In eutrophic waters, what typically happens to dissolved oxygen levels after algal decomposition?

• A. Increase
• B. Decrease
• C. No change
• D. Fluctuate

Answer: (B)

When nutrients are abundant, algae bloom in many waters. After the bloom dies, microbes breakdown the vast amount of algal matter. This decomposition uses a lot of dissolved oxygen through respiration, increasing the biological oxygen demand. If oxygen use outpaces replenishment by mixing and photosynthesis, the dissolved oxygen concentration falls, sometimes becoming very low and stressing or killing aquatic life. Daytime photosynthesis can temporarily raise oxygen, but the overall effect after substantial algal decay is a decrease in dissolved oxygen. So, the typical outcome is a decrease.