Despite being surrounded by stressed coral reefs, an island in Indonesia’s Spermonde Archipelago has continued to grow. A new study by researchers from the Leibniz Centre for Tropical Marine Research (ZMT) in Bremen, Germany, indicates that reef islands can dynamically respond to environmental changes affecting their reef systems. Scientists analyzed the composition of Langkai, an inhabited reef island near Makassar, and mapped its coastline changes since 1999. The findings have been published in the journal Sedimentary Geology.

The calcareous skeletons and shells of corals, mussels, snails and calcareous algae are important constituents of tropical reef islands. Their sediments form the substance of reef islands and keep them stable or even allow them to grow – at least if the surrounding reef systems are healthy. For decades, however, coral reef ecosystems have been under the influence of strongly changing environmental conditions – for example due to climate change and the global rise in water temperatures, which leads to coral bleaching, or local human intervention in the ecosystems or through intensive fishing.

Impact of Coral Reef Damage on Sediment Production and Reef Island Stability

How does the damage to coral reefs affect the production of calcareous sediment and thus the behaviour of reef islands? An international research team around Yannis Kappelmann, doctoral candidate in the Geoecology and Carbonate Sedimentology working group at ZMT, asked this question in their latest study. In addition to ZMT scientists, researchers from the Federal Institute for Geosciences and Natural Resources (BGR) in Hanover, Germany, and the Marine Research Centre at Hasanuddin University in Makassar, Indonesia, were also involved in the work.

The scientists focused their research on the island of Langkai in the Indonesian Spermonde Archipelago off the coast of Makassar. Various studies in recent years have repeatedly described the condition of the reef systems around this island off southwest Sulawesi as poor.

„We wanted to know how this severe damage to the reef systems is reflected in the island’s sediment,“ says Kappelmann, first author of the study. „Can we also see these reef changes in the composition of the sediments? Is less sediment being produced for the island?“

To answer these questions, the scientists took more than 50 sediment cores from the island, which they examined in the laboratory for their composition and changes over time. The results were initially not that surprising.

„We actually found fewer pieces of coral skeletons in the sediments that had been deposited along the coast of Langkai over the last few decades,“ reports Kappelmann.“Instead, the proportion of the calcifying green algae Halimeda was significantly higher – a sign that the reefs around Langkai are now dominated by algae as a result of the changed environmental conditions.“

Satellite Analysis Reveals Unexpected Reef Island Growth

This finding became particularly interesting for the researchers when they used satellite images to evaluate the change in the coastline of Langkai over the last 24 years (1999 – 2023).

„The coastline reconstruction showed that the area of the reef island has increased by a total of 13 percent,“ says Kappelmann. „It looks like it initially makes no difference to island growth whether coral or algae skeletons provide the constituents for the sediment. Islands that are located in the vicinity of altered coral reefs – in this case reef systems that are characterized by algal dominance – can continue to obtain sufficient sediment for their coastlines from these ecosystems.“

This suggests that changes in the sediment composition of reef islands do not necessarily lead to destabilisation of the islands.

„The study underlines the highly dynamic nature of these landforms,“ says Hildegard Westphal, head of the Geoecology and Carbonate Sedimentology working group at ZMT and Professor of Tropical Geology at the University of Bremen. „Degradation of reefs and increased growth of calcifying macroalgae have been observed worldwide in recent years, a trend that is likely to continue in the wake of ocean warming and other stress factors. Our findings show that the medium-term consequences of these developments for sediment production need not be negative. However, this does not mean at all that they have no negative consequences for reef islands, because coral reefs act as breakwaters and prevent the erosion of loose sediment along coasts in the long term. If the reefs disappear, the islands are eroded very quickly. Reef protection therefore remains a key concern for the conservation of reef islands.“

Yannis Kappelmann adds: „In recent years, a number of projects have been developed by and with the local population to protect marine ecosystems. Healthy coral reef ecosystems are not only the most effective way to preserve reef islands, but also provide more productive fishing grounds for the local people.“