Depth-dependent irradiance from sunrise to sunset across the shallow to mesophotic depth gradient for three coral morphologies from a backward Monte Carlo ray-tracing model

Abstract

Mesophotic coral reefs, defined as deep reefs between 30 and 150 m, are found worldwide and are largely structured by changes in the underwater light field. Additionally, it is increasingly understood that reef-to-reef variability in topography, combined with quantitative and qualitative changes in the underwater light field with increasing depth, significantly influence the observed changes in coral distribution and abundance. Here we take a modeling approach to examine the effects of the inherent optical properties of the water column on the irradiance that corals are exposed to along a shallow to mesophotic depth gradient. In particular, the roles of reef topography including horizontal, sloping and vertical substrates are quantified as well as the differences between mounding, plating and branching colony morphologies. Downwelling irradiance and reef topography interact such that for a water mass of similar optical properties the irradiance reaching the benthos varies significantly with topography (i.e., substrate angle). Corals with different morphologies also interact with these benthic irradiances; model results show that isolated hemispherical colonies consistently “see” greater irradiances across depths, and throughout the day, compared to plating and branching morphologies. The differences in the photoautotrophic potential of different coral morphologies, based on the changes in irradiance modelled here, are not, however, consistent with depth-dependent distributions of these coral morphotypes. Other factors (e.g., heterotrophy) arguably contribute, but irradiance driven patterns are a strong proximate cause for the observed differences in mesophotic communities on sloping versus vertical reef substrates.

For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/841083