United States
Environmental Protection ottlceotNA
Agency MC4304T
Office of Water
EPA820-S-13-001
May 2013
Impacts of Climate Change on the Occurrence of
Harmful Algal Blooms
Summary
Climate change is predicted to change many
environmental conditions that could affect the
natural properties of fresh and marine waters both in
the US and worldwide. Changes in these factors
could favor the growth of harmful algal blooms and
habitat changes such that marine HABs can invade
and occur in freshwater. An increase in the
occurrence and intensity of harmful algal blooms
may negatively impact the environment, human
health, and the economy for communities across the
US and around the world. The purpose of this fact
sheet is to provide climate change researchers and
decision-makers a summary of the potential impacts
of climate change on harmful algal blooms in
freshwater and marine ecosystems. Although much
of the evidence presented in this fact sheet suggests
that the problem of harmful algal blooms may
worsen under future climate scenarios, further
research is needed to better understand the
association between climate change and harmful
algae.
Microcystis bloom in Lake Neatahwanta, NY,
August, 2010. Courtesy of James Hyde, NYSDOH.
Background
Algae occur naturally in marine and fresh waters.
Under favorable conditions that include adequate
light availability, warm waters, and high nutrient
levels, algae can rapidly grow and multiply causing
"blooms." Blooms of algae can cause damage to
aquatic environments by blocking sunlight and
depleting oxygen required by other aquatic
organisms, restricting their growth and survival.
Some species of algae, including golden and red
algae and certain types of cyanobacteria, can produce
potent toxins that can cause adverse health effects to
wildlife and humans, such as damage to the liver and
nervous system. When algal blooms impair aquatic
ecosystems or have the potential to affect human
health, they are known as harmful algal blooms
(HABs).
In recent decades, scientists have observed an
increase in the frequency, severity and geographic
distribution of HABs worldwide. Recent research
suggests that the impacts of climate change may
promote the growth and dominance of harmful algal
blooms through a variety of mechanisms including:
• Warmer water temperatures
• Changes in salinity
• Increases in atmospheric carbon dioxide
concentrations
• Changes in rainfall patterns
• Intensifying of coastal upwelling
• Sea level rise
Temperature
Harmful algae typically bloom during the warm
summer season or when water temperatures are
warmer than usual. As temperatures become warmer
due to climate change, the growth of harmful algae
may be favored over other non-harmful algae
through a combination of mechanisms:
Warmer temperatures create a competitive
advantage for certain types of harmful algae.
As seen with the toxin-producing cyanobacteria
Microcystis, certain harmful algae grow faster
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than other non-harmful algae at relatively high
temperatures, some of them at temperatures above
77°F. This competitive advantage increases the
likelihood of HAB events.
Warmer temperatures increase thermal
stratification, favoring the growth of some harmful
algae.
The density of water is strongly influenced by
temperature and varies with depth in the water
column. Surface waters are usually warmer and less
dense compared to bottom waters that are usually
colder and denser. During summer months, warming
of surface waters can create a physical force strong
enough to resist the wind's ability to mix the water.
These layers can restrict the vertical movement of
oxygen and nutrients in water. This process is known
as thermal stratification.
The warming of surface waters increases the
frequency, strength, and duration of stratification,
which favors the growth of harmful algal blooms.
Cyanobacteria and dinoflagellates have the ability to
control their depth in water. In stratified waters, these
algae can float or swim upwards to form dense surface
blooms that block the sunlight for other algae and
aquatic organisms that live in deeper layers and are
unable to migrate to the surface. Under stratified
conditions, these algae may also encounter less
competition for nutrients in surface water, increasing
their competitive advantage. Most bloom-forming
cyanobacteria can form gas vesicles, regulating their
vertical position and migrating up and down to obtain
nutrients from deeper waters while returning to the
surface as blooms.
Warmer temperatures decrease the viscosity of water,
favoring the growth of harmful algal blooms.
Viscosity is a description of the thickness of fluids.
Fluids that have a higher viscosity like honey, are
thicker, and therefore do not flow as easily as liquids
with a lower viscosity, such as water.
Warmer temperatures reduce the viscosity of water,
which increases the speed at which small aquatic
organisms can vertically migrate. This makes it easier
for small cyanobacteria to float to the surface to form
harmful algal blooms. On the other hand, a decrease
in viscosity promotes the sinking of larger algae and
organisms that are not capable of migrating towards
the surface. This may increase the competitive
advantage of cyanobacteria over other algae.
