Research Article:
Corals-Providing a Baseline for Climate Change
The Potential Effect of Human Activities on Climate
Dr. Jerry Wellington is interested in how climates are changing and whether
recent climate changes are a result of human activities. Climate is different
from weather. Weather refers to the atmospheric conditions (rain, heat,
sunshine, clouds, snow, etc.) at a given place and time. Climate refers to the
weather conditions at a given place over a period of time.
In recent years, scientists have become concerned that human activities may
be increasing the carbon dioxide in the earth's atmosphere and depleting the
ozone in the stratosphere. The stratosphere is one of the layers in the earth's atmosphere. The ozone there acts as a shield that keeps too much light from
reaching the earth.
Changes to the earth's atmosphere may affect the climate in various locations.
Increased carbon dioxide, for example, could increase temperatures on earth.
Depletion of stratospheric ozone could let more sunlight reach the earth.
Climatic changes could significantly affect human life. For example, if the
average temperature on earth rises, glaciers might melt and raise sea levels.
How Scientists Study Climate
Are the current trends in climate a result of natural variations or human
activities? To answer this question, we need to know how climate has varied
in the past. Records of temperature and rainfall in the past provide
information about climate. Unfortunately, these records go back no more
than 100 years. To find out what the climate was like more than 100 years ago,
scientists study various organisms that were alive a long time ago. Some of
these organisms are still alive. Ancient trees and coral, for example, live
hundreds or even thousands of years. Many are still alive today; others died a
long time ago and now are fossils. Body structures (e.g., wood, skeletons) and
components (e.g., pollen) of ancient organisms provide clues about what the
climate was like when the organisms were alive.
Dr. Wellington uses coral to determine past climate changes. To understand
how coral provides clues about climate, you first have to understand what
coral is.
What Is Coral?
Coral polyps are small marine organisms that flourish in consistently warm,
shallow ocean waters with high salinity (saltiness). They grow in colonies that form coral heads, consisting of:
- a thick base, up to hundreds of meters (hundreds of yards) deep,
composed of the calcified remains of the skeletons of dead polyps, and
- a very thin (usually less than 5-millimeter) layer on top of the base. This
layer consists of live coral polyps. Each coral head is composed of hundreds
to thousands of living coral polyps and the remains of millions more.
Polyps are coelenterates-a group of primitive organisms that includes
jellyfish. What makes polyps different from jellyfish is that they secrete
limestone to form a cup, in which the polyp lives. Each cup becomes glued to
its neighbor by a process known as calcification to form a structure called an
exoskeleton. ("Exo-" means "on the outside.") When the coral polyps
eventually die, their cups decompose and add to the base of the coral head.
New coral polyps establish themselves on top of the decomposing shells of
dead polyps and build their own exoskeletons.
Over time, the decomposed matter left by the dead polyps becomes cemented
together by a process called lithification to form a rock called limestone. In
this way, a coral head grows from the inside out. Only the thin outside layer
of a coral head is alive.
During the day, coral polyps usually stay inside their cups, safe from
predators. Most people think of coral as being like rock. That's because they
usually see coral heads during the day, when the polyps are retracted. Even
when their polyps are retracted, corals are still vulnerable. They can be
damaged by natural forces (for example, dirt or silt deposited by weather) or
human activities-including boat anchors, dredging, and the touch of divers,
snorkelers, and swimmers. If you touch the coral, slime comes out. That
slime is actually part of the living coral polyp! Even the pressure of a human
finger is enough to cut the polyp against its own cup, and even destroy it.
As corals grow, they form distinctive ring patterns, much like those of trees.
In fact, just as in trees, each ring represents one year's growth. Dr. Wellington
counts the number of rings to find out how old the coral is. He measures the
size of each ring to determine how much the coral grew during that year and
how this growth compares to that of other years. Dr. Wellington also
measures the amount and kind of carbon molecules in the coral exoskeleton,
which provides clues about climate conditions at the time the exoskeleton
was forming. To understand how carbon provides clues about climate
conditions, you have to know how coral gets its food.
The Two Feeding Methods of Corals
Corals have two methods of feeding, and they change from one to another
according to the amount of sunlight available. At night or under very cloudy
conditions, corals actively gather food. But unlike other hunting species, the
coral polyp can't leave its home-the crystalline cup it secretes around itself. Instead, coral polyps feed by extending tentacles out into the water to catch
zooplankton floating by. The tentacles have special stinging cells (called
nematocysts) to capture zooplankton. Corals extend their tentacles only when
it is dark because they are more vulnerable to predators under brighter
lighting conditions-and because more zooplankton are present at night.
