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This talk will focus on four areas: a general overview of the
problem presented, structure of the coral animal, how seawater can
buffer itself to maintain life, and how CO2 affects seawater and in
turn, the coral animals of the reef.CO2 emissions have changed the
chemistry of the atmosphere and are beginning to change the
chemistry of the surface ocean. These changes could have a direct
effect on the physiology of marine organisms.CO2 may increase rates
of photosynthesis in regions where nutrients are not limitingCO32-
may decrease rates of calcification because the alkalinity may drop
to dangerous levels. Alkalinity helps to maintain pH and buffer
seawater in order for calcium to be incorporated into the skeletons
of coral.
If these CO2 emissions produced were big enough, the balance
between photosynthesis and calcification could be upset. Coral
reefs could be the first ecosystems to feel the effects because of
an altering to the carbonate buffering system of seawater as well
as the following problems.
Less able to keep up with sealevel rise. When global
temperatures increase, it will eventually cause polar icecaps to
melt which will cause seas to rise. With higher seas, less light
will be able to reach coral reefs, which will decrease the rate and
amount of photosynthesis. (Langdon, 2001)
2. Less able to compete for space with faster growing algae.
When carbonate buffering systems are not working properly, nuisance
algae will grow and out compete corals for nutrients, calcium, and
light. (Gerin and Edmumds, 2001)
3. More susceptible to boring organisms and storm damage. Corals
will not be able to grow a thick skeleton, which will lead to
brittle animals. Storms will break limbs of coral easily, as well
as boring invertebrates can puncture the coral skeleton much
easier. (Marubini, et. al., 2003)
The graph on the left indicates how levels of CO2 have
fluctuated over the last 300000 years. As you can see, we are
currently experiencing an increase in CO2 production because of a
variety of impacts from humans.
The graph on the right indicates the amount of CO2 produced in
the last 1000 years. Recent human actions have caused a spike in
the amount of CO2 produced. As you can see, it is higher than at
any time in human history with no indications of slowing or
decreasing. The elevated CO2 will cause devastating effects on our
oceans, in particular, the coral reefs.As greenhouse gas emissions
have increased since 1850 (the industrial revolution), CO2 has been
the main contributor to these emissions. As with the previous
slide, the amount of CO2 is at unprecedented levels as seen through
human history.What we see as a large head (or colony) of coral is
actually made up of thousands of tiny individual animals called
polyps. These polyps range in diameter from about 1 mm to 1 cm or
more. Each polyp looks somewhat like a small sea anemone, which is
not surprising, as corals and sea anemones are cousins. Another
member of the corals family is the jellyfish. All three of these
groups contain stinging cells called nematocysts. Most corals
nematocysts are not strong enough to penetrate human skin, so we
cannot feel them sting, but they are able to use their nematocysts
to defend against some predators and to catch food.
Before we can talk about coral reefs, we need to start with a
basic questionwhat is a coral? Is it animal, vegetable or
mineral?The correct answer is that it can be all three. Corals are
animals that may have a special relationship with a microscopic
plant and that can make an external limestone (or mineral)
skeleton.
This relationship between corals and algae is a type of
symbiosis. Both the coral and the algae (known as zooxanthellae)
benefit from the partnership, so this is an example of a
mutualistic symbiosis. The zooxanthellae are safe from predators
inside the coral tissue (remember, corals have stinging cells) and
the coral provides them with nutrients in the form of excreted
nitrogen and phosphorus. In return, the zooxanthellae provide the
coral with sugar compounds that they make when they
photosynthesize.Most corals appear brown in color. This is a result
of the millions of brown zooxanthellae that are found in their
cells. Occasionally, when corals are stresses by high water
temperature or other factors, the zooxanthellae will be lost,
leaving the corals appearing almost pure white. This phenomenon is
known as coral bleaching. The coral may die following bleaching,
but often it will recover and become repopulated with
zooxanthellae.
By adding CO2 to water, it quickly converts to carbonic acid and
dissociated to bicarbonate.
So, as more CO2 dissolves, more protons are released and this
acidifies the water. The carbonate combines with the protons and
produces bicarbonate which decreases carbonate concentration.
(Gattuso, et al, 1999)Dissolution of calcium carbonate in seawater
is influenced by three major factors: temperature, pressure and
partial pressure of carbon dioxide (CO2). The easiest way to
understand calcium carbonate (CaCO3) dissolution is to recognize
that it is controlled, in large part, by the solubility of CO2:
CaCO3 + H20 + CO2 Ca++ + 2HCO3- The more CO2 that can be held in
solution, the more CaCO3 that will dissolve. Since more CO2 can be
held in solution at higher pressures and cooler temperatures, CaCO3
is more soluble in the deep ocean than in surface waters. Finally,
as CO2 is added to the water, more CaCO3 can dissolve. The result
is that, as more CO2 is added to deep ocean water by the
respiration of organisms, the more corrosive the bottom water
becomes to calcareous shells.
Leads to a less stable reef structure and the dissolving of
calcium carbonate.
