Emerging filamentous red algae blooms: A rising threat to coral reef restoration

Abstract

Introduction: The in-situ cultivation of acroporid corals in Culebra, Puerto Rico, has been impacted by blooms of filamentous red algae in recent years. Since 2022, the onset of this bloom has occurred from March to the end of April, with a consistent recurrence pattern. The filamentous algae become entangled with corals both in nursery structures and after outplanting to the reef, often leading to partial or complete tissue smothering.

Objective: Document, for the first time, the recurrence and ecological impact of a filamentous red algal bloom observed since 2022 in the coral nurseries and restoration sites of Culebra, Puerto Rico.

Results: The mortality rate resulting from each bloom event, including corals on farms and transplants, ranged from 5 % to 20 %. The specific algal species or assemblage remains unknown; however, visual inspection is consistent with species of the genus Cottoniella, characterized by fine, red filamentous thalli, profuse branching, and the formation of dense, entangling mats.

Conclusions: The environmental factors that promote these blooms and their origin remain to be elucidated. Furthermore, the origin and the geographical distribution of these blooms have not been determined; at present, it is yet unclear whether they are a local or regional phenomenon. It is vital to note that these events, when considered in conjunction with other environmental disturbances, have the potential to compromise the success of coral reef restoration and conservation efforts.