海洋生态与环境科学重点实验室知识库

工业革命以来，人类活动使得大气CO₂含量持续上升，导致海洋酸化。酸化会对海洋生态系统产生严重影响。pH作为指示海水酸碱度变化的重要参数，受到国内外学者的广泛关注。本学位论文在系统分析物理因素(特别是温度、盐度)及化学组成、生物过程等与海水pH关系的基础上，提出了用海水pH_{in situ}(25°C)表征现场海水酸碱度的新思路，选择近海富营养化的长江口邻近海域和大洋寡营养的夏威夷ALOHA站为研究对象，对2015~2016年长江口邻近海域四个季节pH及其他参数的调查数据以及夏威夷ALOHA站1992~2016年海水pH及其他参数的长期监测数据进行分析，并与传统的pH_{in situ}进行对比。综合运用相关分析、回归分析及时间序列分析等方法探讨了富营养高生产力的长江口邻近海域及寡营养低生产力海域ALOHA站海水pH的季节变化特征、影响因素及其长期发展趋势。获得了如下认识：

1. 提出了pH_{in situ}(25°C)这一参数，用以表示原位温度下海水的酸碱度。pH_{in situ}(25°C)随温度变化趋势与pH_{in situ}相反，即温度越高，pH_{in situ}(25°C)越高，海水碱性越强。这与碳酸钙饱和度随温度升高而升高的事实相耦合。海水的盐度与碳酸盐体系相对组成对当前海水pH的绝对值(7.5~8.3)具有决定性作用。

pH_{in situ}是海水氢离子绝对浓度的负对数。水的pH随温度升高显著降低，但依然为中性。因此pH_{in situ}不能指示原位温度下海水的酸碱度。pH_{in situ}(25°C)是将原位温度下的pH进行氢离子浓度的标准化，即将氢离子浓度按比例转换至25°C对应的氢离子浓度，以此衡量温度对海水酸碱度的影响。pH_{in situ}的温度校正系数约为-0.0127 pH/°C，而pH_{in situ}(25°C)约为+0.0032 pH/°C。pH_{in situ}(25°C)与温度的关系完全改变了以往有关温度对海水pH或海水酸碱度的认识。温度升高，pH_{in situ}(25°C)增大，海水碱性增强；且温度对海水pH_{in situ}(25°C)的影响程度降低至温度对pH_{in situ}影响的四分之一。

盐度虽为保守性参数，其对海水pH同样具有重要影响。实验及计算结果表明，当大洋表层海水盐度为0而碳酸盐体系组成不变时其pH将高达9.4。相对于海水碳酸盐体系组成，海水的盐度同样控制着当前海水pH的绝对值。

1. 富营养高生产力的长江口邻近海域与寡营养低生产力的ALOHA站表层海水pH_{in situ}(25°C)的季节变化特征一致，均为夏季最高，冬季最低。主要与海-气CO₂交换、浮游植物季节性生长及温度对海水酸碱度的影响有关。但二者表层海水pH_{in situ}的季节变化特征相反，长江口邻近海域表层海水pH_{in situ}夏季高、冬季低；低生产力海区则为夏季低、冬季高。二者相反的pH_{in situ}季节变化特征体现了两种海区pH_{in situ}季节变化的不同调控机制，高生产力海区pH_{in situ}的季节变化受控于浮游植物生长的季节性变化，而低生产力海区pH_{in situ}的季节变化受控于温度的季节性差异。

长江口邻近海域春、夏、秋、冬表层海水pH_{in situ}(25°C)分别为7.81 ± 0.15、8.18 ± 0.14、7.83 ± 0.09、7.79 ± 0.09，夏季明显高于冬季。ALOHA站表层海水pH_{in situ}(25°C)在夏季11月达到最高值，冬季4月为最低值。由于夏季温度高，CO₂溶解度低；浮游植物生长旺盛，光合作用吸收CO₂；且pH_{in situ}(25°C)本身与温度呈正相关。因此，高生产力的长江口邻近海域与低生产力的夏威夷ALOHA站表层海水pH_{in situ}(25°C)均为夏季高、冬季低。

