Last Thursday, Pamela Hallock Muller, professor of biogeological oceanography at the University of South Florida, delivered a lecture entitled “Coral Reefs in the 21st Century: Is the Past the Key to the Future?” The lecture, part of a series sponsored by the geosciences department, highlighted the principal threats to coral reefs worldwide: ecological phenomena such as ocean acidification, rising sea surface temperatures, increased exposure of the earth’s surface to ultraviolet radiation and exposure of coastal waters to nutrient-rich surface runoff.
In her lecture, Muller referred to lessons learned from studying the ancient geologic record. She noted that an understanding of what will likely happen to shallow-water coral reefs can be derived from a “mass extinction event” that occurred during the Eocene period.
The concentration of human populations along coastlines contributes greatly to the infusion of plant nutrients such as nitrogen and phosphorous from fertilizers into the delicate ecosystems of coral reefs through runoff. Coral are designed to flourish and form reefs in marine environments where such nutrients are scarce. They derive 90 percent of their energy from photosynthesis efficiently performed by symbiotic algae. The nutrients deposited in great quantities by agricultural runoff promote excess algal blooms by other species, which can obscure light streaming through the usually clear water of a coral reef environment, thus interrupting the photosynthesis of the coral’s symbiotic algae.
“It’s like pouring water on a cactus in the desert,” Muller said. “It doesn’t help the cactus, which is eventually replaced by grass, changing the ecosystem entirely.” In excessively large quantities, these nutrient deposits can encourage devastating algal blooms that kill off other species competing for nutrients. The algal blooms also deplete the dissolved oxygen content in the area, thereby creating a dead zone in which no other marine animals can survive. Such areas exist in parts of Chesapeake Bay and in the Gulf of Mexico at the Mississippi River basin, where the dead zone occupies a space the size of New Jersey.
Muller delineated other factors endangering the survival of the world’s remaining coral reefs, including increased exposure to ultraviolet radiation, a result of the depletion of the ozone layer and rising ocean temperatures, a result of the general trend toward global warming. Taken together, these phenomena contribute to an event known as coral bleaching. “This bleaching represents the loss of color in animals that host algal symbionts. It results from the inability of the algae to tolerate high temperatures and intense irradiation,” said Muller. “The presence of these factors is now relatively common. Bleaching thresholds are exceeded annually. The intensity of UVB radiation that in 1960 was present only around the summer solstice is now what we are exposed to from April through August.”
The photo-oxidative stress experienced by the coral’s symbiotic algae can lead to its death or to its expulsion by the coral. Without the presence of symbiotic algae, the coral loses its primary energy source and is perilously weak. Bleached coral is susceptible to disease, virtually incapable of reproduction and lacking sufficient energy to build onto its aragonite shell and skeleton.
This final implication of coral bleaching becomes especially important when the effects of an increasingly acidic ocean environment are taken into consideration. Humans produce carbon dioxide as a byproduct of industrial practices such as the combustion of fossil fuels and the production of cement. “You are breathing in 30 percent more carbon dioxide today than your great-grandmother did,” Muller said. “By 2065 it is predicted that the concentration of atmospheric carbon dioxide will be double what it was in the pre-industrial era.”
The oceans absorb increasing levels of carbon dioxide as more is released into the atmosphere. This means that more molecules will react with water to form carbonic acid, thus decreasing the pH of the environment, which also decreases the extent to which the ocean environment is saturated with the carbonate ion. When the ocean environment is under-saturated with this ion, structures made of calcium carbonate in all its forms (including the aragonite found in coral) are vulnerable to dissolution.
Muller gravely concluded the lecture by saying, “we are likely the last human generations to see shallow-water reefs.” She attributed the loss of such coral reefs in the 21st century to a combination of all of the factors she described. “The human population has exceeded the carrying capacity of the planet,” Muller noted. “We produce 400 times the annual energy production of photosynthesis in the burning of fossil fuels.”