Submarine Groundwater Discharge Alters Benthic Community Composition and Functional Diversity on Coral Reefs

Abstract: Coral reefs experience numerous natural and anthropogenic environmental gradients that alter biophysical conditions and affect biodiversity. While many studies have focused on drivers of reef biodiversity using traditional diversity metrics (e.g., species richness, diversity, evenness), less is known about how environmental variability may influence functional diversity. In this study, we tested the impact of submarine groundwater discharge (SGD) on taxonomic and functional diversity metrics in Mo‘orea, French Polynesia. SGD is the expulsion of terrestrial fresh or recirculated seawater into marine environments and is associated with reduced temperatures, pH, and salinity and elevated nutrient levels. Using a regression approach along the SGD gradient, we found that taxon and functional-entity richness displayed unimodal relationships to SGD parameters, primarily nitrate + nitrite and phosphate variability, with peak richness at moderate SGD for stony coral and the full benthic community. Macroalgae showed this unimodal pattern for functional-entity but not taxonomic richness. Functional community composition (presence and abundance of functional entities) increased along the gradient, while taxonomic composition showed a nonlinear relationship to SGD-related parameters. SGD is a common feature of many coastal ecosystems globally and therefore may be more important to structuring benthic functional diversity than previously thought. Further, studying community shifts through a functional-trait lens may provide important insights into the roles of community functions on ecosystem processes and stability, leading to improved management strategies. Keywords: biodiversity; coral reefs; functional diversity; nutrient enrichment; submarine groundwater discharge摘要：珊瑚礁面临诸多自然及人为环境梯度，这些梯度会改变生物物理条件并影响生物多样性。既往多数研究依托传统多样性指标（如物种丰富度、多样性、均匀度）聚焦珊瑚礁生物多样性的驱动因子，但针对环境变异如何影响功能多样性的认知仍较为匮乏。本研究以法属波利尼西亚的莫雷阿岛（Mo‘orea）为研究区域，探究了海底地下水排泄（submarine groundwater discharge, SGD）对分类学与功能多样性指标的影响。海底地下水排泄是指陆地淡水或再循环海水向海洋环境的排放过程，该过程会伴随水温、pH值与盐度的降低，以及营养盐水平的升高。通过沿SGD梯度开展回归分析，我们发现：石珊瑚与完整底栖生物群落的类群及功能实体丰富度与SGD参数（主要为硝酸盐+亚硝酸盐与磷酸盐的变异水平）呈单峰关系，在中等SGD强度下达到丰富度峰值；大型藻类的功能实体丰富度呈现该单峰模式，但分类学丰富度并未表现出该特征。功能群落组成（即功能实体的存在与丰度）随SGD梯度升高而显著提升，而分类学组成与SGD相关参数则呈非线性关系。SGD是全球众多沿岸生态系统的常见特征，因此其对底栖生物功能多样性的塑造作用可能远超此前认知。此外，从功能性状视角解析群落结构变化，可为理解群落功能在生态系统过程与稳定性中的作用提供重要洞见，进而助力优化珊瑚礁管理策略。 关键词：生物多样性；珊瑚礁；功能多样性；营养盐富集；海底地下水排泄