Juliano Morais: What is the fate of coral colonies after death? An Investigation into the erosion of dead coral colonies

In our latest post, Juliano Morais—a PhD candidate at James Cook University, Australia—presents his new work ‘On the fate of dead coral colonies’. In this blogpost, Juliano discusses the importance of erosion for coral reef survival and encourages non-native language speakers to be bold and start a career in ecology.  

About the paper

Carbonate budgets dynamically balance production and loss of calcium carbonate (CaCO3) on coral reefs. To sustain or expand the coral reef framework, CaCO3 production by calcifying organisms must be higher than erosion. The physical structure of reefs is vital for maintaining the abundance, diversity, and ecosystem functioning of reef-associated communities. In addition to this, reefs also provide key services to people, such as coastal protection from inundation during storms. However, the robustness of reef structures may be at risk due to the increasing impact of climate change on coral reefs. Furthermore, while we know that bleaching can result in coral death, important questions about what happens to coral skeletons after death remain largely unexplored. For example, how long does the colony structure on the reef last and what are the factors involved in the erosion of coral colonies? During our study, Lizard Island was hit by two bleaching events (2016 and 2017) that resulted in mass coral death. This made Lizard Island a perfect place to follow the fate of dead coral colonies and investigate erosion rates and factors associated with this process.

In this paper, we examinate the erosion rates of dead coral colonies of complex growth forms. Specifically, we investigated how long coral skeleton structure persisted for, as well as quantified the erosion rates of individual dead coral colonies. We also examined how factors commonly believed to underpin erosion rates on reefs related to variability in erosion rates of dead corals. Importantly, it is believed that dead coral colonies are an integral component in the formation of the reef matrix. However, contrary to these expectations, we found intense dead coral colony erosion with approximately 80% of dead colonies completely disintegrating within 60 months. We also found that neither estimated parrotfish erosion, nor exposure level or coral growth form, were capable of explaining the observed variation in coral erosion rates. These results suggest we may need to re-evaluate the role of complex growth form corals in reef building and other associated processes.

The lead author, Juliano Morais, photographing coral colonies. Field work for this study was done at Lizard Island, on the Great Barrier Reef, Australia (credit: Sterling Tebbett)

The idea for this paper came when I was working in the first chapter of my PhD where I followed the fate of the living tissue area of Acroporas and Massive Porites until their death. During the data processing of this article, an important question arose: what happens to these colonies after death? This question helped us to realize that we had an important dataset on our hands, and so we decided to explore that data and write this paper.

Parrotfish (Chlorurus microrhinos) biting and removing calcium carbonate from the reef structure (credit: Victor Huertas).

The novelty of this work is highlighted by two points. First, there is a frequent assumption of functional studies on coral reefs that presence is equal to function, or, in other words, that ecosystem functions are considered to be homogeneously distributed through space and time. Based on this assumption, bioerosion rates have been estimated based on surveyed parrotfish abundances and body size data with species-level trait data compiled from the literature, such as average bite rate and bite volume, for example. However, numerous studies using reef fishes as model organisms have shown a disconnect between animal presence and their presumed function. They have also showed that functions are delivered to the reef by fishes in a highly heterogenous manner. Indeed, there may be a mismatch in terms of how we estimate bioerosion on the reef and how this function is actually delivered to the reef. Our estimates of coral colony erosion represent the measurement of an actual process—accounting for variation in space and time—while the estimates of parrotfish bioerosion merely represents an estimate of a potential process.

Dead coral colonies after the 2016 bleaching event on Lizard Island, Australia (credit: Sterling Tebbett)

Second, our study is part of a group of recent work that has been showing good results in modelling and predicting colony volume (3D) based on planar area (2D). This allows us to obtain coral colony volume from simple bird’s-eye-view photos without using photogrammetric approaches via structure-from-motion—which can be a relatively expensive technique, particularly from a temporal point of view, as 3D models may require many hours or days to build on average computer processors.

About the research

Besides providing estimates of erosion and disappearance of coral colonies after death, our paper sheds light on a very important area of discussion about how we might erroneously estimate erosion in coral reefs based mainly on the abundance of parrotfish. We discussed many important points about the methodology and the assumptions that most published works have been built on regarding this topic. In the end, we invite readers to reflect on a possible reassessment of the role of complex-structured corals in reef building and associated processes. We also suggest that there is a limitation in our current understanding of parrotfish bioerosion on coral reefs, especially in terms of the erosion of recently killed complex coral colonies.

Coral colony of Acropora hyacinthus in the process of erosion after bleaching event-related mortality (credit: Juliano Morais)

These findings surprised us quite a bit as there is an extensive body of literature identifying parrotfish as a major reef bioeroder, and our results did not show parrotfish having an effect on coral erosion in our locations. Furthermore, the fact that nearly 80% of the dead coral colonies in our study have completely disappeared was something we did not expect.

Erosion on coral reefs is a process that involves a wide range of biological organisms as well as physical and chemical factors. Given the complexity of this process, many questions are still open and need to be further explored. This is especially urgent due to our current predicament where climate change impacts the entire process and agents involved in the erosion of coral reefs.

About the author

Juliano Morais during fieldwork at the Lizard Island research station (credit: Sterling Tebbett)

When I was in the high school, a group of ecologists were investigating how pesticides from soy crops were impacting a population of White-winged nightjars (Hydropsalis candicans) at a national park located near my hometown. This group visited my classroom and chose a student to accompany them to a night field trip to collect some birds and investigate their vomit in order to look for signs of pesticides in the flying insects that they ate at night. I was the chosen student and that night I was bombarded with information about the ecology and ecological processes of nocturnal birds, insects, predation, competition for food and habitat, etc… And, that night, at the age of 15, I decided to become an ecologist. Just a few years later, I entered into my undergraduate biological sciences course. Currently, I’m a PhD candidate at James Cook University in Townsville,  Australia, and I have been working with my PhD supervisor, Prof. David Bellwood, at the Reef Functional Hub and the Centre of Excellence in Coral reef studies. We are investigating the impact of climate change and bleaching events on coral communities. My inspiration at the academy is all the mentors I’ve had along my academic journey. I am very lucky to have worked with great scientists since my undergraduate and master’s degree in Brazil, right through to now with my PhD in Australia. I must mention: Prof. Christiano P. Coelho and Frederico A. Guilherme (undergraduate supervisors), Prof. Braulio A. Santos (master’s supervisor), and Prof. David Bellwood and Renato A. Morais (PhD supervisors). Their commitment and respect for science have contributed greatly to my career and they inspire me to be a better scientist every day.

I faced a few barriers when I decided to do my PhD outside of Brazil and in an English-speaking country. The fact that English is not my first language closed some doors for me at the beginning of my search for a position as a PhD student. However, after living in Australia for a while and having daily contact with English, these barriers were left behind and I was able to progress my career in science. I would like to direct my advice to those students like myself from non-English speaking developing countries who dream of taking a part of their education outside of their home country. There will be a lot of people telling you that it is impossible and/or that you are not good enough to get into a good university. Don’t listen to these negative and discouraging comments. Work hard, do your best, and do not give up. It is hard, but not impossible, and it will be very rewarding and positive for your career.

Enjoyed the blogpost? Read the research here!

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