Eutrophication is the process of excessive algal growth in lakes, ponds, and coastal waters due to the increased availability of one or more limiting factors needed for photosynthesis: sunlight, nutrients, or carbon dioxide. While eutrophication can occur naturally as a result of lake sedimentation, it is exacerbated by manmade nutrient contamination. The acceleration of eutrophication as a result of human activities is known as cultural eutrophication.

In freshwater, algal blooms grow, die, and sink to the bottom. Bacteria then consume the oxygen in the water as they decompose the dead algae, rapidly reducing the dissolved oxygen content and creating hypoxic dead zones. In coastal waters, algal blooms also prevent oxygen from the atmosphere from effectively diffusing into the upper marine layers, further exacerbating the problem.