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Glass sand-restored corals still growing!

Our Medalla Light-sponsored project using recycled glass sand in coral reef restoration is reporting over 50% survivorship for all experimental plots.

Growing elkhorn coral fragment, photo by H. Ruiz

Monitoring surveys conducted in September 2022 and January 2023 have allowed us to continue evaluating the growth and overall health of the outplanted corals at our experimental site in the south of the island. Read more about the project in our past posts here and here.

Although there has been mortality (which is expected), the survivorship overall has been relatively high. All three of the experimental plots are located at least 200 m apart, which ensures that they are 1) being subjected to the same water quality conditions overall and 2) far enough apart to not impede each other over time. They were restored to areas of the Cayo Maria Langa reef that were in need of restoration and where Elkhorn coral had historically been present as large, old colonies.

Map showing the general location of the experimental plots

For reference, there were four different cement mixtures that we tested - three experimental and one control. The ratios refer to the amount of cement/marmolina/RGS that were used (see graph below). Recall that marmolina is crushed marble and is used to slow the hardening process to give us time to work with the material underwater. The control (2:1:0) is the traditional mix that involves two parts cement and one part marmolina (indicated by the star in the graph). The other mixtures either include marmolina or not, such as the 2:0:1 mix which is just two parts cement to one part RGS.

We chose these ratios after first experimenting in the lab to see what percentage of RGS caused the cement to lose its integrity (crumble, or not harden). These ratios provided a usable mixture to serve our purpose. Now, over time, we have been observing how well the corals are growing and comparing that growth by mixture.

Check out the three images below to see a progression of growth. The first image is the area before outplanting, the second image is the outplanted corals on the day we cemented them to the reef in April 2022 and the third image is their growth as of March 2023, almost a year later. Cue in on the orange "squares", those are the coral fragments.

When we compare survivorship of the corals based on the experimental ratios, we see that the ratio that included RGS and marmolina had the same survivorship as the control (the star above the mixture ratio in the table below). The second highest survivorship belonged to the ratio that contained just cement and RGS (73%), while the last ratio was a higher percentage of RGS combined with cement (67% survivorship).

We were never expecting 100% survivorship of any of these ratios, including the control, as that is an unrealistic expectation in coral reef restoration and rarely occurs when the effort is monitored in the long-term beyond the initial outplanting. Overall, these percentages do indicate relatively high success of the restoration effort and so far they also indicate that RGS is not having a negative effect on coral growth.

However, we will perform one last monitoring session in June in order to finish up this study. At that point, we will have three measurements of growth and survivorship and will be able to calculate a growth rate for each of the mixtures. We are hoping to find that 1) the use of RGS contributed to faster growth rates or 2) there was no difference in growth rates between the control and the mixtures containing RGS. Based on what we have seen so far, we do not anticipate to observe that RGS had any negative impacts on coral growth.

Stay tuned to learn how this project ends and to hear more about the next phase of restoration work using recycled glass sand!


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