Coral reef bleaching

Coral reefs are in trouble Sea sick
Corals are really communities of tiny animals called polyps. They make bony frameworks, like tiny houses, in which they live. These frameworks build up to make huge coral reefs, home to myriads of colourful organisms. The biggest of these is the Australian Great Barrier Reef. Corals also soak up some of the carbon dioxide humans are pumping into the atmosphere from cars and industry, rather like undersea trees.

danger - poisons Coral polyps cannot survive without tiny plants called algae. The algae live inside the polyp and provide it with food which they can make by photosynthesis from the sun. In turn, the polyp provides a safe haven for its algal friends... until humans' pollution starts to mess things up. This comes into the sea from polluted rivers and makes the algae leave the coral polyps so that the animals turn white and die. This is bleaching and it's caused by several things:

abnormally warm waters (global warming)
polyps get choked with silt from big rivers after heavy rains have swept soil from ploughed or deforested land
pollutants like fertiliser run-off and pesticides from farming
human sewage (which acts as a fertiliser)

Poisonous algae may also grow rapidly creating algal blooms ('red tides') and killing lots of fish. Human pollution also causes tumours in turtles and these wastes can suppress large organisms' immune systems and make them sick like the corals.

[1] Science News, 30/1/99, 72-74