About 97% of the world's water is salty and is found in our oceans and seas. But, as we can't drink seawater, how can it be important? Every part of our seas and oceans contains an amazing number of animals and fish that live at different ocean depths. Most of the different species of animals and fish depend on simple plants for their food. These simple plants called algae   drift near the surface of the ocean and use sunlight to turn carbon dioxide and water into food and oxygen. In fact, algae produce over half of the oxygen people breathe. How important seawater is! Each plant or animal in our seas and oceans is an important link in a food chain. The algae are eaten in large amounts by microscopic animals, which are in turn consumed by larger animals. These food chains are delicately balanced. The bad news about the food chains in the oceans is that they are under threat because of man. People once thought that the oceans were so big that it didn't matter if we dumped rubbish into them or caught huge quantities of fish and whales for food. But we now know this is not true and fish stocks in the oceans have started to drop. Thankfully, the world is taking steps to protect the future of our oceans by introducing international agreements to protect marine habitats. Most countries have introduced fishing restrictions to protect fish stocks in the oceans and new techniques are being pioneered to cope with pollution. Finally, the importance of protecting oceans is being made known to more people. This is just the beginning of a long process to protect the oceans for our future. We depend on the oceans for fish which are an important part of the human diet. How important seawater is! People used to think that the rubbish thrown into the sea   _  .
wouldn't harm the fish in the sea would change the balance of the food chain would be broken down in the sea wouldn't do much harm to the sea