Oceanic Food Webs & Nutrient Productivity

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  • 0:01 Not All Water Is Equal
  • 1:00 Creating & Recycling Nutrients
  • 2:00 Temperature & Currents
  • 4:05 Food Webs & Total Productivity
  • 5:15 Lesson Summary
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Lesson Transcript
Instructor: Kevin Newton

Kevin has edited encyclopedias, taught middle and high school history, and has a master's degree in Islamic law.

In this lesson, we take a look at the nutrient productivity of the world's oceans, as well as the food webs that connect all the organisms that live in them. Afterwards, you can take a brief quiz to check your understanding.

Not All Water Is Equal

Lucky you, you're on vacation! Okay, maybe not, but just stick with me. Think about a nice tropical resort, complete with white sand, clear seas, and massive shrimp cocktails just waiting for your attention and appetite! After all, you're at the beach - doesn't it make sense to eat something caught close by?

However, I've got some bad news. Chances are those shrimp have been flown in from someplace with much cooler water, like the Gulf of Mexico or even the New England coast. The simple fact is that shrimp, and really most seafood and fish we eat, aren't caught in tropical waters. This lesson will explain why.

'But wait,' you say, 'that can't be!' You just had a snorkeling expedition and saw an abandoned ship that was just teeming with life! Surely there must be a giant school of tasty shrimp just begging to be dunked in cocktail sauce. Surely your hotel didn't fly them in from Cajun country. So, how can I be so sure of this?

Creating and Recycling Nutrients

Essentially, life in the oceans is very similar to life on land. Sure, great white sharks don't have lungs, and it's probably a good thing that they don't, but the same general theme of producer, consumer, bigger consumer still holds. In fact, many of the same basic nutrients, like phosphate and nitrogen, are needed for aquatic producers as well. However, it doesn't end there.

Eventually, everything dies. Eventually, everything that dies rots. And, eventually, everything that rots gives nutrients to something new. When things on land die, they tend to die on the ground; that means all those nutrients go to the ground. Now, when things at sea die, they tend to sink; that's one of the reasons that your shipwreck had so much life - it was at the bottom, so there were plenty of nutrients to go around. Coral reefs also have plenty of nutrients since they are on the seabed, and since they are relatively close to the surface, they get plenty of sunlight. However, further out, dead stuff just sinks further.

Temperature and Currents

Here's where there are some pretty big differences between oceans. In the tropics, it's usually warm and sunny and the water is really nice. Now, if you'll remember back to earlier science classes, you know that warmer things tend to float on top of cooler things. Water in oceans is no exception. This is why it was probably a few degrees cooler diving that shipwreck than it was hanging out on the beach, right? I thought so. In the tropics, the water at the top is always warmer than the water down below. That's a problem, since the water at the bottom of the ocean has most of the nutrients in it. Since it stays cool, it rarely comes up to enrich the upper layers of the ocean.

Meanwhile, more temperate zones do have cooler weather. Water is a pretty unique liquid in that it is not at its densest point when it is freezing. In fact, water is at its densest when it's about 4 degrees Celsius. This is why ice floats. For us, that means that when water gets cold on top, it sinks down and pushes up the now warmer water, complete with all those nutrients. Like any producer, the producers of the ocean need two things: nutrients and sunlight. It's actually the temperate areas of the world's oceans that get the sweet spot of enough of both to support big populations.

Think about the plants that exist in the ocean. Most of what we'd easily identify as plants are in that area with plenty of sunlight and plenty of nutrients, especially in shallow water. The sea bed of deeper water, on the other hand, is pretty barren. Instead, the biggest producers of the ocean are among its smallest life forms.

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