An algal bloom is a rapid increase in the population of algae in water bodies, often due to excess nutrients like nitrogen and phosphorus. These nutrients can come from sources such as agricultural runoff, wastewater, and fertilizers. When conditions are right, algae can grow quickly, leading to dense green, blue, or red patches on the water's surface. While some algal blooms are harmless, others can produce toxins that are harmful to aquatic life, animals, and humans. These toxic blooms can lead to hypoxia, a condition where oxygen levels in the water drop, causing fish and other organisms to suffocate.