The movement of water can have a dramatic effect on the organisms that live in it. In fact, take for example the river dolphins that have evolved to sleep in one hemisphere of their brain at a time. It's a totally bizarre phenomenon that allows them to live in these waters with currents. If they were to sleep in both hemispheres simultaneously, they probably get smashed around by the water currents, which could end up being fatal. So, this is an adaptation based on the movement of water. Now the movement of water is also going to affect nutrient availability, and we're going to take a look at that in just a second.

I do want to quickly mention plankton, which are these small floating organisms in water, mainly made up of diatoms, as you can see here, and protozoans as well as crustaceans. And these guys are going to be a very important food source in aquatic biomes. Now speaking of that nutrient availability, the deeper waters tend to be richer in nutrients because all of that dead organic matter is going to sink to the benthic zone, and so that water is going to have more nutrients. Ocean upwelling is this interesting phenomenon where nutrient-rich water from the depths will rise up and replace surface water that's moving away from coasts, thereby adding more nutrients into the system.

Thermoclines are pretty, I guess, you could say bizarre phenomena. They kind of go against our common experience, you know. If we're looking at a body of water we would kind of expect that the temperature would just go down with depth. However, you can sometimes have these distinct layers called thermoclines that will actually separate deeper colder waters from warmer surface waters. And in the thermocline, you're going to have this really abrupt temperature change. Notice here that our y-axis on this chart is depth, and our x-axis is going to be temperature. Notice that in these deeper depths, for example, as we go down, there's very little variation in temperature. Right? It's just like from there to there, a very small variation in temperature. However, up here, you have this rapid drop in temperature. You barely go down in depth at all, and yet you're experiencing a massive temperature change. That is a thermocline. This horizontal line that I've drawn across here is representing our thermocline. It's that band where an abrupt temperature change happens, and it's going to separate these warmer surface waters up here from these colder waters of the depths.

Now, water temperature can actually also lead to nutrient-rich water from the bottom coming up and enriching the water closer to the surface. This is actually something that's going to happen in lakes; it's an event known as lake turnover and it's going to be due to seasonal temperature changes. So in the winter, let's just say this is a smaller body of water, like a smaller lake, it will have ice covering the top. Surprisingly, that's actually going to be colder than the water below. The water below will actually be a little warmer than that surface water. Now, during the spring we'll have our first turnover event because as the ice melts into the water it's going to cool it and that cold water is going to sink. That warmer water that was at the bottom, that nutrient-rich water is going to rise up and move towards the surface. So we're going to have some mixing of nutrients. Now in the summer, you're going to have a much more dramatic temperature gradient through your lake. It's going to be fairly warm at the top and roughly unchanged at the bottom. But what's going to happen during the fall is, as that surface water cools, it's going to sink, move down, and cause some of that nutrient-rich water from the bottom to come up. So this seasonal turnover will actually happen both in fall and spring, and it's going to cause some mixing of the more nutrient-rich water from the bottom with the surface waters at the top, and we're getting this cycling of water due to these temperature changes.

So with that, let's go ahead and flip the page.