One of the more colorful and complicated threats to our water supply is algal bloom. When phosphates are introduced into water systems, higher concentrations cause increased growth of algae and plants. Algae tend to grow very quickly under high nutrient availability, but each alga is short-lived, and the result is a high concentration of dead organic matter which starts to decay. The decay process consumes dissolved oxygen in the water, resulting in hypoxic conditions. Without sufficient dissolved oxygen in the water, animals and plants can begin to die off in large numbers.
Algal blooms are generally the result of an excess of nutrients, such as phosphates. Algal blooms can originate from household cleaning products containing phosphorus. These nutrients can then enter watersheds through water runoff. Excess carbon and nitrogen have also been suspected as causes. Another 
source of phosphates are fertilizers that are applied to land for agricultural or recreational purposes. This is likely the case in Lake Erie, which is experiencing an unprecedented algal bloom. In the summer of 2011, western Lake Erie turned a noxious green, as a massive algae bloom coated the surface and lapped up in mats along the shore. In this case, agricultural practices have changed in the past few years, allowing for more fertilizer to run off from fields in the surrounding region and end up in the lake after heavy rainstorms, which increase the amount of total runoff into the lake. The fertilizer is carried into the lake water and begins the vicious algal cycle. Runoff into Lake Erie was made worse by explosive spring storms in 2011, which dropped a lot of rain in a very short period of time. In the lake, above-average temperatures and a lack of strong winds, which prevented the lake from being mixed up, also contributed to the bloom.
Treating algal bloom varies by region and the availability of treatment options. Some of the more common treatment methods include copper (where permitted by law), aluminum, iron, calcium salts, and aluminum sulfate. Aluminum sulfate (alum) is the most commonly used nutrient inactivation chemical for lake projects. Eliminating an algal bloom can be a lengthy, expensive, and time consuming process; when the public water supply is threatened, water treatment operators have their work cut out for them.
