31.05.2011 Ocean Acid­i­fic­a­tion and Coral Reefs

Ocean Acidification and Coral Reefs

Nat­ural car­bon di­ox­ide (CO<sub>2</​sub>) seeps in Papua New Guinea have given sci­ent­ists rare in­sights into what trop­ical coral reefs could look like if hu­man-in­duced at­mo­spheric CO<sub>2</​sub> con­cen­tra­tions con­tinue to rise un­abated. At present rates of in­crease, the In­ter­gov­ern­mental Panel on Cli­mate Change (IPCC) fore­casts at­mo­spheric CO<sub>2</​sub> levels of about 750ppm or more by 2100.
 
About a third of this ex­tra at­mo­spheric CO<sub>2</​sub> is ab­sorbed by the world’s oceans. As a con­sequence, pH levels will drop from 8.1 to 7.8, res­ult­ing in in­creased ocean acid­i­fic­a­tion which will severely im­pact mar­ine eco­sys­tems. The coral reefs, hav­ing the highest biod­iversity of all mar­ine eco­sys­tems, are prob­ably the most vul­ner­able.

Dr Kath­ar­ina Fab­ri­cius, sci­ent­ist at the Aus­tralian In­sti­tute of Mar­ine Sci­ence (AIMS) has led two re­search ex­ped­i­tions, with re­search­ers from six coun­tries in­clud­ing Ger­many, USA and Papua New Guinea. Among the in­ter­na­tional team of sci­ent­ists was a group from the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy in Bre­men, ex­perts in the field of mi­cro­sensors, a valu­able tool for study­ing coral reefs.

The team of sci­ent­ists stud­ied three nat­ural CO<sub>2</​sub> seeps in Milne Bay Province, Papua New Guinea. This unique loc­a­tion was dis­covered by chance dur­ing a field trip to doc­u­ment biod­iversity and is the only presently known cool CO<sub>2</​sub> seep site in trop­ical wa­ters con­tain­ing coral reef eco­sys­tems. The study has given sci­ent­ists un­pre­ced­en­ted in­sights into what coral reefs would look like if green­house gas emis­sions and res­ult­ing ocean acid­i­fic­a­tion con­tin­ues to in­crease at present rates. At the seeps, streams of CO<sub>2</​sub> bubbles em­an­ate from the ocean floor due to vol­canic activ­ity.

This week a sci­entific pa­per on the first res­ults of this study is pub­lished in the pres­ti­gi­ous, in­ter­na­tional sci­entific journal Nature Cli­mate Change. It is the first sci­entific pa­per to present data on trop­ical coral reef eco­sys­tems that are nat­ur­ally ad­ap­ted and ac­cli­mat­ised to el­ev­ated CO<sub>2</​sub>. “Our re­search showed us there will be some win­ners, but many more losers, when trop­ical coral reefs are ex­posed to ocean acid­i­fic­a­tion,” said prin­cipal in­vest­ig­ator Dr. Fab­ri­cius. “In the past, we have re­lied on short-term labor­at­ory ex­per­i­ments to tell us what hap­pens to mar­ine or­gan­isms ex­posed to ocean acid­i­fic­a­tion,” she said. “Those ex­per­i­ments in­dic­ated de­le­ter­i­ous ef­fects on the per­form­ance of many spe­cies.”
While labor­at­ory ex­per­i­ments are im­port­ant, Dr Fab­ri­cius said the nat­ural CO<sub>2 </​sub>seeps in Milne Bay provided a more com­plete pic­ture about the eco­lo­gical con­sequences for coral reef com­munit­ies when ex­posed to higher levels of CO<sub>2 </​sub>for many dec­ades. This nat­ural set­ting al­lowed sci­ent­ists to com­pare coral reef com­munit­ies along a gradi­ent from nor­mal present day to low pH.

“We found that as pH de­creases the num­ber and types of cor­als mak­ing up coral reefs are much re­duced. Di­versity of cor­als drops by 40 per cent, and the reef be­comes dom­in­ated by one form of cor­als, massive boulder cor­als (Por­ites).
“The cover of the more del­ic­ate tab­u­late, fo­li­ose and branch­ing cor­als was re­duced three-fold near the CO<sub>2 </​sub>seeps. Sim­il­arly, the abund­ance of soft cor­als and sponges were also sig­ni­fic­antly re­duced. Most im­port­antly we found that reef de­vel­op­ment ceased be­low pH level 7.7.”

One of the MPI co-au­thors, Mar­tin Glas said: “Not only did coral abund­ance change, also other cal­ci­fiers like fo­raminifera and cal­ci­fy­ing al­gae were strongly re­duced un­der el­ev­ated CO<sub>2 </​sub>levels. This is dis­turb­ing news as they rep­res­ent key-spe­cies for the form­a­tion of a healthy coral reef and con­trib­ute sig­ni­fic­antly to the reefs cal­cium car­bon­ate pro­duc­tion.”

Amongst the few win­ners at higher levels of CO<sub>2 </​sub>were seagrasses which showed in­creased cover with three to four times more shoots and roots than un­der nor­mal con­di­tions.

Dr Fab­ri­cius said the study showed that ocean acid­i­fic­a­tion leads to pro­found changes in coral reefs eco­sys­tems. “The de­cline of the struc­tur­ally com­plex cor­als means the reef will be much sim­pler and there will be less hab­itat for the hun­dreds of thou­sands of spe­cies we as­so­ci­ate with today’s coral reefs.
“They would not be the richly di­verse and beau­ti­ful hab­it­ats we cur­rently see in places such as the Great Bar­rier Reef.”
“There are also fewer ju­ven­ile cor­als in areas with high CO<sub>2 </​sub>levels, there­fore coral reefs in those en­vir­on­ments face greater chal­lenges re­cov­er­ing from dis­turb­ances such as trop­ical storms.
“Ul­ti­mately, what we ob­served was that the di­versity of reefs pro­gress­ively de­clines with in­creas­ing CO<sub>2</​sub>. At con­cen­tra­tions sim­ilar to those pre­dicted for the end of this cen­tury at a ‘busi­ness as usu­al’ emis­sions scen­ario, the “coral reef” ob­served was de­pau­per­ate and lacked the struc­tural com­plex­ity of present healthy trop­ical coral reefs. These changes are simply due to ocean acid­i­fic­a­tion, i.e., even without the pro­jec­ted +2°C warm­ing of the oceans as­so­ci­ated with rising green­house gases. The 0.5°C warm­ing we have already ob­served in the trop­ics in the last 50 years has already caused mass coral bleach­ing events and de­clin­ing coral cal­ci­fic­a­tion.”