Low dissolved oxygen in outdoor systems is caused by which condition?

• A. time of day (early AM), algae die off, cloudy water
• B. time of day (afternoon), heavy algae bloom
• C. location of pond
• D. clear water

Answer: (A)

Dissolved oxygen in outdoor systems is controlled by the balance between oxygen produced by photosynthesis and oxygen consumed by respiration and decomposition. During the day, algae and aquatic plants add oxygen to the water, but at night photosynthesis stops while respiration continues, so DO falls and is often lowest just before dawn. If a bloom dies off, the decay uses even more oxygen, driving DO down further. Cloudy water reduces light penetration, limiting photosynthesis and keeping oxygen production low. So a scenario that combines early morning timing (when production is minimal and consumption remains), algal die-off (which increases oxygen demand), and cloudy water (which suppresses production) best explains low dissolved oxygen. By contrast, location or clear water alone don’t inherently cause low DO, and a heavy bloom tends to raise DO during daylight rather than create low DO unless followed by decay and nighttime respiration.