|关键词||硝酸盐氮稳定同位素 细菌反硝化法 同化吸收 硝化过程 中国近海|
在南黄海的四条断面开展现场调查，春、秋季分别采集28和27个站位多个水层的样品。结果显示，调查期间陆源输入是南黄海表层NO3的重要来源，受浮游植物同化吸收作用，NO3浓度降低而δ15NNO3值升高。该过程在春季较秋季明显。远离近岸海域的深层水中存在一个冷水团，春季规模大于秋季，春、秋季的平均δ15NNO3值分别为7.57 ‰和5.27 ‰。由于春季δ15NNO3高于秋季而NO3浓度低于秋季，推测冷水团中可能存在内部硝化过程产生硝酸盐。在黄海冷水团站位开展同位素稀释实验验证这一推论，结果显示春季冷水团内以硝化过程为主，速率为0.27 μmol N·L-1·h-1；而在秋季内部硝酸盐以消耗为主，硝酸盐的净吸收速率为0.52 μmol N·L-1·h-1。
在长江口及邻近海域，沿三条断面采集不同深度的海水样品并分析各项指标的时空变化特征。结果显示，长江冲淡水是调查区域浅层水体（<10m）NO3的主要来源，其δ15NNO3值的范围为3.21–3.55‰。长江口及邻近海域的部分深层水体（>30m）受到黑潮次表层水的影响，其δ15NNO3值为6.03–7.6‰，略高于台湾东北处的黑潮次表层水特征值。浅层水体中浮游植物对硝酸盐的同化吸收作用存在季节性差异。由于春季适宜的温度和充足的营养盐条件使得浮游植物大量繁殖，进而引发有害藻华灾害爆发。因此，春季的浮游植物同化吸收硝酸盐强于秋季水平。浮游植物同化吸收过程引起氮同位素分馏，春季和秋季的同化吸收分馏系数分别为4.57‰和4.41‰，与实验室培养和其他现场调查的结果较为接近。研究还发现，长江口及邻近海域深层水体中存在较明显的硝化过程，该过程在秋季强于春季水平。硝化过程的分馏系数范围在24–25‰之间，与先前发现的Nitrosospira tenuis分馏结果较为相似。通过同位素稀释法测定硝酸盐同化吸收过程的速率，发现在春、秋季邻近长江口站位的表层水体中速率分别为0.26–1.17 μmol N·L-1·h-1和0.16–0.17 μmol N·L-1·h-1，调查区域内的硝化过程则强于以往的调查水平。
在中国东海南部及台湾东部海域（仅春季）的调查中，春季和秋季分别采集了6条断面共36个站位和6条断面共29个站位的不同深度的海水样品，分析了各项指标的时空变化特征。结果显示，黑潮次表层水入侵中国东海并在陆架分为两个分支：一支为黑潮近岸分支，另一支则为黑潮远岸分支。春季入侵较强，近岸分支可达到浙江沿岸海域；而秋季黑潮水的入侵较弱，仅北移至27.5°N、122°E附近。在春季，台湾东部的黑潮次表层水的硝酸盐浓度相对稳定，而在台湾东北处收到深层水的上升流补偿。在黑潮分支进入东海陆架后，内部持续发生硝化作用，产生硝酸盐的同时逐渐消耗黑潮近岸分支底层的DO。这一过程可能是导致浙江沿岸低氧区产生的重要原因。结合δ15ONO3的变化特征，发现在春季黑潮近岸分支内部氮氧同位素分馏比（18ɛ:15ɛ）为1.18，说明水团内部硝化过程发生的同时还存在同化吸收过程。我们借助数值模拟计算了春季黑潮近岸分支的水团通量和NO3收支，发现自DH9断面以北硝化作用产生至少~0.52 kmol·s-1的NO3，而在DH4断面则有至少~0.11 kmol·s-1的NO3被消耗。基于Rayleigh模型，分析黑潮分支与长江水对东海南部海域的初级生产力的贡献，发现大部分区域受到黑潮输送NO3的影响发生同化吸收过程，引起δ15NNO3出现5.5‰的分馏；仅在调查海域北部的部分近岸区域，同化吸收过程由黑潮水和长江水形成的沿岸流共同控制。
|其他摘要||As an advanced and efficient pretreatment for nitrogen isotopic analysis of nitrate, the denitrifier method was established and completed in this thesis based on related references. The accuracy and precision were tested via measuring international standard substances. According to the results, correlation coefficients are better than 0.997, and the mean standard deviation is 0.26‰. The results were verified repeatedly. The analytical accuracy and precision were excellent and were satisfied to the demand of seawater analysis. Based on this technique, investigations were conducted in the southern Yellow Sea (SYS), the Changjiang River estuary and adjacent waters (CREAW) and the East China Sea (ECS) during May to June (spring) and Octorber to November (autumn). Samples were collected for analyzing the nitrate nitrogen isotopes (δ15NNO3), nitrogen and phosphorus nutrients, dissolved oxygen (DO) and chlorophyll-a (chl-a). Physical parameters, such as salinity, temperature and density were measured instantly via the CTD recorder. The isotope dilution in situ experiments were handled in significant regions of the Yellow Sea Cold Water Mass (YSCWM) and the CREAW. These experiments were applied for measuring the key biogeochemical processes of nitrate. This thesis aims to illustrate the sources and biogeochemical processes of nitrate systemically and comprehensively. The specific outcomes are as follows.|
In situ investigations were conducted along 4 transects in the SYS. Samples were collected from varying layers of 28 sites during spring and of 27 sites during autumn. The samples were measured and the spatial and temporal distributions were analyzed. Results show that terrigenous input was the main source of NO3 in the SYS during investigated period. NO3 were assimilated by phytoplankton in the surface water, which was significant during spring and weak in autumn. The YSCWM, which located in the offshore deep waters, was larger in spring than in autumn. The mean value of δ15NNO3 in the YSCWM was 7.57 ‰ in spring and was 5.27 ‰ in autumn. Combined with the lower NO3 concentrations in spring than that in autumn, it indicated that nitrification process probably exsited in the YSCWM. The isotope dilution experiments were conducted in the sites of YSCWM for verification. The results demonstrated that nitrification was the dominant process in spring, which had an actual rate as 0.27 μmol N·L-1·h-1. However, NO3 was consumed during autumn, and the actual rate of uptake was 0.52 μmol N·L-1·h-1.
