Resilience of Octocoral Forests to Catastrophic Storms

H R Lasker,L Bramanti,L Bramanti,L Bramanti,L Bramanti,P J Edmunds,Á Martínez-Quintana

doi:10.1038/s41598-020-61238-1

Abstract

After centuries of human-mediated disturbances, Caribbean reef communities are vastly different from those described in the 1950s. Many are functionally dominated by macroalgae, but this community state represents only one of several possibilities into which present-day coral reefs can transition. Octocorals have always been abundant on Caribbean reefs, but increases in their abundance over the last few decades suggest that arborescent octocorals have the potential to expand their populations on reefs that hitherto had been dominated by scleractinians. Here we show that octocoral-dominated communities at three sites on the fringing reefs of St. John, US Virgin Islands, were resilient to the effects of two Category 5 hurricanes in 2017. We describe the dynamics of octocoral communities over five years at three sites on shallow reefs (~9-m depth), and test for the effects of Hurricanes Irma and Maria. The hurricanes depressed the densities of juvenile and adult octocoral colonies as much as 47%. However, there were only weak effects on species richness and the relative abundances of the octocoral species. The hurricanes did not alter patterns of spatial variability in octocoral community structure that existed among sites prior to the storms. The density of octocoral recruits (individuals ≤ 5 cm high) was reduced in the year following the hurricanes, mainly due to a decline in abundance of recruits <0.5 cm, but returned to pre-storm densities in 2019. Persistently high octocoral recruitment provides a mechanism supporting ecological resilience of these communities. Continuing environmental degradation is a threat to all tropical marine communities, but the reefs of St. John illustrate how “octocoral forests” can persist as the structurally dominant community on Caribbean reefs.

Highlights

Most present-day coral reefs greatly differ from the reefs described by ecologists in the 1950s and 1960s1–4, and are strikingly different from those encountered by European explorers in the 15th Century[5]
Octocorals have always been abundant on Caribbean reefs[25,26], but unlike scleractinians, which have undergone marked declines in abundance, octocorals have increased in abundance on shallow reefs (i.e.,
The presence of tropical reefs from the Pleistocene on which the scleractinian fauna mirrors the taxonomic composition of 20th century reefs[35,36] is testimony to the historic ability of coral reefs to recover from disturbance

Summary

Introduction

Most present-day coral reefs greatly differ from the reefs described by ecologists in the 1950s and 1960s1–4, and are strikingly different from those encountered by European explorers in the 15th Century[5] These changes have been attributed to a diversity of natural and anthropogenic disturbances[6], but the emergence of climate change and ocean acidification[7] has created the possibility that reefs dominated by scleractinian corals might cease to exist within decades[8]. In this study we report on the resistance and resilience of octocorals on three reefs on the south shore of St. John, U.S Virgin Islands (Fig. 1), following the passage of Hurricanes Irma and Maria in September 2017. If present-day octocoral-dominated communities in the Caribbean are resistant and resilient to disturbance, the trend towards community dominance by octocorals may create a “new normal” for shallow reefs in the region: a persistent transition from scleractinian reefs to octocoral forests (sensu Rossi et al.[50])

Methods

Results

Conclusion