CRUSTOSE CORALLINE ALGAE INCREASE FRAMEWORK AND DIVERSITY ON ANCIENT CORAL REEFS

Crustose coralline algae (CCA) are calcitic red algae that are key producers of carbonate sediment in the ocean and on reefs today. Given their importance in modern systems, CCA are assumed to play an essential role on fossil reefs as well; however, the global, long-term relationship of CCA with reef-building corals in the fossil record has not been quantitatively assessed. This study focuses on the correlation between CCA abundance and function on the diversity and framework of fossil coral reefs. Reef data were compiled from the Paleobiology Database, the Paleoreefs Database, and a comprehensive literature review. A four point “Algae Score” was implemented to designate the level of involvement of CCA in a reef setting; for example, reefs in which CCA were of volumetric importance were scored higher than reefs with a low volume of CCA.

Our statistical analysis demonstrates a strong association between CCA and the framework building capacity of fossil coral reefs, as well as the generic diversity of fossil corals and other reef dwellers. A higher involvement of CCA was associated with true reefs as opposed to a reef mound or biostrome. True reefs are defined as having framework (wave resistant intergrowth of biomineralizers), thus more abundant CCA correlated with a stronger reef structure. This echoes what is seen in the modern, where CCA act as important binders and cementers, making reefs more resistant to physical disturbances, particularly in higher-energy environments. Coral reefs with higher amounts of CCA also had higher coral and reef dweller generic diversity. On modern reefs, CCA are known to induce the settlement of larvae of corals and other reef dwellers, and so play a key role in maintaining biodiversity. Our analysis shows that CCA increase the building capacity and biotic resistance of coral reefs; CCA thus play a critical role in the preventing reef collapse (decreased carbonate production and loss of biodiversity). The significance of the association between CCA and corals in the fossil record is consistent with similar observations on modern reefs; CCA are critical components of Cenozoic coral reef systems. These results confirm that Cenozoic coral reefs had similar relationship with CCA as modern corals do, and underscores the potential for past reef collapse as an analogue for current/future reef stresses.