Listen to part of the lecture in the marine biology class.

And the sea is teeming with tiny organisms, but they don't get as much popular attention as say, whales.Microscopic algae just aren't as exciting I suppose.

海洋里满是微生物，但他们得到的关注远不及鲸鱼等生物。也许微藻就没那么令人兴奋吧。

And yet those organisms are the foundation of the bulk of the marine food chain.Without plankton, which is the global term for these tiny organisms, there will be no whales.Plankton is found both in fresh water and marine environments.Again, it's a term we use for any small organisms that float along with the current, either because they are too small or weak to swim against it, or because they don't have any capacity at all to move by themselves.

但是，这些微生物正是绝大部分海洋食物链的基础。浮游生物是这些微生物的统称。没了他们，就不会有鲸鱼。在淡水和海洋中都有浮游生物的存在。毕竟，浮游生物指的就是随着水流漂浮的微生物。它们要么太小太弱，无法对抗水流的力量，要么没有自行移动的能力。

Plants and plant-like plankton are called phytoplankton while animals and animal-like plankton are called zooplankton.For over a century now, researchers have been trying to solve the mystery about zooplankton.You see, some species of zooplankton migrate are...um...not the way birds do when the seasons change. But daily, in the phenomenon we call Diel Vertical Migration or DVM.In the Diel Vertical Migration, sole plankton swim up near the surface of the water during the night and swim down to deeper water during the day.Depending on the species and region, this can be a round trip of between 100 and 400 meters.For a tiny microscopic organism, that's a huge distance.

植物和类植物浮游生物被称为浮游植物，动物和类动物浮游生物被称为浮游动物。一个世纪以来，研究人员们一直试图解开浮游动物的谜团。你看，有些浮游生物，呃，不像鸟类那样按季节迁徙，而是每天迁徙。这种这种现象我们称之为昼夜垂直迁徙，或简称DVM。在昼夜垂直迁徙模式中，浮游生物每天晚上会上游到水面附近，白天则下沉到更深的水域。不同种类的浮游生物在不同地区的迁徙长度不同，全程可达100到400米。这种长度对微生物来说称得上是远了。

Remember now, zooplankton can't swim very well and DVM requires a lot of energy.So, there must be an important benefit to these daily up-and-down commuting.We're not exactly sure what this benefit is.Though there are several compelling theories.I'll talk about them in a moment, but first I want to talk about what we do know or rather what we are pretty sure we know.

记住，浮游生物的游泳能力不佳，因此昼夜垂直迁徙需要耗费很多能量。所以，每天这样上上下下的通勤一定是有极大好处的。目前，我们还不知道好处在哪。但我们有几个有力的理论。接下来我会讲这几个理论，但首先，我想先讲一下我们目前知道什么，或者更确切地说，讲一下我们比较有把握的东西。

So, researchers generally agree that the stimulus for zooplankton DVM is light.Zooplankton tend to swim away from sunlight into deeper water where the sun's rays barely penetrate.At night, when the sun no longer illuminates shallower water, zooplankton head back toward the surface.Now, why would light cause zooplankton to expend all that energy in migrate?

总体来说，研究人员普遍认可浮游动物进行昼夜垂直迁移的刺激因素是光线。浮游生物倾向于避开日光，游向更深的水域。太阳的光线很难触及那里。到了晚上，太阳不再照亮浅水，浮游生物就调头朝水面行进。说到这，为什么光会让浮游生物耗费这么多能量在迁徙上呢？

One popular theory is that zooplankton are hiding during the day from visual predators, uh...those animals that hunt by sight.The darkness provides safety during the day.Then at night after migrating upward, they have an opportunity to feed on phytoplankton that float at the surface.Make sense, doesn't it?

有一个流行理论认为：浮游动物在白天这样做，是为了躲避那些“视觉捕食者”，呃，也就是依赖视觉捕食的动物。在白天，黑暗保护着这些浮游动物。晚上上浮之后，它们就能享用浮在水面的浮游植物。挺有道理的是吧？

But what do we do with the data showing that many kinds of zooplankton don't dive deep enough during the day to become invisible to predators or that others dive deeper than it's necessary to escape hunters' eyes.And that some zooplankton are bioluminescent, which means they have special organs that light up and make them visible even at great depth.

可数据表明，许多浮游动物下沉的深度不足以在白天躲避捕食者的视线，其他一些浮游生物的下潜深度更远超必要。这我们怎么解释呢？同时，一部分浮游生物是生物发光的。这意味着，它们身上特殊的器官所发出的光让他们在极深的水域都可见。

Well, despite all these, we believe predator avoidance is a possible explanation because of studies done in fresh water lakes.It turns out there is a correlation between the presence or absence of vertical migration, and the presence or absence of fish that find their prey by sight.But what are some other possible explanations?

好吧，尽管有这些出入，在淡水湖里开展的研究仍然让我们相信躲避捕食者这一动机可以解释浮游生物的迁徙。研究表明，依赖视觉捕食的鱼的存在与否与浮游生物是否垂直迁徙有关系。但其他的解释是怎么样的呢？

Some researchers suggest that zooplankton migrate to avoid the sun's ultra-violet light.That would explain why some zooplankton are found at such great depth.Visible light may not penetrate very far down, but ultra-violet light can.And we know that some zooplankton have special pigments that protect them from the damage ultra-violet light can cause.That could be why some zooplankton are able to stay closer to the surface during daylight hours.

部分研究人员认为浮游动物迁徙是为了躲避太阳的紫外线。这能解释为什么有些浮游生物会下沉到极深的水域。可见光穿透水的能力有限，但紫外线的穿透能力极强。同时，我们知道，有些浮游动物有特殊色素，可以保护他们免受紫外线的伤害。一部分浮游动物也许就是因此能在白天更加靠近水面。

And there is a third theory.Although it takes a lot of energy for the zooplankton to migrate, they conserve energy while floating in deeper colder water.So, while they're not feeding, they are quietly digesting in cooler water.

同时，还有第三个理论。迁徙固然要花费很多能量，但浮游动物在更深更冷的水中漂浮可以节省能量。因此，当浮游动物不进食的时候，它们都在更冷的水里静静地消化。

But remember, zooplankton consist of any number of different organisms from microscopic worms to crab larvae to tiny fish.And they are found in a large range of marine habitats: cold water, warm water, shallow water, deep water.So, there may be different reasons for different species.

但要记得，浮游动物种类繁多，从微蠕虫到螃蟹幼虫，再到小鱼，不一而足。它们生活的水域也有好几种：冷水域、热水域、浅水域、深水域。因此，不同种类的浮游生物的迁徙原因可能不同。