Changing environmental conditions on the health of two dominant Acropora species at Ningaloo Reef

Variations in coral health indices, reflecting autotrophic activity (chlorophyll a (chl a) and zooxanthellae density), metabolic activity (RNA/DNA ratio and protein) and energy storage (lipid ratios) for two dominant Acropora species (A. digitifera (Dana, 1846) and A. spicifera (Dana, 1846)) were investigated at Ningaloo Reef (North-Western Australia). Health indices were measured before and after spawning in austral autumn and winter 2010 and austral summer 2011, at six stations. Physico-chemical-biological parameters were also measured simultaneously to determine the key environmental factors impacting coral health. In addition, the suitability of these health measures as short-term and long-term indicators of coral health was investigated by comparing diel (throughout the day), daily (between days) and seasonal patterns. Coral health indices showed clear seasonal and species-specific differences but negligible spatial differences across a reef section (2-5 km). For both species, indices of metabolic activity showed highest and optimal values during autumn (at 26-28°C), while autotrophic indices were highest in winter and summer. The unexpected high autotrophic indices and low metabolic indices and energy stores during summer 2011 are related to the La Niña event. In this case high sea surface temperatures did not result in bleaching since high plankton concentrations in the water led to low light levels in the reef lagoon, counteracting the impact of high seawater temperatures and resulting in a shift towards high coral symbiont densities, but low metabolic activities and energy levels of the coral host. During this time feeding modes shifted for both species, with an increased importance of planktonic food for metabolic activity of A. digitifera, but light and plankton-dependence for A. spicifera. Under normal conditions metabolic indices for A. spicifera correlated with nitrogen supply, temperature and zooplankton concentrations while for A. digitifera, light as well as picoplankton concentrations were important. This can possibly be explained by higher energy requirement for lipid synthesis and storage in A. digitifera since lipid ratios were higher for this species and changed throughout the year, while they were stable for A. spicifera. The impact of the broadcast spawning event on coral health indices at Ningaloo Reef occurred only as a modest shift against a backdrop of massive seasonal changes in coral physiology. Overall, results show that the two species vary physiologically and react differently to extreme events. This knowledge can be used to make predictions for how changes in environmental factors may impact the health of corals in the future.