We examined the diurnal to seasonal dynamics of the sea surface partial pressure of carbon dioxide (pCO₂) in a subtropical nearshore estuarine system, Hangzhou Bay, adjacent to the Changjiang Estuary in the vicinity of the East China Sea, based on data collected between July 30, 2010 to September 20, 2011 by a surface buoy equipped with an autonomous pCO₂ system along with hydrological and other chemical sensors. The study site (122.37° E, 30.55° N) is influenced by the river plumes of both the Changjiang and Qiantang River and is characterized by strong tidal circulation and highly turbid waters. The amplitude of pCO₂ changes increased from winter to summer over both diurnal and spring-neap tidal cycle timescales. The average surface water pCO₂ was slightly undersaturated with respect to the atmosphere in winter (382 ± 18 μatm), but supersaturated in spring (500 ± 56 μatm) and summer (687 ± 110 μatm). Overall the study site was a source of atmospheric CO₂ with an average sea to air flux of 14 ± 9 mmol C m⁻² d⁻¹ from January to October 2011. We revealed factors controlling the pCO₂ dynamics at different timescales. Over seasonal timescales, temperature and estuarine mixing dominated the seawater pCO₂ variability. Over spring-neap tidal timescales in winter and spring, the major drivers were similarly water mass mixing and temperature. However, in summer, biological activity and air-sea exchange became the two principal factors controlling the variations in surface seawater pCO₂. Our mass balance models further suggested that biological processes impacted surface pCO₂ differently during different tidal phases. Respiration was revealed to promote the increase in pCO₂ during spring tide in August, but in neap tides of the same month biological production was evident and resulted in the drawdown of pCO₂. This is because photosynthesis was generally limited by light in summer at the study site due to high turbidity, except during neap tides when turbidity was dramatically drawn down, triggering high biological productivity. At the diurnal timescale, sea surface pCO₂ was primarily controlled by tidal mixing, except during neap tides in summer when sea surface pCO₂ was greatly influenced by biological metabolism. This study also revealed significant inter-summer differences between 2010 and 2011, showing lower sea surface pCO₂ in August 2010 as compared to August 2011, which was likely due to the enhanced biological uptake as a result of the relatively low turbidity caused by weak tidal currents and enhanced river flow in August 2010. Our study highlights a highly dynamic system primarily driven by tidal mixing, which not only modulates water mass mixing but also affects turbidity, which subsequently controls biological production. These processes led to a synergy of CO₂ dynamics in a tidally driven and highly turbid nearshore system, where high frequency time-series observations are essential to reveal the complex controls of CO₂ dynamics.

我们根据收集的数据，研究了东海附近亚热带近岸河口系统杭州湾与长江口相邻处的二氧化碳海面分压（p CO ₂）的昼夜变化。 2010年7月30日至2011年9月20日，一个浮标配备了自主的p CO ₂系统以及水文和其他化学传感器。研究地点（东经122.37°，北纬30.55°）受长江和钱塘江两河的羽流影响，其特征是潮汐环流强，水质浑浊。p CO ₂的振幅在昼夜潮汐周期时间尺度上，冬季到夏季的变化增加。在冬季（382±18μatm），相对于大气，平均地表水p CO ₂略微饱和，但在春季（500±56μatm）和夏季（687±110μatm），过饱和。总体而言，从2011年1月到10月，研究地点是大气CO ₂的来源，海到空气的平均通量为14±9 mmol C m ^-2 d ^-1。我们揭示了在不同时间范围内控制p CO ₂动态的因素。在季节性的时间尺度上，温度和河口混合占主导地位的海水p CO ₂变化性。在冬季和春季的春季潮汐时间尺度上，主要的推动因素类似地是水质混合和温度。然而，在夏季，生物活性和海气交换成为控制地表海水p CO ₂变化的两个主要因素。我们的质量平衡模型进一步表明，在不同的潮汐阶段，生物过程对表面p CO _2的影响不同。在八月份的春季潮汐中，呼吸被发现促进了p CO ₂的增加，但是在同月的潮汐潮汐中，明显的生物生产并导致p CO ₂的下降。。这是因为在夏季，由于高浊度，光合作用通常受到光的限制，除了在潮汐期间，浊度被急剧降低，从而触发了高生物生产力之外，这是在潮汐期间。在昼夜尺度上，海面p CO ₂主要受潮汐混合控制，除了在夏季的潮汐潮时，海面p CO ₂受到生物代谢的影响很大。这项研究还揭示了2010年至2011年夏季之间的显着差异，显示出较低的海面p CO ₂2010年8月与2011年8月相比，可能是由于弱潮流和2010年8月河水流量增加导致的相对较低的浊度导致生物吸收增加所致。我们的研究强调了一个主要由潮汐驱动的高度动态系统混合不仅可以调节水的质量混合，还可以影响混浊度，从而控制生物生产。这些过程在潮汐驱动且高度浑浊的近岸系统中导致了CO ₂动力学的协同作用，其中高频时间序列观测对于揭示复杂的CO ₂动力学控制至关重要。