What is the primary cause of air binding in a filter?

The primary cause of air binding in a filter is the reduction of pressure that results from operating under negative head. When a filter operates under negative head, the pressure inside the filter is lower than the atmospheric pressure, which can create conditions conducive to the formation of air pockets or bubbles within the filter media.

These air pockets can obstruct the flow of water, reducing the filter's efficiency and causing operational issues. When the filter media becomes saturated with air, it can no longer effectively trap contaminants, leading to increased bypass flow or even potential damage to the filtration system.

The other choices address potential factors that might contribute to filter performance issues but do not directly cause air binding in the same systemic manner. An increase in water temperature can affect the overall performance of the filter but does not directly result in air binding. Similarly, while algae may release oxygen, this is typically not a significant factor in the air binding phenomenon relative to pressure conditions.

Therefore, the clear connection between negative pressure and the creation of air pockets in the filter explains why this is the correct answer.