长江口水域富营养化的形成、演变与特点研究

【作者】 江涛；

【导师】 俞志明； 宋秀贤；

【作者基本信息】 中国科学院研究生院（海洋研究所） ， 环境科学， 2009， 博士

【摘要】 本文以我国长江口水域富营养化为研究对象,对长江水体溶解态无机氮、磷分布特点和通量变化进行了分析,基于长江流域氮“输入-输出”关系模型探索了水体氮的来源;分析了长江口水域富营养化长期演变及特点;探索了长江口海域低氧区的发生机制。结果如下:长江水体中NO₃^--N、NH₄⁺-N、DIN和DIP浓度从上游往下游呈增加趋势,但存在季节差异。长江流域从上游往下游的DIN输送通量的变化主要受水流量的影响,但从上游往下游单位面积年产N量逐渐升高;DIP输送通量从上游往下游呈增加趋势,同时也主要受水流量控制,但从季节变化来讲,DIP的月输送通量受其浓度的控制更加明显。自20世纪60年代来,长江水体中NO₃^--N、NO₂^--N、DIN和DIP浓度都处于缓慢上升趋势,但到80年代上升速度明显加快;不同历史时期DIN和DIP的季节变化特点也不尽相同,反映出其来源的差异。同时,本研究采用长江流域氮“输入-输出”关系模型（污染负荷统计模型）对长江水体氮来源进行了分析,估算了各种氮源对水体氮的贡献率。结果表明,2006年向长江流域输入氮的总量为17.6 Tg,其中20%的输入氮转移到了水体（3.5 Tg）。本年度长江大通水文站实测氮输送通量为1.8 Tg,表明约50%的氮在水体输送过程中发生了生物、化学、物理损耗。对于长江水体氮的来源来讲,饲养牲畜粪便氮流失和大气干/湿沉降氮的贡献率较大,分别为26%和25%;农业氮肥流失和城市生活污水排放的贡献率相同,都为17%;农村人口粪便氮流失和工业废水排放的贡献率分别为6%和9%。自20世纪60年代以来,长江口口门内和外海（盐度>30psu）水体中营养盐浓度增加显著。在表层水体盐度大于22psu海域DIN: PO₄^3--P值表现出了明显升高的历史变化趋势。SiO₃: PO₄^3--P值从1959年到1985-86年显著下降,然后到2003-06年有所上升。根据SiO₃: PO₄^3--P值和DIN: PO₄^3--P值的长期变化趋势,可以推出,SiO₃: DIN值从20世纪50-60年代以后呈现下降趋势。在长江口海域,随着营养盐浓度的增加,浮游生物量的大幅度升高在本研究中得到证实。同时,长江口水域浮游植物种群结构对营养盐结构的长期变化产生响应,研究结果表明,硅藻种类比重从1985-86年84.6%下降到2004-05年69.8%;年均硅藻丰度占浮游植物总丰度比重在1985-86年达到99.5%,但到2004-05年降低为75.5%,而甲藻丰度比重则由0.7%增大到25.4%。底层水体DO浓度与Delta S（底层水体与表层水体的盐度之差）和Delta T（表层水体与底层水体的温度之差）成显著负相关,这表明了水体层化或者垂直水体交换是控制长江口水域底层水体溶解氧变化的主要因素,但水体温度层化要比盐度层化在控制低氧区形成上起到更大的作用。上升流在该海域低氧的形成和分布上起到很重要的作用,显著影响低氧水团的垂直分布,也显著影响到溶解氧的水平分布。现场生产的浮游植物可能是低氧区的形成的生物基础,日益增加的叶绿素a浓度和大规模的有害藻华可能是长江口低氧区逐渐增大的原因。本研究认为,此海域低氧区的形成主要受长江冲淡水、台湾暖流的入侵、地形、尤其是温跃层的形成和现场生产的有机物质控制。更多还原