In the investigations of the CREAW, we collected water samples from different depths along three transects and analyzed the vertical distribution and seasonal variations. Results show that the Changjiang River diluted water (CDW) was the main factor affecting the shallow waters (above 10 m) of the CREAW, and CDW tended to influence the northern areas in June and the southern areas in November. δ15NNO3 values in CDW ranged from 3.21–3.55‰. In contrast, the deep waters (below 30 m) were affected by the subsurface water of the Kuroshio Current, which intruded into the waters near 31°N in June. The δ15NNO3 values of these waters were 6.03–7.6‰, slightly higher than the values of the Kuroshio Current. Nitrate assimilation by phytoplankton in the shallow waters of the study area varied seasonally. Because of the favorable temperature and nutrient conditions in June, abundant phytoplankton growth resulted in harmful algae blooms (HABs). Therefore, nitrate assimilation was strong in June and weak in November. The δ15NNO3 fractionations caused by assimilation of phytoplankton were 4.57‰ and 4.41‰ in the shallow waters in June and November, respectively. These results are consistent with previous laboratory cultures and in situ investigations. Nitrification processes were observed in some deep waters of the study area, and they were more apparent in November than in June. The fractionation values of nitrification ranged from 24–25‰, which agrees with results for Nitrosospira tenuis reported by previous studies. The isotope dilution method measured the rates of nitrate assimilation in the surface waters near the Changjiang River estuary，which were 0.26–1.17 μmol N·L-1·h-1 during spring and 0.16–0.17 μmol N·L-1·h-1 during autumn. The rates of nitrification were stronger than past investigations.
In the southern ECS and east of Taiwan (only in spring), seawater samples were collected from 36 stations of 6 transects and 29 stations of 6 transects in spring and autumn, respectively. Spatial and temporal distributions were analyzed after key parameters were determined. Results show that the Kuroshio subsurface water intrudes into the ECS and separates into two branches on the continental shelf: the nearshore Kuroshio branch current (NKBC) and the offshore Kuroshio branch current (OKBC). The NKBC extended to coastal area of Zhejiang Province in spring while it reached to 27.5°N, 122°E in autumn. In spring, the nitrate concentration in the Kuroshio subsurface water was relatively stable east of Taiwan, supplied by upwelling currents northeast of Taiwan. Continuous nitrification occurred in the NKBC after intrusion into the ECS, revealed by the gradual consumption of DO in the bottom water of the NKBC. This process may significantly contribute to the hypoxia zone near the coast of Zhejiang Province, China. Our results also indicated that NKBC assimilation is likely where nitrification occurs because the isotope fractionation ratio of oxygen and nitrogen (18ɛ:15ɛ) in NO3 was 1.18 in spring. We calculated the flux and NO3 budget in the NKBC in spring via numerical simulations and demonstrated that nitrification has occurred at a rate of at least ~0.52 kmol NO3·s-1 since the DH9 transect and that ~0.11 kmol NO3·s-1 was consumed in the coastal DH4 transect. According to the Rayleigh model, primary production was supported by the intrusion of the Kuroshio subsurface water into the southern ECS, causing 5.5‰ isotope fractionation. Moreover, at partial nearshore stations north of the region investigated, the assimilated nitrate originated from the mixing of NKBC and coastal currents from the Changjiang diluted water (CDW).
|王文涛. 基于氮稳定同位素方法的中国近海硝酸盐关键生物地球化学过程研究[D]. 北京. 中国科学院大学,2016.|