Coral reefs are the marine ecosystem we are looking at :).
Coral reefs are made from calcium carbonate, which is secreted by coral. Corals are colonies of polyps (the animal secreting calcium carbonate). When a polyp dies, then the coral reef suffers. If all the polyp die, the coral reef dies too. This is called a dead zone as the ecosystem has collapsed. The coral will look white if it is dead otherwise it will be very colourful and bright.
Corals are only found in specific conditions. Most are found near the equator and between the tropics of cancer and capricorn. This is because the water needs to be moderately warm (between 24 and 26 degrees Centigrade) and clean - not much sediment so that the light hits the coral quicky. The coral reef is fully dependant on ligh levels because it needs to photosynthesize to live. Therefore, they need to be quite close to the surface and not too deep (25m or less) to be able to us the light energy for photosynthesis. They also need salty water, which should have a balance to the amount of salt they need in the water to grow.
Unfortunately, human activity is driving coral reefs to very low numbers. The main activities are: overfishing, diving, tourism, waste, pollution, trawl fishing. But there are some natural causes too, which are: temperatures rising, sea levels rising, natural hazards (tsunamis and hurricanes), crown of thorns starfish (these eat the coral). When the coral dies because the light has been blocked, it is called bleaching. Siltation (water mixes with silt, sedement) can cause bleaching because the sediment can form a layer over the coral reef, which would block the light from being recieved by the coral reef. Waste can change the pH of the water, which can damage the polyps in the coral reef. As a result of increasing CO2 emmisions, more is being dissolved into the oceans - which is decreasing the water's pH. It is expected that by 2050, the pH level will drop from 8.2 to 7.9. Bleaching is also caused by the…