toxic but do cause disruption to the grazer community and
thus to trophic transfer and the food web in general. Such
blooms have been coined ecosystem disruptive algal
blooms, EDABs (Sunda et al., 2006).
Adding to the confusing terminology associated with
HABs, some HABs are technically not “algae” at all, but
rather small animal-like microbes that obtain their nutrition by grazing on other small algae or bacteria; they either
do not photosynthesize at all or only do so in conjunction
with grazing. These complex and diverse nutritional strategies are described in more detail below. Other “HABs”
are more bacteria-like. These are the cyanobacteria
(CyanoHABs), some of which have the ability to “fix”
nitrogen from the atmosphere as their nitrogen source.
All of these complexities underscore that the term
“HAB” is simply an operational term, recognizing the
group of species (not all of which are strictly algae) that
can cause harm – to health, to the environment, or to the
economy.
Common species groups and harmful properties
Of the thousands of species of marine phytoplankton from
hundreds of genera, only a few can be highlighted here.
Diatoms are the most common organisms associated
with “spring blooms.” Diatoms are capable of rapid
growth rates. They have a silicate shell and thus have
a unique requirement for this element among the algae.
Most diatoms are not harmful, but large spring blooms
are associated with hypoxia or “dead zones” when the
biomass sinks to deeper waters where it decomposes in
oxygen-consuming processes. The annual development
of summer hypoxia in Chesapeake Bay, for example, is
due to these large spring bloom events (Kemp et al.,
2005).
Diatoms may also be toxic, as is the case of Pseudonitzschia spp. that produce domoic acid that is responsible
for the human illness called amnesic shellfish poisoning
(Trainer et al., 2012). Exposure to this HAB group is
now being linked to seizure and memory loss in laboratory
animals and to premature births and strandings in animals
such as sea lions (Johnson et al., 2010; Bargu et al., 2012).
Dinoflagellates are among the more common toxic
HABs as well as NABs. Dinoflagellates all possess two
dissimilar flagella; they often display complex life cycles
and typically have much slower growth rates than
diatoms. Many produce toxins that can kill fish directly or
that intoxicate seafood with toxins that can be passed onto
human consumers. One toxic dinoflagellate that causes significant human health effects is Karenia brevis, which produces large blooms along the coast of the Gulf of Mexico.
This species produces a neurotoxin, brevetoxin, that is
responsible for the human illness called neurotoxic
shellfish poisoning (Backer and McGillicuddy, 2006).
Several dinoflagellate species, including Alexandrium
spp. and Pyrodinium bahamense (var. compressum), produce saxitoxin, responsible for Paralytic Shellfish Poisoning (PSP) (Backer and McGillicuddy, 2006). PSP has
been particularly problematic in Southeast Asia, where
many human fatalities have been reported over the past several decades (Azanza and Taylor, 2001). Another example
of a toxigenic dinoflagellate is Karlodinium veneficum.
This species has been implicated in fish-kill events in the
Chesapeake Bay area (Adolf et al., 2008) as well as
in coastal waters of Southwest Africa, Europe, United
States, Western Australia, and other temperate coastal
environments.
Many HAB dinoflagellates develop large-scale blooms
but do not produce toxins that have significant human
effects. Prorocentrum minimum is one such globally distributed species. This species can produce high biomass
blooms that affect the food web by altering ingestion rates
and/or growth rates of consumers especially at the larval
stages (Heil et al., 2005; Glibert et al., 2008). Benthic species of this genus are known toxin producers, however
(Glibert et al., 2012).
Another significant group of HABs is the
prymnesiophytes. Many, if not most, Prymnesium species
are toxic to gill-breathing organisms and thus are responsible for many fish kills around the world, especially in
eutrophic waters. For example, Chrysochromulina
polylepis has been the cause of fish kills along the
Norwegian coast, and Prymnesium parvum (Carter) is
responsible for reoccurring fish kills in coastal and inland
waters worldwide (reviewed by Edvardsen and Paasche,
1998; Edvardsen and Imai, 2006; Roelke et al., 2007).
Along the North Sea coast, blooms of Phaeocystis
spp. are common. Because they are mucilage rich, water
can turn viscous, and beaches can be drenched in foam
from decaying blooms (Lancelot, 1995).
The raphidophytes are yet another common HAB
group distributed worldwide. These organisms have often
caused large-scale fish mortalities, both in aquaculture
settings and in natural environments. Among the more
common HAB raphidophytes are Heterosigma akashiwo
and Chattonella spp. The fish-killing properties of these
species are due to their production of neurotoxins or to
their production of reactive oxygen or other hemolytic
agents (Edvardsen and Imai, 2006).
The CyanoHABs are increasingly an important nuisance and toxic HAB group affecting both freshwaters
and estuarine and coastal systems worldwide. The most
common toxins associated with this group of HABs are
hepatotoxins, such as microcystin, nodularin, and
cylindrospermopsin, but some species may also produce
neurotoxins, such as anatoxin and saxitoxin (O’Neil
et al., 2012). The world’s largest estuary, the Baltic Sea,
is now annually affected by massive CyanoHAB blooms,
including species such as Nodularia, Anabaena, and
Aphanizomenon. One of the most common HAB types
in freshwater as well as in upper estuaries and one exemplifying the effect of both direct and indirect contact is
Microcystis spp. Exposure to water with this HAB or its
toxin can cause skin irritation or respiratory irritation,
but prolonged, repeated, or intensive exposure to the
HAB toxin has been associated with tumor promotion,
8
ALGAL BLOOMS
thus to trophic transfer and the food web in general. Such
blooms have been coined ecosystem disruptive algal
blooms, EDABs (Sunda et al., 2006).