长江口邻近海域表层海水pH_{in situ}夏季高于冬季，ALOHA站则相反。长江口邻近海域表层海水生产力高，Chl a浓度季节变化较大，pH_{in situ}的季节变化受控于浮游植物的季节性生长。长江口邻近海域浮游植物生长旺季为夏季，因而夏季表层海水pH_{in situ}高于冬季。而ALOHA站表层海水营养盐含量低，生产力低，pH_{in situ}的季节变化则主要受温度对海水碳酸盐解离常数的影响，温度越高，pH_{in situ}越低，因而ALOHA站表层海水pH_{in situ}冬季高于夏季。

1. ALOHA站表层海水盐度增加加剧了表层海水的酸化，而温度的升高略微减缓了表层海水的酸化趋势。ALOHA站次表层海水pH降低速率明显高于表层，主要与真光层内增加的有机质在次表层分解产生更多的CO₂以及次表层水体较弱的酸碱缓冲能力有关。

1992~2016年ALOHA站表层(0-10 m)海水温度以0.013°C/yr的速率升高，盐度以0.010 /yr的速率增加。盐度及温度对pH_{in situ}(25°C)降低的贡献值分别为9.1%和-0.2%。近30年来，ALOHA站次表层(235-265 m)海水pH(25°C)以-0.0027 pH/yr的速率降低，降低速率约为表层海水的2倍。次表层海水较高的酸化速率与其较高的CO₂增加速率及较弱的缓冲能力有关。次表层海水nDIC(指溶解无机碳DIC标准化至盐度为35的结果)的升高速率约为1.61 µmol/kg/yr，明显高于表层(1.08 µmol/kg/yr)。表层海水nDIC平均值为1978.8 µmol/kg，而次表层海水的nDIC平均值为2082.8 µmol/kg，次表层较高的nDIC意味着较弱的酸碱缓冲能力。

Over the past 250 years, more and more anthropogenic CO₂ have been released into the atmosphere, which is partially eased by oceanic uptake. However, this process causes “ocean acidification”. Ocean acidification will have a serious impact on marine ecosystem. As a key parameter indicating the acidity of seawater, pH has received extensive attention from scholars around the world. Based on systematically analysis of the role of physical factors (especially temperature and salinity), chemical composition and biological processes played on seawater pH, we propose a new pH parameter named pH_{in situ}(25°C) to represent the acidity of seawater. This, we analyzed pH and other parameters of station ALOHA in the North Pacific Ocean from 1992 to 2016 and the adjacent waters of the Changjiang Estuary at all seasons from 2015 to 2016, and discussed pH seasonal changes, influencing factors and long-term acidification trends of the eutrophic adjacent waters of the Changjiang Estuary and the oligotrophic station ALOHA with correlation analysis, regression analysis and time series analysis. The main results are as follows:

(1) A new parameter named “pH_{in situ}(25°C)” was defined to indicate the acidity of seawater at in situ temperature. The relationship between pH_{in situ}(25°C) and temperature is opposite to the pH_{in situ}, that is, the higher the temperature, the higher the pH_{in situ}(25°C), which is coupled to the fact that calcium carbonate saturation increases with temperature rising. Salinity is of vital importance for the absolute value of the current seawater pH which ranges from 7.5 to 8.3.

pH_{in situ} is the negative logarithm of the absolute concentration of hydrogen ions in seawater. The pH of water decreases significantly with increasing temperature, and water remains neutral. Therefore, pH_{in situ} can not indicate the acidity of seawater at in situ temperature. pH_{in situ}(25°C) is a parameter normalize the hydrogen ion concentration at in situ temperature to 25°C, it can indicate the effect of temperature on the acidity of seawater. The temperature coefficient of pH_{in situ} is approximately -0.0127 pH/°C, the one of pH_{in situ}(25°C) is about +0.0032 pH/°C. This completely changed previous understanding of the effect of temperature on seawater pH or acidity. As the temperature increases, the seawater is more alkaline, and the effect of temperature on pH_{in situ}(25°C) is reduced to a quarter of the effect of temperature on pH_{in situ}.

