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An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in fresh water or Ocean water systems. It is often recognized by the discoloration in the water from the algae's pigments. The term algae encompasses many types of aquatic Photosynthesis organisms, both macroscopic multicellular organisms like seaweed and microscopic unicellular organisms like cyanobacteria. (2025). 9781439867334, CRC Press. ISBN 9781439867334 Algal bloom commonly refers to the rapid growth of microscopic unicellular algae, not macroscopic algae. An example of a macroscopic algal bloom is a kelp forest.
Algal blooms are the result of a nutrient, like nitrogen or phosphorus from various sources (for example fertilizer runoff or other forms of nutrient pollution), entering the aquatic system and causing excessive growth of algae. An algal bloom affects the whole ecosystem.
Consequences range from benign effects, such as feeding of higher trophic levels, to more harmful effects like blocking sunlight from reaching other organisms, causing a depletion of oxygen levels in the water, and, depending on the organism, secreting toxins into the water. Yet, algae also play a crucial role by producing about Oxygen, which supports terrestrial life. Blooms that can injure animals or the ecology, especially those blooms where toxins are secreted by the algae, are usually called "harmful algal blooms" (HAB), and can lead to fish die-offs, cities cutting off water to residents, or states having to close fisheries. The process of the oversupply of nutrients leading to algae growth and oxygen depletion is called eutrophication.
Algal and bacterial blooms have persistently contributed to driven by global warming in the geologic past, such as during the end-Permian extinction driven by Siberian Traps volcanism and during the biotic recovery following the mass extinction (by delaying the recovery).
Blooms are the result of a nutrient needed by the particular algae being introduced to the local aquatic system. This growth-limiting nutrient is typically nitrogen or phosphorus, but can also be iron, vitamins, or amino acids. There are several mechanisms for the addition of these nutrients to the water. In the open ocean and along coastlines, upwelling from both winds and topographical ocean floor features can draw nutrients to the Photic zone, or sunlit zone of the ocean. (2025). 9789231039485, UNESCO. ISBN 9789231039485 Along coastal regions and in freshwater systems, agricultural, city, and sewage runoff can cause algal blooms.
Algal blooms, especially large algal bloom events, can reduce the transparency of the water and can discolor it. The photosynthetic pigments in the algal cells, like chlorophyll and photoprotective pigments, determine the color of the algal bloom. Depending on the organism, its pigments, and the depth in the water column, algal blooms can be green, red, brown, golden, or purple. Bright green blooms in freshwater systems are frequently a result of cyanobacteria (colloquially known as "blue-green algae") such as Microcystis. Blooms may also consist of macroalgal (non-phytoplanktonic) species. These blooms are recognizable by large blades of algae that may wash up onto the shoreline.
Once the nutrient is present in the water, the algae begin to grow at a much faster rate than usual. In a mini bloom, this fast growth benefits the whole ecosystem by providing food and nutrients for other organisms.
Of particular note are the harmful algal blooms (HABs), which are algal bloom events involving toxic or otherwise harmful phytoplankton. Many species can cause harmful algal blooms. For example,
The reduction of phosphorus inputs is required to mitigate blooms that contain cyanobacteria. In lakes that are stratified in the summer, autumn turnover can release substantial quantities of bio-available phosphorus potentially triggering algal blooms as soon as sufficient photosynthetic light is available. Excess nutrients can enter Drainage basin 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 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 decompose. Natural decomposers present in the water begin decomposing the dead algae, consuming dissolved oxygen present in the water during the process. This can result in a sharp decrease in available dissolved oxygen for other aquatic life. Without sufficient dissolved oxygen in the water, animals and plants may die off in large numbers. This may also be known as a dead zone.
Blooms may be observed in freshwater aquariums when fish are overfed and excess nutrients are not absorbed by plants. 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.
Cyanobacteria are able to retain high phosphorus uptake in the absence of nutrients which help their success in oligotrophic environments. Cyanobacteria species such as D. lemmermannii are able to move between the hypolimnion which is rich in nutrients such as phosphates and the nutrient-poor metalimnion which lacks phosphates. This causes phosphates to be brought up to the metalimnion and give organisms an abundance of phosphates, exacerbating the likelihood for algal blooms.
Satellites reveal the location and abundance of phytoplankton by detecting the amount of chlorophyll present in coastal and open waters—the higher the concentration, the larger the bloom. Observations show blooms typically last until late spring or early summer, when nutrient stocks are in decline and predatory zooplankton start to graze. The visualization on the left immediately below uses NASA SeaWiFS data to map bloom populations. Super Blooms NASA Visualization Explorer, 8 May 2012.
The NAAMES study conducted between 2015 and 2019 investigated aspects of phytoplankton dynamics in ocean ecosystems, and how such dynamics influence atmospheric aerosols, clouds, and climate.
In France, citizens are requested to report coloured waters through the project PHENOMER. This helps to understand the occurrence of marine blooms.
can cause phytoplankton blooms via oceanic deposition of wildfire aerosols.
HAB has been proved to be harmful to humans. Humans may be exposed to toxic algae by direct consuming seafood containing toxins, swimming or other activities in water, and breathing tiny droplets in the air that contain toxins. Because human exposure can take place by consuming seafood products that contain the toxins expelled by HAB algae, food-borne diseases are present and can affect the nervous, digestive, respiratory, hepatic, dermatological, and cardiac systems in the body.
Beach users have often experienced upper respiratory diseases, eye and nose irritation, fever, and have often needed medical care in order to be treated. Ciguatera fish poisoning (CFP) is very common from the exposure of algal blooms. Water-borne diseases are also present as our drinking waters can be contaminated by cyanotoxins.
If the HAB event results in a high enough concentration of algae the water may become discoloured or murky, varying in colour from purple to almost pink, normally being red or green. Not all algal blooms are dense enough to cause water discolouration.
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Within control, there are mechanical, biological, chemical, genetic and environmental controls. Mechanical control involves dispersing clay into the water to aggregate with the HAB leading to less of these HAB to go through the process of sedimentation. Biological control varies largely and can be used through pheromones or releasing sterile males to reduce reproduction. Chemical control uses toxic chemical release. However, it may cause problems of mortality of other non targeted organisms. Genetic control involves genetically engineering species in their environmental tolerances and reproduction processes. However, there are problems of harming indigenous organisms. For environmental control, it can use water circulation and aeration.
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HABs have a large effect on the Great Lakes St. Lawrence River Basin. Invasive zebra and quagga mussels are positively correlated with their impact on the environment. These mussels increase the cycling of phosphorus which therefore increases harmful algae blooms in areas they are present. Harmful algae blooms continue to infect water supplies at the Binational Great Lakes Basin and due to the world’s recovery from the Covid-19 Pandemic, solving the issue has become a low priority. This economical problem has become part of politics in the United States, whereas in allied countries such as Canada there is low concern.
The impact of harmful algae blooms on the environment have a substantial effect on marine life. For example, in August 2024 the growth of the toxic algae, Pseudo-nitzschia, along California coasts were making sea lions sick and aggressive to beach goers. Scientists claim this is a seasonal occurrence. The growth of Pseudo-nitzschia leads to the production of a dominic acid which accumulates in fishes such as sardines, anchovies, and squids. This directly affects the food web and the primary food source of sea lions. Once the toxins are transferred via consumption, they can cause seizures, brain damage, and death to the animal. During this surge, people reported bites and unpredictable, aggressive behavior from the infected sea lions. In this sickened state, the sea lions are scared and act out of fear in order to protect themselves. Pregnant sea lions are most vulnerable to toxic algae poisoning and are more likely to die from the effects.
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