|Place of Conferral||北京|
碳、氮、磷、硅和氧等生源要素是海洋中物质循环和能量流动的基本要素，在海洋生态系统中发挥着重要作用。近些年，随着沿海地区城镇化进程的快速推进以及近海养殖业的迅猛增长，我国近岸海域出现一系列环境问题，如水体富营养化、缺氧、酸化、赤潮和绿藻频发等，这对生态系统的健康构成严重威胁，给国民经济带来巨大的损失。研究近岸海域生源要素的地球化学循环及其主控因素对了解区域物质循环、环境演变、近海环境保护等方面具有重要的科学和现实意义。本学位论文以养马岛附近海域为研究对象，通过两年（2016 ~ 2017年）的连续调查，系统分析了水体营养盐的含量、分布特征及其影响因素，对夏季底层海水低氧分布特征及影响机制进行初步探讨，同时对水体及沉积物中的碳循环过程进行研究，旨在丰富对人类活动影响下近岸海域生源要素的生物地球化学过程的认识，以期为区域陆海相互作用研究提供科学依据。通过上述研究，获得了一系列新的结果和认识：
养马岛附近海域水体营养盐浓度相对较低，其中溶解无机氮（DIN）和活性磷酸盐（PO43-）浓度符合一类海水标准。受到复杂的水文和生化条件的影响，营养盐的时空变化规律不明显。调查期间，DIN主要以硝酸盐（NO3-）为主，占DIN浓度的28% ~ 74%（平均为58%），其次是铵盐（NH4+），占DIN浓度的21% ~ 64%（平均为38%）。潜在富营养化评价模型和富营养化状态指数结果显示，研究海域水体处于潜在贫营养化（I）水平和中等营养化状态。
与营养盐有所差异，海水中溶解氧（DO）、无机碳（DIC）、有色溶解有机物（CDOM）以及表层沉积物有机质（SOM）表现出明显的时空变化特征。在5 ~ 11月，海水DO含量呈现先降低、后升高的季节变化特征，其最小值出现在8月底层海水。在夏季，研究海域出现底层水体低氧现象，其低氧区主要分布在近岸海域，其生消特征如下：底层水体低氧现象在6 ~ 7月开始萌生，至8月出现大面积DO<94 μmol L-1的低氧区，9月低氧现象消失。同样的，表层海水DIC也随季节呈现先降低、后升高的变化趋势，其最小值出现在8月，而底层海水DIC的季节变化特征与之相反。空间分布而言，海水DIC整体（除3月外）呈现近岸高于远岸的分布特征。在5 ~ 11月，研究海域整体上是大气CO2的净源，其向大气释放CO2的通量分别为7.95 ± 6.94（5月）、3.58 ± 3.56（6月）、18.98 ± 10.26（7月）、12.34 ± 11.85（8月）、27.94 ± 21.21（9月）和4.77 ± 2.93 mmol C m-2 day-1（11月）；而3月则是净汇，从大气中吸收CO2的通量为3.25 ± 6.04 mmol C m-2 day-1。与海水DO的季节变化相反，海水DOC、CDOM及SOM的最高值出现在8月。空间分布来看，夏季海水CDOM以及秋季SOM含量呈现近岸高、远岸低的分布特征；而春季和夏季SOM含量呈现远岸高于近岸的分布特征。
养马岛附近海域生源要素（海水营养盐、DO、DIC、DOM及SOM）的地球化学特征主要受到复杂的物理和生化过程的影响。然而不同生源要素，其主控因素有所差异。对于海水营养盐，浮游植物繁殖、大气沉降、与邻近海域水体交换、有机质分解、贝类养殖活动以及沉积物-水界面交换是影响其时空变化的重要因素。相对而言，河流淡水输入对营养盐的贡献相对较小。营养盐收支结果显示，海水DIN主要来源于大气沉降和养殖贝类排泄，分别占总DIN的49.3% ~ 63.5%（平均为56.4%）和27.4% ~ 38.1%（平均为32.8%）；海水PO43-主要来源于养殖贝类排泄和沉积物释放，分别占总PO43-的51.5% ~ 54.4%（平均为53.0%）和23.6% ~ 25.0%（平均为24.3%）；而DSi主要来源于沉积物释放，占总DSi的94.7% ~ 95.0%（平均为94.8%）。此外，每年约41.3 × 106 ~ 74.7 × 106（平均为58.0 × 106）、4.74 × 106 ~ 5.03 × 106（平均为4.89 × 106）和205.5 × 106 ~ 206.3 × 106（平均为205.9 × 106）mol的DIN、PO43-和DSi转化为其他形式（被浮游植物、藻类等吸收、通过贝类收获移除等）。对于底层水体DO，温盐跃层和海水锋面的形成是低氧维持及发展的重要物理条件。研究海域温盐跃层在夏季（6 ~ 8月）出现，秋季消失。在空间分布上，水体层化的密集区与底层低氧区分布基本吻合。DO收支实验结果表明，底层水体有机质的耗氧分解是低氧形成的主要生化因素，占总耗氧的84.6%，而沉积物耗氧占15.4%。夏季表层水体浮游植物的生长繁殖是底层水体耗氧有机质的主要来源。
海水中的碳酸盐体系受到水团的物理混合、温度变化、生物呼吸作用和光合作用等因素显著影响。在夏季（6 ~ 8月），高初级生产力导致表层海水DIC含量降低。此外，DIC在底层含量明显高于表层，这主要是底层水体有机质氧化分解所导致；在6月、7月和8月，底层水体中约42.5、62.5和62.4 μmol kg-1的DIC可能来自有机质的分解，分别占DIC浓度的1.91%、2.77%和2.83%。
对于海水CDOM，其主要来源于海洋自生和微生物分解。在表层约11.6% ~ 35.2%的CDOM来源于浮游植物。对于CDOM荧光组分，浮游植物对类蛋白组分C1和C2的贡献分别为9.0% ~ 37.4%和9.1% ~ 37.4%，而对类腐殖质组分C3和C4的贡献为7.8% ~ 18.7%和11.4% ~ 19.9%。底层水体有机质的分解是类腐殖质组分C3和C4的重要来源之一。对于C3组分，底层有机质分解在7 ~ 9月分别贡献了9.1%、18.7%和48.5%；对于C4，底层有机质分解在6 ~ 9月分别贡献了14.6%、16.3%、18.0%和26.6%。
Essential biogenic elements such as oxygen, carbon, nitrogen and phosphorus are the basic substances of material circulation and energy flow in the marine, and play key roles in marine ecosystems. In recent years, due to the rapid advancement of urbanization and rapid growth of aquaculture in coastal areas, a number of environmental problems have emerged, e.g. coastal eutrophication, hypoxia, seawater acidification, red and green tides, which bring a serious threat to the healthy development of the marine ecosystem and causes huge losses to the national economy. It is thus important to explore the biogeochemical cycle of biogenic elements and its main controlling factors, which has important scientific and practical significance to the regional material cycling and environmental evolution and