Some species form colonial stages during parts of their
life cycle that have implications for grazers or may deter or
protect species from viral or bacterial infection (Lancelot
et al., 2002). Many HAB species are motile, and under
certain environmental conditions, their swimming
behavior or buoyancy may result in the formation of
high-density patches (e.g., Franks, 1992; Kamykowski
et al., 1998). Some cyanobacterial species are able to regulate their vertical positioning by synthesis and collapse of
gas vesicles. Vertical movement by cells in a stratified
environment may help to maximize encounter frequencies
for sexual reproduction, minimize grazing losses, and
allow cells to obtain nutrients at depth and light at the surface. All of these behaviors have important implications
for species and bloom success and serve to underscore
the deep complexity of the biology of these seemingly
“simple” organisms.
Trophic interactions
The proliferation of HABs reflects the metabolism and
specific growth of the “algal” cells, but also the dynamics
of the species that are co-occurring with the HAB. It has
long been argued that production of allelopathic exudates
allows some harmful species to outcompete co-occurring
phytoplankton (e.g., Granéli and Johansson, 2003). The
ability of a HAB species to build their population under
specific conditions is therefore related to the availability
of sufficient nutrients to sustain it and to reduction in mortality rates. The response of zooplankton and other grazers
to toxic algae is often species specific in terms of behavioral responses and toxin susceptibility. Many HAB
species produce more toxins under stress, thereby
allowing them to avoid predation and competition by killing their predators and the competing algal species
(Granéli and Johansson, 2003). Fish and zooplankton
avoid dense concentrations of certain HAB species, and
some toxic species are rejected by predators or grazers.
Grazing control of HABs can also depend on the population density of the harmful algae, as demonstrated for
the brown tides in Narragansett Bay, USA, where suppression of grazing occurs above a threshold concentration
(Tracey, 1988). A threshold effect may also occur if the
daily production of new harmful cells becomes large
enough to saturate the ingestion response of the grazers
and the ability of grazers to increase their populations.
In that case, population growth can accelerate dramatically (Donaghay, 1988).
Bacteria play an important role in controlling many
HABs and regulating their impacts, including their toxicity. Bacteria may also interact with HABs in a positive
manner by stimulating their growth. CyanoHABs, in
particular, establish mutually beneficial consortia of
microorganisms (Paerl and Millie, 1996). A different type
of bacterial interaction with HAB species was described
by Bates et al. (1995) who showed that the toxicity of
the diatom Pseudo-nitzschia was dramatically enhanced
by the presence of bacteria in laboratory cultures.
Likewise, viruses are also now known to have significant
impacts on the dynamics of marine communities, and
some have been found to infect algae and have been implicated in the demise of red or brown tide blooms (Fuhrman
and Suttle, 1993). The extent to which any of the above
interactions occur in natural waters and affect HAB
dynamics is not well known and represents an important
line of inquiry.
Physical dynamics
The physical environment is also intrinsically intertwined
with the biology and ecology of algae. Many large-scale
features have direct relevance to HABs or their likelihood
for formation, including such features as tidal fronts,
coastal jets, and upwelling. Some HABs tend to aggregate
subsurface in thin layers. For example, in the Baltic Sea,
the heterotrophic dinoflagellate Dinophysis may only be
found in a 1–2 m layer, but at a depth of 20–25 m
(Gisselson et al., 2002). As another example, it has been
found off the coast of France that some subsurface layers
are comprised of up to 100 % dinoflagellates, most of
which are harmful and all of which are mixotrophic,
a so-called magic carpet of toxic HABs (Gentien et al.,
2008 and references therein). Stratification and mixing
have pronounced effects on the distribution and success
of HABs. Turbulence, also, has significant consequences
for the growth and decline of HABs through its influence
on the transport of nutrients, the mixing of phytoplankton
through gradients of light, and even through direct impairment of growth. Many questions remain about the specific
adaptations of cells to these local environments and the
interplay between physics and biology in maintaining
these microstructures.
Global expansion of algal blooms, HABs,
and their effects
Both toxic and nuisance HABs are increasing throughout
much of the world. For example, global occurrences of
PSP increased dramatically over the three-decade period
from 1970–2000 (Figure 1, Glibert et al., 2005a). Dead
zones, another effect of high biomass algal blooms, are
also increasing worldwide. The number of dead zones
from excessive algal production has doubled each decade
since the 1960s. Some systems have shown a progression
from episodic to seasonal hypoxia and then, with
increased nutrient enrichment, to more and more sustained
hypoxia. Dead zones are now found in waters across the
globe (Diaz and Rosenberg, 2008).
Although some of the factors contributing to the global
expansion are natural, such as biological species dispersal,
many others are considered to be a result of human
activities, among which nutrient pollution is the most
important. The exploitation of natural fish stocks has also,
in some cases, led to a decrease in the control of HAB
species by removal of the primary grazers through
trophic cascade effects. Global climate change may also
be important in the increase in HABs.
