Resumen de Hydrodynamic conditions that favor the settlement of Diadema antillarum to a western Caribbean coral reef

Ma. Julieta Maldonado Sánchez, Teresa Herrera Dorantes, Pedro Luis Ardisson

• The reduction in the Diadema antillarum Philippi, 1845 population across the Caribbean region in the 1980s had adverse ecological consequences, including coral-algal phase-shifts. Reduced larval supply, and subsequent low recruitment success, has hampered population recovery. To examine larval density and the influence of hydrodynamics on settlement of D. antillarum, monthly plankton tows were conducted from May 2014 to April 2015 in the lagoon and fore reef zones of a western Caribbean coral reef (Xcalak, Mexico). A moored Nortek acoustic waves and currents (AWAC-Nortek) oceanographic instrument recorded wave height, wave velocity, and suspended particles in the water column. To assess settlement of D. antillarum, collectors were placed along the lagoon and fore reef, and inspected monthly. Monthly samples had a mean density of 0.06 larvae per 15 m3 from June to August corresponding to two developmental stages. Five settlers with test diameter approximately 1 mm were collected from August to December. During August, Doppler profiles recorded via the AWAC-Nortek instrument showed the lowest wave height values (<0.3 m) and a low bottom shear stress of <0.15 N m–1. Acoustic signals indicated that the particle displacement was driven primarily by wave action and that currents played a secondary role. During settlement of D. antillarum, wave orbital velocity (Uw ) was below the threshold of motion (Uwr ), allowing deposition of particles at the seabed. These hydrodynamic conditions suggest a window of opportunity for larval transport and benthic settlement. Ours is the first field study documenting the mechanisms that link hydrodynamics to the settlement of D. antillarum to a Caribbean reef.