Evidence for systematic declines in growth rates of corals from the Florida reef tract
Peter K. Swart1), R.E Dodge2) and H.D. Hudson3)
1) MGG/RSMAS, University of Miami, 4600 Rickenbacker Causeway, Miami Fl 33149, U.S.A. 2) NOVA Southeastern University, Dania, Fl 33004, U.S.A. 3) Key Largo National Marine Sanctuary, P.O. Box 1083, Key Largo, Fl 33037
Over the past twenty years corals from the Florida reef tract have been heavily sampled to study changes in growth rate and geochemical proxies which might be indicative of global climate change resulting from increases in the concentration of atmospheric CO2 or other anthropogenic processes. In total over 300 individuals have been studied, principally the species Montastraea faveolata, from the Dry Tortugas to West Palm Beach, a distance of over 300 km. The extension rates have been measured on all of these specimens, the density on three individuals, and the stable oxygen and carbon isotopic composition on six. Although there is a great deal of variability in the extension rate of these corals, our studies have revealed (i) systematic changes in extension rate which can correlated over the entire distance of the study area, (ii) systematic decreases in extension rate since in start of the 18th century, (iii) in two individuals which have been studied to date, decreases in the rate of calcification rate since the 18th century. Although these data are consistent with the suggestion that increases in the pCO2 of the atmosphere which have occurred over the last 200 years have reduced the saturation state of the oceans sufficiently to cause a decline in the calcification rate of corals, alternative influences such as local changes must also be considered. In addition carbon isotopic analyses of these corals clearly show the 13C-Suess effect, resulting from the addition of isotopically depleted carbon to the atmosphere and oxygen isotopic compositions in all the corals show a slight decrease towards the modern day, consistent with an increase in water temperature. Although significantly more work remains to be carried out to establish unequivocally that these changes are related to global change, the trends observed in this unique data set may provide the evidence that changes in pCO2 are related to the decrease in calcification in marine organisms.
