Where in the world are our strongest corals?

Seventy Islands, Palau - One of several Rock Islands chains in Palau
Cohen's Lab
The Palauan archipelago is a composed of more than 250 islands. Among these are about 200 known as the Rock Islands, due their tough limestone composition. These islands form a series of semi enclosed bays south of Palau's most populated island, Koror. These bays are thriving with wildlife and the coral cover and diversity is spectacular, especially when considering the environmental conditions in these areas.

Reef-building corals, build make their skeleton from calcium carbonate (limestone), specifically the aragonite mineral form of calcium carbonate. How difficult it is for them to make aragonite is largely dependent on a quantity called the aragonite saturation, which essentially corresponds to how favorably this mineral can form under the current conditions and is very sensitive to the pH of the water.

Decades of research on coral health and optimal conditions for coral growth indicate that corals can make their skeletons more easily and more quickly when the pH is more basic as opposed to more acidic, or correspondingly when the aragonite saturation is higher. The relationship between coral growth and pH is particularly relevant in the context of global climate change and rising atmospheric concentrations of CO2 - as CO2 concentrations in the atmosphere increase, more CO2 becomes dissolved in the oceans, a process that decreases the pH of the water (commonly referred to as ocean acidification), and decreases the aragonite saturation. Combined with other stressors such as increased temperatures, increased pollution and nutrient input, impacts from overfishing and coastal developments, corals face an increasing number of threats.

Corals in Nikko Bay, Palau
H. Barkley

In Palau's Rock Islands, the pH and the aragonite saturation is much lower than in the surrounding outer reefs, however, the corals in these bays show no reduction in growth despite living in conditions we would normally deems stressful for their growth. Our lab made this discovery a few years ago and has been studying the corals in these bays ever since, trying to understand just how they are able to defy our expectations. Why can they thrive under these conditions? Have they always been able to do so? Are the populations in these bays different from those in other regions of Palau? Is there genetic exchange between these resistant corals and other populations in Palau's outer reefs and beyond?

In our current expedition will have a rare opportunity to reach some of the islands farthest from Palau's mainland, such as Helen Reef, the southernmost of Palau's reefs and Kayangel, the northern most. We will characterize environmental conditions at these new sites - for instance, are there other low pH sites we've yet to uncover? Furthermore, we also want to study connectivity between different areas of Palau. By taking tissue samples from corals at multiple locations, we can use genetic techniques to study gene flow between different regions, this can tell us if corals in a particular environment, such as the low pH Rock Islands, are genetically isolated from other populations and have perhaps adapted to these conditions over time. If they are not isolated, it can tell us where these resistant populations could have dispersed to in the past, indicating those areas may show resistance when conditions become stressful in the future. Knowledge that is very important in making adequate management decisions.

We will tour Palau for almost three weeks, stopping at fifteen sites. We are quite fortunate to have use of the vessel M/V Alucia, for our work during this expedition, without which access to the farther reefs like Helen and Kayangel would have been unfeasible.

Anchors away.

Merand is the Palauan word for coral. The Cohen's Lab current expedition to Palau seeks to understand why some
of Palau's coral can thrive in low pH conditions.


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