Reconstructing ecosystem baselines using Pleistocene data: insights from an Egyptian coral reef

Worldwide, coral reefs are declining due to a combination of local and global stressors. In the Red Sea, these pressures have affected hard coral cover, altered community composition, and reduced coral colony size. However, the lack of long-term historical data in this region makes it difficult to accurately quantify the extent of reef degradation. Establishing regional baselines that represent pre-anthropogenic, pristine reef conditions is therefore essential. Pleistocene reefs provide a unique opportunity in this regard, offering insights into reef community structure and composition in the absence of recent anthropogenic disturbance. In this study, we conduct, for the first time, a quantitative comparison between a Pleistocene (Marine Isotope Stage 5e; ~125,000 years ago) fossil reef and an adjacent modern reef in northern Egypt to assess the current state of the modern reef. Our results show that the fossil reef had larger colony size in four of the five most abundant genera and the overall community composition differed significantly between the fossil and modern reef. Our findings also suggest that massive corals may be more suitable than branching corals for comparisons of coral colony size distributions due to their better preservation potential in the fossil record. While some observed differences may stem from taphonomic processes, time-averaging and environmental differences, we argue that most of the disparity reflects genuine degradation in the modern reef.