Adaptive responses and local stressor mitigation drive coral resilience in warmer, more acidic oceans

Abstract

Coral reefs have great biological and socioeconomic value, but are threatened by ocean acidification, climate change and local human impacts. The capacity for corals to adapt or acclimatize to novel environmental conditions is unknown but fundamental to projected reef futures. The coral reefs of Kāne'ohe Bay, Hawai'i were devastated by anthropogenic insults from the 1930s to 1970s. These reefs experience naturally reduced pH and elevated temperature relative to many other Hawaiian reefs which are not expected to face similar conditions for decades. Despite catastrophic loss in coral cover owing to human disturbance, these reefs recovered under low pH and high temperature within 20 years after sewage input was diverted. We compare the pH and temperature tolerances of three dominant Hawaiian coral species from within Kāne'ohe Bay to conspecifics from a nearby control site and show that corals from Kāne'ohe are far more resistant to acidification and warming. These results show that corals can have different pH and temperature tolerances among habitats and understanding the mechanisms by which coral cover rebounded within two decades under projected future ocean conditions will be critical to management. Together these results indicate that reducing human stressors offers hope for reef resilience and effective conservation over coming decades.

Keywords: acclimatization; adaptation; climate change; ocean acidification; resilience; super corals.

Figure 1.

Effects of pH and temperature after three months of exposure on survivorship (d–f), and calcification rate (g–i) for the three coral species examined in this study (a–c), shown according to collection location. Seasonal maximum temperatures (26.7, 28.2, 29.7°C) were maintained for the first five weeks, after which temperatures were lowered by 1.5°C in all treatments (25.2, 26.7, 28.2°C) to mean annual temperatures, and corals were exposed to these conditions for an additional nine weeks. High-pH, low-temperature conditions occur at the Waimānalo Bay collection location, while the Kāne‘ohe Bay location experiences low-pH, high-temperature conditions. Where bars are not evident for Poc. acuta from Waimānalo Bay in (d), it is because all individuals in those treatments died (survivorship = 0). Sample size for survivorship data, n = 10–12 for most treatments (see the electronic supplementary material, figure S5). Corals which experienced partial mortality or died were excluded from the calcification analysis; data shown for treatments with n = 2–12 survivors. All data reported as mean ± s.e.m. Photos courtesy of Keoki and Yuko Stender. (Online version in colour.)

Figure 2.

Reaction norms under elevated temperature (a,c,e) and reduced pH (b,d,f) for each coral species and collection location. Temperature tolerance distributions were estimated from changes in survivorship under elevated temperature, while pH tolerance distributions were estimated from changes in calcification under reduced pH, based on data shown in figure 1. Arrows indicate significant differences in tolerance among conspecifics between the two collection locations. (Online version in colour.)