protection. This study focused on the coastal area near the Yangma Island. The concentration, distribution characteristics and the influencing factors of nutrients in the water were analyzed systematically through a continuous survey of two years (2016 ~ 2017). In addition, the characteristics of low-oxygen distribution in the bottom water in summer and its influence mechanism was analyzed preliminary, and the carbon cycle process in the water and sedimentary environment was studied. This work aims to enrich the understanding of the biogeochemical processes of typical marine source elements under human disturbance, and provide the scientific basis on the study of regional biogeochemical processes and land-sea interaction. A series of results and viewpoints were presented as follows:
(1) The biogeochemical characteristics of biogenic elements in the coastal waters near the Yangma Island
The nutrient concentrations in the coastal waters near the Yangma Island were relatively low, and the inorganic nitrogen (DIN) and phosphate (PO43-) values meet the first-class seawater standards. The spatiotemporal variations of nutrients were not obvious due to the influence of complex hydrological and biochemical conditions. For nitrogen compounds, DIN was mainly dominated by NO3-, which accounted for 28% ~ 74% (mean 58%) during the study period, followed by NH4+, which accounted for 21% ~ 64% (mean 38%). According to the potential eutrophication and trophic index (TRIX) assessment model, the nutrition level was characterized in oligotrophic level, and the trophic status was rated at the medium level.
It was different from the nutrients, the dissolved oxygen (DO), inorganic carbon (DIC), chromophoric dissolved organic matter (CDOM) in the water and the sedimentary organic matter (SOM) exhibited obvious spatiotemporal variations. From May to November, the monthly variation of DO was firstly reduced and then increased with the lowest values appeared in August. In the study area, the hypoxia phenomenon in the bottom water appeared in summer, and the low-oxygen area was mainly distributed in the inshore area. The characteristics are as follows: the low DO characteristics began to emerge from June to July, and a large area of DO < 94 μmol L-1 occurred in August, and then disappeared in September. It was similar to DO, the DIC showed a trend of decreasing first and then increasing with the month, with the lowest values appeared in August. From May to November, the study area was the net source of atmospheric CO2. The flux of seawater to the atmosphere were 7.95 ± 6.94 (May), 3.58 ± 3.56 (June), 18.98 ± 10.26 (July), 12.34 ± 11.85 (August), 27.94 ± 21.21 (September) and 4.77 ± 2.93 mmol C m-2 day-1 (November). However, it was a net sink in March, and the flux of CO2 absorbed was 3.25 ± 6.04 mmol C m-2 day-1. As for the spatial distribution of DIC, it was higher in the nearshore area and decreased towards the offshore area. It was opposite to the seasonal variation of DO, the highest values of DOC, CDOM in the water and the SOM were found in August. Spatially, the CDOM in summer and SOM in autumn were higher in the nearshore area and decreased towards the offshore area. However, the relatively high values of SOM appeared in the offshore areas in summer and spring.
