Corals under climate change: Hawai’i’s winners and losers

The beauty of a healthy, thriving coral reef community is astonishing. These ‘rainforests of the sea’ are unique and their beauty is unmatched. While coral reefs only occupy less than 1% of the world’s ocean floor, they support more than 25% of all marine species. An estimated 85% of the United States’ reef area is located within the Hawaiian Archipelago that holds the largest marine sanctuary in the world, the Papahānaumokuākea Marine National Monument. Coral reef communities within these waters are of particular concern because of the high proportion of endemic marine species (>25% for most taxa). These isolated subtropical coral reef communities were once safe from conditions that have ravaged similar ecosystems in other parts of the world’s oceans. Corals reefs of the Hawaiian Islands are currently experiencing the growing impacts of climate change with more frequent and severe bleaching events occurring in 1996, 2002, 2014 and 2015. The severity and extent of these bleaching events may pose a significant threat to the integrity of this unique ecosystem. 

Reefs throughout the world are undergoing significant ecological change due to climate change. The effects of burning fossil fuels are two fold: warming and acidification. Increases in atmospheric carbon dioxide create a green house effect causing the planet and its oceans to warm, also known as “global warming”. Offshore water temperatures of Hawai’i have already increased by 1.15°C over the past 58 years. Corals are also faced with another CO₂ problem: ocean acidification. Carbon dioxide dissolves into the ocean, which causes the pH of the ocean to decrease. Ocean acidity has increased by approximately 25% since pre-industrial times (circa 1750-1850) and has resulted in severe negative effects on many calcifying marine organisms. Since these two processes are expected to increase in unison, it is important that we understand how corals respond to these stressors simultaneously.

Previous work has shown ocean warming and acidification reduces coral growth and increases coral bleaching (through the loss of algal symbionts); however, responses are not uniform within and between coral species. This suggests some individuals may not be affected by end-of-century climate change scenarios. To gain a better understanding of the future of coral reefs in Hawaiʻi, I conducted a series of laboratory experiments to determine the relative susceptibility of five abundant species including the endemic finger coral (Porites compressa), lace coral (Pocillopora damicornis), rice coral (Montipora capitata), crust coral (Leptastrea purpurea) and mushroom coral (Fungia scutaria) to future climate change conditions. I did this by exposing corals to warming temperatures of 2°C or more, as well as ocean acidification by decreasing the pH of the water. I exposed corals to these factors separately and together for an eight-week period.

Overall, I found that ocean warming had a greater negative impact on coral growth compared to ocean acidification. The crust coral showed no detectable response in growth to increased temperatures and acidity, revealing this species may be more resistant to climate change. Ocean acidification also did not appear to effect growth in rice corals. However, the other four coral species all had lower growth rates under ocean warming conditions, and the endemic finger coral was found to be the most susceptible coral species to these climate change stressors.

Climate change effects on coral species are generally negative, but the magnitude and variability of the response varied among individual corals and species. Climate change is also affecting corals on a backdrop of many other anthropogenic stressors such as sedimentation, overfishing, trampling by snorkelers and divers, and nutrient loading. It is important that we understand why some corals species are more susceptible than others to these climate change stressors to gain a better idea of what future reefs may look like. Studies such as mine also are useful for providing a framework to model future species composition, abundance, and available habitat for future coral reefs. With these models, scientists, managers, and policy makers can prioritize areas that are subject to the most immediate and dangerous threats and discuss strategies to minimize climate change effects in those areas.  There are conservation and management actions that can be implemented now to help sustain coral reefs such as water conservation, pollution reduction, reducing carbon footprints, and when diving and enjoying reefs to practice safe snorkeling techniques.

Lastly, Hawaiʻi has a statewide reporting system, Hawaiʻi Eyes of the Reef, which enables all community members to contribute to long-term protection of our local reefs. The public is welcome to submit photo-documentation and reports of coral bleaching to the Eyes of the Reef Hawaiʻi network.

Keisha Bahr is a Ph.D. Candidate at the University of Hawaiʻi Mānoa (Pacific Islands CSC) in the Coral Reef Ecology Lab at the Hawaiʻi Institute of Marine Biology.

For more information and results, check out Keisha's recently published paper: Relative sensitivity of five Hawaiian coral species to high temperature under high-pCO2 conditions in the journal Coral Reefs.

Check out how much life one cubic foot of a reef supports at Biocubes: Life In Once Cubic Foot.