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

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.

中深水珊瑚礁，定义为深度介于30至150米之间的珊瑚礁，分布全球，其结构主要受水下光场变化的影响。此外，随着对珊瑚礁地形、深度增加导致的水下光场数量和质量变化的认识日益加深，人们逐渐认识到珊瑚礁之间地形变化的多样性对珊瑚分布和丰度的观察变化产生了显著影响。本研究采用建模方法，探讨水体柱本身的物理光学特性对珊瑚在浅水至中深水梯度中暴露于下的辐照度的影响。特别是，量化了包括水平、倾斜和垂直底质在内的珊瑚礁地形的作用，以及隆起、板状和分支殖民地形态之间的差异。向下辐照度与珊瑚礁地形相互作用，使得在具有相似光学特性的水团中，到达底层的辐照度因地形（即底质角度）而显著变化。不同形态的珊瑚也与这些底栖辐照度相互作用；模型结果表明，孤立半球形殖民地相对于板状和分支形态，在深度和全天的辐照度上始终“感知”到更高的值。然而，基于此处模拟的辐照度变化的不同珊瑚形态的光合自养潜力与这些珊瑚形态型的深度依赖性分布并不一致。其他因素（例如异养作用）可能有所贡献，但辐照度驱动的模式是观测到斜坡与垂直礁底质上中深水群落差异的直接原因。