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Evidence for a decadal-scale decline in the growth rates of juvenile scleractinian corals
Juvenile life stages play critical roles in the population dynamics of virtually all organisms, and therefore precise estimates of juvenile growth and survival are important for accurate demographic analyses. For tropical reef corals, the contribution of juveniles to population dynamics is strongly determined by their growth rates, which are inversely proportional to the duration of this life stage and the risks of mortality, yet empirical estimates of this important trait are surprisingly rare. Based largely on results published before 1990, it is often assumed that juvenile corals ≤50 mm diameter grow ~10 to 34 mm yr-1, and therefore are ~1.5 to 5.0 yr old. In contrast, results presented here show that juvenile corals (≤40 mm diameter) in St. John, US Virgin Islands, have grown at much slower rates on shallow reefs (<9 m depth) where annual censuses have been completed for 9 yr (1996 to 2005). For nearly a decade, juvenile corals in this location have maintained overall mean growth rates of only 3 mm yr-1, or 6 mm yr-1 for the subset of colonies that grew ≥ 0 mm yr-1. Therefore, most of these juvenile corals have grown at rates consistent with an upper age estimate of 7 to 13 yr, which is 1.4 to 8.7 times older than estimates derived from often-cited growth rates. This discrepancy has important implications, because it suggests that the recruitment dynamics of coral populations may function over time scales longer than are usually considered. Conceivably, these time scales may now extend over lengthier periods than once was the case, at least as can be determined from sparse results distributed through >32 yr of peer-reviewed studies that reveal a gradual decline in the growth rates of juvenile corals. The correspondence of this decline with rising seawater temperature and depressed aragonite saturation state raises the possibility that the effects of global climate change have already reduced the growth of juvenile corals.