Algal bloom is a result of …

An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in freshwater or marine water systems. Algal blooms are the result of a nutrient, like nitrogen or phosphorus from fertilizer runoff, entering the aquatic system and causing excessive growth of algae. An algal bloom affects the whole ecosystem; it can have benign results like simply feeding higher tropic levels to more harmful effects like blocking the sunlight from reaching other organisms, causing a depletion of oxygen levels in the water, and, depending on the organism, secreting toxins into the water. The process of the oversupply of nutrients leading to algae growth and oxygen depletion is called eutrophication. Blooms that can injure animals or the ecology are called "harmful algal blooms" (HAB), and can lead to fish die-offs.