Adding to the confusing terminology associated with
HABs, some HABs are technically not “algae” at all, but
rather small animal-like microbes that obtain their nutrition by grazing on other small algae or bacteria; they either
do not photosynthesize at all or only do so in conjunction
with grazing. These complex and diverse nutritional strategies are described in more detail below. Other “HABs”
are more bacteria-like. These are the cyanobacteria
(CyanoHABs), some of which have the ability to “fix”
nitrogen from the atmosphere as their nitrogen source.
All of these complexities underscore that the term
“HAB” is simply an operational term, recognizing the
group of species (not all of which are strictly algae) that
can cause harm – to health, to the environment, or to the
economy.
Common species groups and harmful properties
Of the thousands of species of marine phytoplankton from
hundreds of genera, only a few can be highlighted here.
Diatoms are the most common organisms associated
with “spring blooms.” Diatoms are capable of rapid
growth rates. They have a silicate shell and thus have
a unique requirement for this element among the algae.
Most diatoms are not harmful, but large spring blooms
are associated with hypoxia or “dead zones” when the
biomass sinks to deeper waters where it decomposes in
oxygen-consuming processes. The annual development
of summer hypoxia in Chesapeake Bay, for example, is
due to these large spring bloom events (Kemp et al.,
2005).
Diatoms may also be toxic, as is the case of Pseudonitzschia spp. that produce domoic acid that is responsible
for the human illness called amnesic shellfish poisoning
(Trainer et al., 2012). Exposure to this HAB group is
now being linked to seizure and memory loss in laboratory
animals and to premature births and strandings in animals
such as sea lions (Johnson et al., 2010; Bargu et al., 2012).
Dinoflagellates are among the more common toxic
HABs as well as NABs. Dinoflagellates all possess two
dissimilar flagella; they often display complex life cycles
and typically have much slower growth rates than
diatoms. Many produce toxins that can kill fish directly or
that intoxicate seafood with toxins that can be passed onto
human consumers. One toxic dinoflagellate that causes significant human health effects is Karenia brevis, which produces large blooms along the coast of the Gulf of Mexico.
This species produces a neurotoxin, brevetoxin, that is
responsible for the human illness called neurotoxic
shellfish poisoning (Backer and McGillicuddy, 2006).
Several dinoflagellate species, including Alexandrium
spp. and Pyrodinium bahamense (var. compressum), produce saxitoxin, responsible for Paralytic Shellfish Poisoning (PSP) (Backer and McGillicuddy, 2006). PSP has
been particularly problematic in Southeast Asia, where
many human fatalities have been reported over the past several decades (Azanza and Taylor, 2001). Another example
of a toxigenic dinoflagellate is Karlodinium veneficum.
This species has been implicated in fish-kill events in the
Chesapeake Bay area (Adolf et al., 2008) as well as
in coastal waters of Southwest Africa, Europe, United
States, Western Australia, and other temperate coastal
environments.
Many HAB dinoflagellates develop large-scale blooms
but do not produce toxins that have significant human
effects. Prorocentrum minimum is one such globally distributed species. This species can produce high biomass
blooms that affect the food web by altering ingestion rates
and/or growth rates of consumers especially at the larval
stages (Heil et al., 2005; Glibert et al., 2008). Benthic species of this genus are known toxin producers, however
(Glibert et al., 2012).
Another significant group of HABs is the
prymnesiophytes. Many, if not most, Prymnesium species
are toxic to gill-breathing organisms and thus are responsible for many fish kills around the world, especially in
eutrophic waters. For example, Chrysochromulina
polylepis has been the cause of fish kills along the
Norwegian coast, and Prymnesium parvum (Carter) is
responsible for reoccurring fish kills in coastal and inland
waters worldwide (reviewed by Edvardsen and Paasche,
1998; Edvardsen and Imai, 2006; Roelke et al., 2007).
Along the North Sea coast, blooms of Phaeocystis
spp. are common. Because they are mucilage rich, water
can turn viscous, and beaches can be drenched in foam
from decaying blooms (Lancelot, 1995).
The raphidophytes are yet another common HAB
group distributed worldwide. These organisms have often
caused large-scale fish mortalities, both in aquaculture
settings and in natural environments. Among the more
common HAB raphidophytes are Heterosigma akashiwo
and Chattonella spp. The fish-killing properties of these
species are due to their production of neurotoxins or to
their production of reactive oxygen or other hemolytic
agents (Edvardsen and Imai, 2006).
The CyanoHABs are increasingly an important nuisance and toxic HAB group affecting both freshwaters
and estuarine and coastal systems worldwide. The most
common toxins associated with this group of HABs are
hepatotoxins, such as microcystin, nodularin, and
cylindrospermopsin, but some species may also produce
neurotoxins, such as anatoxin and saxitoxin (O’Neil
et al., 2012). The world’s largest estuary, the Baltic Sea,
is now annually affected by massive CyanoHAB blooms,
including species such as Nodularia, Anabaena, and
Aphanizomenon. One of the most common HAB types
in freshwater as well as in upper estuaries and one exemplifying the effect of both direct and indirect contact is
Microcystis spp. Exposure to water with this HAB or its
toxin can cause skin irritation or respiratory irritation,
but prolonged, repeated, or intensive exposure to the
HAB toxin has been associated with tumor promotion,
8
ALGAL BLOOMS