Different resiliencies in coral communities over ecological and geological time scales in American Samoa

C Birkeland,A Lawrence,M Vaeoso,D Fenner,A Green,G Coward

doi:10.3354/meps13792

Abstract

In 1917, Alfred Mayor surveyed a 270 m transect on a reef flat on American Samoa. Eleven surveys were conducted on the transect from 1917 to 2019. The coral community on the reef crest was resilient over the century, occasionally being seriously damaged but always recovering rapidly. In contrast, the originally most dense coral community on the reef flat has been steadily deteriorating throughout the century. Resilience of coral communities in regions of high wave energy on the reef crests was associated with the important binding function of the crustose coralline alga (CCA) Porolithon onkodes. Successful coral recruits were found on CCA 94% of the time, yet living coral cover correlated negatively with CCA cover as they became alternative winners in competition. Mayor drilled a core from the transect on the surface to the basalt base of the reef 48 m below. Communities on Aua reef were dominated by scleractinians through the Holocene, while cores on another transect 2 km away showed the reef was occupied by alcyonaceans of the genus Sinularia, which built the massive reef with spiculite to the basalt base 37 m below. Despite periods of sea levels rising 9 to 15 times the rate of reef accretion, the reefs never drowned. The consistency of scleractinians on Aua reef and Sinularia on Utulei Reef 2 km away during the Holocene was because the shape of the bay allowed more water motion on Aua reef. After 10700 yr of reef building by octocorals, coastal construction terminated this spiculite-reef development.

Highlights

Much attention has been given to investigating the rate at which many reef communities are deteriorating under stress from the increasing activities and resource needs of a growing human population, and rising temperatures and changing seawater chemistry from increasing greenhouse gases
Considering the regular rapid recovery of coral communities on the reef crest or on the limestone blocks on the reef flat, and considering the continuous deterioration of the coral communities on the reef flat (Fig. 1) which was mostly covered by rubble, we concluded that the resilience of coral communities along the Aua transect was a result of substratum stability
Rates of calcium carbonate deposition, the effectively binding Porolithon onkodes, and the dissolution-resistant dolomite in its skeleton are associated with high-energy reef crests and reef fronts where there is usually reliable recruitment and rapid recovery from disturbances

Summary

Introduction

Much attention has been given to investigating the rate at which many reef communities are deteriorating under stress from the increasing activities and resource needs of a growing human population, and rising temperatures and changing seawater chemistry from increasing greenhouse gases. Attention should be given to coral communities. American Samoa is a challenging environment for reef-building corals. Since 2009, when the islands of American Samoa began sinking more rapidly, the annual sea level rise relative to the islands is about 5 times the global average (Han et al 2019), likely allowing more rapid reef accretion as evidenced by especially rapid reef growth during periods of rapid sea level rise in other locations (Eisenhauer et al 1993, Kan & Kawana 2006, Hongo & Kayanne 2011)

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Conclusion