241
Seeing the Light
Unfortunately, at the same time I was “seeing the light” as the necessity for thriving
coral reefs, the “light” itself was becoming a problem. Visible light is a component
of the energy spectrum that ranges from high-energy gamma rays, X-rays, and ultraviolet (UV) radiation, through visible (light) radiation, to infrared (heat) radiation,
to low energy radio waves. The high-energy gamma rays, X-rays, and most energetic UV known as UV-C and UV-B, carry sufficient energy to cause genetic damage. They also carry sufficient energy such that, when they degrade to infrared
radiation, substantial heat is released. Fortunately for life on Earth, sunlight reaching the Earth is almost half visible, slightly over half infrared, and only about 2% UV.
A rather puzzling characteristic of zooxanthellate corals is that they commonly
live at temperatures very near their maximum thermal tolerances. For example, corals commonly are stressed when exposed to temperatures 2°C above those typical
of their seasonal highs. In full tropical sunlight, the process of photosynthesis by the
algae within the corals becomes overwhelmed. As a result, in addition to producing
oxygen molecules (O 2 ), the process produces an excess of oxygen radicals (charged
oxygen ions, O
−
) that can break down the proteins and enzymes of the algae and
their hosts (humans use antioxidant foods and supplements to prevent damage from
oxygen radicals that are naturally produced in our bodies during metabolism). In
zooxanthellate corals, production of excess oxygen radicals in sunlight is known as
“photo-oxidative stress” and is amplified as the temperature rises. Because the corals are adapted to respond by expelling some of their algal cells, if the coral host
suffers such stress over days to weeks, the result is visible color loss known as
“bleaching” (Fig. 14.8). In more extreme cases, mass coral-bleaching events occur.
The U.S. National Oceanic and Atmospheric Administration Coral Reef Watch
posts probable bleaching intensities by monitoring sea-surface temperatures using
data from satellites.
Prior to 1982, coral bleaching was relatively rare and mass bleachings were
unknown. The 1982–1983 bleaching event coincided with unusually warm seasurface temperatures associated with a major El Niño event (El Niño and La Niña
refer to periodic changes in the strength of winds across the Pacific Ocean that influence weather globally). This mass bleaching caused extensive coral mortality in the
eastern tropical Pacific. There, coral reefs have historically been exposed to elevated
nutrients and cooler waters associated with an oceanographic process known as
upwelling.
To understand why El Niño events occur and why they create such dramatic
events on land, ranging from severe droughts and fires in some regions to massive
rain-induced flooding and mudslides in otherwise desert regions, and of course in
the oceans, events such as failures in fish catches, seabird die-offs, and coral bleaching events, one needs to understand how temperature and dissolved nutrients are
distributed in the oceans. Only about 2% of the ocean is warm, representing a thin
warm surface layer that does not reach into polar waters. The warm surface layer is
mixed by the winds, usually down to 10s of meters, sometimes as deep as 150 m or
14 What Is Happening to the World’s Coral Reefs?
Seeing the Light
Unfortunately, at the same time I was “seeing the light” as the necessity for thriving
coral reefs, the “light” itself was becoming a problem. Visible light is a component
of the energy spectrum that ranges from high-energy gamma rays, X-rays, and ultraviolet (UV) radiation, through visible (light) radiation, to infrared (heat) radiation,
to low energy radio waves. The high-energy gamma rays, X-rays, and most energetic UV known as UV-C and UV-B, carry sufficient energy to cause genetic damage. They also carry sufficient energy such that, when they degrade to infrared
radiation, substantial heat is released. Fortunately for life on Earth, sunlight reaching the Earth is almost half visible, slightly over half infrared, and only about 2% UV.
A rather puzzling characteristic of zooxanthellate corals is that they commonly
live at temperatures very near their maximum thermal tolerances. For example, corals commonly are stressed when exposed to temperatures 2°C above those typical
of their seasonal highs. In full tropical sunlight, the process of photosynthesis by the
algae within the corals becomes overwhelmed. As a result, in addition to producing
oxygen molecules (O 2 ), the process produces an excess of oxygen radicals (charged
oxygen ions, O
−
) that can break down the proteins and enzymes of the algae and
their hosts (humans use antioxidant foods and supplements to prevent damage from
oxygen radicals that are naturally produced in our bodies during metabolism). In
zooxanthellate corals, production of excess oxygen radicals in sunlight is known as
“photo-oxidative stress” and is amplified as the temperature rises. Because the corals are adapted to respond by expelling some of their algal cells, if the coral host
suffers such stress over days to weeks, the result is visible color loss known as
“bleaching” (Fig. 14.8). In more extreme cases, mass coral-bleaching events occur.
The U.S. National Oceanic and Atmospheric Administration Coral Reef Watch
posts probable bleaching intensities by monitoring sea-surface temperatures using
data from satellites.
Prior to 1982, coral bleaching was relatively rare and mass bleachings were
unknown. The 1982–1983 bleaching event coincided with unusually warm seasurface temperatures associated with a major El Niño event (El Niño and La Niña
refer to periodic changes in the strength of winds across the Pacific Ocean that influence weather globally). This mass bleaching caused extensive coral mortality in the
eastern tropical Pacific. There, coral reefs have historically been exposed to elevated
nutrients and cooler waters associated with an oceanographic process known as
upwelling.
To understand why El Niño events occur and why they create such dramatic
events on land, ranging from severe droughts and fires in some regions to massive
rain-induced flooding and mudslides in otherwise desert regions, and of course in
the oceans, events such as failures in fish catches, seabird die-offs, and coral bleaching events, one needs to understand how temperature and dissolved nutrients are
distributed in the oceans. Only about 2% of the ocean is warm, representing a thin
warm surface layer that does not reach into polar waters. The warm surface layer is
mixed by the winds, usually down to 10s of meters, sometimes as deep as 150 m or
14 What Is Happening to the World’s Coral Reefs?