Estuaries are hotspots where terrestrially originated dissolved organic matter (DOM) is modified in molecular composition before entering marine environments. However, very few research has considered nitrogen (N) modifications of DOM molecules in estuaries, limiting our understanding of dissolved organic nitrogen (DON) cycling and the associated carbon cycling in estuaries. This study integrated optical, stable isotopes (delta 15N and delta 13C) and molecular composition (FT-ICR MS) to characterize the transformation of DOM in the Yangtze River Estuary. Both concentration of dissolved organic carbon (DOC) and DON decreased with increasing salinity, while their delta 13C and delta 15N increased with the increasing salinity. A significant positive correlation was found between delta 15N and delta 13C during the transportation of DOM to marginal seas, indicating that the behavior of both DOC and DON are primarily controlled by the mixing of freshwater and the seawater in the YRE. During the mixing process, the DON addition was observed using the conservative mixing curves. In the view of molecular composition, DOM molecules became more aromatic as the number of N atoms increased. Spearman correlations reveal that DOM molecules with fewer N atoms exhibited a higher enrichment in protein-like components, while those with more N atoms were more enriched in humic-like components. In addition, the delta 15N and delta 13C tended to increase as the N content of DOM decreased. Therefore, DON molecules with fewer N atoms were likely to be transformed into those with more N atoms based on the isotopic fractionation theory. This study establishes a linkage between the molecular composition and the delta 15N of DOM, and discovers the N transformation pattern within DOM molecules during the transportation to marginal seas.

Marginal seas play a crucial role in the cycling of dissolved organic nitrogen (DON) between the terrestrial and marine environments. However, very few studies have considered the molecular transformation of DON in marginal seas, leaving the DON molecular modifications in its cycling largely unknown. Therefore, this study examined DON cycling in the Bohai Sea and Yellow Sea, two semi-closed marginal seas in northern China, using stable isotopes (δ15N and δ13C), optical characteristics, and molecular compositions. Compared to the Yellow Sea, the Bohai Sea had a weaker exchange with the open ocean, resulting in higher concentrations, lower δ15N, and more recalcitrant properties in DON. The DON cycling showed significant differences inside and outside the Yellow Sea Cold Water (YSCW). Degradation was the major sink of DON in the YSCW, during which more highly unsaturated compounds and carboxyl-rich alicyclic molecules were produced. Nitrogen atoms were found to be removed from the molecules with more N atoms to those with fewer ones during the DON degradation. This study discovered the molecular modifications in DON cycling and highlighted the intrinsic mechanisms in the cycling of DON in marginal seas. © 2023 Elsevier Ltd

An ensemble-based method for the observation system simulation experiment (OSSE) is employed to design optimal observation stations and assess the present observation stations in the northeastern South China Sea (SCS). We employed the 20-year (1992-2012) sea surface height (SSH) data to design an array to monitor the intraseasonal to interannual variability. The results show that the most key region was found located at the northwest of Luzon Island (LI) where the energetic Luzon cyclonic gyre (LCG) occurs; other key regions include the edge of the LCG, the northwest of the Luzon Strait (LS), and the southwest of Taiwan, China. By contrast, we found that the present observation stations might oversample at the northwest of the LS and undersample at the northwest of LI. In addition, the optimal stations perform better in a larger area than the present stations. In vertical direction, the key layer is located within the upper 200-m depth, of which the surface and subsurface layers are most valuable to the observing system.

通过对马里亚纳—雅浦海沟通道走航CTD、ADCP观测数据和历史潜标观测数据进行分析，以及构建三维高分辨率MITgcm深渊混合模型和HYCOM深渊环流模型，我们发现，在马里亚纳海沟142.5°E断面上盐度、流速均存在季节变化,4000-5000米及6000米以深均为气旋式环流，计算得到深渊中净的LCDW输运量为-1.6 Sv,比前人利用少量CTD观测估算得到的-1 Sv大了60%。雅浦—马里亚纳海沟通道140.32°E断面上，在3000米以下主要分布着UCDW和LCDW两个深层水团,4000米深度为UCDW和LCDW深层水团的分界线。利用Thorpe参数化方案和LADCP的观测数据得到了该通道的耗散率和扩散率，其中3500米至底的平均耗散率为9.7×10-10 m2s-3,平均扩散率为3.9×10-3 m2s-1。利用三维非静力、高分辨率的MITgcm模式，对马里亚纳海沟及其周边海域的主要四大分潮M2、S2、K1、O1内潮进行模拟研究，发现在这一海区,M2分潮的内潮能通量远大于其他三个分潮。从内潮能通量分布看，内潮主要在马里亚纳岛弧生成，然后向外传播，最强的内潮能通量达6 kw/m。结合混合参数化公式估算了该海区由于内潮作用引起的耗散及垂向混合扩散率的量级，结果显示在海沟内部，其耗散的量级也可达10-8 W/kg,垂向混合扩散率的值也在10-3 m2s-1左右。 By analysing CTD and ADCP in-situ observation data of the passage of the Mariana-Yap Trench and historical mooring observation data,as well as building up the three-dimensional high-resolution MITgcm abyssal mixing model and HYCOM abyssal circulation model,seasonal variations in salinity and ocean current in the 142.5°E section of the Mariana Trench are depicted.Two cyclonic circulations are found at the depth of 4000-5000 m and below 6000 m.The net LCDW transport volume in the abyss,calculated to be-1.6 Sv,is 60%larger than previous-1 Sv which is estimated by using a small amount of CTD observations.On the 140.32°E section of the passage of the Yap-Mariana Trench,two deep water masses of UCDW and LCDW are mainly distributed below 3000 meters,and the depth of 4000 meters is the boundary between UCDW and LCDW.Using the observation data of LADCP in this passage,combined with the Thorpe parameterization scheme,calculation of the dissipation rate and diffusivity shows that the average dissipation rate from 3500 meters to the bottom is 9.7×10-10 m2s-3 while the average diffusivity rate is 3.9×10-3 m2s-1.A three-dimensional non-hydrostatic,high-resolution MITgcm numerical model is used to simulate the four main tidal components M2,S2,K1,O1 tides in the Mariana Trench and its adjacent ocean.It was found that in the model domain,the M2 internal tidal energy flux is much larger than the others.The distribution of internal tidal energy flux shows that the internal tide is mainly generated in the Mariana Island arc and propagates outward subsequently.The strongest internal tidal energy flux is 6 kw/m.Combined with the mixed parameterization scheme,we estimated the magnitude of the dissipation and diffusivity rate caused by the internal tide that the magnitude of the dissipation in the bottom of trench can reach 10-8 W/kg and the magnitude of the diffusivity is around 10-3 m2s-1,which is an order of magnitude higher than the required vertical mixing level that maintaining deep sea stratification.