Blog

How does ocean acidification affect the internal carbonate chemistry of corals?

The specific mechanisms of how ocean acidification (OA) inhibits coral calcification are still poorly understood, which, in part, is due to the difficulty of accessing the site of calcification. I use coral skeletal boron isotopes and B/Ca ratios to overcome this issue and reconstruct the pH and DIC concentration at the site of calcification. This also has important implications for the boron isotope seawater pH proxy in corals.

I currently collaborate with Drs Rob Toonen and Chris Jury from the Hawaii Institute of Marine Biology as well as my former PhD supervisor Dr Andréa Grottoli to assess the impact of environmental conditions (i.e., temperature, pH, flow) on the internal carbonate chemistry of Hawaiian corals. Furthermore, I will investigate whether corals adapted to naturally lower seawater pH have specific geochemical mechanisms that make them more resistant to ocean acidification.

Can corals acclimate and/or adapt to climate change?

I am currently investigating whether the high heat tolerance of Kimberley corals is the result of acclimatization and/or adaptation. Preliminary results of a short-term reciprocal transplant experiment suggest that adaptation likely plays a greater role in shaping coral heat tolerance than acclimatization. This will be further investigated in collaboration with Dr Luke Thomas from Stanford University.

Mechanisms of heat tolerance in corals from naturally extreme environments

Australia’s remote Kimberley region harbors coral populations that are extremely stress-tolerant, and therefore serves as an ideal natural laboratory to study the mechanisms and limits of coral thermal tolerance. I conducted a heat stress experiment at the Kimberley Marine Research Station at Cygnet Bay Pearl Farm which showed that extreme temperature fluctuations (up to 7°C daily) resulted in increased heat stress resistance but that Kimberley corals were nevertheless not immune to bleaching.

These findings were ultimately confirmed by the first large-scale bleaching event in this region in 2016. Colleagues and I conducted extensive in situ monitoring prior, during and after the bleaching event. Furthermore, I will conduct biogeochemical and genetic analyses to study why some corals were more resistant to bleaching than others, and why some recovered whereas others died.

See Publications and In the News for publications and media coverage.

Photo credit: Chris Cornwall.

Are coral bleaching events recorded in the coral skeleton?

I used Caribbean corals from a controlled culturing experiment to investigate whether coral skeletal carbon isotopes reliably record bleaching events in the coral skeleton. Since this was not the case, I tested the validity of a proposed correction which showed that it cannot be universally applied to improve proxy records of light levels and cloud coverage.

This led me to investigate whether other geochemical proxies, such as coral skeletal boron isotopes or Sr/Ca ratios could serve as potential novel bleaching proxies. Using the same Caribbean coral species, I found that boron isotopes were not affected by significant changes in coral physiology due to short-term bleaching, and that trace element ratios did not consistently record changes in either temperature or coral physiology.

See Publications for more details.

How does combined ocean acidification and warming affect coral fitness?

As part of my PhD research, I also studied how combined ocean acidification (OA) and warming affects coral calcification and energy reserves. I found that some coral species were more resistant to OA and warming than previously expected, and did not consume their energy reserves to maintain calcification under these conditions.

I further used this project to demonstrate that two common techniques to measure coral calcification (i.e., buoyant weighing and alkalinity depletion incubations) resulted in comparable growth rates. This is important because this means that the sensitivity of coral calcification to OA can be predicted reliably, and is thus independent of the technique that was used to measure calcification rates.

See Publications for more details.

Can corals cope with annual coral bleaching?

For my PhD dissertation under the supervision of Dr. Andréa Grottoli at the Ohio State University, I studied the impacts of annual coral bleaching on the bleaching susceptibility and recovery capacity of three Caribbean coral species. As the world’s oceans continue to warm, bleaching events will increase in both frequency and intensity, eventually becoming annual events.

I showed that susceptibility to single bleaching is a poor predictor of susceptibility to repeat bleaching, and that cumulative bleaching stress can promote acclimation in some species while eroding resilience in others. These findings have important implications for predicting the frequency and severity of future bleaching events and are critically needed as, for example, Hawaii and other locations have only recently experienced annual bleaching for the first time.

See Publications and In the News for publications and media coverage.

Prey selection of the coral-eating snail Drupella cornus in the Red Sea

For my master thesis, I studied the prey preferences and microhabitat use of the coral-eating snail Drupella cornus in the northern Red Sea.

Population outbreaks of this coral predator have the potential to destroy wide areas of coral reef and have caused significant damage on many reefs including Ningaloo Reef in Western Australia. During a 2-month fieldtrip to the Egyptian Red Sea, I analyzed for the first time if prey preferences are independent of coral availability using resource selection ratios and extensive Scuba surveys.

I found that both juvenile and adult snails prefer specific species of Acropora independent of their abundance, and that an ontogenetic habitat shift (and thus also shift in prey selectivity) occurs at a size of 2 cm. As a consequence, the two life stages differ in their ecological impact on coral communities. Using mesocosm experiments, I also provided the first experimental evidence that intraspecific attraction plays a major role in prey selection of D. cornus, which may contribute to the formation of population outbreaks.

See Publications and In the News for publications and media coverage.