When the thousands of individuals in a hard coral head extend their
tentacles, they create an efficient "net" for capturing zooplankton. If you live
near water containing coral formations, you can use a bright flashlight to see
corals "hunting" for food at night. Shine the light down onto the coral
structure, and hold it still for a couple of minutes. This usually will attract lots of zooplankton. Then slowly move the light near a coral head with
expanded polyps. You will see the coral tentacles grab up the zooplankton just
like miniature anemone tentacles. You can even hear it-the polyps' feeding
sounds like breakfast cereal crackling and popping when you pour milk on it!
Besides the active feeding described above, corals have a second, more passive
method of getting food. When there is sufficient sunlight, corals get food
through a symbiotic relationship with algae that live inside the lining of the
coral's gut. Algae are one-celled plants; the particular algae that live inside coral are zooxanthellae. They contain chlorophyll-the substance that gives
plants their green color and allows them to make nutrients through
photosynthesis. The zooxanthellae produce food in the form of simple
sugars, on which the coral polyps feed. The polyps also receive oxygen, a by-
product of photosynthesis, from the algae. Zooxanthellae also give coral its
brilliant color-without the zooxanthellae, coral would be transparent! The
algae benefit from this symbiotic relationship too; from the corals, the
zooxanthellae receive carbon dioxide, ammonia (which they need for protein
synthesis), water, and a secure place to live.
Like all plants, algae need sunlight to make food, so at night or in cloudy or
rainy weather the corals can't rely on the zooxanthellae to produce much
food. The less food the coral receives during the day, the more it must depend
on the active form of feeding at night.
Which Coral Will Dr. Wellington Use for This Study?
In this study, Dr. Wellington will examine the C-13 carbon in parts of the
exoskeleton that were formed at different times in the life of the coral. He will
obtain the coral from Conch Reef, in southern Florida. A coral reef is a
complex, wave-resistant structure built almost entirely by biological activity. Reefs are found in tropical waters that are warm (about 20Ð30¡C, or 68Ð86¡F)
and shallow (less than 30 meters, or about 90 ft, deep). There are three major
types of reefs:
- Barrier reefs extend several kilometers offshore and contain a large lagoon
between the shoreline and the reef.
- Fringing reefs are located farther offshore and form underwater along the
edges of islands that are, or once were, volcanic. A lagoon or channel runs
between the island and the reef.
- Atolls, the oldest type of reef, are fringing reefs whose associated volcanic island has eroded or subsided below sea level over time.
In southern Florida, most reefs are barrier reef, and the most common coral
are moosehorn coral (Acropora palmata)and staghorn coral. Both these
species have long branches that grow close together and then become
cemented together by algae. Once cemented together, they form a strong,
wave-resistant wall. Dr. Wellington also will study star coral (Montastrea
annularis), which is most commonly found on the inshore side of reefs-
protected from waves. In Florida, star coral is found on the Keys side of the
reef.
What Will Dr. Wellington Do?
Dr. Wellington will look for corals that are about 1 meter (3 ft) high. He will use a drill to take samples of coral heads located at different points on these corals. Then he will use a saw to cut sections of the coral head into thin slabs
(7 to 10 millimeters thick). He will clean these slabs with water, then use two techniques to gather information about climate:
- First, he will X-ray the slabs. He'll use the X rays to count the number and
size of growth rings to determine the age of the coral and the rate of growth
each year.
- Then, he'll use a small drill to take a small sample of exoskeleton from
between each of the rings. He'll use a special tool called a spectrophotometer
to determine the amounts of C-12 and C-13 in each of the samples.
Dr. Wellington also is interested in conducting a controlled experiment that
would help him understand more precisely how the relative amounts of C-12
and C-13 in coral relate to climatic conditions. Corals are very difficult to
study in a laboratory because they usually don't remain healthy for very long
outside their natural environment. So Dr. Wellington may use the
AQUARIUS to conduct an experiment on corals in their natural habitat.
AQUARIUS is an underwater laboratory currently located on Conch Reef.
Scientists can live in AQUARIUS for up to 14 days to conduct experiments
on marine organisms in their natural habitats.
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Gene Carl Feldman
(gene@seawifs.gsfc.nasa.gov)
(301) 286-9428
Todd Carlo Viola, JASON Foundation for Education (todd@jason.org)
Revised: 17 Oct 1995