High temperatures tolerated by a diverse assemblage of shallow-water corals in American Samoa

Abstract

Corals in shallow waters are subjected to widely fluctuating temperatures on a daily basis. Using continuous temperature recordings, we examined the temperature regime in one such area, a backreef moat with low tide depths of 1–2 m on Ofu Island in American Samoa. The moat supports a high diversity of 85 coral species [H′_(log2)=3.37] with 25–26% live coral coverage. In one section of the moat, a 4,000-m² pool inhabited by 52 coral species, the mean summer temperature was 29.3 °C, but daily temperatures fluctuated up to 6.3 °C and briefly reached a peak of 34.5 °C. The duration of hot water events, e.g., ≥32 °C, averaged 2.4 h per event (maximum 5 h) and occurred on 35 summer days, although daily mean temperatures did not exceed 30.5 °C and were generally within 0.5 °C of that occurring outside the moat at an exposed coastal area. While there was a previous mortality of many acroporids during a long-term (several month) warming period in 1994, at least nine Acropora species and a diverse range of other taxa withstand the current temperature regime.