Seasonal distribution of particulate organic matters in the South Yellow Sea during late 2017 to 2019

This study focused on two subregions of the South Yellow Ses (SYS): the Subei coast region and the central basin. The Subei coast region is near the Subei Shoal with a hydrological regime driven by vertical mixing and costal currents. Its average depth is &lt;20 m, which means robust resuspension due to strong bottom stress and turbulent kinetic energy. The central basin has average depth of &gt;70 m. Its variability of biogeochemical dynamics is substantial with moderate mixing in winter, phytoplankton blooms in spring/autumn, and stratification in summer. <br> Four field surveys were conducted in the SYS between late 2017 and 2019, covering the four seasons. The winter, spring, and summer cruises were conducted by the R/V “<em>Dongfanghong</em> <em>2</em>” during 18 December 2017 to 18 January 2018, 28 March to 16 April 2018, and 24 July to 8 August 2018, respectively. The autumn survey was performed by the R/V “<em>Lanhai 101</em>” during 10–19 October 2019. Three water depth samples were collected at each station in the coastal region (surface layer: 2–4 m; middle layer: 7–10 m; bottom layer: 13–25 m), and three or four water depth samples were collected in the central region (surface layer: 2–5m; subsurface layer: 15–20 m; middle layer: 30–45 m; bottom layer: 55–75 m). Water samples were collected using an onboard Rosette sample system equipped with a conductivity–temperature–depth (CTD) instrument (Seabird 19 Plus). Additional hydrological parameters including temperature, salinity, turbidity, and depth were also measured simultaneously via the CTD sensors. The total number of samples obtained in this study were 108 (9 in the Subei coast region and 47 in the central basin), 84 (9 in the Subei coast region and 33 in the central basin), 116 (8 in the Subei coast region and 41 in the central basin), and 124 (9 in the Subei coast region and 39 in the central basin) in winter, spring, summer, and autumn, respectively. <br> A certain volume of each sample, extracted for particulate matter analysis (depending on the site: 0.5–2 L in the coastal region; 3–5 L in the central region), was filtered through glass fiber filters (GF/F, 47 mm diameter, Whatman). These filters were preweighed and precombusted in a muffle furnace at 450 °C for 5 h. After filtration, these filters were rinsed three times with ultrapure water. Subsequently, they were folded and wrapped in precombusted aluminum foil and then preserved at −20 °C until required for analysis in the laboratory. Prior to analysis, filters were fully dried in a vacuum freeze dryer for at least 24 h. Each dried filter was weighed and then quantitatively divided into two parts. The first part was used to determine particulate carbon and nitrogen; the second part was fumed with 15 mL of 36% HCl for removal of carbonate and then redried for POC measurement. Determination of these elements was performed using a Costech ECS4024 Elemental Analyzer following the standard CHN analysis method by combusting the filters at high temperature (Sharp, 1991). The standard curve was obtained using standard methionine (NC Technologies). The detection limit for carbon was 0.002 mg C (R² &gt; 0.99) and that for nitrogen was 0.005 mg N (R² &gt; 0.99). A methodological problem is that the protocol for removing carbonate is inefficient for the removal of inorganic N, which means that the residual N after fuming is actually the total nitrogen consisting of both organic and inorganic parts. Hence, the N composition of the suspended POM is hereafter represented by PN. The mass of TSM was taken as the dry weight difference of the filters before and after filtration. <br> Weidong Zhai designed the field surveys and supervised the research. Shu Yang maintained the operation of instrument. Congcong Guo finalized the data set. Jing Lang is appreciated for field sampling and most laboratory determination. Yao Niu contributed to the autumn data collection.

本研究聚焦南黄海（South Yellow Sea, SYS）的两个子区域：苏北近岸海域与中部盆地。苏北近岸海域毗邻苏北浅滩，其水动力环境受垂向混合与近岸流系驱动，平均水深不足20 m，因此底部剪切应力与湍动能较强，会引发显著的沉积物再悬浮。中部盆地平均水深大于70 m，生物地球化学过程季节性变异显著：冬季混合作用中等，春、秋季存在浮游植物水华，夏季则出现层化现象。
本研究于2017年末至2019年间在南黄海开展了4个航次的野外调查，覆盖四季。冬季、春季和夏季航次由“东方红2号”（R/V "Dongfanghong 2"）科考船执行，分别于2017年12月18日至2018年1月18日、2018年3月28日至2018年4月16日、2018年7月24日至2018年8月8日开展；秋季航次由“蓝海101”（R/V "Lanhai 101"）科考船于2019年10月10日至19日完成。近岸海域每个站位采集3个分层水样（表层：2~4 m；中层：7~10 m；底层：13~25 m），中部盆地海域每个站位采集3~4个分层水样（表层：2~5 m；次表层：15~20 m；中层：30~45 m；底层：55~75 m）。水样采集采用船载采水器架（Rosette sampler system），搭载导电率-温度-深度（conductivity-temperature-depth, CTD）仪（Seabird 19 Plus）。通过CTD传感器可同步获取温度、盐度、浊度及水深等水动力参数。本研究各航次获取的总样本数分别为：冬季108个（苏北近岸海域9个，中部盆地47个）、春季84个（苏北近岸海域9个，中部盆地33个）、夏季116个（苏北近岸海域8个，中部盆地41个）、秋季124个（苏北近岸海域9个，中部盆地39个）。
用于颗粒物质分析的每份水样，根据采样区域不同取不同体积：近岸海域取0.5~2 L，中部盆地取3~5 L，经玻璃纤维滤膜（GF/F，直径47 mm，Whatman品牌）过滤。滤膜预先经称重，并在马弗炉中以450℃预灼烧5 h。过滤完成后，用超纯水冲洗滤膜3次，随后折叠并用预灼烧过的铝箔包裹，于-20℃下保存至实验室分析。分析前，将滤膜置于真空冷冻干燥机中完全干燥至少24 h。称量干燥后滤膜的重量，随后将其定量分为两份：第一份用于测定颗粒碳与颗粒氮；第二份用15 mL 36%盐酸熏蒸以去除碳酸盐，之后再次干燥以测定颗粒有机碳（particulate organic carbon, POC）。上述元素的测定采用Costech ECS4024元素分析仪，按照高温燃烧的标准CHN分析方法进行（Sharp, 1991）。标准曲线以标准蛋氨酸（NC Technologies公司）绘制。碳的检出限为0.002 mg C（决定系数R²>0.99），氮的检出限为0.005 mg N（R²>0.99）。本方法存在一定局限：去除碳酸盐的步骤无法有效脱除无机氮，因此熏蒸后的残留氮实际为有机氮与无机氮总和的总氮。因此，后续悬浮颗粒有机物质（particulate organic matter, POM）中的氮组分以颗粒氮（particulate nitrogen, PN）表示。总悬浮颗粒物（total suspended matter, TSM）的质量以滤膜过滤前后的干重差值计算。
本研究的野外调查设计与科研监督由Weidong Zhai完成；Shu Yang负责仪器运维；Congcong Guo完成数据集整理；Jing Lang参与野外采样与多数实验室测定工作；Yao Niu协助秋季航次的数据采集工作。