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.中光带珊瑚礁（Mesophotic Coral Reefs）指深度介于30米至150米的深海礁，广泛分布于全球海域，其群落结构主要由水下光场的变化塑造。此外，学界日益认识到，不同礁体间的地形变异性，结合随深度增加而发生的水下光场在数量与质量上的变化，会显著影响观测到的珊瑚分布与丰度变化。本研究采用建模方法，探究水柱固有光学特性对珊瑚沿浅海至中光带深度梯度所受辐照度的影响。具体而言，本研究量化了包括水平、倾斜与垂直基底在内的礁体地形的作用，同时分析了团块状、平板状与分枝状珊瑚群体形态之间的差异。下行辐照度与礁体地形存在交互效应：在光学特性相近的水团中，抵达底栖生物的辐照度会随地形（即基底角度）发生显著变化。不同形态的珊瑚也会与这些底栖辐照度产生交互作用；模型结果显示，与平板状和分枝状形态相比，孤立的半球形珊瑚群体在各深度及全天时段始终能“感知”到更高的辐照度。然而，基于本研究建模得到的辐照度变化推算的不同珊瑚形态的光合自养潜能差异，与这些珊瑚形态类型的深度依赖性分布并不一致。其他因素（如异养作用）或可产生影响，但辐照度驱动的分布模式是倾斜礁体与垂直礁体上中光带群落差异的重要直接诱因。