Semester of Graduation

August 2020

Degree

Master of Science (MS)

Department

Oceanography & Coastal Sciences

Document Type

Thesis

Abstract

As coral populations on shallow reefs decline globally, mesophotic coral ecosystems (MCE) have been suggested as potential coral refugia in the face of climate changes, leading to the development of a comprehensive deep reef refugia hypothesis. The current study assesses the climate and disease refuge potential of MCEs in the U.S. Virgin Islands (USVI) for the gonochoric, broadcast-spawning species Montastraea cavernosa. Polyp, population, and total habitat fecundities were estimated across the species’ depth range, and changes to population oocyte production over time due to recent ecosystem disturbances were considered. The number of gonads producing oocytes in each polyp and oocyte size decreased significantly with depth, potentially due to energy limitations, although the effect sizes were small. Notably, the population sex ratio was 1:1 on shallow and mid-depth reefs, but it became significantly malebiased (3.6:1) at mesophotic depths. Population-level differences in oocyte production over depth were primarily driven by changes in coral cover and sex ratio. The high area of mesophotic reefs in the relative to shallow reefs USVI make MCEs the primary contributor of oocytes, despite the reduced proportion of females at depth. After Hurricanes Irma and Maria in 2017 and the outbreak of Stony Coral Tissue Loss Disease followed by bleaching in late 2019, shallow and mid-depth M. cavernosa populations experienced coral cover declines, resulting in corresponding declines to population fecundities. Coral cover in MCEs remained relatively undisturbed by these largely shallow water perturbations, and population and total habitat fecundities remained constant as well. Thus, MCEs in the USVI currently appear to be a reproductive refuge for M. cavernosa, but the persistence of that refuge remains in question as disease perturbation begins to affect deeper reefs.

Committee Chair

Daniel Holstein

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