Harmful algal blooms themselves increase water
temperature, further favoring their growth.
Harmful algal blooms absorb light from the sun
which increases the temperature of surface water.
This positive feedback mechanism further favors
the growth of harmful algal blooms and promotes
their competitive advantage in aquatic
ecosystems.
Salinity
Climate change may cause summer droughts to
increase in intensity and duration worldwide. During
a drought, the amount of water flowing into lakes
and reservoirs decreases. Combined with warmer
temperatures that cause more evaporation, water
levels of fresh water bodies decrease. This causes the
salinity, or concentration of salt in the water body, to
increase. Although certain toxin-producing
cyanobacteria are quite salt tolerant, temporary
increases in salinity can also cause salt stress leading
to leakage of cells and the release of toxins. Increases
in salinity during drought conditions can also create
favorable conditions for the invasion of marine algae
into what are usually freshwater ecosystems. This is
currently occurring in our southwestern and south
central US lakes where marine alga, Prymnesium
parvum, or golden algae, has been increasing since
2000, causing significant fish kills in inland waters.
Carbon Dioxide
All algae, including harmful species, require carbon
dioxide (CO2) for photosynthesis. Increases in
atmospheric carbon dioxide will increase the levels
of dissolved carbon dioxide in marine and freshwater
ecosystems, favoring those algae that can grow
faster in elevated dissolved carbon dioxide
conditions. In addition, cyanobacteria that can float
to the surface have a distinct advantage over other
competing algae because they can directly utilize
carbon dioxide from the atmosphere. As atmospheric
carbon dioxide concentrations increase due to human
activities such as the burning of fossil fuels and
deforestation, cyanobacteria that can float to the
surface will have greater access to carbon dioxide for
growth, increasing the occurrence of harmful algal
blooms.
This also could lead to changes in the chemistry of
ambient waters. Higher photosynthesis converts
carbon dioxide into living algal biomass, some of
which dies and settles to the bottom. The eventual
decomposition of this surplus organic material is
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analogous to our own breathing activity because it
consumes oxygen and increases carbon dioxide in
areas with poor circulation. This can contribute to
increases in acidity (i.e., lower pH). This ecological
source of acidification is added to the direct
acidifying effects of atmospheric carbon dioxide,
commonly known as ocean acidification. Like
temperature, these changes in water chemistry can
change the competitive relationships between HABs
and other algae, and can also change the ability of
zooplankton to control HABs through their grazing
activity.
US, upwelling may intensify and deliver more
nutrients to coastal waters, favoring the growth of
both harmful algal blooms and benign species.
For More Information
For more information on water and climate change,
visit http://water.epa.gov/scitech/climatechange/.
To learn more about harmful algal blooms, go to:
http: //go .usa. gov/gYTH
Rainfall
Future climate projections suggest an increase in
extreme weather events. For example, the incidence
of intense storms causing rainfall to occur in more
concentrated bursts followed by long dry periods of
drought may increase. Extreme rainfall could
increase the transport of nutrients from land into
water bodies via runoff. If followed by drought
conditions as is projected, water bodies may retain
those nutrients for longer periods of time, which
increases the potential for F£AB development.
Acknowledgements
EPA gratefully acknowledges the valuable
contributions from Christopher Sibrizzi, George
Washington University, and Dr. Stephanie Moore,
Northwest Fisheries Science Center, West Coast
Center for Oceans and Human Health, NOAA, in
developing this work.
Sea Level Rise
Scientific models predict that sea level could rise
from 18 cm up to one meter by the year 2100. This
would increase the extent of continental shelf areas,
providing shallow, stable coastal waters that may
favor the growth of harmful algae and/or expand
their habitat inland.
Coastal Upwelling
Coastal upwelling is a naturally occurring process, in
which alongshore winds move coastal surface water
offshore which is replaced by deep water that moves
along the ocean floor towards the coast. This
"upwelled" water brings nutrients from the ocean
floor to the surface leading to high productivity.
Harmful algal blooms' growth in upwelling systems
vary due to atmospheric oscillations (upwelling or
downwelling by interannual fluctuations), and is
subject to water-column stratification, nutrient
availability, and the intensity and persistence of
upwelling conditions. Global climate change may
alter the timing and intensity of coastal upwelling.
Along ocean boundaries, such as the west coast of the
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