摘要：Sandy beaches, no sharks, and the clearest water you've ever seen — Excuse me?

Ah, the Great Lakes.

啊，五大湖。

The perfect place to swim.

游泳的绝佳去处。

Sandy beaches, no sharks, and the clearest water you've ever seen — Excuse me?

有沙滩，没有鲨鱼，还有你见过的最清澈的水 ——什么？

How on Earth could clean water be a bad thing?

干净的水怎么会是坏事呢？

Everybody's like, "Wow, the Great Lakes are so clean. This is amazing." But it's actually a kind of a sign of them being unhealthy.

大家都在说：“哇，五大湖的水好干净啊。这太棒了。”但实际上，这反而是湖水生态不健康的一种表现。

The biggest invasive species issue that we have are, you know, the zebra and quagga mussels are cleaning the water so thoroughly that we've actually, in last couple of years, we've been able to see shipwrecks from aerial perspective straight through the water because it's so clean.

我们面临的最严重的外来物种入侵问题是，斑马贻贝和拟斑马贻贝把水净化得太彻底了。在过去几年里，因为水太清澈，我们甚至能从空中直接看到水下的沉船。

So even though these mussels are native to Ukraine and Russia, contaminated water discharge from ships helped them spread worldwide.

虽然这些贻贝原产于乌克兰和俄罗斯，但船只排放的受污染的水帮助它们扩散到了世界各地。

And since they filter the water around them for food particles, they end up cleaning the water when they eat.

由于它们会过滤周围水中的食物颗粒来觅食，所以进食的同时也就净化了水。

In some cases that can actually be helpful.

在某些情况下，这确实可能是有益的。

What's the problem here, then?

那么问题出在哪里呢？

When a lake like this is perfectly clean, you have sunlight filtering through the water and allowing the growth of algae.

当湖水变得异常清澈时，阳光能够穿透水面，促进藻类的生长。

And the algae grows dramatically because the sunlight is feeding it.

藻类会迅速繁殖，因为阳光为它们提供了养分。

But then it dies en masse, like all of the algae blooms, and then it dies.

但随后，它们会大规模死亡，就像所有的藻类爆发一样，最终走向枯萎。

And when it dies, it sinks to the bottom and uses the oxygen in the surrounding area to die and then starts to, like, put out carbon dioxide into the water.

藻类死亡后，会沉入湖底，在分解的过程中消耗周围水域的氧气，然后开始向水中释放二氧化碳。

So the water becomes toxic.

这样一来，水就变得有毒了。

It becomes anoxic, where there's no oxygen.

水中会出现缺氧的情况，也就是没有氧气。

So no animals can breathe in that type of water.

在这样的水域里，任何动物都无法呼吸。

These parts of the waters that are shallow and sensitive, they end up with these enormous dead zones as a result.

最终，这些浅水区和生态敏感区域会形成大片的 “死亡区”。

"Dead zones." Maybe I'll just head to the pool.

“死亡区。”也许我还是去游泳池好了。

来源：英语东