For which reason must a pool be closed?

A pool must be closed due to the presence of algae because it poses a significant health risk to swimmers. Algae can create slippery surfaces and cloudy water, which can lead to accidents and make it difficult to see submerged objects or individuals in distress. Moreover, some types of algae can release toxins that are harmful to people and can lead to skin irritations, respiratory issues, or even gastroenteritis if ingested. Thus, it is critical to maintain proper water quality and treat the pool as needed to prevent algae growth, ensuring a safe swimming environment for all users.

While debris and high wind warnings can also present challenges, they do not generally pose the same health and safety risks. Temperature below 70°F is a concern for swimmer comfort and operational practices, but it does not justify closing the pool for safety reasons. Therefore, the presence of algae is a compelling reason to close a pool until the issue is resolved.