Why does the level of dissolved oxygen go down after sunset?

Answer 1

The level of dissolved oxygen often decreases after sunset due to several factors:

1. Lack of photosynthesis: During daylight hours, photosynthesizing plants and algae produce oxygen as a byproduct. This process increases the level of dissolved oxygen in water. Once the sun sets, photosynthesis ceases, and oxygen production slows down or stops, leading to a decrease in dissolved oxygen levels.

2. Respiration of aquatic organisms: Aquatic organisms, such as fish, insects, and bacteria, continue to respire even after sunset. They consume oxygen for metabolic processes, using up the available dissolved oxygen in the water. As their respiration rates often exceed the oxygen production, the overall oxygen content decreases.

3. Decomposition: Organic matter that enters the water, such as leaves, dead plants, or animal waste, undergoes decomposition. This process is primarily carried out by bacteria, which consume oxygen while breaking down the organic matter. As the decomposition activity increases overnight, oxygen is consumed at a higher rate, leading to a drop in dissolved oxygen levels.

4. Temperature effects: Water temperature can influence the solubility of oxygen. Cooler water can hold more dissolved oxygen compared to warmer water. As night falls, temperatures tend to decrease, causing a decline in the solubility of oxygen, leading to a decrease in dissolved oxygen content.

Overall, the combination of decreased oxygen production, increased oxygen consumption, decomposition activity, and temperature changes contributes to the decrease in dissolved oxygen levels after sunset.