(2) The key process for controlling the changes of biogenic elements in the coastal waters near the Yangma Island
The geochemical characteristics of the biogenic elements (nutrients, DO, DOM, DIC and SOM) in the coastal waters near the Yangma Island were mainly affected by the complex physical and biochemical processes. However, the main controlling factors of different biogenic elements were different. Phytoplankton reproduction, atmospheric deposition, the adjacent sea input, the degradation of organic matter, scallop aquaculture and the exchange between sediment-water interfaces were the main factor affecting its spatiotemporal variations of nutrients. Relatively, the river input contributed less to nutrients. A preliminary estimate of nutrient budgets demonstrated that, the DIN load in the coastal waters near the Yangma Island was mainly from atmospheric deposition and scallop excretion, contributing 49.3% ~ 63.5% (mean 56.4%) and 27.4 ~ 38.1% (mean 32.8%) of the total influx. Scallop excretion and sediment release were the major source of PO43-, contributing 51.5% ~ 54.4% (mean 53.0%) and 23.6% ~ 25.0% of (mean 24.3%) total influx, while the silicate (DSi) mainly came from the sediment release, accounting for 94.7% ~ 95.0% (mean 94.8%) of total influx. From the perspective of the whole year, about 41.3 × 106 ~ 74.7 × 106 mol (mean 58.0 × 106 mol) for DIN, 4.74 × 106 ~ 5.03 × 106 mol (mean 4.89 × 106 mol) for PO43- and 205.5 × 106 ~ 206.3 × 106 mol (mean 205.9 × 106 mol) for DSi were converted to other forms (e.g. phytoplankton reproduction and removed by scallop harvest). As for DO in the bottom water, the formation of the thermohalocline and salinity front was an important condition for the maintenance and development of hypoxia. In this study, the thermohalocline appeared from June to August, and disappeared in September. Spatially, the water-stratified dense area was basically consistent with the distribution of the low-oxygen area. The experiment of DO budget shows that, the aerobic decomposition of the organic matter in the bottom water was the main biochemical factor for the formation of hypoxia, which accounted for 84.6% of the total oxygen consumption, while the sediment oxygen demand contributed to 15.4% of total oxygen consumption. The reproduction of phytoplankton in the surface water in summer was the main source of oxygen-consuming organic matter.
The carbonate system in the water was affected by multiple physical and biogeochemical drivers, i.e. the physical mixing of water mass, temperature changes, biological respiration and photosynthesis. In summer (from June to August), the high primary production in the surface water caused large seasonal decreases in DIC. In comparison, DIC concentrations in the bottom water were higher than those in the surface water, especially in summer, as consequence of marked stratification, restricted mixing and organic matter respiration, and about 42.5 µmol kg-1 in June, 62.5 µmol kg-1 in July and 62.4 µmol kg-1 in August of DIC, which accounting for 1.91%, 2.77% and 2.83%, could come from respiration in the bottom water.
The CDOM in the water was mainly from autochthonous and microbial decomposition. In the surface water, phytoplankton was assessed as the contributor to 11.6% ~ 35.2% of the CDOM, and 9.0% ~ 37.4%, 9.1% ~ 37.4%, 7.8% ~ 18.7% and 11.4% ~ 19.9% of the CDOM were identified as C1, C2, C3 and C4, respectively. In the bottom water, microbial aerobic degradation of sinking biogenic particles is an important source of humic-like components, which contributed 9.1%, 18.7% and 48.5% to C3 in July, August and September, and 14.6%, 16.3%, 18.0% and 26.6% to C4 from June to September, respectively.
The primary productivity in the surface water and the concentration of DO in the bottom water are the main factors affecting the preservation of SOM. In summer, the high primary productivity in the surface water and hypoxia condition in the bottom water led to high SOM values. In comparison, SOM contents were relatively low due to sufficient DO in the bottom water and relatively low supply of organic matter in autumn and spring. Dissolved nutrients could affect the accumulation of autogenous organic matter by impacting upon primary production. In summer, the NO3- in the surface water had the most obvious effects on autogenous organic carbon (AOC) and may be the principal factor of limiting the growth of phytoplankton. In autumn, the NO3- as well as DSi had more effects on AOC storage. However, PO43- had the most obvious influence on AOC storage in spring.
|MOST Discipline Catalogue||理学::海洋科学|
|杨波. 养马岛附近海域生源要素的生物地球化学研究[D]. 北京. 中国科学院大学,2019.|
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