Algae are simple, aquatic plants that grow in water. When their population grows rapidly and forms a visible mass or patch on the surface of the water, it is called an algal bloom. These blooms can occur naturally as part of the seasonal cycle of phytoplankton growth but they may also be caused by human activities such as agricultural runoff, sewage discharge and other pollution sources that release nutrients into bodies of water. Algal blooms are not always harmful, in fact some species produce toxins that can kill or stunt the growth of other organisms, while others provide food for filter-feeding animals like zooplankton. However, when they become excessive and persistent, algal blooms can have negative impacts on aquatic ecosystems by depleting oxygen levels in the water, causing fish kills or harming other marine life.