An algal bloom is a rapid increase or accumulation in the population of algae in freshwater or marine water systems, and is recognized by the discoloration in the water from their pigments. Cyanobacteria were mistaken for algae in the past, so cyanobacterial blooms are also called algal blooms. Blooms which can injure animals or the ecology are called harmful algal blooms, since algae is a broad term including organisms of widely varying sizes, growth rates and nutrient requirements, there is no officially recognized threshold level as to what is defined as a bloom. For some species, algae can be considered to be blooming at concentrations reaching millions of cells per milliliter, bright green blooms in freshwater systems are frequently a result of cyanobacteria such as Microcystis. Blooms may also consist of macroalgal species and these blooms are recognizable by large blades of algae that may wash up onto the shoreline. Such blooms often take on a red or brown hue and are known colloquially as red tides, freshwater algal blooms are the result of an excess of nutrients, particularly some phosphates. The excess of nutrients may originate from fertilizers that are applied to land for agricultural or recreational purposes and they may also 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. Presence of residual sodium carbonate acts as catalyst for the algae to bloom by providing dissolved carbon dioxide for enhanced photosynthesis in the presence of nutrients, when phosphates are introduced into water systems, higher concentrations cause increased growth of algae and plants. Algae tend to very quickly under high nutrient availability, but each alga is short-lived. The decay process consumes dissolved oxygen in the water, resulting in hypoxic conditions, without sufficient dissolved oxygen in the water, animals and plants may die off in large numbers. Use of an Olszewski tube can help combat problems with hypolimnetic withdrawal. Blooms may be observed in freshwater aquariums when fish are overfed and these are generally harmful for fish, and the situation can be corrected by changing the water in the tank and then reducing the amount of food given. A harmful algal bloom is a bloom that causes negative impacts to other organisms via production of natural toxins, mechanical damage to other organisms. HABs are often associated with large-scale marine mortality events and have associated with various types of shellfish poisonings. In studies at the population level bloom coverage has been related to the risk of non-alcoholic liver disease death. In the marine environment, single-celled, microscopic, plant-like organisms naturally occur in the surface layer of any body of water. These organisms, referred to as phytoplankton or microalgae, form the base of the food web upon which all other marine organisms depend
Image: Toxic Algae Bloom in Lake Erie
Algal blooms can present problems for ecosystems and human society.
An algae bloom off the southern coast of Devon and Cornwall in England, in 1999