Question

1. Does it seem likely that any of the seasonal changes noted in Part II, Question, re-oxygenate the bottom waters of the Dead Zone in the autumn and winter? 2. Recall that in the summer the water column in the zone of hypoxia is layered. Figure in Part III shows that the river plume occupied the upper water column. This resulted in a low salinity surface layer, made warm by solar irradiance. Beneath the river plume was the Gulf water. This water had a higher salinity and was cooler. How does temperature and salinity affect the density of water? How does this affect the stability of the water? 3. Let's check your answers with a demonstration. Your instructor will queue up a film clip. Predict what will happen to the water when the barrier is removed from the tank, and explain why. 4. Observe the film clip. Did it confirm your prediction? If not, what did happen and why? 5. To mix a stable water column requires kinetic energy. Can you think of any processes that might supply this energy? Do any of these processes change in intensity with the seasons? 6. What makes the hypoxia disappear in the fall and winter?

Answer 1

Yes, seasonal changes in the fall and winter can re-oxygenate the bottom waters of the Dead Zone. Temperature and salinity affect water density and stability, and processes such as wind can supply kinetic energy for mixing the water column. The hypoxia in the Dead Zone disappears in the fall and winter due to fall turnover.

Step-by-step explanation:

Yes, the seasonal changes of fall and winter can re-oxygenate the bottom waters of the Dead Zone. As air temperatures drop, the temperature of lake water cools, causing fall turnover. This process moves oxygen-rich water from the surface to the bottom of the lake, while nutrients rise to the surface. During winter, decomposers and other organisms use the oxygen at the bottom of the lake. Additionally, the relative transparency of ice allows for the penetration of visible light, enabling photosynthesis by algae.

Temperature and salinity affect the density of water. Cooler water is denser than warmer water, and water with higher salinity is denser than water with lower salinity. These differences in density create layers in lakes and oceans. The stability of the water column is affected by the density differences between the layers. When the lake or ocean is stratified, the bottom layers may become depleted of oxygen because they do not mix with the oxygenated surface water.

The stability of the water column can be altered by processes that supply kinetic energy. Wind is a common process that can mix the water column and supply kinetic energy. Other processes, such as upwelling and currents, can also contribute to mixing. These processes can change in intensity with the seasons, such as stronger winds in the fall and winter.

The hypoxia, or low oxygen levels, in the Dead Zone can disappear in the fall and winter due to fall turnover. During fall turnover, the oxygen-rich water from the surface moves to the bottom of the lake, replenishing the bottom waters with oxygen. Additionally, the colder temperatures during the fall and winter can decrease the metabolic rate of organisms and reduce their oxygen demand.