第20期:认识细菌(20)

Animal wastes and farm chemicals are also sources of pollution. Fertilizers or chemicals farmers use to help crops grow may wash into ponds, where they cause algae to grow quickly.

As algae covers the pond it blocks sunlight. This lack of sunlight can kill the plants that animals rely on to survive.

How do artificial chemicals affect the ocean? Because the ocean is so big it's hard to tell what effect chemicals have on it.

But we can see their effects on some of the largest bodies of fresh water on earth. For example, high levels of chlorine, a chemical use of bleached paper, have been found in Lake Michigan.

Fluorine bromine and other dangerous chemicals have also been found there. And DDT and PCBs two of the most poisonous and long-lasting substances known continue to threaten aquatic wildlife.

The pollutants found in many of the Great Lakes may be an early warning sign of what to expect in the oceans in the future. Pesticides also present a problem.

Farmers use pesticides to kill beetles worms and other animals that damage crops. But the chemicals and pesticides may harm birds and other animals that feed in the fields.

Because agricultural chemicals are usually sprayed over large areas, it's hard to avoid polluting nearby water. And even low levels of chemicals in the water can become harmful, because they build up over time.

But water gets polluted in more direct ways too. People enjoy lakes and rivers for recreation but they also use them as dumping grounds for industrial wastes.

Along the Detroit River stands a series of abandoned industrial sites. After having spilled PCBs and other pollutants into the river in the 1970s, many industries simply moved leaving the contaminated sites behind.