245
absorb the higher energy radiation, which is why tannins in seawater are sometimes
called “sunscreens”. However, as the tannins absorb solar radiation, they are broken
down and the solar energy is degraded to heat. Thus, an increase in the UV-B reaching the sea surface has at least three effects in shallow waters: (1) it increases the
rate of breakdown of tannins, thereby increasing water transparency; (2) this allows
the higher-energy wavelengths (UV-B, UV-A, and blue light) to penetrate deeper
into the water; and (3) additional heat is added to the surface waters. In already clear
waters, a small change in water transparency can significantly increase the radiant
energy reaching the seafloor at reef depths. As a consequence, corals living in waters
with naturally low concentrations of tannins (e.g., outer reef habitats) are those most
affected because an increase in either blue light or UV can increase photo-oxidative
stress. This specific factor at least partly explains why bleaching and disease have
caused the most mortality in corals on offshore reefs, while less decline has been
observed in coral populations on mid-shelf reefs.
So why did mass bleaching emerge as a threat to reef ecosystems in 1982–1983?
The El Niño conditions reduced upwelling, which diminished the supply of nutrients for phytoplankton growth, increasing water transparency. Ocean waters became
warmer and clearer, and ozone depletion following the El Chichon eruption allowed
more UV-B to reach the sea surface. Combined, these changes resulted in increased
photo-oxidative stress on the corals, with the result being mass bleaching and substantial coral mortality.
Unfortunately for coral reefs worldwide, the decline in the stratospheric ozone
has proceeded along with, and contributed to, the increase in sea-surface temperatures. The eruption of Mt. Pinatubo in May 1991 initially injected so much ash and
aerosol into the stratosphere, which reflected solar radiation, that it caused shortterm global cooling. But as the ash settled out, the chemicals in the aerosols continued to degrade the stratospheric ozone. By the mid-1990s, the Earth’s average
stratospheric ozone concentration plummeted to the lowest levels ever recorded.
When another major El Niño event developed in 1997, the consequences were devastating for corals nearly globally.
According to the Scientific Assessment of Ozone Depletion: 2018, there have
been inter-annual fluctuations, but not significant recovery in stratospheric ozone
concentrations since 1997. The recovery of the stratospheric ozone layer to 1980
levels is predicted to occur by 2050. In the meantime, coral bleaching events are
now five times more common than 40 years ago. And between 2015 and 2018, the
world’s reefs experienced three consecutive years of widespread bleaching.
The Effects of Coastal Perturbation and Pollution
Returning to coral ecology, corals are very diverse and different species can live
across a wide range of habitats, with different ecological niches and feeding strategies. Many depend mostly on the food they receive from their symbiotic algae for
day-to-day energy needs, while others are more active feeders. Some corals thrive
14 What Is Happening to the World’s Coral Reefs?
absorb the higher energy radiation, which is why tannins in seawater are sometimes
called “sunscreens”. However, as the tannins absorb solar radiation, they are broken
down and the solar energy is degraded to heat. Thus, an increase in the UV-B reaching the sea surface has at least three effects in shallow waters: (1) it increases the
rate of breakdown of tannins, thereby increasing water transparency; (2) this allows
the higher-energy wavelengths (UV-B, UV-A, and blue light) to penetrate deeper
into the water; and (3) additional heat is added to the surface waters. In already clear
waters, a small change in water transparency can significantly increase the radiant
energy reaching the seafloor at reef depths. As a consequence, corals living in waters
with naturally low concentrations of tannins (e.g., outer reef habitats) are those most
affected because an increase in either blue light or UV can increase photo-oxidative
stress. This specific factor at least partly explains why bleaching and disease have
caused the most mortality in corals on offshore reefs, while less decline has been
observed in coral populations on mid-shelf reefs.
So why did mass bleaching emerge as a threat to reef ecosystems in 1982–1983?
The El Niño conditions reduced upwelling, which diminished the supply of nutrients for phytoplankton growth, increasing water transparency. Ocean waters became
warmer and clearer, and ozone depletion following the El Chichon eruption allowed
more UV-B to reach the sea surface. Combined, these changes resulted in increased
photo-oxidative stress on the corals, with the result being mass bleaching and substantial coral mortality.
Unfortunately for coral reefs worldwide, the decline in the stratospheric ozone
has proceeded along with, and contributed to, the increase in sea-surface temperatures. The eruption of Mt. Pinatubo in May 1991 initially injected so much ash and
aerosol into the stratosphere, which reflected solar radiation, that it caused shortterm global cooling. But as the ash settled out, the chemicals in the aerosols continued to degrade the stratospheric ozone. By the mid-1990s, the Earth’s average
stratospheric ozone concentration plummeted to the lowest levels ever recorded.
When another major El Niño event developed in 1997, the consequences were devastating for corals nearly globally.
According to the Scientific Assessment of Ozone Depletion: 2018, there have
been inter-annual fluctuations, but not significant recovery in stratospheric ozone
concentrations since 1997. The recovery of the stratospheric ozone layer to 1980
levels is predicted to occur by 2050. In the meantime, coral bleaching events are
now five times more common than 40 years ago. And between 2015 and 2018, the
world’s reefs experienced three consecutive years of widespread bleaching.
The Effects of Coastal Perturbation and Pollution
Returning to coral ecology, corals are very diverse and different species can live
across a wide range of habitats, with different ecological niches and feeding strategies. Many depend mostly on the food they receive from their symbiotic algae for
day-to-day energy needs, while others are more active feeders. Some corals thrive
14 What Is Happening to the World’s Coral Reefs?