This video, we're going to talk about seasonal turnover and ocean upwelling. There's important context to understanding these two processes: in lakes and oceans, nutrients have a tendency to continuously sink towards the bottom, leaving the surface waters nutrient-depleted. On the other hand, oxygen gas tends to become depleted at the bottom of lakes and oceans. Seasonal turnover and ocean upwelling are two processes that help to replenish nutrients in surface waters and oxygen gas in deeper waters. Seasonal turnover refers to the vertical mixing of water layers in a lake or an ocean, specifically during the spring and fall seasons.
Seasonal turnover occurs most effectively in parts of the world that experience the four classical seasons - cold winters, hot summers, and transitional spring and fall seasons. It's crucial to remember that the turnover actually happens during the spring and the fall. This mixing of water layers is driven by changes in water density, which are due to seasonal temperature fluctuations. As the water temperature changes, so does the density of the water. When the density of the water is quite similar, the water mixes very well.
However, when the temperature of the water differs, the densities also differ, causing the waters not to mix well. This fundamental concept underpins the turnover process. Let's visualize with an example focusing on lake turnover through the four classical seasons.
Starting with the winter season: unsurprisingly, we have 0 degrees Celsius and frozen ice at the top of the water. The maximum density of fresh water is around 4 degrees Celsius. As you go deeper, there's a temperature gradient, which also means there's a density gradient, preventing the water from mixing well.
When spring arrives, the turnover process can begin. The sun heats all the water to the same temperature, 4 degrees Celsius at the top and bottom, allowing the densities to match and mix well. Now, nutrients from the bottom can resurface at the top, replenishing them, and oxygen at the top can be brought down to replenish the bottom. Perfect.
During the summer, the sun creates a temperature gradient where the surface waters are hotter, creating a drastic change in temperature over a short distance, known as a thermocline. This results in a density gradient, and the waters do not mix well. However, as fall returns, the waters cool to the same temperature, 4 degrees at the top and bottom, enabling effective mixing like in the spring.
The cycle then returns to winter, completing the process. That's spring turnover for you. Now, let's discuss ocean upwelling. Ocean upwelling is the process where deep, cold, nutrient-rich water rises to replace surface waters moving along the coast. Prevailing winds push the surface waters along the coast, giving an opportunity for nutrient-rich waters to rise and replace the water being blown by the wind to the right, also allowing the surface water to bring oxygen down to deeper waters. This concludes our lesson, and I'll see you all in our next video.
