Studies conducted in the waters of Florida and the Caribbean have shown that there have been declines in the coral populations which have coincided with increases in the presence of macroalgae. The causes of this decline still aren’t agreed upon. Various things, some of human influence and some of natural occurrence, such as water quality and diseases, or storms and temperature rises, have been suggested as possible reasons for the decreasing numbers in coral populations.

In ocean acidification the increase in the quantities of Carbon Dioxide (CO₂₎ present in the water decreases the amount of Calcium Carbonate (CaCO_3­) available. Because Calcium Carbonate is used by various organisms to make skeletons, ocean acidification affects the calcification rates in corals. This subsequently interferes with their skeletal growth and sometimes even results in the dissolution of their skeletons in conditions where pH levels were lower than what the organisms were accustomed to. Decreased rates of skeletal growth can have negative effects on coral survival. For instance, thinner more fragile skeletons decrease coral ability to resist erosion. Decreases in skeletal growth also changes the age at which they reach sexual maturity.

Coral and macroalgae compete for space. So a decrease in coral population is beneficial for their main competitors. Macroalgae thrive in environments with no herbivores, high quantities of nutrients and slow coral growth. Ocean acidification slows coral growth and therefore favors the spread of macroalgae over coral.

Further studies have shown that increase in contact and interaction between corals and macroalgae increased the rates of disease spread in coral populations. One particular infection known as the white plague which has spread from colonies in the Florida Keys all throughout the Caribbean. Results of an experiment conducted to test the veracity of this claim demonstrated that after two weeks of exposure to Halimeda Opuntia, a type of algae, coral colonies tested on displayed symptoms of infection. And after a month, they had fully contracted the disease.

The effects of decrease in coral populations and increases in macroalgal populations have repercussions on other species as well. The presence of coral reefs provides physical protection to the creatures that inhabit shallower coastal waters. Their erosion would leave these creatures and their homes exposed to the elements.

Lower coral presence can also lead to decline in biodiversity, as the coral reefs become unable to offer adequate habitat for the species to which they provide shelter. While a direct relation has yet to be demonstrated, large fish presence in various areas has usually corresponded with low quantities of macroalgae. And large quantities of macroalgae tend to coincide with low coral population. “As soon as they move in, there goes the neighborhood.”

Sources:

D. Lirman, Competition between macroalgae and corals: effects of herbivore exclusion and increased algal biomass and growth. 9/4/2011

Nancy Knowlton, Jeremy B.C. Jackson, Shifting baselines, Local impacts, and Global Change on coral reefs. 9/4/2011

Maggy M. Nugues, Garriett W. Smith, Ruben J. Von Hooidonk, Maria I. Seabra, Rolf P. M. Bak, Algal contact as a trigger for coral disease. 9/4/2011

http://www.springerlink.com/content/u0481van807pkncm/fulltext.pdf

http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.0060054

http://onlinelibrary.wiley.com/doi/10.1111/j.1461-0248.2004.00651.x/full

With the accelerating phenomenon of ocean acidification caused by increased atmospheric CO2 levels, there is a pressing need for evidence delineating the effects on different types of oceanic ecosystems. The majority of on-site studies to date on ocean acidification have been on coral reef ecosystems. Researchers from the University of Miami examined three coral reef sites in Papa New Guinea. At each site, there were volcanic fissures on the seabed that constantly poured out CO2, simulating the effects of atmospheric CO2 dissolving into the ocean. The pH level dropped from the normal level of 8.1 to 7.7, a more than 250% increase in the concentration of hydrogen ions. An oceanic pH of 7.7-7.8 is projected for the year 2100 in the business-as-usual model, where humans do nothing to curb the rates of CO2, being emitted into the atmosphere.

The glimpse into what our coral reefs could become in less than a century was certainly shocking. The coral diversity at the sites dropped by a staggering 40 percent, due to the marked decline of the key calcifiers such as the zooxanthellate corals and macroalgae. These species suffer when greater amounts of CO2 are dissolved into the ocean because the gas reacts with H2O to produce greater concentrations of hydrogen ions, driving the pH down. As pH decreases, CO3- (Carbonate ion), a key ingredient for calcification, becomes more scarce. As a result, many corals suffer a decreased rate of calcification, eventually dying off because they are unable to adapt to the changing ocean chemistry. Though, it has to be noted that not all species are harmed from ocean acidification, namely sea grasses and non-calcifying macroalgae. In fact, the researchers discovered that sea grasses extended their area on the seabed by three or four times after the volcanic leakage of CO2, demonstrating that unaffected species capitalize on the decline of calcifying organisms. Access to sunlight is crucial for the survival many coral reef species, explaining why the sea grasses were quick to colonize into the recently vacated areas. The changes in the coral reef ecosystem are better described by a “regime-shift” of species rather than mass extinction.

Still, the losers from an acidified ocean greatly outnumber the winners. As calcifying organisms begin to disappear, the complexity of coral structures also diminishes, leaving a more flattened habitat. This then cascades into the decline of other aquatic organisms such as schools of fish or bottom-dwelling crabs that rely on the coral structures to live in, hide, or hunt from. The level of diversity across the board dwindles significantly from the adverse effects of increased amounts of CO2 released into the environment that eventually dissolves into the ocean. If this trend continues for rest of the century, perhaps coral reefs will no longer be described as the “rainforest” among oceanic ecosystems.

Sources:
http://www.physorg.com/news/2011-06-ocean-acidification-coral-reefs.html
http://www.earthtimes.org/nature/ocean-acidification-threatens-coral-reefs/936/