Benthic Characterization of Mesophotic Communities Based on Optical Depths in the Southern Mexican Pacific Coast (Oaxaca)

Abstract

The distinction between shallow coral reefs and mesophotic coral ecosystems (MCEs) has not been fully clarified yet. The original definition of MCEs, by depths of 30–150 m, fixes their bathymetrical limits and fails to accommodate environmental and biological variation. Recent studies have indicated that water transparency and light availability may explain why MCEs do not occur at fixed depths but vary among localities. This study aimed to evaluate the presence and distribution of MCEs, along the central coast of Oaxaca, through optical depths and the associated benthic community. Using MODIS-Aqua satellite data (Kd490), we estimated the mesophotic optical depths monthly (z10%, z1%, z0.1%), over the last four years. In addition, to characterize benthic community structure, we conducted underwater photo quadrat surveys at two locations on the southern Mexican Pacific coast from 10 to 55 m depth. Significant differences between depths and locations were found in benthic communities. Furthermore, the lower distribution of photosynthetic taxa was different between the two locations but indicative to the z10% and z1% in both cases. Those differences were associated with the upwelling season, which reduces, drastically and differentially, the light availability for benthic communities between the two locations and limits the development of MCEs on the central coast of Oaxaca.

Effects of surface geometry on light exposure, photoacclimation and photosynthetic energy acquisition in zooxanthellate corals

“…Overall, the combination of water optical properties [16,57,61], substrate type and landscape architecture [42,62], determines the balance between direct and diffuse components in the local light field at specific depths. The vertical attenuation coefficient for downwelling irradiance (K d ), a proxy of the water optical properties, is influenced by the concentration and composition of dissolved and particulate materials, which can absorb and scatter light as it travels through the water.…”