Primary Production in Aquatic Ecosystems: Light & Nutrient Limitations

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  • 0:00 Underwater Producers
  • 0:30 The Problem of Depth &…
  • 1:39 Nutrient Cycles in Water
  • 2:38 The Sweet Spot of Production
  • 4:05 Saltwater vs. Freshwater
  • 5:10 Lesson Summary
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Lesson Transcript
Instructor: Kevin Newton

Kevin has edited encyclopedias, taught history, and has an MA in Islamic law/finance. He has since founded his own financial advice firm, Newton Analytical.

Production requires sunlight and nutrients, whether its on land or in the water. In this lesson, we look at how producers in aquatic ecosystems acquire both sunlight and nutrients in order to allow life to exist in water.

Underwater Producers

Chances are when you learned about food chains, food webs, and nutrient cycles that you focused mainly on organisms that lived on land. After all, it's pretty simple to follow the chain from acorn to deer to bear. However, the same thing happens under water. Still, there are some challenges to being a producer underwater. In this lesson, we're going to look at how these underwater producers access sunlight and nutrients so the rest of the aquatic ecosystem can flourish.

The Problem of Depth & Sunlight

With few exceptions, land-based producers have at least a decent amount of access to sunlight. Sure, you could argue that rain forests and the shadows of taller structures limit the amount of sunlight that reaches the surface, but the fact is that there is still some amount of sunlight that can sneak through the cracks.

However, the same cannot be said about the bottom of the ocean. As a result, the bottom of the world's oceans is pretty barren. There simply isn't enough light to make it to the bottom. Because of this difficulty, much of the biomass of the world's oceans lives in the upper levels of the seas.

Anyone who has ever snorkeled or dived in places like the Caribbean or the Great Barrier Reef may immediately think of coral reefs. There's no other word for it, these places simply teem with life. However, these are found in the upper level of the water and have something else going for them. The first thing you notice about these places is just how bright they are. There is plenty of sunlight that can make it to the sea floor. So, is sunlight the magic ingredient for underwater life? It's one half of the recipe.

Nutrient Cycles in Water

But we've got to look at the other half of the producer equation: nutrients. The desert gets plenty of sunlight, for example. However, there is a real lack of nutrients, namely water. But back to our discussion of aquatic ecosystems.

Nutrients also move through a cycle. While it's not romantic to think about it, when things die, they fall down. When they fall down, they rot. And when things rot, they return to being nutrients. But where did these things fall? Down. That's right.

In the water, something that has died may float along the surface for a few days, but it will end up sinking below the waves and plummeting to the seabed. Once it gets there, it rots as well. As such, the nutrients of the world's oceans, lakes, and other bodies of water are largely concentrated at the bottom. That means that in places like coral reefs where the water is relatively shallow, plenty of sunlight gets to the area where the nutrients are found - in the seabed.

Finding the Sweet Spot of Production

So we've got a problem. The nutrients of the world's bodies of water, whether it's a lake or the Pacific Ocean, like to sink to the bottom where they are broken down. However, the sunlight that makes life possible is all gathered at the top of the ocean. So how do these nutrients meet so that life can thrive in deeper areas of water where the seabed is farther away from the sunlight?

Luckily for us, water is weird. Most substances become more and more dense as they cool down. Water, on the other hand, is at its densest point at around 4 degrees Celsius. That means anything colder and water expands. This is why ice floats - the most dense parts of the water are actually a little bit warmer than freezing.

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