An algal bloom is a rapid increase
or accumulation in the population of algae (typically microscopic) in an
aquatic system. Algal blooms may occur in freshwater as well as marine
environments. Typically, only one or a small number of phytoplankton species
are involved, and some blooms may be recognized by discoloration of the water
resulting from the high density of pigmented cells. Although there is no
officially recognized threshold level, algae can be considered to be blooming
at concentrations of hundreds to thousands of cells per milliliter, depending
on the severity. Algal bloom concentrations may reach millions of cells per
milliliter. Algal blooms are often green, but they can also be other colors
such as yellow-brown or red, depending on the species of algae.
Bright green blooms are 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.
Of particular note are harmful
algal blooms (HABs), which are algal bloom events involving toxic or otherwise
harmful phytoplankton such as dinoflagellates of the genus Alexandrium and
Karenia, or diatoms of the genus Pseudo-nitzschia. Such blooms often take on a
red or brown hue and are known colloquially as red tides.
Freshwater algal blooms
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. 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 photo synthesis 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 decay. 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.
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.
Harmful algal blooms
An algae bloom off the southern
coast of Devon and Cornwall in England, in 1999
Satellite image of phytoplankton
swirling around the Swedish island of Gotland in the Baltic Sea, in 2005
A harmful algal bloom (HAB) is an
algal bloom that causes negative impacts to other organisms via production of
natural toxins, mechanical damage to other organisms, or by other means. HABs
are often associated with large-scale marine mortality events and have been
associated with various types of shellfish poisonings.
Background
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.
Harmful algal blooms have been
observed to cause adverse effects to varying species of marine mammals and sea
turtles, with each presenting specific toxicity-induced reductions in
developmental, immunological, neurological, and reproductive capacities. A mass
mortality event of 107 bottlenose dolphins occurred along the Florida panhandle
in the spring of 2004 due to ingestion of contaminated menhaden with high levels
of brevetoxin. Manatee mortalities have also been attributed to brevetoxin but
unlike dolphins, the main toxin vector was endemic seagrass species (Thalassia
testudinum) in which high concentrations of brevetoxins were detected and
subsequently found as a main component of the stomach contents of manatees.
Additional marine mammal species,
like the highly endangered North Atlantic Right Whale, have been exposed to
neurotoxins by preying on highly contaminated zooplankton. With the summertime habitat
of this species overlapping with seasonal blooms of the toxic dinoflagellate
Alexandrium fundyense, and subsequent copepod grazing, foraging right whales
will ingest large concentrations of these contaminated copepods. Ingestion of
such contaminated prey can affect respiratory capabilities, feeding behavior,
and ultimately the reproductive condition of the population]
Immune system responses have been
affected by brevetoxin exposure in another critically endangered species, the
Loggerhead sea turtle. Brevetoxin exposure, via inhalation of aerosolized
toxins and ingestion of contaminated prey, can have clinical signs of increased
lethargy and muscle weakness in loggerhead sea turtles causing these animals to
wash ashore in a decreased metabolic state with increases of immune system responses
upon blood analysis. Examples of common harmful effects of HABs include:
the production of neurotoxins which cause mass mortalities in fish,
seabirds, sea turtles, and marine mammals
human illness or death via consumption of seafood contaminated by toxic
algae
mechanical damage to other organisms, such as disruption of epithelial
gill tissues in fish, resulting in asphyxiation
oxygen depletion of the water column (hypoxia or anoxia) from cellular
respiration and bacterial degradation
Due to their negative economic and
health impacts, HABs are often carefully monitored.
HABs occur in many regions of the
world, and in the United States are recurring phenomena in multiple geographical
regions. The Gulf of Maine frequently experiences blooms of the dinoflagellate
Alexandrium fundyense, an organism that produces saxitoxin, the neurotoxin
responsible for paralytic shellfish poisoning. The well-known "Florida red
tide" that occurs in the Gulf of Mexico is a HAB caused by Karenia brevis,
another dinoflagellate which produces brevetoxin, the neurotoxin responsible
for neurotoxic shellfish poisoning. California coastal waters also experience
seasonal blooms of Pseudo-nitzschia, a diatom known to produce domoic acid, the
neurotoxin responsible for amnesic shellfish poisoning. Off the west coast of
South Africa, HABs caused by Alexandrium catanella occur every spring. These
blooms of organisms cause severe disruptions in fisheries of these waters as
the toxins in the phytoplankton cause filter-feeding shellfish in affected
waters to become poisonous for human consumption.
