Department of Biology, California State University, Northridge, CA 91330, USA
J Exp Biol. 2020 Nov 6;223(Pt 21):jeb225045. doi: 10.1242/jeb.225045.
Coral reefs, one of the most diverse ecosystems in the world, face increasing pressures from global and local anthropogenic stressors. Therefore, a better understanding of the ecological ramifications of warming and land-based inputs (e.g. sedimentation and nutrient loading) on coral reef ecosystems is necessary. In this study, we measured how a natural nutrient and sedimentation gradient affected multiple facets of coral functionality, including endosymbiont and coral host response variables, holobiont metabolic responses and percent cover of colonies in Mo'orea, French Polynesia. We used thermal performance curves to quantify the relationship between metabolic rates and temperature along the environmental gradient. We found that algal endosymbiont percent nitrogen content, endosymbiont densities and total chlorophyll content increased with nutrient input, while endosymbiont nitrogen content per cell decreased, likely representing competition among the algal endosymbionts. Nutrient and sediment loading decreased coral metabolic responses to thermal stress in terms of their thermal performance and metabolic rate processes. The acute thermal optimum for dark respiration decreased, along with the maximal performance for gross photosynthetic and calcification rates. Gross photosynthetic and calcification rates normalized to a reference temperature (26.8°C) decreased along the gradient. Lastly, percent cover of colonies decreased by nearly two orders of magnitude along the nutrient gradient. These findings illustrate that nutrient and sediment loading affect multiple levels of coral functionality. Understanding how local-scale anthropogenic stressors influence the responses of corals to temperature can inform coral reef management, particularly in relation to the mediation of land-based inputs into coastal coral reef ecosystems.
珊瑚礁是世界上生物多样性最丰富的生态系统之一，但正面临着来自全球和地方人为压力源的日益增加的压力。因此，需要更好地了解变暖以及陆地输入（如沉积物和营养负荷）对珊瑚礁生态系统的生态影响。在这项研究中，我们测量了自然养分和沉积物梯度如何影响珊瑚功能的多个方面，包括共生体和珊瑚宿主的反应变量、共生体的代谢反应以及在法属波利尼西亚莫雷阿岛的珊瑚群体的覆盖率。我们使用热性能曲线来量化代谢率与环境梯度之间的关系。我们发现，藻类共生体的氮含量、共生体密度和总叶绿素含量随着养分输入的增加而增加，而每个细胞中的共生体氮含量却减少了，这可能代表着藻类共生体之间的竞争。养分和沉积物的输入降低了珊瑚对热胁迫的代谢反应，表现为热性能和代谢率过程。黑暗呼吸的急性热最佳值降低了，最大光合作用和钙化率的表现也降低了。相对于参考温度（26.8℃）归一化的总光合作用和钙化率沿着梯度下降。最后，珊瑚群体的覆盖率沿着养分梯度下降了近两个数量级。这些发现表明，养分和沉积物的输入会影响珊瑚功能的多个层次。了解地方尺度的人为压力源如何影响珊瑚对温度的反应，可以为珊瑚礁管理提供信息，特别是在调节陆地输入对沿海珊瑚礁生态系统的影响方面。
