Phytoplankton increase the oxygen content of the water in a pond.

• A. It depends on light levels
• B. They have no effect
• C. True
• D. False

Answer: (C)

Phytoplankton produce oxygen through photosynthesis, using light, carbon dioxide, and water to make sugars and release O2 as a byproduct. In a pond with adequate light, this photosynthetic activity adds dissolved oxygen to the water, supporting fish and other aerobic organisms. The amount of oxygen produced depends on light availability and duration, so during the day the oxygen content typically rises, while at night photosynthesis stops and respiration can use up oxygen. In very large blooms, decomposition after death can later consume oxygen and reduce levels, but under normal daylight conditions the presence of phytoplankton increases the water’s oxygen content, making the statement true.