上升流区生物活动对上层水体碳酸钙饱和度的影响：以印尼爪哇南部海域为例

The upwelling process has two opposite effects on carbonate calcium saturation state (Ω). On one hand, the upwelling system is at high risk of ocean acidification (OA) due to upwelling of cold subsurface waters with high CO2 concentrations and low Ω levels (physical transport); on the other hand, fuelled by abundant nutrients from subsurface the upwelling system usually hosts high primary production, which absorbs CO2 and increases Ω. However, decreased Ω and enhanced OA by upwelling-induced transport is stressed in a number of studies, and in contrast increased Ω and alleviated OA by upwelling-fuelled biological activity has not been highlighted sufficiently yet. Thus, this point will be explored in a representative eastern boundary upwelling system driven by monsoon -the southern coast off Java, Indonesia, which is susceptible to climate change, and is also a typical coral reef ecosystem. Since the two opposite effects on Ω associated with upwelling, i.e., physical transport by upwelling and biological activity, exist simultaneously, the key to stressing this point is to separate their contributions. In this program, at least two cruises will be conducted in this region during the early and peak periods of upwelling to examine the status of Ω and to quantitatively identify biological contribution on Ω as well as its response to upwelling stages by separating the two effects of upwelling using a two end-member mixing model. This program can help understand the OA mechanism in the upwelling system.

上升流过程对碳酸钙饱和度（Ω）的影响具有双重性。一方面，次表层高CO2和低Ω冷水的涌升（物理输运）使上升流区成为最易酸化的海域之一；另一方面，上升流也给上层水体带来了大量营养盐，由此促进浮游植物活动，造成海水中CO2浓度的下降和Ω的增加。然而，目前的研究格外强调上升流造成的Ω的降低及其对酸化的加剧，而对上升流引起的强生物活动能够增加Ω、进而缓解酸化这一过程却不够重视。要准确理解这一过程，关键是分离上升流的物理输运和生物活动这两种作用相反的过程对Ω的贡献。本项目选择印尼爪哇南部海域这一由季风驱动且易受气候变化影响的东边界上升流系统（也是典型的珊瑚礁生态系统）作为研究对象，拟于上升流的初期和鼎盛期对该海域的Ω状况进行研究，通过两端元混合模型，分离出上升流的物理输运和生物活动对Ω的贡献，定量评估生物活动对Ω的影响及其对上升流发展阶段的响应。本研究将有助于厘清上升流区海洋酸化的调控机制。

海洋酸化对海洋生物乃至整个海洋生态系统都有重大影响。探讨典型生态系统中碳酸钙饱和度的主控过程对于预测未来海洋酸化的趋势具有重要的意义。基于在印尼爪哇岛南部海域调查得到的碳酸盐参数和相关水文数据，调查了该海域pH和文石饱和度（Ωarag）的分布及南爪哇上升流（季风上升流）对其影响。调查结果显示，上升流影响区表层pH和Ωarag低于周围非上升流影响区。上升流的物理输运至少造成了海表层盐度增加0.4个单位，溶解无机碳（DIC）增加110 μmol kg-1，pH降低~0.2个单位，Ωarag降低~0.8个单位；同时，在上升流区，生物作用较高，Chl a浓度大于0.4 mg m-3，生物活动使得DIC的降低量可达70 μmol kg-1，pH的增加量可达0.15个单位，Ωarag的增加量可达0.6个单位。总之，该研究区域的物理输运作用大于生物作用，总体效应表现为DIC的增加及pH和Ωarag的降低。同上升流的物理输运作用和生物作用相比，上升流引起的温度和盐度的改变对pH的影响较小。另外，发现在十年尺度上印度洋偶极子（IOD，印度洋的独特气候现象）对Ωarag的影响可能要大于由大气CO2增加引起的海洋酸化的影响。该研究对于厘清上升流区海洋酸化的控制机制有重要意义。