Coral percent of the Earth’s surface, yet
Reefs: Mapping the Decline
November 20, 2017
Coral reefs are biodiverse ecosystems that
support 25 percent of marine organisms. They contribute economically by
generating jobs and tourism revenue, stimulating medical research, providing
food sources to millions of people, and protecting coastal communities from
erosion. Although they provide these benefits, they are faced with increasing
threats to their health and productivity. Unfortunately, most of these threats
are human-related and include, but are not limited to, increasing sea surface
temperatures and ocean acidification caused by climate change, pollution,
sedimentation, unsustainable fishing practices, and increased tourism. Emerging
technologies will allow us to better track changes in coral reefs over time.
Remote sensing, one of these technologies, is allowing us to monitor and manage
coral reefs on a larger scale, while also providing us access to areas we could
not normally access. With remote sensing, we can make more informed decisions
on distributing resources to reefs most in need.
Table of Contents
Reef Importance?. 1-4
Economical value. 1-2
Shoreline protection. 2-4
Food source. 4
Medical research. 4
Reefs: Today’s Threats
climate change. 4-5
pollution and sedimentation. 5
increased tourism. 5
unsustainable fishing practices. 6
Remote Sensing and Why is it Useful for Coral Reef Monitoring?
List of Figures
Figure 1. Economic Values of Coral
Reefs by Category. 2
Figure 2. Wave Energy and Height
Reduction Based on Reef Environment. 3
Figure 3. Reef Environment
Illustration with Corresponding Percentages. 3
Figure 4. Sources of Pollution and
Sedimentation with Regards to Coral Reefs. 5
Figure 5. NASA’s CORAL Project Uses
Remote Sensing to Monitor Coral Reef Health. 7
Coral reefs are considered one of the most diverse
ecosystems in the world, rivaling tropical rainforests. They cover less than 2
percent of the ocean floor and less than 1 percent of the Earth’s surface, yet
they provide food and shelter to approximately 25 percent of marine organisms
(Frost, 2012). Not only are they responsible for sheltering and feeding many
organisms, they also contribute billions of dollars economically, protect
coastal shorelines, act as a food source for millions of people, and contribute
to ongoing medical research.
Although a precious commodity, coral reef ecosystems
are facing many threats today. The most critical threat is global warming, but
increasing tourism, pollution, sedimentation, and unsustainable fishing
practices are also causing many problems. Since coral reefs are on the decline,
we must find new ways to monitor the changes over time in coral health so that
proper resources can go to those most at risk. Remote sensing technology is
increasing in popularity as being a preferred method of accomplishing this goal.
Coral reefs are often called the rainforest of the sea
as they are one of the most biodiverse ecosystems in the world. They are home
to over one million species, rivaling, and perhaps exceeding, the biodiversity
of rainforests. Scientists have found that 32 of the 34 recognized animal Phyla
are found on coral reefs, while only 9 of the 34 are found in rainforests
(Spalding, 2001). This biodiversity is important for driving medical research,
providing food, and generating revenue as will be discussed.
Perhaps not immediately recognizable, coral reefs are
very important economically as they generate billions of dollars per year. One
report estimates approximately $30 billion per year, as shown in Figure 1
1. Economic Values of Coral Reefs by Category.
This figure shows the different categories related to coral reef economics and
the approximate amount of money they generate in billions of dollars per year
(NOAA CRCP, 2016).
Most of this money is generated through the tourism
industry as the beauty of corals draws in tourists and divers for recreational
activities such as snorkeling. The tourism industry also provides jobs that
help build healthy local economies. Another major contributor is coastal
protection, discussed next, coming in at approximately $9 billion per year. Fishing
generates close to $6 billion per year and provides income for fisherman in
communities where other jobs are nonexistent. Without the biodiversity of reef
systems, none of this would be possible. For instance, fish would lack places
to hide when growing from juveniles to adults, thus affecting the fishing
industry. It would also remove coral reefs as a source of Marine Natural
Products (MNPs) used to find cures for human illnesses through medical
Coral reefs protect shorelines, homes and businesses,
and other ecosystems, from damage due to storms, waves, and flooding. They are
estimated to protect approximately 40 percent of the world’s population (Ferrario,
et al., 2014) living along 150,000 km of coastlines in over 100 different
countries (Burke, 2011). Out of 255 studies, whole reef systems were found to
reduce wave energy by 97 percent, with 86 percent being reduced by reef crest
alone (Ferrario, et al., 2014). Wave height was also significantly impacted,
with whole reef systems reducing wave height by 84 percent (Ferrario, et al.,
2014). These percentages are shown in Figure 2 below.
