Bleaching in reef corals: Physiological and stable isotopic responses

Area of Science:

• Marine Biology
• Coral Reef Ecology
• Climate Change Impacts

Background:

• Caribbean reef corals underwent a significant bleaching event in late summer/fall 1987.
• Coral bleaching, a discoloration event, was widespread but did not lead to widespread mortality as initially predicted.

Purpose of the Study:

• To assess the impact of the 1987 coral bleaching event on coral energy input.
• To investigate the relationship between coral physiological parameters and environmental conditions during the bleaching event.

Main Methods:

• Analysis of delta(13)C values in skeletal aragonite to estimate carbon assimilation by zooxanthellae.
• In situ photosynthesis-irradiance measurements to quantify photosynthetic activity.
• Measurement of tissue biomass parameters for Montastraea annularis and Agaricia lamarcki.
• Examination of delta(18)O signals in M. annularis skeletons to reconstruct past water temperatures.

Main Results:

• Despite survival, bleached corals showed decreased energy input from zooxanthellae.
• Reduced energy input was evidenced by lower delta(13)C values in coral skeletons.
• Photosynthesis-irradiance data and tissue biomass confirmed reduced coral energy status.
• Delta(18)O analysis indicated that the bleaching event coincided with abnormally elevated sea surface temperatures.

Conclusions:

• The 1987 coral bleaching event in the Caribbean led to reduced coral energy reserves, despite high survival rates.
• Elevated sea temperatures were identified as the primary environmental driver of the bleaching and subsequent physiological stress.
• This study highlights the subtle but significant impacts of thermal stress on coral reef ecosystems.