Introduction

The oceans cover most of the planet - so possibly our planet should be called "Ocean", not "Earth"! The continents and islands divide all this water into 5 major Oceans, called, the Pacific, Atlantic, Indian, Southern and Arctic. However, they are all linked together and form one continuous expanse of water.

Seawater contains over 96% pure water, 3% is common salt and the balance is made up of more than 80 other elements, including gold.

Cold, cold water

The temperature of seawater varies - the surface waters are the warmest (they are warmed by the sun), and the lower you go the colder it gets. The temperature and salinity of water increase the density of water - thus cold and salty water tends to sink under warm, less salty water. This "sinking", together with steady and continuous winds, are the two major forces in causing ocean currents.

Life in the Ocean

The wide variety of plants and animals living in the oceans are adapted to living in one of three regions:

• the sunlight zone - near the surface,

• plenty of light the twilight zone - the mid waters, less light

• the midnight zone - the deep ocean, no light

Apart from these three zones differing in the amount of light reaching them, the temperature, pressure and nutrient supply of each differ as well.

The sunlight zone

This zone supports more than 90% of all marine species. As it is the only zone able to support plant life, it is very important. Plants need light in order to photosynthesize. Photosynthesis is a process which results in the production of food in the form of 0 carbohydrates, protein and fat. Marine plants are important because through the process of photosynthesis they:

• form the foundation of the marine food web (see simplified diagram on opposite page);

• Produce vast quantities of oxygen (oxygen is a by product of photosynthesis);

• produce carbon (carbon is incorporated into food in the process of photosynthesis).

Mid-waters in the twilight zone- the murky

No plants grow in this zone due to the lack of light. Some of the animals such as shrimp and squid have special light producing organs. It is possible that these lights may attract prey, identify Potential mates or surprise predators.

The midnight zone - the deep dark sea

Scientists estimate that only 1% of all ocean species live in this zone- with a temperature that is close to freezing, a total lack of light and the tremendous pressure of water, it is easy to understand why!

Along the coast and close to shore

The area where land and sea meet and interact is known as the coastal zone. Tides, waves and wind are the most important factors that shape the coast and influence life in this area.

Rocky pools, beaches, estuaries and mangroves are a few of the habitats in which a variety of uniquely adapted creatures live.

People and Oceans

The oceans have long been a valuable resource relied upon for recreation, food, minerals (such as gas and oil), water (through desalinization) and, sadly, as a dumping ground for a variety of wastes. Although people have gained a tremendous amount from the oceans and coasts of the world, this has not been without cost.

Problems facing the oceans include pollution, overfishing, coastal erosion, pollution and infilling of estuaries, stripping of rocklife and the development of coastal areas without regard for their unique ecological importance.

Further Reading:

1. The Living Shores of Southern Africa. Margo and George Branch. Struik, Cape Town, 1981.

2. The Gaia Atlas of Planet Management. Norman Myers (ed). Pan Books, London, 1985. 3. Oceans of Life off Southern Africa. A. Payne and R. Crawford (eds). Vlaeberg, Cape Town, 1989.

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Useful Address:

1. Sea Fisheries Institute. Private Bag X2, Roggebaai, 8201

2. SAAMBR. The SA Association for Marine Biological Research, P O Box 10712, Marine Parade, Durban 4056. Tel: 031 373536 Fax: 031 372132

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