associated with nutrient enrichment is the dinoflagellate
Prorocentrum minimum. Globally, it is found in regions
of the world where the coasts are receiving elevated nutrients from anthropogenic sources (Glibert et al., 2008).
In the Chesapeake Bay, blooms of this species now appear
to be 10- to 100-fold higher in maximum density than
blooms recorded a few decades ago, and these increases
track the nutrient load to the bay over the past several
decades (Heil et al., 2005). In Puget Sound, Washington,
a striking correlation has been found between the growth
in documented cases of PSP over four decades and
the growth in human population, based on US census
statistics, strongly indicative of nutrient loading and eutrophication as the causative agent of change (Trainer et al.,
2003). Based on analyses of frustules preserved in cores,
blooms of the diatom Pseudo-nitzschia spp. in the Gulf
of Mexico were also rare prior to the 1950s, but have
increased significantly in abundance and frequency
since then as nutrient loads from the Mississippi River
have risen (Parsons et al., 2002). The Baltic Sea, the
Aegean Sea, the Northern Adriatic, and the Black Seas
have all experienced increases in HABs coincident with
increases in nutrient loading (Granéli et al., 1999; Heil
et al., 2005). In Northern European waters, blooms of
the mucus-forming HAB species Phaeocystis globosa
have been shown to be directly related to the excess nitrate
content of riverine and coastal waters, that is, the
nitrate remaining after other species of algae deplete silicate (Lancelot, 1995). One region where expansion of
eutrophication-related HABs has been particularly pronounced has been along the Asian coast, where blooms
have expanded in recent years in areal extent (from square
kilometers to tens of square kilometers), in duration (days
to months), in species, and in harmful impacts (Furuya
et al., 2010). These increases all parallel the increase in
the use of anthropogenic fertilizers and the accelerated
development of China.
On shorter time scales, there are also examples of
HABs responding rapidly to injections of nutrients
from pulsed events. Beman et al. (2005) reported, during
a 5-year study, a strong positive relationship between
nitrogen-rich agricultural runoff to the Gulf of California
and the development, within days, of extensive phytoplankton blooms. Similarly, Pseudo-nitzschia pseudodelicatissima was found a week after elevated ammonium
levels were reported in these waters (Trainer et al.,
2007), and in Chesapeake Bay, blooms of Prorocentrum
minimum have been found to follow within days of elevated levels of urea following agricultural applications
(Glibert et al., 2001).
Another important consideration in nutrient effects is
the relatively recent phenomenon of changing stoichiometry of nutrient supplies. In many parts of the developed
world, phosphorus reductions have been undertaken as
a means to reduce or control algal blooms (e.g., in sewage
effluents and laundry detergents), whereas nitrogen
loads often are allowed to remain elevated. Thus, not only
have many systems undergone eutrophication, but many
are showing signs of reversal due to this single nutrient
reduction. The consequence is that many receiving waters
are now not only enriched with nutrients, but these nutrients are in proportions that differ from those of decades
past – and also diverge considerably from those that
have long been associated with phytoplankton growth
(Glibert and Burkholder, 2011). Many types of harmful
algae appear to be able to thrive when nutrient loads are
not in classically defined ideal proportions. Not only are
many HABs able to access nutrients not available to competitors through mixotrophy, some species increase toxin
production when growing in a state of nutrient imbalance.
As examples, toxin production by the flagellates
Prymnesium parvum and Chrysochromulina polylepis
increases under both P and N stress, relative to toxin production in more nutrient-balanced growth conditions
(Granéli and Flynn, 2006). As another example, in the
dinoflagellate Alexandrium tamarense, the production of
saxitoxin has been shown to increase by three to fourfold
under phosphorus deficiency (Granéli and Flynn, 2006).
Adding to the complexity of nutrient effects, in some
cases anthropogenic nutrients may not directly stimulate
HABs, but may become linked to their growth and
abundance following biogeochemical processing or following the stimulation of other components of the food
web on which they may depend, or anthropogenic
nutrients may be displaced in time and space leading to
blooms that are displaced from their nutrient sources.
It has recently been found, for example, that Noctiluca,
the species responsible for classic “red” water, may well
be a coastal or offshore manifestation of eutrophication,
a mixotroph responding to successional planktonic
changes in nutrient availability (Harrison et al., 2011).
Relatedly, regulation of single nutrients (e.g., controlling
phosphorus without controlling nitrogen) may lead to
a situation where an estuary or inshore coastal environment is effectively phosphorus limited and blooms are
controlled, but the nitrogen is displaced downstream
where it eventually may help to support offshore blooms.
Such effects have been documented for the Neuse River
Estuary, the mid-region of the Chesapeake Bay, and the
southern Baltic Sea region; all of these regions have experienced either episodic or sustained reductions in inshore
blooms, but parallel increases in offshore blooms
(Glibert et al., 2011 and references therein).
Links to climate
Climate ultimately controls the fundamental parameters
regulating algal growth, including water temperature,
nutrients, and light, and thus can be expected to result in
changes in the species composition, trophic structure,
and function of marine ecosystems. Examples can be
found through the US and European coasts, where correlations between shifts in HAB species and the timing of their
outbreaks and increases in mean water temperature have
been documented. In addition, changing greenhouse
gases, pH, as well as temperature are related to changes
12
ALGAL BLOOMS
Prorocentrum minimum. Globally, it is found in regions
of the world where the coasts are receiving elevated nutrients from anthropogenic sources (Glibert et al., 2008).
