DO RUBBLE BEDS HOLD POTENTIAL FOR REEF DEVELOPMENT?

The structural complexity of coral reefs plays an important role in supporting biodiversity. Natural and anthropogenic stressors continue to exacerbate reef erosion leading to the development of rubble beds which have lasting effects on reef communities. Recent studies have shown that while rubble beds have low surface complexity, they can provide a dynamic ecological framework that supports a diverse community. Additionally, consolidated rubble can promote reef recovery, which has led to a growing interest in the stabilization of rubble beds as a coral restoration method. Our understanding of the natural processes involved in coral recovery within rubble beds is limited. This study aims to investigate the community membership and reef development within ancient rubble frameworks found within the Late Triassic (ca. 210 Ma) Dachstein platform. Our quantitative assessment shows that 67.4% of collected samples were composed of reef rubble and 29% were composed of in-situ reef patches. The abundance of reef rubble suggests that physical disturbances acted as a major control on taxonomic composition and reef development. Based on modern ecological studies, reef recovery in this storm driven environment would require the stabilization and lithification of rubble followed by the colonization of important frame-building organisms. Our study shows that organisms known for their ability to bind and stabilize sediments are prominent within the study localities. The abundance of borings within the matrix points to early lithification. A linear regression shows that there is a strong positive correlation (R²=0.65, p-value<0.001) between sponge abundances and coral abundances through time suggesting that sponges could play a role in creating substrate favorable to coral settlement in rubble beds. Despite the evidence of rubble consolidation and the colonization of frame-building organisms, coral reefs were not able to build large complex structures and instead were restricted to structurally simplistic reef patches. This could be due to the frequency of physical disturbances. This study highlights the ecological importance of sponges within rubble beds and how frequently recurring storms can hinder the development of structurally complex reefs, which are otherwise common along the Dachstein platform.