Nutrient-rich submarine groundwater discharge increases algal carbon uptake in a tropical reef ecosystem Frontiers in Marine Science

Submarine groundwater discharge (SGD) in high volcanic islands can be an important source of freshwater and nutrients to coral reefs. High inorganic nutrient content is generally thought to augment primary production in coastal systems but when this is delivered via a freshwater vector as is the case with SGD in this study, the effects on productivity are unclear. In the current literature, there is limited evidence for a direct association between SGD and primary productivity of reefs. To elucidate the response of primary productivity to SGD, we conducted spatially and temporally explicit in situ benthic chamber experiments on a reef flat along a gradient of SGD. We found significant quadratic relationships between C-uptake and SGD for both phytoplankton and the most abundant macroalga, Gracilaria salicornia, with uptake maxima at SGD-derived salinities of ~21−22 (24.5−26.6 μmol NO3-L−1). These results suggest a physiological tradeoff between salinity tolerance and nutrient availability for reef primary producers. Spatially explicit modeling of reefs with SGD and without SGD indicate reef-scale G. salicornia and phytoplankton C-uptake decreased by 82% and 36% in the absence of SGD, respectively. Thus, nutrient-rich and low salinity SGD has significant effects on algal C-uptake in reef systems. Grant no. NA14OAR4170071 Grant no. NA22OAR4170108

高火山岛的海底地下水排泄（Submarine groundwater discharge, SGD）是向珊瑚礁输送淡水与营养盐的重要途径。学界普遍认为，沿岸系统中较高的无机营养盐含量可促进初级生产力，但当此类营养盐通过淡水载体（即本研究中的海底地下水排泄）进行输送时，其对生产力的影响尚不明确。现有文献中，关于海底地下水排泄与珊瑚礁初级生产力之间直接关联的研究证据十分有限。为阐明初级生产力对海底地下水排泄的响应机制，我们沿海底地下水排泄梯度，在珊瑚礁礁坪上开展了空间与时间显式的原位底栖箱式培养实验。研究发现，浮游植物与优势大型藻类厚叶江蓠（Gracilaria salicornia）的碳摄取量与海底地下水排泄强度均呈现显著的二次相关关系，其碳摄取峰值出现在由海底地下水排泄主导的盐度约21~22的环境中（对应硝酸根浓度为24.5~26.6 μmol·L⁻¹）。上述结果表明，珊瑚礁初级生产者在盐度耐受能力与营养盐可利用性之间存在生理权衡。我们通过对有无海底地下水排泄的珊瑚礁开展空间显式模拟，结果显示：在无海底地下水排泄的情况下，礁坪尺度下厚叶江蓠（Gracilaria salicornia）与浮游植物的碳摄取量分别下降82%与36%。由此可见，富营养低盐的海底地下水排泄对珊瑚礁系统中的藻类碳摄取具有显著影响。本研究受以下项目资助：资助号NA14OAR4170071、资助号NA22OAR4170108。