Rain dissolves fertilizer in the soil. Runoff carries it away. The fertilizer ends up in bodies of water, from ponds to oceans. The nitrogen is a fertilizer in the water bodies. Since there is a lot of nitrogen it causes algae to grow out of control. Figure below shows a pond covered with algae. Algae may use up so much oxygen in the water that nothing else can grow. Soon, even the algae die out. Decomposers break down the dead tissue and use up all the oxygen in the water. This creates a dead zone. A dead zone is an area in a body of water where nothing grows because there is too little oxygen. There is a large dead zone in the Gulf of Mexico. You can see it Figure below .
Mark's grandparents owned some land in the country and he remembered fishing there when he was young. Mark decided to go visit the place and do some fishing. He was disappointed when he got there and saw the pond covered in algae and there were very few fish. Since he had come all this way he decided to try fishing anyway but didn't catch any fish all day.
If the algae dies will the oxygen increase or decrease?