World Builders™
World Builders™



Session Five  --  Seaweeds
Session Five  --  Seaweeds



The Continental Shelves
The Continental Shelves

The edges of the continents that we live on are under water.

When we read about the structure of the oceans we saw that the continents float on top of the mantle of the earth.  The continents are not entirely above sea level, however,  when you look at the maps, you can see that the margins of the continents are .covered by the oceans.  These water covered parts of our continents are called the continental shelves.  

     A continental shelf goes out to where the water is about 220 yards or 200 meters deep.  The shelves may be narrow, less than 50 miles in width, or they may extend out a long way, perhaps as much as 500 miles.   At the edge of the continental shelf there is a steep drop to much deeper waters, and the cold darkness of the lightless sea.

     These are richly productive areas of the ocean  Rivers bring dissolved minerals and decaying vegetation from the land.  These chemicals help the seaweeds and detritovores to grow and reproduce, so there is lots of food for animals to eat on the continental shelves.

     If you check with the Photic Zone  you can see that light has the potential to penetrate much of the water above the continental shelves.  Light does not penetrate as deeply as possible into the water here because the water contains silt from the land and hosts of  phytoplankton.  There may be oil on the water, and debris from the land, which also reduce the penetration of light.  

    Several of the aquatic biomes that we are studying are located on the continental shelves.  Corals are found at depths above 75 feet, and anchored algae such as kelp may be anchored by their holdfasts as deeply as 100 feet (30 meters).  

     Below the levels with enough light to support algae growth the environment belongs to animals, which depend on the photosynthesizers above them as the base of their food chain.  Some animals go up to feed on the algae, some eat the pieces of algae that float down to them.  

     Of course, the animals eat each other as well.  Have you ever noticed that fishes have dark colored backs and light colored undersides?  This helps them to escape notice from predators.  When the predator looks down, it sees the dark depths and the dark back of the fishes.  When the predator looks up, it sees the light shining into the water and the light colored underside of the fishes above it.  This is a kind of protective camouflage called protective coloration.

    The oceans have not been at their present levels throughout the history of the earth.  Sometimes, when the global climate was colder, and more of the earth's water had been stored as ice, the ocean levels have been lower.  We know this because archeologists who are also divers have found the remains of cities below the water level, with the stone streets and ruins of buildings still intact there.  These sunken cities are near the land, and on the continental shelf.

      During low levels of water in the sea, parts of continental shelves that are submerged today are above water, and so create land bridges, like the one which joined what is now Russia to Alaska about 10,000 years ago.  New animals and also people walked across this land bridge from Asia and spread out through the Americas.  There were also land bridges in other parts of the world.

     Scientists predict that global warming will cause melting of polar, and especially antarctic, ice, which will result in a rise in sea level.  Then low-lying areas that are dry land right now will be flooded.  Parts of Florida and Louisiana, and most of Bangladesh, will be below sea level, and many coastal cities will have to deal with various degrees of flooding.  As you can imagine, rising sea levels will pose many problems for people and societies.

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© 1996,1997, 1998, 1999, 2000, 2002, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at .