(Barker et al, 2003)
Rising atmospheric CO2 leads to increased dissolution of CO2
into sea water. This changes the carbonate equilibrium in the
oceans resulting in a decrease in alkalinity. Alkalinity is an
important factor in coral skeletal growth. Alkalinity is a
measuring of buffering potential and the amount of carbonate
available to bind with calcium to deposit into coral skeletons.
Alkalinity also buffers pH to keep seawater at a pH between 8.0 and
8.4. Lower alkalinity reduces the CaCO3 saturation state of water
making it harder for coral reefs to grow. If there is not enough
carbonate in the system, then calcium will not be pulled out of the
water and deposited into the coral skeleton.
(Langdon, 2001)The graph illustrates past relationships and well
as possible future affects on seawater carbonate chemistry. As the
concentration of CO2 increased, the saturation state of aragonite,
which controls coral skeleton calcification, decreased. As the
author extrapolates for possible future CO2 increases, the amount
of calcium that is available for corals as diminishing. This study
was completed assuming the normal state of seawater which has a
salinity of 35 ppt and a total alkalinity of 3.5 meq/L
(Langdon, 2001)Data taken from the Hawaiian Ocean Times Series
Station that depicts the decrease of the saturation state of
aragonite which leads to coral calcification. As the alkalinity
decreases because of excessive CO2, the amount of calcium that is
available for coral skeletons has decreased.
(Langdon, 2001)In this experiment, Langdon altered the amount of
CO2 in the atmosphere to determine the affects of CO2 on seawater
carbonate chemistry and its relationship to seawater pH. He varied
the pCO2 from roughly 200 uatm to 400 uatm, and 700 uatm. The
results indicate that bicarbonate ions increase as CO2 increases
because of the shift in equilibrium of the buffering system. The
amount of carbonate ion decreases as well as pH when CO2 increases.
This limits the alkalinity of the seawater system which ultimately
affects coral skeleton calcification.
(Langdon, 2001)Graphs shows the affects of increased amount of
atmospheric CO2 on the calcification of a coral reef community. As
the amount of CO2 increases, the rate of calcification also
decreases. This will eventually cause coral skeletons to become
very thin and fragile, or to stop coral growth altogether.
(Langdon,2001)
Another study showing the relationship between atmospheric CO2
and calcification. In this study, the CO2 was increased from 200
uatm to 700 uatm over a range of light intensities. The data show
that the coral species Porites compressa had a lower rate of
calcification when the amount of CO2 was elevated.Langdon, 2001
lists an accumulation of various other studies to show a doubling
of CO2 on the rate of calcification. Multiple types of corals, and
other marine invertebrates were used in these studies, they all
showed a decrease in calcification when CO2 was increased. This
indicates that the increased CO2 concentration in the earths
atmosphere will have a deleterious effects on coral reefs and the
marine invertebrates that inhabit these ecosystems.Caldiera and
Wickett 2003 estimated what would happen if mankind were to burn
all known stocks of fossil fuels as atmospheric CO2 would exceed
1,900 parts per million around the year 2300. This PCO2 would
result in a maximum pH reduction at the ocean surface of 0.77
units. Coral reefs, calcareous plankton and other organisms whose
skeletons or shells contain calcium carbonate would be affected.
While a seemingly small change in pH, ice-core records and other
paleoclimate data suggest that over the past 300 Myr there is no
evidence that ocean pH was more than 0.6 units lower than today.
Unabated CO2 emissions over the coming centuries may produce
changes in ocean pH that are greater than any experienced in the
past 300 Myr.
Photosynthesis and calcification are not tightly coupled with
respect to their response to rising CO2.CO2 Calcif.There is a
nonlinear relationship between calcification and pCO2for 200 to 280
matm pCO2 Calcif. 34%for 350 to 700 matm pCO2 Calcif. 58%
As more CO2 is added to the atmosphere, the amount and rate of
calcification will decline which will have devastating affects of
coral reef ecosystems. Coral skeletons will not be able to grow or
recover from damage or attack. In addition, the loss of
zooxanthellae will cause a decline in photosynthesis which will
cause bleaching of corals. Once the bleaching has occurred, it is
extremely difficult to keep the coral animal alive.
Data are consistent with the hypothesis that saturation state
controls the calcification of the coral reef system, several
species of coral, calcareous red algae, and a natural community
dominated by green algae.Consequences of reduced calcification to
corals and coral reefs reduced ability to compete for space and
lightreduced ability to keep up with sealevel riseincreased
susceptibility to erosion and damage by fish, boring organisms and
storms.
From a geochemical point of view the reduction in the ratio of
calcification to net photosynthesis as CO2 rises provides a
negative feedback on atmospheric CO2 levels. What occurs is that
more CO2 will be released into the atmosphere because the carbonate
buffering system cannot handle the decomposition of calcium
carbonate. The calcium carbonate will leach back into the seawater
and eventually be released as CO2 into the atmosphere, therefore
causing an even high level of CO2 and creating an even greater
warming affect on the planet.
