habitats produce organisms that are often uniquely adapted to survive in extreme environments.  Without these habitats the oceans may indeed be a watery desert.  Though most people will never personally experience many of these places, having a basic understanding of the ocean’s habitats makes them less mysterious and a joy to experience.


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