Impacts of Hurricanes Irma and Maria on Coral Reef Sponge Communities in St. Thomas, U.S. Virgin Islands

Abstract

Many studies have evaluated the impacts of hurricanes on coral communities, but far less is known about impacts, recovery, and resilience of sponge communities to these extreme events. In September 2017, St. Thomas, U.S. Virgin Islands, was impacted by two Category 5 hurricanes within 2 weeks: Hurricanes Irma and Maria. Such extreme events occurring in such rapid succession are virtually unprecedented. Pre-hurricane (2015, 2016) surveys of permanent transects at six sites around St. Thomas were compared with those at 10 weeks post-hurricanes (December 2017) to evaluate storm impacts on sponges and on benthic coral reef constituents. These surveys also established a baseline for evaluating future recovery. Percent cover of sponges declined by 24.9% post-hurricanes. In contrast, sponge density increased by 43.9% from 2015 to 2016 and declined slightly after the hurricanes. Overall sponge volume did not vary over time, and whereas sponge diversity was similar in 2015 and 2016, it increased post-hurricanes. Sponge morphologies were differentially affected by the hurricanes; the proportion of upright sponges declined by 36.9%, while there was a 24.4% increase in encrusting sponges. Coral and macroalgal cover did not change significantly over the sampling period, while percent cover of epilithic algae increased and non-living substrata decreased from 2015 to 2016 but did not change further post-hurricanes. At all sites, recruitment and/or regrowth of sponges was observed within 10 weeks post-hurricanes, indicating potential resilience in Caribbean sponge communities. Whether these sponge communities return to pre-hurricane conditions and how long that will take remains to be seen.