Temperature tolerance of the coral Porites lutea exposed to simulated large amplitude internal waves (LAIW)

Large-amplitude internal waves (LAIW) cause intense drops in sea water temperature (6-9 °C) and acidity (0.6 pH units) within minutes, posing a major disturbance factor for shallow water corals in the Andaman Sea. During periods of elevated temperatures, however, they may provide a vital source of relief to heat-stressed corals. To assess the effect of LAIW on the thermal tolerance of the massive coral Porites lutea, we carried out a one-month reciprocal LAIW simulation experiment with coral nubbins collected from the LAIW-exposed and LAIW-sheltered sides of the Similan Islands, an offshore archipelago near the Thai Andaman shelf break (distance of sampling locations < 3 km). Corals from either origin were subjected to (1) ambient (29 °C, pH 8.39) vs. elevated (31.5 °C) temperatures; and (2) ambient temperatures with simulated LAIW (30 minute drops to 23.8 °C, pH 7.9, twice per day) vs. elevated temperatures with LAIW (30 minute drops to 24.9 °C, pH 7.9, twice per day). Simulated LAIW reduced heat stress responses in all corals. However, nubbins from the LAIW-exposed site showed a higher tolerance to the heat stress than sheltered corals, i.e. lower bleaching- and mortality rates, higher photosynthetic performances and higher protein contents at the end of the experiment, despite identical algal symbiont genotypes (ITS-2, C15). Our results suggest that LAIW-induced nutrient availability, enhanced heterotrophic feeding and thermal variability increased the upper thermal resistance of P. lutea, supporting the notion that LAIW-exposed reefs might be important refugia for coral survival in a warming ocean.