Although salinity is a conservative parameter, it also has an important influence on seawater pH. The experimental and calculation results show that when the sea surface salinity is 0 and the composition of the carbonate system does not change, the pH will be as high as 9.4, which reveals the significant role of salinity played on seawater pH.

(2) Seasonal change characteristics of surface pH_{in situ}(25°C) of the high-productivity Changjiang Estuary adjacent waters and low-productivity surface layer water at station ALOHA is coincident，which are the highest in summer and lowest in winter. It is mainly related to the exchange of sea-gas CO₂, the seasonal growth of phytoplankton and the effect of temperature on seawater acidity. Surface pH_{in situ} of Changjiang Estuary adjacent waters is higher in summer and lower in winter; while it is on the contrary in the low-productivity sea area. Seasonal change of surface pH_{in situ} in the high productivity sea area is relative to the seasonal variation of concentration of Chl a, while surface pH_{in situ} in the low-productivity sea area is significantly affected by temperature.

Surface pH_{in situ}(25°C) of the waters adjacent to the Changjiang Estuary was 7.81 ± 0.15 in spring, 8.18 ± 0.14 in summer, 7.83 ± 0.09 in autumn and 7.79 ± 0.09 in winter, which was higher in summer and lower in winter. pH_{in situ} (25°C) of the surface water of station ALOHA reached the highest value in summer and the lowest in winter. Lower CO₂ solubility at higher temperature and biological drawdowm effect on pCO₂ lead to lower concentration of carbon dioxide in summer. Moreover, higher temperature means higher pH_{in situ}(25°C). All of these processes lead to higher pH_{in situ}(25°C) in summer than that in winter whether it is high-productivity sea area or low-productivity sea area.

Surface pH_{in situ} of the waters adjacent to the Changjiang Estuary is higher in summer and lower in winter, and it is the opposite at station ALOHA. The seasonal change of pH_{in situ} in the adjacent waters of the Changjiang Estuary is controlled by the seasonal growth of phytoplankton because of its high production. While the seasonal change of surface pH_{in situ} at station ALOHA depends more on seasonal temperature changes, which will affect the dissociation constant of seawater carbonate and then change pH.

(3) The increasing salinity of the surface water at station ALOHA accelerates the pH reducing, while the increasing sea surface temperature slightly eases the acidification at station ALOHA. The pH reduction rate of subsurface water (235-265 m) at station ALOHA was significantly higher than surface layer. It is mainly caused by the higher increase rate of CO₂ and the weaker buffering capacity.

From 1992 to 2016, the sea surface temperature of station ALOHA increased at a rate of 0.013°C/yr, and the salinity increased at a rate of 0.010/yr. The increase in salinity contributed approximately 9.1% to the decrease of pH_{in situ}(25°C), and the contribution of temperature was -0.2%. In the past 30 years, the subsurface (235-265 m) pH(25°C) of station ALOHA has been reduced at a rate of -0.0027 pH/yr, which is about twice as much as the pH decrease rate of surface water. The higher acidification rate of subsurface seawater is related to its higher CO₂ increase rate and weaker buffer capacity. The rate of increase of nDIC (nDIC = 35DIC/Salinity) in subsurface seawater is about 1.61 μmol/kg/yr, compared to 1.08 μmol/kg/yr in surface water. The nDIC average value of subsurface seawater was 2082.8 μmol/kg, compared to 1978.8 μmol/kg in surface layer. The higher nDIC means higher CO₂ and weaker buffer capacity.