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Animal wastes and farm chemicals are also sources of pollution.
动物粪便和农用化学品也是污染源。
Fertilizers or chemicals farmers use to help crops grow may wash into ponds, where they cause algae to grow quickly.
农民用来帮助作物生长的肥料或化学品可能会被冲入水塘，导致藻类快速生长。
As algae covers the pond it blocks sunlight.
藻类覆盖住池塘，挡住阳光。
This lack of sunlight can kill the plants that animals rely on to survive.
缺乏阳光会杀死动物赖以生存的植物。
How do artificial chemicals affect the ocean?
人造化学物质如何影响海洋？
Because the ocean is so big it's hard to tell what effect chemicals have on it.
因为海洋太大了，很难说化学物质对它的影响。
But we can see their effects on some of the largest bodies of fresh water on earth.
但我们可以看到它们对地球上最大的淡水水体的影响。
For example, high levels of chlorine, a chemical use of bleached paper, have been found in Lake Michigan.
例如，在密歇根湖，人们发现了大量的漂白纸中所含有氯。
Fluorine bromine and other dangerous chemicals have also been found there.
还发现了氟溴和其他危险化学品。
And DDT and PCBs two of the most poisonous and long-lasting substances known continue to threaten aquatic wildlife.
滴滴涕和多氯联苯是已知的两种毒性和持久性最强的物质，它们继续威胁水中的野生动物。
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.
在许多大湖中发现的污染物可能是预警信号，预示着未来海洋会出现的情况。
Pesticides also present a problem.
杀虫剂也是一个问题。
Farmers use pesticides to kill beetles worms and other animals that damage crops.
农民用杀虫剂杀死甲虫、蠕虫和其他破坏农作物的动物。
But the chemicals and pesticides may harm birds and other animals that feed in the fields.
但是这些化学物质和杀虫剂，可能会伤害鸟类和其他在田里觅食的动物。
Because agricultural chemicals are usually sprayed over large areas, it's hard to avoid polluting nearby water.
由于农用化学品通常大面积喷洒，很难避免污染附近的水。
And even low levels of chemicals in the water can become harmful, because they build up over time.
即使是水中含量低的化学物质也会有害，因为它们会随着时间的推移而累积。
But water gets polluted in more direct ways too.
但是水也会以更直接的方式受到污染。
People enjoy lakes and rivers for recreation but they also use them as dumping grounds for industrial wastes.
人们喜欢在湖泊和河流中做各种娱乐活动，但也把它们作为工业废物的倾倒场。
Along the Detroit River stands a series of abandoned industrial sites.
底特律河沿岸矗立着一系列废弃的工业区。
After having spilled PCBs and other pollutants into the river in the 1970s, many industries simply moved leaving the contaminated sites behind.
在上世纪70年代，多氯联苯和其他污染物排入河流后，许多行业搬离了这些受污染的地方。
As the site's continued to disintegrate, the pollutants find their way into the river.
随着的这些地区中的物质不断分解，污染物进入河流。
