Algal blooms are caused both naturally and unnaturally by an increase in nutrients. The University of Florida notes that nitrogen, phosphorous and light enable algae growth. Nutrient increase can be caused by runoff, especially runoff that contains a high chemical or mineral count. Runoff naturally results from storms and precipitation and is influenced unnaturally by human waste and fertilization systems.
The Environmental Protection Agency remarks that precipitation, hurricanes, tropical storms, drought and temperatures have increased in the world in recent decades. Wet weather affects runoff and erosion, two factors that increase nutrients in bodies of water. Drought precipitates low water levels which, mixed with an elevated amount of sediment, can increase favorable conditions for algae growth. Warmer temperatures further the growth of algae by providing more sunlight and oxygen.
The way human societies live possibly contributes to algal blooms. Activities such as shipping, dams, watering and fertilization of crops and dredging have an impact on algae. The Department of Sustainability and Environment (DSE) in Victoria, Australia, explains that dredging removes material from the bottom of a body of water to "create new or deeper access or berths for vessels" among other things. The sediment at the bottom of the water holds high levels of minerals that algae need for a bloom. Dredging stirs that sediment up for the algae to reach.
Bigelow Laboratory for Ocean Sciences notes that grazers, or aquatic species that eat algae, also affect algal blooms. If naturally balanced, an aquatic region has the correct amount of algae consumers to control the algae population. If there are too few of such creatures, the algae can grow and take over. Over-fishing, non-native species and toxic pollution contribute to a low count of algae consumers.