Harmful algal bloom
A harmful algal bloom, or excessive algae growth, sometimes called a red tide in marine environments, is an algal bloom that causes negative impacts to other organisms by production of natural algae-produced toxins, water deoxygenation, mechanical damage to other organisms, or by other means. HABs are sometimes defined as only those algal blooms that produce toxins, and sometimes as any algal bloom that can result in severely lower oxygen levels in natural waters, killing organisms in marine or fresh waters. Blooms can last from a few days to many months. After the bloom dies, the microbes that decompose the dead algae use up more of the oxygen, generating a "dead zone" which can cause fish die-offs. When these zones cover a large area for an extended period of time, neither fish nor plants are able to survive.
It is sometimes unclear what causes specific HABs as their occurrence in some locations appears to be entirely natural, while in others they appear to be a result of human activities. In certain locations there are links to particular drivers like nutrients, but HABs have also been occurring since before humans started to affect the environment. HABs are induced by eutrophication, which is an overabundance of nutrients in the water. The two most common nutrients are fixed nitrogen and phosphate. The excess nutrients are emitted by agriculture, industrial pollution, excessive fertilizer use in urban/suburban areas, and associated urban runoff. Higher water temperature and low circulation also contribute.
HABs can cause significant harm to animals, the environment and economies. They have been increasing in size and frequency worldwide, a fact that many experts attribute to global climate change. The U.S. National Oceanic and Atmospheric Administration predicts more harmful blooms in the Pacific Ocean. Potential remedies include chemical treatment, additional reservoirs, sensors and monitoring devices, reducing nutrient runoff, research and management as well as monitoring and reporting.
Terrestrial runoff, containing fertilizer, sewage and livestock wastes, transports abundant nutrients to the seawater and stimulates bloom events. Natural causes, such as river floods or upwelling of nutrients from the sea floor, often following massive storms, provide nutrients and trigger bloom events as well. Increasing coastal developments and aquaculture also contribute to the occurrence of coastal HABs. Effects of HABs can worsen locally due to wind driven Langmuir circulation and their biological effects.
Description and identification
HABs from cyanobacteria can appear as a foam, scum, or mat on or just below the surface of water and can take on various colors depending on their pigments. Cyanobacteria blooms in freshwater lakes or rivers may appear bright green, often with surface streaks that look like floating paint. Cyanobacterial blooms are a global problem.Most blooms occur in warm waters with excessive nutrients. The harmful effects from such blooms are due to the toxins they produce or from using up oxygen in the water which can lead to fish die-offs. Not all algal blooms produce toxins, however, with some only discoloring water, producing a smelly odor, or adding a bad taste to the water. Unfortunately, it is not possible to tell if a bloom is harmful from just appearances, since sampling and microscopic examination is required. In many cases microscopy is not sufficient to tell the difference between toxic and non-toxic populations. In these cases, tools can be employed to measure the toxin level or to determine if the toxin-production genes are present.
Terminology
In a narrow definition, harmful algal blooms are only those blooms that release toxins that affect other species. On the other hand, any algal bloom can cause dead zones due to low oxygen levels, and could therefore be called "harmful" in that sense. The usage of the term "harmful algal blooms" in the media and scientific literature is varied. In a broader definition, all "organisms and events are considered to be HABs if they negatively impact human health or socioeconomic interests or are detrimental to aquatic systems". A harmful algal bloom is "a societal concept rather than a scientific definition".A similarly broad definition of HABs was adopted by the US Environmental Protection Agency in 2008 who stated that HABs include "potentially toxic species and high-biomass producers that can cause hypoxia and anoxia and indiscriminate mortalities of marine life after reaching dense concentrations, whether or not toxins are produced".
Red tide
Harmful algal bloom in coastal areas are also often referred to as "red tides". The term "red tide" is derived from blooms of any of several species of dinoflagellate, such as Karenia brevis. However, the term is misleading since algal blooms can widely vary in color, and growth of algae is unrelated to the tides. Not all red tides are produced by dinoflagellates. The mixotrophic ciliate Mesodinium rubrum produces non-toxic blooms coloured deep red by chloroplasts it obtains from the algae it eats.File:Karenia brevis Anatomy.png|thumb|Karenia brevis, a dinoflagellate that caused a HAB event in the Gulf of Mexico
As a technical term, it is being replaced in favor of more precise terminology, including the generic term "harmful algal bloom" for harmful species, and "algal bloom" for benign species.
