Question

Most aquatic organisms are very sensitive to concentrations of dissolved oxygen. Briefly discuss factors that deplete dissolved oxygen in natural waters, and what processes re-aerate them, considering how fast the aeration process can replenish dissolved oxygen in water.

Answer 1

Oxygen and Biological Productivity :-

Extensive research has been conducted to document the relationship between the biological productivity of a lake and the amount of oxygen in the water.

As a result of this work, there are a few generalizations that can be made. For example, oligotrophic lakes seem to experience relatively small changes in oxygen concentrations over a 24-hour period. This can be attributed to the fact that lakes with low productivity experience less photosynthetic activity and also less respiration (due to the smaller number of aquatic organisms within the waterbody).

On the other end of the spectrum, more productive waterbodies, such as eutrophic and hypereutrophic lakes, have been found to experience large fluctuations in oxygen concentrations over a 24-hour period. This is attributed to the fact that lakes with lots of aquatic plants and animals tend to experience high levels of photosynthetic activity and respiration; there's simply a lot more going on within the system. These waterbodies also happen to have the greatest potential for oxygen-related problems.

Factors that can decrease dissolved oxygen in water :-

Large-scale loss of algae or plants can deplete oxygen in much the same way. This can be a problem in lakes where algicides or herbicides are used to eliminate unwanted vegetation. If algae or aquatic plants die quickly and sink to the bottom, decomposition increases, accelerating oxygen consumption, and a fish kill can result. During warm weather when dissolved-oxygen concentrations are already low, chances of such problems increase. This is why chemical treatments of excess vegetation in a waterbody should be staggered over time; so large amounts of plants do not die all at once. See Section 4 for more information on developing management plans.

Altitude and Dissolved Oxygen :-

Altitude is not really a factor in dissolved oxygen in Florida, as most of the state is barely above sea level. However, for lakes in northern latitudes (and higher altitudes), the amount of oxygen in a lake or waterbody decreases as altitude increases. This is because there is less atmospheric pressure at higher altitudes to push oxygen molecules into the water.