Localized Impacts of Hurricane Irma on Diadema antillarum and Coral Reef Community Structure

Abstract

Strong physical disturbance from hurricanes can disrupt coral reef ecosystems and precipitate a regime shift toward algal dominance, particularly in the absence of grazing pressure to regulate algal growth post-storm. Here, we examine the influence of Hurricane Irma on a keystone grazer, Diadema antillarum, and the surrounding coral reef benthic community in the Florida Keys. D. antillarum densities and test diameters, as well as percent cover of coral reef benthic groups, were measured at 10 sites in the middle and upper Keys before and after Irma. Significant decreases in mean D. antillarum density and median test diameter were observed following the storm. There was a correlation between the magnitude of decline in D. antillarum density and the magnitude of sediment deposition on reefs, suggesting that abrasion or burial from sediment transport may have contributed to D. antillarum mortality. We detected significant decreases in the percent cover of sponges and hydrocorals following the storm, but no change in scleractinian coral cover, which was very low (3% mean cover) at the onset of the study. Macroalgal cover increased at sites in the upper Keys and decreased at sites in the middle Keys. There was no relationship between post-storm D. antillarum density and the change in percent cover of macroalgae or turf-algal-sediment matrix (TAS), likely due to low overall abundance of the grazer. We predict that coral reefs will remain in an algal-dominated ecosystem state due to, among other factors, increasing frequency of strong hurricanes that impact the D. antillarum population.