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.
Red tides
A red tide
Red tide is a term often used to
describe HABs in marine coastal areas, as the dinoflagellate species involved
in HABs are often red or brown, and tint the sea water to a reddish color. Red
tides can also be caused by bioluminescent dinoflagellates like Noctiluca
scintillans. The more correct and preferred term in use is harmful algal bloom,
because:
these blooms are not associated with tides
not all algal blooms cause reddish discoloration of water
not all algal blooms are harmful, even those involving red discolouration
Causes of HABs
It is unclear what causes HABs;
their occurrence in some locations appears to be entirely natural,while in
others they appear to be a result of human activities. Furthermore, there are
many different species of algae that can form HABs, each with different
environmental requirements for optimal growth. The frequency and severity of
HABs in some parts of the world have been linked to increased nutrient loading
from human activities. In other areas, HABs are a predictable seasonal
occurrence resulting from coastal upwelling, a natural result of the movement
of certain ocean currents. The growth of marine phytoplankton (both non-toxic
and toxic) is generally limited by the availability of nitrates and phosphates,
which can be abundant in coastal upwelling zones as well as in agricultural
run-off. The type of nitrates and phosphates available in the system are also a
factor, since phytoplankton can grow at different rates depending on the
relative abundance of these substances (e.g. ammonia, urea, nitrate ion). A
variety of other nutrient sources can also play an important role in affecting
algal bloom formation, including iron, silica or carbon. Coastal water
pollution produced by humans and systematic increase in sea water temperature
have also been suggested as possible contributing factors in HABs. Other
factors such as iron-rich dust influx from large desert areas such as the
Sahara are thought to play a role in causing HABs. Some algal blooms on the
Pacific coast have also been linked to natural occurrences of large-scale climatic
oscillations such as El Niño events. While HABs in the Gulf of Mexico have been
occurring since the time of early explorers such as Cabeza de Vaca, it is
unclear what initiates these blooms and how large a role anthropogenic and
natural factors play in their development. It is also unclear whether the
apparent increase in frequency and severity of HABs in various parts of the
world is in fact a real increase or is due to increased observation effort and
advances in species identification technology.
Researching Solutions
The decline of filter-feeding
shellfish populations, such as oysters, likely contribute to HAB occurrence. As
such, numerous research projects are assessing the potential of restored
shellfish populations to reduce HAB occurrence.
Since many Algal blooms are caused
by a major influx of nutrient-rich runoff into a water body, programs to treat
wastewater, reduce the overuse of fertilizers in agriculture and reducing the
bulk flow of runoff can be effective for reducing severe algal blooms at river
mouths, estuaries, and the ocean directly in front of the river's mouth.
Notable occurrences
Lingulodinium polyedrum produces brilliant displays of bioluminescence
in warm coastal waters. Seen in Southern California regularly since at least
1901.
In 1972 a red tide was caused in New England by a toxic dinoflagellate
Alexandrium (Gonyaulax) tamarense.
In 2005 the Canadian HAB was discovered to have come further south than
it has in years prior by a ship called The Oceanus, closing shellfish beds in
Maine and Massachusetts and alerting authorities as far south as Montauk (Long
Island, NY) to check their beds. Experts who discovered the reproductive cysts
in the seabed warn of a possible spread to Long Island in the future, halting
the area's fishing and shellfish industry and threatening the tourist trade,
which constitutes a significant portion of the island's economy.
In 2009, Brittany, France experienced recurring algal blooms caused by
the high amount of fertilizer discharging in the sea due to intensive pig
farming, causing lethal gas emissions that have led to one case of human
unconsciousness and three animal deaths.
In 2010 dissolved iron in the ash from the Eyjafjallajökull volcano
triggered a plankton bloom in the North Atlantic.
In 2013, an algal bloom was caused in Qingdao, China, by sea lettuce.
In 2014, Myrionecta rubra (previously known as Mesodinium rubrum), a
ciliate protist that ingests cryptomonad algae, caused a bloom in southeastern
coast of Brazil.
Red, orange, yellow and green
represent areas where algal blooms abound. Blue patches represent nutrient-poor
zones where blooms exist in low numbers.
The US Coast Guard Cutter Healy
ferried scientists to 26 study sites in the Arctic, where blooms ranged in concentration
from high (red) to low (purple).
Researcher David Mayer of Clark
University lowers a video camera below the ice to observe a dense bloom of
phytoplankton.
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