Oceans in the eastern tropical Pacific are relatively acidic because strong upwelling forces bring up carbon-dioxide-rich waters from the depths below, where decomposition of dead organisms is taking place. Now researchers in the US have found that corals in the region contain low levels of inorganic cement and exhibit high erosion rates.
The finding could provide an insight into the future of coral reefs worldwide as manmade carbon dioxide emissions increase and cause ocean acidification.
“The eastern Pacific is in essence a natural laboratory to study how coral reef ecosystems are structured and function under these acidic conditions,” Derek Manzello of NOAA told environmentalresearchweb. “This is key because laboratory studies are limited in the timescale over which they can be conducted, so we as a research community don’t yet understand the full ramifications of acidification on whole-reef structure and function.”
Panama reef frame
Panama reef frame
Acidification is expected to reduce the rate at which corals build their skeletons. According to the researchers, their findings imply that it could also decrease the precipitation of binding cements, making coral reefs more susceptible to erosion. “This is of concern because it has been found that healthy corals build reef structures at rates only slightly greater than they are ultimately destroyed by physical, biological and chemical erosive processes,” said Manzello. “Thus, any declines in the rate of skeletal production, or in the rates of binding cement precipitation implies that coral reefs will become net erosional with increasing acidification.”
Inorganic cements are a secondary limestone precipitate thought to be vital in the binding and construction of reef framework materials and structures. Manzello and colleagues from the University of Miami, National Center for Atmospheric Research, and National Oceanic and Atmospheric Administration found that the abundance of the cements appeared to track carbon dioxide levels in the oceans of the eastern tropical Pacific. The compounds were nearly non-existent in coral reef framework components from the Galapagos, where carbon dioxide levels are highest.
“The abundance [of the cements] appears to inversely relate to the rate of bioerosion previously documented for these reefs,” said Manzello. “In other words, where these cements are non-existent, bioerosion rates are the highest documented in the entire world. These cements appear to act like a glue that may minimize the rates at which erosive processes occur on reefs.”
Manzello reckons there is still much to be learned about the relationship between the cementation process and seawater chemistry. Now the team hopes to compare the highly-cemented reefs in the Caribbean with uncemented reefs in the eastern Pacific to understand how entire reef systems will respond to ongoing global ocean acidification.
“I hope to add some western Pacific sites to my studies and have a few reef site candidates already in mind that are also impacted by upwelling processes,” he added.
The researchers reported their work in PNAS.
environmentalresearchweb, 29 July 2008. Article.
[The terminology used may be misleading to the general public. The definition of “acidic” is “having the properties of an acid, or containing acid; having a pH below 7“. Eastern tropical Pacific water are obviously not acidic (pH > 7). “Relatively acidic” here means that their pH is lower than elsewhere in the ocean due to upwelling of deep, CO2 rich, waters.
Jean-Pierre Gattuso.]