【Abstract】 In order to study the eutrophication of the Changjiang Estuary, the distribution and fluxes of dissolved inorganic nitrogen （DIN） and phosphorus （DIP） in the Changjiang River were investigated in this paper. And also, this study estimated the sources of riverine N in the Changjiang River drainage according to a N budget model based on the N input to watersheds and N transported into waterbodies. Moreover, the long-term ecological interactions between nutrients concentration and structure, and phytoplankton community from the 1950s in the Changjiang Estuary were analyzed, disclosing some patterns of eutrohpication. Last but not the least, we also studied the occurrence, distribution and seasonal variation of hypoxia in the susceptible area of the Changjiang Estuary. The main results are as follows:The results suggested that the concentrations of NO₃^--N, NH₄⁺-N, DIN and DIP increased from upstream to downstream in the waterbodies of the Changjiang River, while seasonal differences were also observed. According to our data, DIN flux was mainly controlled by water discharge, whereas DIP flux was greatly influenced by its concentration when seasonal variations were considered. The areal yields of DIN were obviously increased downstream. The concentrations of NO₃^--N, NO₂--N, DIN and DIP increased gradually since the 1960s, while the rising-speeds were greatly enhanced after 1980. The results of the work also revealed that the seasonal variations of DIN and DIP were quite different in different periods since the 1960s, indicating the elemental sources were not the same. A N budget model, based on N input to watersheds and N transported into waterbodies, was established to assess the contribution of different sources to riverine N in the Changjiang River drainage. The results of our estimate disclosed that the total N input to the Changjiang drainage was 17.6 Tg in 2006, 20% of which （3.5 Tg） was transported to the waterbodies. Meanwhile, the field investigation at Datong hydrological station recorded the riverine TN flux was approximately 1.8 Tg, indicating that about 50% of total N in waterbodies was consumed by watershed process before reaching Datong. Of the total N transported into waterbodies, excretion N from raised animals accounted for 26%; atmospheric wet/dry deposition N provided 25%; synthetic fertilizer N was quite similar to domestic sewage wastes, being 17%; excretion N from the rural population and industrial wastewater N produced only 6% and 9%, respectively.Both nitrate （NO₃^--N） and soluble reactive phosphate (PO₄^3--P) concentrations at the freshwater end-member inside the mouth of the Changjiang River and in the outer sea area with surface salinity>30 psu increased obviously since 1960s. Values of DIN to PO₄^3--P ratio in the area where the surface seawater salinity>22 psu showed a clear increasing trend. In contrast, an overall historical change of SiO₃: PO₄^3--P ratio showed a reverse trend in this area. Based on the long-term change of SiO₃: PO₄^3--P and DIN: PO₄^3--P ratios, we could make a conclusion that an overall historical change of SiO₃: DIN ratios was decreasing in this area from 1950-60s to 2000s. This work provided an evidence that the increasing nutrient contents resulted in the increment of planktonic biomass in the Changjiang Estuary. Meanwhile, the analysis on the response of phytoplankton community to the long-term changes of nutrient structure showed that the annual averaged percentage of diatom species in the Changjiang Estuary decreased from 84.6% during 1985-86 to 69.8% during 2004-05. Furthermore, the yearly averaged percentage of diatom abundance in the Changjiang Estuary decreased from 99.5% during the former period to 75.5% during the latter period, while that of dinoflagellates increased dramatically from 0.7% to 25.4%.The occurrence, distribution and seasonal variations of hypoxia in the Changjiang Estuary were studied and discussed in this paper. The results suggested that thermal stratification played a more important role in controlling the occurrence of hypoxia than saline stratification. The inflow of Taiwan Warm Current （TWC） from the southeast brought low-oxygen waters to the subsurface and made a great contribution to water stratification, which inhibited vertical mixing and associated aeration of the subsurface waters. Meanwhile, upwelling would contribute to a strong thermal and saline stratification, which further prevented the bottom water from exchanging with the surface water. In addition, upwelling would push the hypoxic waters upward and influence the vertical and horizontal distribution of hypoxia. Overall, the formation of hypoxia was controlled by a combination of several factors, especially the thermal stratification and in situ production, and the Changjiang diluted water, the inflow of TWC and the topography are included as well.更多还原