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Substratum stability and coral reef resilience: insights from 90 years of disturbances on a reef in American Samoa

Author(s): Birkeland, C.E | Green, A. | Fenner, D. | Squair, C. | Dahl, A.L.

Journal: Micronesica
ISSN 0026-279X

Volume: 2013;
Issue: 6;
Start page: 1;
Date: 2013;
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Keywords: coral | resilience | recovery | substrata | rubble | zonation

n 1917, Alfred Mayor recorded rich coral communities in distinct zones on the reef flat along a permanent transect at Aua village in Pago Pago Harbor, American Samoa. In the 1950s-1980s, this area was seriously degraded by chronic pollution from two tuna canneries and fuel spills in the inner harbor and by coastal development. By the 1970s, coral communities had declined substantially. Mayor provided a map and photographs of the transect in his 1924 report and so we were able to repeat surveys along the same transect in 1973, 1980, 1995, 1998, 1999, 2000, 2004 and 2007. In 1992, a large pipe was installed to export wastewater from the tuna canneries to the harbor mouth. Management of coastal development and fuel spills had improved by the early 1990s. We found that since then, there has been a significant recovery of coral communities on the reef crest and outer reef flat where there is consolidated reef substratum (up to 30 m back from the reef crest). In contrast, we found that recovery has been substantially slower or absent behind the reef crest, where the substratum is primarily loose rubble. In particular, the Acropora zone recorded on the outer reef flat in 1917 (120–140 m behind the reef crest) had disappeared completely by the 1990s. Recovery is now proceeding in this zone by the slow accumulation of Acropora muricata colonies that are large enough to become established on the loose substratum. The recovery of coral communities on a few large stable blocks of reef rock scattered across the zone of loose rubble was similar to the recovery on the solid reef crest. The large stable blocks provided a natural experiment that demonstrated resilience (rates of recovery) of the coral communities after chronic or acute disturbance was determined largely by stability of the substratum.
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