In the Chesapeake Bay, blooms of this species now appear
to be 10- to 100-fold higher in maximum density than
blooms recorded a few decades ago, and these increases
track the nutrient load to the bay over the past several
decades (Heil et al., 2005). In Puget Sound, Washington,
a striking correlation has been found between the growth
in documented cases of PSP over four decades and
the growth in human population, based on US census
statistics, strongly indicative of nutrient loading and eutrophication as the causative agent of change (Trainer et al.,
2003). Based on analyses of frustules preserved in cores,
blooms of the diatom Pseudo-nitzschia spp. in the Gulf
of Mexico were also rare prior to the 1950s, but have
increased significantly in abundance and frequency
since then as nutrient loads from the Mississippi River
have risen (Parsons et al., 2002). The Baltic Sea, the
Aegean Sea, the Northern Adriatic, and the Black Seas
have all experienced increases in HABs coincident with
increases in nutrient loading (Granéli et al., 1999; Heil
et al., 2005). In Northern European waters, blooms of
the mucus-forming HAB species Phaeocystis globosa
have been shown to be directly related to the excess nitrate
content of riverine and coastal waters, that is, the
nitrate remaining after other species of algae deplete silicate (Lancelot, 1995). One region where expansion of
eutrophication-related HABs has been particularly pronounced has been along the Asian coast, where blooms
have expanded in recent years in areal extent (from square
kilometers to tens of square kilometers), in duration (days
to months), in species, and in harmful impacts (Furuya
et al., 2010). These increases all parallel the increase in
the use of anthropogenic fertilizers and the accelerated
development of China.
On shorter time scales, there are also examples of
HABs responding rapidly to injections of nutrients
from pulsed events. Beman et al. (2005) reported, during
a 5-year study, a strong positive relationship between
nitrogen-rich agricultural runoff to the Gulf of California
and the development, within days, of extensive phytoplankton blooms. Similarly, Pseudo-nitzschia pseudodelicatissima was found a week after elevated ammonium
levels were reported in these waters (Trainer et al.,
2007), and in Chesapeake Bay, blooms of Prorocentrum
minimum have been found to follow within days of elevated levels of urea following agricultural applications
(Glibert et al., 2001).
Another important consideration in nutrient effects is
the relatively recent phenomenon of changing stoichiometry of nutrient supplies. In many parts of the developed
world, phosphorus reductions have been undertaken as
a means to reduce or control algal blooms (e.g., in sewage
effluents and laundry detergents), whereas nitrogen
loads often are allowed to remain elevated. Thus, not only
have many systems undergone eutrophication, but many
are showing signs of reversal due to this single nutrient
reduction. The consequence is that many receiving waters
are now not only enriched with nutrients, but these nutrients are in proportions that differ from those of decades
past – and also diverge considerably from those that
have long been associated with phytoplankton growth
(Glibert and Burkholder, 2011). Many types of harmful
algae appear to be able to thrive when nutrient loads are
not in classically defined ideal proportions. Not only are
many HABs able to access nutrients not available to competitors through mixotrophy, some species increase toxin
production when growing in a state of nutrient imbalance.
As examples, toxin production by the flagellates
Prymnesium parvum and Chrysochromulina polylepis
increases under both P and N stress, relative to toxin production in more nutrient-balanced growth conditions
(Granéli and Flynn, 2006). As another example, in the
dinoflagellate Alexandrium tamarense, the production of
saxitoxin has been shown to increase by three to fourfold
under phosphorus deficiency (Granéli and Flynn, 2006).
Adding to the complexity of nutrient effects, in some
cases anthropogenic nutrients may not directly stimulate
HABs, but may become linked to their growth and
abundance following biogeochemical processing or following the stimulation of other components of the food
web on which they may depend, or anthropogenic
nutrients may be displaced in time and space leading to
blooms that are displaced from their nutrient sources.
It has recently been found, for example, that Noctiluca,
the species responsible for classic “red” water, may well
be a coastal or offshore manifestation of eutrophication,
a mixotroph responding to successional planktonic
changes in nutrient availability (Harrison et al., 2011).
Relatedly, regulation of single nutrients (e.g., controlling
phosphorus without controlling nitrogen) may lead to
a situation where an estuary or inshore coastal environment is effectively phosphorus limited and blooms are
controlled, but the nitrogen is displaced downstream
where it eventually may help to support offshore blooms.
Such effects have been documented for the Neuse River
Estuary, the mid-region of the Chesapeake Bay, and the
southern Baltic Sea region; all of these regions have experienced either episodic or sustained reductions in inshore
blooms, but parallel increases in offshore blooms
(Glibert et al., 2011 and references therein).
Links to climate
Climate ultimately controls the fundamental parameters
regulating algal growth, including water temperature,
nutrients, and light, and thus can be expected to result in
changes in the species composition, trophic structure,
and function of marine ecosystems. Examples can be
found through the US and European coasts, where correlations between shifts in HAB species and the timing of their
outbreaks and increases in mean water temperature have
been documented. In addition, changing greenhouse
gases, pH, as well as temperature are related to changes
12
ALGAL BLOOMS