2. Wave Energy and Height Reduction Based on Reef Environment.
This figure shows the average percent of wave energy and height reductions
provided by different parts of coral reef environments (Ferrario, et al.,
An illustration of reef environments and correlating
percentages is depicted in Figure 3.
3. Reef Environment Illustration with Corresponding Percentages.
This figure shows how waves interact with different parts of the reef and the
percentage of wave energy and height reduction that corresponds to the whole
reef system (Ferrario, et al., 2014).
These calculations conclude that reefs act as natural
buffers to erosion, which protects people and structures. This coastal
protection is estimated to be valued at approximately 9 billion dollars per
year, only further validating how economically important reefs can be.
Coral reefs are directly connected to fisheries. They
provide shelter to many species that are sought after in the fishing industry.
Without reef habitats for protection, these species fall victim to predation,
leaving less to harvest for fishermen. Many coastal communities rely on
fisheries not only as a source of income, but also as an important source of
animal protein (Reef Resilience Network, 2017).
In the United States, fisheries generate $100 million per year, with
over half of its fisheries being dependent upon coral reefs in some way (Reef
Resilience Network, 2017). On a global scale, 9 to 12 percent of fisheries are
found to be dependent upon coral reefs (Spalding, 2001).
Coral reefs don’t only protect humans from erosional
processes and provide food to millions of people. They also supply cures to
common human illnesses. Many of the species that inhabit reefs have evolved
chemical compounds for protection, some of which have been found to be
medically relevant (Burke, et al, 2011). These chemical compounds are also
known as Marine Natural Products (MNPs) and they provide the basis for marine
pharmacology research (Glaser, 2009).
For example, corals use an enzyme called secosteroids
to protect themselves from disease. This enzyme is also used in treatment of
arthritis and asthma (The Nature Conservancy, n.d.). In 2004, ziconotide, also
known as Prialt and used for treating chronic pain, was approved by the FDA
(Glaser, 2009). Ziconotide is derived from the venom of a marine snail, Conus magus, that is an inhabitant of
coral reefs (McGivern, 2007). Another example of important pharmaceuticals
derived from coral reefs is the anticancer agent, Ara-C, that is derived from
sponges found in Caribbean reef systems (Bruckner, n.d.). There are numerous
known medicines that are harvested from coral reefs directly or through
organisms that rely on healthy reefs for survival, and there are most certainly
unknown discoveries awaiting in the future.
Reefs: Threats of Today
Coral reef health has been declining over the years.
It’s projected that 60 percent of the world’s reefs will be destroyed over the
next 30 years if we don’t implement changes to better manage and protect them
(Burke, 2013). These threats are largely triggered by humans and include
pollution and sedimentation, increased tourism, unsustainable fishing
practices, and perhaps the most important, climate change.
Climate change, a result of an increase of carbon
dioxide in the atmosphere, causes higher sea surface temperatures and
acidification of the ocean. Higher sea surface temperatures act as a heat
stress on corals, subjecting the corals to the equivalent of a persistent
fever. Stress begins with only 1°C increase in temperature relative to the
highest summertime temperature of the water (Glynn and D’Croz, 1990). This is
known as the “bleaching threshold” as it puts the corals at risk for bleaching.
Corals contain zooxanthellae, or microscopic algae,
living in their tissue and providing them with a source of food and energy,
while also contributing to their color (NOAA, 2010). Heat stress causes the
coral to expel their algae, removing their food source and color, while also leaving
it defenseless against diseases (NOAA, 2010). Ocean acidification is another
problem related to climate change as the ocean absorbs more carbon dioxide.
Since corals are calcareous, acidification hinders their growth rates and even
causes dissolution in severe cases.
Pollution and sedimentation, shown in Figure 4, are
also human-related threats to reefs. For the most part, excess nutrients,
pathogens, and sediments, runoff into streams and rivers where they are
transported to the ocean. When sediment transported to the ocean loses energy,
it settles out, often smothering coral reefs. Coral reefs may also be poisoned
by toxins, increasing susceptibility to disease and hindering their ability to
function properly. Algal blooms caused by excess nutrients can block sunlight
needed for reef systems and their inhabitants to survive.
4. Sources of Pollution and Sedimentation with Regards to Coral Reefs.
This figure shows the different pathways taken by pollution and sedimentation
to coral reefs. (NOAA, 2017).
