Abstract
Mesophotic coral ecosystems are extensive light-dependent habitats that typically form between 30 – 150 m depth in the tropical oceans. The forces that structure the benthic communities in these ecosystems are poorly understood but this is rapidly changing with technological advances in technical diving and remote observation that allow large-scale scientific investigation. Recent observations of southeastern Puerto Rican Shelf of the US Virgin Islands have shown that this Caribbean mesophotic coral ecosystem has distinct habitats within the same depth ranges and across small horizontal distances (25%. High-resolution bathymetric mapping of the shelf edge revealed a topographically distinct semi-continuous 71 km-long relict barrier reef bank system. The purpose of this study was to characterize the pattern of mesophotic habitat development of the shelf edge and use this data to narrow the potential long-term and large-scale structuring forces of this mesophotic coral ecosystem. We hypothesized from limited preliminary observations that the shelf edge coral cover was limited in shallower portions of the bank and on the seaward orientation. Through stratified random surveys we found that increasing depth and decreasing wave driven benthic orbital velocities were positively related to coral abundance on the shelf edge. In addition, low coral cover habitats of the shelf edge contrasted strongly with adjacent on shelf banks surveyed previously in the same depth range, which had relatively high coral cover (>30%). Predictions of benthic orbital velocities during major storms suggested that mechanical disturbance combined with low rates of coral recovery as a possible mechanism structuring the patterns of coral cover, and these factors could be targets of future research.
Highlights
Mesophotic coral ecosystems of the Caribbean are poorly known environments beyond the depth ranges of historical coral reef investigation (30–100 m depth; Ginsburg, 2007; Hinderstein et al, 2010)
Recent observations have shown that bank mesophotic coral ecosystems, areas of hardbottom raised above the seafloor and supporting stony corals, show horizontal variations in coral community structure as well (Bridge et al, 2011, 2012), sometimes over scales
We modeled benthic orbital velocities using the CariCOOS Nearshore Wave Model (CNWM), which is an operational model based on the Simulating Waves Nearshore (SWAN) spectral wave model (Booij et al, 1999)
Summary
Mesophotic coral ecosystems of the Caribbean are poorly known environments beyond the depth ranges of historical coral reef investigation (30–100 m depth; Ginsburg, 2007; Hinderstein et al, 2010) These environments support light dependent stony corals and can be important as reservoirs of biodiversity and, potentially, as refuges or refugia from local and global stressors. The scientific focus on mesophotic coral ecosystems has recently turned from descriptive studies of community structure to specific investigations of the factors controlling their geomorphological and community structure Much of this focus has been on vertical zonation of steep slopes or walls (Grigg, 2006; Bongaerts et al, 2015), with the largest structuring forces suggested as light intensity and quality (Kahng and Kelley, 2007; Lesser et al, 2009), heterotrophic food supply (Leichter and Genovese, 2006) and temperature (Kobluk and Lysenko, 1994; Wolanski et al, 2004; Bak et al, 2005). Unknown forces acting along horizontal gradients are important in structuring these communities
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