Surface growth rings of Porites lutea microatolls accurately track their annual growth
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Microatolls are disc-shaped coral colonies, having perimeters of live coral surrounding dead centres, that are limited in their upward growth by exposure at extreme low tide. We looked at surface annuli of microatolls of Porites lutea on the open ocean reef flat of Abaiang Atoll, in the Republic of Kiribati, as potential external indicators of microatoll yearly growth pattern. We measured surface annuli width and disc diameters of 10 microatolls, from March 1998 to October 2000, and found a mean growth rate of 2.2 cm/year. X-radiography was used to establish a direct connection between surface annuli and internal growth bands. Comparison of the growth pattern of surface annuli to observed monthly mean lower low waters showed that these corals closely track fluctuations in local mean sea level. This suggests that there is a potential for using the surface annuli of Porites lutea microatolls on well drained, open ocean reef flats as an ongoing, low cost, non-destructive method of assessing trends in local mean sea level.