Three decades of degradation lead to diminished impacts of severe hurricanes on Caribbean reefs

Abstract

Major tropical storms are destructive phenomena with large effects on the community dynamics of multiple biomes. On coral reefs, their impacts have been described for decades, leading to the expectation that future storms should have effects similar to those recorded in the past. This expectation relies on the assumption that storm intensities will remain unchanged, and the impacted coral reef communities are similar to those of the recent past; neither assumption is correct. This study quantified the effects of two category five hurricanes on the reefs of St. John, U.S. Virgin Islands, where 31 yr of time‐series analyses reveal chronic coral mortality, increasing macroalgal abundance, and five major hurricanes that caused acute coral mortality. Contextualized by these trends, the effects of the most recent storms, Hurricanes Irma and Maria (September 2017), on coral cover were modest. While mean absolute coral cover declined 1–4% depending on site, these effects were not statistically discernable. Following decades of increasing abundance of macroalgae, this functional group responded to the recent hurricanes with large increases in abundance on both absolute and relative scales. Decades of chronic mortality have changed the coral assemblages of St. John to create degraded communities that are resistant to severe storms.

The demography of hurricane effects on two coral populations differing in dynamics

“…Coral abundance from March 1988 to November 2011 on reefs dominated by Orbicella annularis at (A) Yawzi Point (9 m depth) and (B) Tektite (14 m depth). Data from 1988 to 2013 are subsets of annual time‐series (Edmunds , ) that have been used to support demographic analyses at 5‐yr increments (Edmunds and Elahi , Edmunds ), and are combined with three samplings over 12 months from July 2017 that quantify the effects of Hurricanes Irma and Maria (September 2017). Black = mean (± SE ) combined coral cover (%, left ordinates) of which O. annularis complex (sensu Knowlton et al.…”

“…Leading reasons for the limited underwater effects of Hurricanes Irma and Maria on coral reefs is that colonies of more delicate scleractinians had already been killed prior to September 2017 (Zhang et al. ), allowing these reefs to acquire resilience through adversity and the low coral abundance that it favors (Edmunds ). Regardless of the causes of this resilience, statistical power in detecting changes in low coral cover (i.e., for rare organism) is likely to be low in time‐series analyses that were designed for coral‐dominated reefs (i.e., where coral is common; e.g., Cunningham and Lindenmayer ).…”

“…In both cases, 200 random dots were superimposed on each image, with the number of dots falling on each substratum expressed as a proportion of the dot population providing a measure of percentage cover (Menge ). The present analysis reports the percentage cover of all corals and O. annularis , with a full description of changes in benthic communities over nearly three decades summarized elsewhere (Edmunds , , , Edmunds and Lasker ).…”

“…Over the last 50 yr, studies of hurricanes on coral reefs have been published with regularity (Woodley et al. , Harmelin‐Vivien , Edmunds ), and their consequences have been codified in ecological theory addressing community diversity, stability, and resilience (Connell ). Yet unexpected and nuanced effects of hurricanes on coral reefs continue to be reported.…”

“…For example, throughout the Florida Reef Tract in 2005, multiple hurricanes cooled seawater through wind-driven vertical mixing, thus alleviating the consequences of bleaching (Manzello et al 2007), and throughout the Caribbean, clonal proliferation of the columnar-lobate colonies of O. annularis is accentuated on steep reef slopes in the paths of frequent hurricanes (Foster et al 2013). In St. John, Hurricanes Irma and Maria impacted the island within 14 d as Category 5 storms, yet the effects were small on reefs at 7-14 m depth on the south shore (1-4% change in absolute coral cover) because decades of degradation had created low cover coral communities that were resilient to disturbances (Edmunds 2019). The modest effect of the recent hurricanes on coral reefs in St. John is, in part, a product of the earlier removal of delicate species such as Acropora spp.…”