Increased tourism, while positive for the economy, can
have negative impacts on reefs. Reefs are often damaged by careless tourists
who walk on them even though they are asked not to. Boat anchors also cause
destruction when they are dropped on coral reefs.
Unsustainable fishing practices are major threats to coral
reefs. Overfishing reduces productivity of reefs and revenue generated by
fisheries, but consequences of destructive fishing practices are even worse.
Dynamite, or blast, fishing destroys large areas of reefs when explosives are
used to stun or kill large quantities of fish for easy collection. This
practice is outlawed in most places, but is often used illegally in Southeast
Asia and East Africa (Burke, 2011).
Together, these threats create unique problems facing
the coral reef systems of the world. We must be diligent in our efforts to
better monitor and protect these fragile ecosystems. There are many methods
being implemented today to achieve this, with remote sensing satellite imagery
being perhaps the most important.
is Remote Sensing and Why is it Useful for Coral Reef Monitoring?
With coral ecosystems facing severe threats every day,
more efficient monitoring methods are highly sought after to ensure that
high-risk reefs receive the necessary resources to delay and ultimately prevent
destruction. Mapping reefs provides a way to collect important data about reef
characteristics, stressors, and health. It also provides a point of reference
that is required to track changes in reefs over time.
A compilation of methods is used to monitor coral reef
health. These methods include, but are not limited to, photo and video mapping
via towed underwater devices, field observations made by scuba divers, seafloor
topography maps generated by LIDAR, GIS, swath acoustic seabed mapping systems,
ground-truthing, and remote sensing satellite imagery (USGS, 2017). Although no
one method is best, and all must be used to reinforce each other, remote
sensing has gained increasing favorability over the years.
“Remote sensing”, as defined by NOAA, “is the science
of obtaining information about objects or areas from a distance, typically from
aircraft or satellites” (NOAA, 2009). With remote sensing, sensors are attached
to aircrafts or satellites and they capture electromagnetic energy that is
reflected, usually via sunlight, from the earth’s surface back to the sensor.
Information about that specific energy, or wavelength, is recorded and sent to
a receiving station to be processed into an image that is usable by scientists
The increase in popularity of this technology is
mostly attributed to its ability to easily repeat observations over very large
areas, making it easy to track changes on the earth over time. It is also
non-invasive and non-destructive as contact is not made with the object of
interest. Additionally, it allows scientists access to areas that are not
physically accessible. However, remote sensing is not as accurate as field
observations, nor can it be used at night or with heavy cloud coverage. It
requires a well-lit, clear view of the object of interest.
Since objects reflect
light differently, remote sensing can be used to detect the state of health in
coral reefs. Sand, algae, live coral, and dead coral, as shown in Figure 5, all
reflect different wavelengths of energy. This data, which is later validated by
ground-truthing, or field-based observations, is useful in determining reef
characteristics and health status.
Figure 5. NASA’s CORAL Project Uses Remote
Sensing to Monitor Coral Reef Health. This figure shows the
method used by NASA’s CORAL Project. PRISM (Portable Remote Imaging
Spectrometer) is used to collect spectral image data (Earth Science Airborne
Remote sensing also gives scientists a range of other
relevant data that is pertinent to monitoring reef health. This data includes
four basic types: color of the water, temperature of the water, how rough the
water surface is, and how far away the water surface is. With this, sea floor
topography, sediment load, wave height and length, sea level, turbidity, ocean
current direction and speed, and more, can be determined. Since coral bleaching
is a foremost threat to coral reefs today, sea surface temperature monitoring
is extremely important. It lets us know which reefs are susceptible to
bleaching events and allows us to plan accordingly. With the many advantages of
remote sensing discussed above, it is easy to see why it has become perhaps the
most preferred method for monitoring the precious coral reef ecosystems of our
Coral reefs are biodiverse ecosystems that support
millions of marine species. They are economically valuable, protect coastal
areas from storm and wave erosion, provide important sources of protein to
coastal communities, and may indirectly provide cures to diseases, leading to
important ongoing medical research. Although irreplaceable, coral reefs are
declining rapidly due to factors that are mostly human driven. These factors
include climate change, increased tourism, unsustainable fishing practices,
pollution, and sedimentation. Better policies, management, and conservational
efforts are needed if we want to protect these valuable ecosystems and preserve
them for the future. Remote sensing technology is one of the most important
ways these systems are monitored, making it easier for scientists to allocate
resources to reefs most in need.