Coral reefs are uniquely complex ecosystems in that they are defined by the underlying geologic structures ("reefs") that are constructed primarily by calcifying organisms (mostly coral and algae). Coral-reef habitats are one of the most important ecosystems on Earth. They harbor the highest biodiversity of any known marine ecosystem and provide critical habitat for many fish and invertebrate species that are of global commercial importance. Coral reefs also provide numerous important economic benefits that help sustain a large and ever-growing coastal human population. However, a recent increase in anthropogenic and climatic stresses has resulted in degradation and near collapse of many coral communities worldwide. Recent reports have identified ocean acidification as a potential major stressor to coral reefs and the various calcifying organisms that build them. Improved understanding and information are needed to guide policies and best-management practices effectively in order to preserve and restore coral-reef resources for future generations.
As atmospheric carbon dioxide (CO2) continues to increase, primarily as a result of anthropogenic activities, complex changes to the carbon cycle take place on land and in the oceans. Oceans are the ultimate sink for most of the additional CO2, which has significant impacts on seawater chemistry. As the amount of CO2 dissolved in seawater increases, pH decreases (hence the term "ocean acidification"), which in turn decreases the availability of carbonate ions (CO32-). The decrease in carbonate ions lowers the saturation state with respect to aragonite, which is an important mineral for shell and skeleton formation in calcifying marine organisms. Estimates indicate that ocean surface waters may have already undergone a reduction of 0.1 pH units since the beginning of industrial times. As a result, complex interactions between seawater, calcifying organisms, and surrounding carbonate sediments are expected to occur in coastal-marine ecosystems.
Calcification rates are expected to decrease in response to decreasing pH for several major groups of calcifying marine organisms in coastal and open-ocean environments. Numerous predictions based on both in situ experiments and computer-model simulations indicate that large decreases (as much as 50 percent) in calcification and an associated loss of coral-reef ecosystems could occur within the next few decades to centuries. In contrast, some researchers have concluded that, despite a decrease in ocean pH and aragonite saturation, calcification in corals may increase, owing to an increased metabolic response driven by warming associated with increased anthropogenic CO2. Although such findings remain controversial, they emphasize the fact that critical gaps exist in our knowledge of how coastal tropical-marine ecosystems, such as coral reefs, will respond to global changes brought about by increased atmospheric CO2.
|Above left: X-radiograph positive of a coral skeleton. A graphical overlay of relative skeletal density (white line) shows the alternating high- and low-density bands. Each couplet corresponds to 1 year of coral growth. Image by Kevin Helmle and Ryan Moyer. Above center: USGS scientists Nate Smiley (left) and Ryan Moyer use a submersible hydraulic drill to extract a coral core from a large colony ofMontastraea faveolata off Puerto Rico. Photograph by Chris DuFore. Above right: Ryan Moyer applies a cement-and-epoxy cap to the core hole in a large colony of Montastraea faveolata. The cap will allow coral tissue to grow over the affected area. Photograph by Nate Smiley.|
Currently, U.S. Geological Survey (USGS) scientist Ryan Moyer is trying to understand the effects of ocean acidification on coral growth and calcification by measuring growth and geochemical variations in coral cores collected in the western Atlantic and Caribbean Sea region. Moyer is a Mendenhall Postdoctoral Research Fellow at the USGS science center in St. Petersburg, Florida. His current research builds on his earlier Ph.D. work, which focused on using geochemical tracers in coral skeletons to understand local carbon cycling in tropical mountainous watersheds and how processes on land affect the biogeochemistry of the tropical coastal ocean and coral reefs.
Corals are excellent recorders of environmental change: they deposit calcium carbonate (aragonite) skeletons in distinct couplets of annual bands and can grow for several hundred years. Cyclic variations in skeletal density within the growth record of corals are evident on X-radiographs and can be combined with isotope and (or) trace-metal geochemistry of the skeleton to serve as proxies for a host of paleoenvironmental events and conditions. Several of these proxies have direct relevance to research addressing the question of coral response to ocean acidification. Variations in the ratio of stable isotopes of boron (δ11B) record changes in seawater pH, and skeletal density as inferred from X-radiographs records relative changes in growth and calcification over the lifespan of the coral. Thus, Moyer hypothesizes that the information recorded in skeletons of modern corals that have grown over the past century (or longer) should provide critical information on how corals have responded in terms of growth and calcification to seawater pH changes that are already known to have occurred since preindustrial times, owing to excess anthropogenic CO2. Coral-based paleo-pH records have been successfully produced by using a Porites coral from the Great Barrier Reef; the authors of that study concluded that "Additional paleo-pH records are required from a range of coral reef ecosystems to improve our understanding of the physical and biological controls on reef-water pH, and the long-term impacts of future ocean acidification." (From "Preindustrial to Modern Interdecadal Variability in Coral Reef pH" by Carles Pelejero and others, published 2005 in Science, v. 309, p. 2204-2207 [http://dx.doi.org/10.1126/science.1113692].)
In July 2009, Moyer, Nate Smiley, and Chris DuFore (all USGS, St. Petersburg) traveled to Puerto Rico to collect cores from corals growing on the reefs off La Parguera. Coral coring is a labor-intensive process that involves the use of a surface-supplied hydraulic drill by a trained team of scientific divers. The drill is fitted with a 4-inch-diameter diamond-bit core barrel that is used to drill into the coral skeleton. Extension bars allow the team to collect cores as long as 3 m, which could correspond to 300-plus years of coral growth. The USGS team collected eight cores from large colonies of three species: Montastraea faveolata, Diploria strigosa, and Siderastrea siderea. After each colony was cored, the borehole was filled with loose reef rubble, fitted with a cement cap, and sealed with an underwater epoxy. This "plug" creates a surface for the surrounding coral tissue to grow over. Monitoring studies have shown that complete coral recovery is possible within a few years after coring.
A preliminary assessment of the Puerto Rico cores indicates that they include at least two 100-year-plus records of coral growth. Further processing of the cores is currently being conducted by Moyer and collaborators Kevin Helmle of the National Oceanic and Atmospheric Administration (NOAA) and Richard Dodge of the National Coral Reef Institute (NCRI). The cores will be cut longitudinally, planed into parallel-sided slabs, and X-radiographed to reveal an accurate coral-growth history. Moyer and his collaborators will then use the X-radiographs to determine changes in growth rate and calcification over the lifespan of the coral. Moyer will also make measurements of δ11B within the skeleton in cooperation with Bärbel Hönisch at the Lamont-Doherty Earth Observatory. The growth and calcification data will then be combined with the δ11B paleo-pH proxy data to determine how coral growth and calcification have responded to changes in surface-ocean pH since the beginning of industrial times. This work is also being conducted on coral cores collected off Florida and Tobago, whereby Moyer hopes to better understand the natural variability and local- to regional-scale impacts of ocean acidification on coral growth in the Caribbean and western tropical Atlantic Ocean regions.