10
ALGAL BLOOMS
life cycle that have implications for grazers or may deter or
protect species from viral or bacterial infection (Lancelot
et al., 2002). Many HAB species are motile, and under
certain environmental conditions, their swimming
behavior or buoyancy may result in the formation of
high-density patches (e.g., Franks, 1992; Kamykowski
et al., 1998). Some cyanobacterial species are able to regulate their vertical positioning by synthesis and collapse of
gas vesicles. Vertical movement by cells in a stratified
environment may help to maximize encounter frequencies
for sexual reproduction, minimize grazing losses, and
allow cells to obtain nutrients at depth and light at the surface. All of these behaviors have important implications
for species and bloom success and serve to underscore
the deep complexity of the biology of these seemingly
“simple” organisms.
Trophic interactions
The proliferation of HABs reflects the metabolism and
specific growth of the “algal” cells, but also the dynamics
of the species that are co-occurring with the HAB. It has
long been argued that production of allelopathic exudates
allows some harmful species to outcompete co-occurring
phytoplankton (e.g., Granéli and Johansson, 2003). The
ability of a HAB species to build their population under
specific conditions is therefore related to the availability
of sufficient nutrients to sustain it and to reduction in mortality rates. The response of zooplankton and other grazers
to toxic algae is often species specific in terms of behavioral responses and toxin susceptibility. Many HAB
species produce more toxins under stress, thereby
allowing them to avoid predation and competition by killing their predators and the competing algal species
(Granéli and Johansson, 2003). Fish and zooplankton
avoid dense concentrations of certain HAB species, and
some toxic species are rejected by predators or grazers.
Grazing control of HABs can also depend on the population density of the harmful algae, as demonstrated for
the brown tides in Narragansett Bay, USA, where suppression of grazing occurs above a threshold concentration
(Tracey, 1988). A threshold effect may also occur if the
daily production of new harmful cells becomes large
enough to saturate the ingestion response of the grazers
and the ability of grazers to increase their populations.
In that case, population growth can accelerate dramatically (Donaghay, 1988).
Bacteria play an important role in controlling many
HABs and regulating their impacts, including their toxicity. Bacteria may also interact with HABs in a positive
manner by stimulating their growth. CyanoHABs, in
particular, establish mutually beneficial consortia of
microorganisms (Paerl and Millie, 1996). A different type
of bacterial interaction with HAB species was described
by Bates et al. (1995) who showed that the toxicity of
the diatom Pseudo-nitzschia was dramatically enhanced
by the presence of bacteria in laboratory cultures.
Likewise, viruses are also now known to have significant
impacts on the dynamics of marine communities, and
some have been found to infect algae and have been implicated in the demise of red or brown tide blooms (Fuhrman
and Suttle, 1993). The extent to which any of the above
interactions occur in natural waters and affect HAB
dynamics is not well known and represents an important
line of inquiry.
Physical dynamics
The physical environment is also intrinsically intertwined
with the biology and ecology of algae. Many large-scale
features have direct relevance to HABs or their likelihood
for formation, including such features as tidal fronts,
coastal jets, and upwelling. Some HABs tend to aggregate
subsurface in thin layers. For example, in the Baltic Sea,
the heterotrophic dinoflagellate Dinophysis may only be
found in a 1–2 m layer, but at a depth of 20–25 m
(Gisselson et al., 2002). As another example, it has been
found off the coast of France that some subsurface layers
are comprised of up to 100 % dinoflagellates, most of
which are harmful and all of which are mixotrophic,
a so-called magic carpet of toxic HABs (Gentien et al.,
2008 and references therein). Stratification and mixing
have pronounced effects on the distribution and success
of HABs. Turbulence, also, has significant consequences
for the growth and decline of HABs through its influence
on the transport of nutrients, the mixing of phytoplankton
through gradients of light, and even through direct impairment of growth. Many questions remain about the specific
adaptations of cells to these local environments and the
interplay between physics and biology in maintaining
these microstructures.
Global expansion of algal blooms, HABs,
and their effects
Both toxic and nuisance HABs are increasing throughout
much of the world. For example, global occurrences of
PSP increased dramatically over the three-decade period
from 1970–2000 (Figure 1, Glibert et al., 2005a). Dead
zones, another effect of high biomass algal blooms, are
also increasing worldwide. The number of dead zones
from excessive algal production has doubled each decade
since the 1960s. Some systems have shown a progression
from episodic to seasonal hypoxia and then, with
increased nutrient enrichment, to more and more sustained
hypoxia. Dead zones are now found in waters across the
globe (Diaz and Rosenberg, 2008).
Although some of the factors contributing to the global
expansion are natural, such as biological species dispersal,
many others are considered to be a result of human
activities, among which nutrient pollution is the most
important. The exploitation of natural fish stocks has also,
in some cases, led to a decrease in the control of HAB
species by removal of the primary grazers through
trophic cascade effects. Global climate change may also
be important in the increase in HABs.
10
ALGAL BLOOMS