Holocene coral assemblages reveal similarities to living communities in Singapore's urban reef environment

Coral reefs in Southeast Asia represent some of the most diverse marine systems in the world. These reefs have also experienced high levels of anthropogenic impacts from coastal development and climate change. Understanding historical baselines of reefs in the region can help guide analyses and projections of how corals may respond to different environmental changes. Here, we extracted 20 reef cores from five sites in Singapore to characterise the growth, diversity and community structure of coral assemblages through the Holocene and for comparison with modern reefs. Based on radiocarbon dating and analysis of corals in the cores, we established that reef initiation began approximately 8000 years before present (BP) with nearly all corals dated to between that time and about 4000 years BP. Past coral assemblages reconstructed from the core material were dominated by stress-tolerant genera Porites, Goniopora and Euphyllia, but these were lower in relative abundances compared to modern communities dominated also by Merulinidae and Pachyseris. Past assemblages also hosted lower coral richness compared to modern assemblages. Grain size analysis of the sediment matrix showed a shift towards finer material (i.e. fine sand, silt and clay; <500 μm) downcore. Transitions in coral assemblages were also observed, but no specific depth-related patterns were apparent. Together, these results indicate that reef assemblages in Singapore have persisted throughout the Holocene in a turbid environment, dominated by stress-tolerant corals with limited community change through the millennia. Despite this long-term persistence, observed losses of corals in recent decades suggest that those adapted to turbid reefs remain vulnerable to excessive environmental change and require active intervention to halt the current decline trajectory.