Mesophotic coral refuges following multiple disturbances

Abstract

As coral populations on shallow reefs decline globally, mesophotic coral ecosystems (MCE) have been proposed as potential coral refugia from thermal, storm, and anthropogenic disturbances in the face of climate change. The current study assesses the refuge potential of MCEs in the U.S. Virgin Islands (USVI) for Montastraea cavernosa by extrapolating reproductive potential through depth-stratified coral loss following regional storm, disease, and bleaching perturbations. Fecundity of this depth-generalist coral from 4 to 40 m was measured histologically, and polyp, population, and total habitat fecundities were then extrapolated across the species’ depth range. The number of reproductively active female gonads per polyp and oocyte size experienced a significant, though small, decrease with depth, potentially due to energetic limitations. Notably, the population sex ratio was not different from 1:1 on shallow and mid-depth reefs, but it became significantly male-biased (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 large spatial extent of mesophotic reefs relative to shallow reefs in the USVI makes MCEs the primary contributor of oocytes, despite the reduced proportion of females at depth. Following Hurricanes Irma and Maria in 2017, the outbreak of Stony Coral Tissue Loss Disease in 2019, and a bleaching event in 2019, shallow and mid-depth M. cavernosa populations experienced severe coral cover declines. Shallow and mid-depth population fecundities were predicted to decline correspondingly. Coral cover in MCEs remained relatively stable following these largely shallow water perturbations, and predicted population and total habitat fecundities remained constant as well. Thus, MCEs in the USVI currently appear to be reproductive refuges for M. cavernosa, but the persistence of that refuge remains in question as disease perturbation begins to affect deeper reefs.