Corals: environmental records of the past

Humans are wonderful creatures; our intellect has allowed us to thrive on this planet through impressive technological and scientific advances. However, this success has come at a cost. For example, most marine ecosystems are now under pressure from a variety of stressors mostly linked to human activities. In the case of coral reefs, these stressors include a combination of global pressures in the form of warming and ocean acidification, and local pressures which include overfishing, habitat destruction, pollution, increased sedimentation, to name just a few. This pressure has caused a worldwide loss and degradation of coral reefs; however, there are still questions about the full scale of these changes and on the ability of corals to withstand them. For example, we know that extreme heat can kill corals, but it is not fully understood how quickly corals can increase their tolerance to thermal stress, or how local and global stressors interact with each other. The latter is crucial because while fixing global warming and ocean acidification requires a global effort, but local governments and managers can implement policies to remove or reduce local stressors and give corals a better fighting chance in the face of climate change.

Massive colony of Sidereastrea siderea colony in Martinique. Photo: Jean-Philippe Maréchal (2021).

One of the reasons for current knowledge gaps is the lack of long-term temporal observations of key environmental parameters, such as temperature, salinity, or the pH of seawater. Historically, we either lacked the technology to monitor these parameters, access to such technology was expensive or there was no clear understanding of the need to monitor these parameters. Therefore, to put current changes in perspective, accounting for natural variability, it is necessary to find ways to fill knowledge gaps, and corals can help us to do so!

Reef forming corals precipitate a calcium carbonate skeleton that forms the reef foundation.  These skeletons come in all shapes and sizes, from the delicate and intricate branching corals to the massive types that are named as such due to their stable ball- or boulder-shaped skeleton. The skeletons of corals are attractive to paleoceanographers because they can tell us about the past, if you know how to read the stories recorded in their skeletons. This is because corals continuously deposit new layers on top of their older skeleton, thus growing bigger and bigger every year and creating a record of their history. Some massive corals can become real life giants as the can live for several centuries, never ceasing to grow. The oldest massive corals are believed to be nearly a thousand years old. But corals are living organisms that respond to their environment as we do, and their skeleton records these changes in a similar way to trees forming rings. In winter, when the waters are cold, corals tend to grow slower and form a denser skeleton while in summer, when it is warmer, corals usually grow faster and form a less dense skeleton. Therefore the skeleton of a massive coral records this seasonal rhythm in the form of annual bands that resemble the rings in the trunk of a tree.

X-ray image from a core of a massive coral showing the annual density bands.

If a year is unsually cold or warm this influences the average growth rate for that particularly year. Therefore, if we measure the distance between these annual bands, we can reconstruct the growth history of a coral throughout its life. The incorporation of elements present in seawater into the skeleton of the corals is also influenced by changes in the environment. For example, the incorporation in strontium into the skeleton varies with temperature; therefore the changes in strontium in the skeleton can be used to reconstruct the changes in the seawater temperature through time. Similarly the amount of barium in the skeleton of corals appears to respond to changes in the amount of terrestrial runoff reaching the reef.

As part of the MaCoBioS project we are planning to analyse the growth and chemistry of the skeleton of corals from Martinique and Bonaire the Caribbean. These analyses will be used to reconstruct temporal changes in terrestrial inputs into the reef, and to determine how this is linked to land use change and pollution over time. In addition, we will investigate how different corals control their internal chemistry to facilitate the formation of the skeleton and how this mechanism responds to ocean acidification and thermals stress. For this second goal we will compare corals from the tropical Caribbean with corals from the temperate Mediterranean. These projects aim to provide managers and policy makers with better information on how corals respond to changes in their environment over long periods of time so that they can take the right steps to manage the reef and help preserve these magnificent ecosystems.

The Freie Universität Berlin team (Dr. Georg Heiß, Dr. Juan Pablo D’Olivo Cordero and Dr. Moshira Hassan) collecting a coral core in Martinique. Photo: Dr. Diego Kersting

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