Types
There are three main types of phytoplankton which can form into harmful algal blooms: cyanobacteria, dinoflagellates, and diatoms. All three are made up of microscopic floating organisms which, like plants, can create their own food from sunlight by means of photosynthesis. That ability makes the majority of them an essential part of the food web for small fish and other organisms.Cyanobacteria
Harmful algal blooms in freshwater lakes and rivers, or at estuaries, where rivers flow into the ocean, are caused by cyanobacteria, which are commonly referred to as "blue-green algae", but are in fact prokaryotic bacteria, as opposed to algae which are eukaryotes. Some cyanobacteria, including the widespread genus Microcystis, can produce hazardous cyanotoxins such as microcystins, which are hepatotoxins that harm the liver of mammals. Other types of cyanobacteria can also produce hepatotoxins, as well as neurotoxins, cytotoxins, and endotoxins. Water purification plants may be unable to remove these toxins, leading to increasingly common localised advisories against drinking tap water, as happened in Toledo, Ohio in August 2014.In August 2021, there were 47 lakes confirmed to have algal blooms in New York State alone. In September 2021, Spokane County's Environmental Programs issued a HAB alert for Newman Lake following tests showing potentially harmful toxicity levels for cyanobacteria, while in the same month record-high levels of microcystins were reported leading to an extended 'Do Not Drink' advisory for 280 households at Clear Lake, California's second-largest freshwater lake. Water conditions in Florida, meanwhile, continue to deteriorate under increasing nutrient inflows, causing severe HAB events in both freshwater and marine areas.
HABs also cause harm by blocking the sunlight used by plants and algae to photosynthesise, or by depleting the dissolved oxygen needed by fish and other aquatic animals, which can lead to fish die-offs. When such oxygen-depleted water covers a large area for an extended period of time, it can become hypoxic or even anoxic; these areas are commonly called dead zones. These dead zones can be the result of numerous different factors ranging from natural phenomenon to deliberate human intervention, and are not just limited to large bodies of fresh water as found in the great lakes, but are also prone to bodies of salt water as well.
Dual-stage life systems of algal species
Many of the species that form harmful algae blooms will undergo a dual-stage life system. These species will alternate between a benthic resting stage and a pelagic vegetative state. The benthic resting stage corresponds to when these species are resting near the ocean floor. In this stage, the species cells are waiting for optimal conditions so that they can move towards the surface. These species will then transition from the benthic resting stage into the pelagic vegetative state - where they are more active and found near the water body surface. In the pelagic vegetative state, these cells are able to grow and multiply. It is within the pelagic vegetative state that a bloom is able to occur - as the cells rapidly reproduce and take over the upper regions of the body of water.The transition between these two life stages can have multiple effects on the algae bloom, such as rapid termination of the HAB as cells convert from the pelagic state to the benthic state. Many of the algal species that undergo this dual-stage life cycle are capable of rapid vertical migration. This migration is required for the movement from the benthic area of bodies of water to the pelagic zone. These species require immense amounts of energy as they pass through the various thermoclines, haloclines, and pycnoclines that are associated with the bodies of water in which these cells exist.
Diatoms and dinoflagellates (in marine coastal areas)
The other types of algae are diatoms and dinoflagellates, found primarily in marine environments, such as ocean coastlines or bays, where they can also form algal blooms. Coastal HABs are a natural phenomenon, although in many instances, particularly when they form close to coastlines or in estuaries, it has been shown that they are exacerbated by human-induced eutrophication and / or climate change. They can occur when warmer water, lower salinity, and nutrients reach certain levels, which then stimulates their growth. Most HAB algae are dinoflagellates. They are visible in water at a concentration of 1,000 algae cells/ml, while in dense blooms they can measure over 200,000/ml.Diatoms produce domoic acid, another neurotoxin, which can cause seizures in higher vertebrates and birds as it concentrates up the food chain. Domoic acid readily accumulates in the bodies of shellfish, sardines, and anchovies, which if then eaten by sea lions, otters, cetaceans, birds or people, can affect the nervous system causing serious injury or death. In the summer of 2015, the state governments closed important shellfish fisheries in Washington, Oregon, and California because of high concentrations of domoic acid in shellfish.
In the marine environment, single-celled, microscopic, plant-like organisms naturally occur in the well-lit surface layer of any body of water. These organisms, referred to as phytoplankton or microalgae, form the base of the food web upon which nearly all other marine organisms depend. Of the 5000+ species of marine phytoplankton that exist worldwide, about 2% are known to be harmful or toxic. Blooms of harmful algae can have large and varied impacts on marine ecosystems, depending on the species involved, the environment where they are found, and the mechanism by which they exert negative effects.