TOEFL Listening Practice: Lecture18
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|Okay! Today I’d like to talk about an interesting phenomenon on Earth called ocean mixing. Ocean mixing happens when the upper layer of the ocean mixes with the lower layer of the ocean because of the movements of the ocean currents.
In the previous lecture, we learned that winds can vary considerably from week to week, but over longer periods the wind will usually blow more often from one direction than from any other. We also learned that a wind that constantly blows more from one direction is known as a prevailing wind. As the winds sweep across the ocean surface, they drive the ocean surface currents. Over periods of months to years, they set up a global circulation of surface currents, which reflects the patterns of the prevailing winds.
Now the question is how does this mixing occur? You see when the wind blows across the ocean surface, it pushes the water away. The water then rises up from beneath the surface to replace the water that was pushed away. This process is known as “upwelling”. It usually occurs in the open ocean and along coastlines. Now remember cold water has a higher density than warm water. And water gets colder with depth because cold, salty ocean water sinks to the bottom of the ocean basins. Therefore, water that rises to the surface as a result of upwelling is typically colder and is rich in nutrients. These nutrients fertilize surface waters, meaning that these surface waters often have high biological productivity. That’s why good fishing grounds typically are found where upwelling is common. A similar mixing process driven by the tides also occurs when the ocean current moves up and down.
Over the past century, scientists have attributed the driving forces of ocean mixing to the wind and the tides. Both create ocean currents which contribute to the mixing in the ocean, but according to new research, there might be another crucial force in ocean circulation that scientists haven’t accounted for. That is the billions upon billions of small marine animals that live in its depths.
Throngs of tiny organisms called zooplankton inhabit the ocean—everything from microscopic protozoans to krill to jellyfish. Many of these animals live deep underwater during the day to avoid predators and migrate to the surface to feed at night. The new research suggests that the zooplanktons’ daily collective movements may have a profound influence on ocean dynamics by mixing up its waters.
To mimic the zooplanktons’ migration in the ocean, a group of researchers devised an automated laser robot that had the capability of moving blue light through a water tank filled with thousands of brine shrimp. The shrimp known as sea monkeys followed the laser light as it swept from the bottom of the tank to the top. And interestingly, as they swam, they kicked back water behind them.
Individually, a sea monkey’s kick doesn’t move much water, but as the researchers discovered, their collective migration created large eddies. The researchers suspect that when an untold number of zooplankton migrate up and down the ocean every day, they may have a substantial effect on the circulation of the ocean. Many oceanographers are skeptical of this theory, particularly since the zooplanktons’ migration is much harder to measure in the real world than it is to measure such regimented things like the wind and tides. In my opinion, the ocean is much bigger than the tank in the lab so it has billions and billions of them opposed to the tank with only a few thousand of these organisms.