Seasonal variations in energy levels and metabolic processes of two dominant Acropora species (A. spicifera and A. digitifera) at Ningaloo Reef

Seasonal variations in coral health indices reflecting autotrophic activity (chlorophyll a and zooxanthellae density), metabolic rates (RNA/DNA ratio and protein) and energy storage (ratio of storage: structural lipids or lipid ratios) were examined for two dominant Acropora species [Acropora digitifera (AD) and Acropora spicifera (AS)] at Ningaloo Reef (north-western Australia). Such detailed investigation of metabolic processes is important background, with regard to understanding the vulnerability of corals to environmental change. Health indices in AD and AS were measured before and after spawning in austral autumn and winter 2010, and austral summer 2011 at six stations. Health indices showed seasonal and species-specific differences but negligible spatial differences across a reef section. For AD, autotrophic indices were negatively correlated with lipid ratios and metabolic indices. Metabolic indices were significantly higher in AS than AD. No correlation was observed between RNA/DNA ratios and lipid ratios with any autotrophic indices for AS. Lipid ratios were stable throughout the year for AS while they changed significantly for AD. For both species, indices of metabolic activity were highest during autumn, while autotrophic indices were highest in winter and summer. Results suggest that the impact of the broadcast spawning event on coral health indices at Ningaloo Reef occurred only as a backdrop to massive seasonal changes in coral physiology. The La Niña summer pattern resulted in high autotrophic indices and low metabolic indices and energy stores. Our results imply different metabolic processes in A. digitifera and A. spicifera as well as a strong impact of extreme events on coral physiology.