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河口既是陆地与海洋的连接通道,也是海岸带的重要组成部分,沉积环境与生态系统受到海洋和陆地的共同作用,构成海洋与陆地相互影响的敏感区域。河口区域的沉积物是流域各种化学元素迁移转化的积蓄库和归宿地。河口区域的沉积物不仅记录了受人类活动影响以前各化学元素环境背景值,同时还记录了人类活动影响后的污染物的供给、迁移和沉淀历史。因此成为研究流域人类活动强度和海洋环境变化的重要载体[1]。沉积物中的N与P作为生源要素,不仅直接影响着海洋的初级生产力,而且N、P与C的关系密切,直接记录了古环境的演变历程。一般来说, 沉积记录中N、P与C初级生产力异常大幅增加的历史时期与水体富营养化时期存在较好的对应关系。流域的人类活动会引起径流中营养盐数量的变化,这种变化会带来河口及邻近海湾的营养盐浓度水平及自然结构的改变,进而造成河口区域一系列的生态环境问题。因此,河口及邻近海湾对流域人类排放的N、P的响应及其机制,成为国内外研究和迫切需要解决的科学热点[2-4]。
总有机碳(TOC)、总氮(TN)、总磷(TP)的变化常被用来解译河口沉积物形成时的古环境演变,而碳氮比(C/N)、碳磷比(C/P)、氮磷比(N/P)可以被用来探索沉积物中不同来源有机质的贡献[5-6]。
在长江口、珠江口、渤海等海域生源要素的研究起步较早,N、P与C带来水体富营养化的研究已相对较为系统[7-10],在北黄海和鸭绿江口,水体富营养化的研究工作尚显薄弱。但限于鸭绿江口特殊的政治地缘环境,野外调查限制很多,河口水体富营养化的系统性研究欠缺,制约了鸭绿江口及其邻近的北黄海海域富营养化形成机制及生态响应的深入探索,还涉及到海洋国土安全或海洋权益区分等一系列敏感问题。
虽然已有研究呈现了鸭绿江口西汊道沉积物中生源要素的沉积特征[11],但因其反映的是近百年来已经脱离径流影响的鸭绿江西汊道沉积环境的变化,生源要素主要来源于附近海岸及相邻海水养殖区域。目前鸭绿江口的主水道已由西汊道转移至东汊道,对东汊道生源要素沉积记录的研究才能全面反映鸭绿江流域的营养盐物源状况及其指示的生态环境变化过程。本文通过分析鸭绿江东汊道A4柱的沉积记录,利用TOC、TN和TP等沉积指标来追溯鸭绿江口及邻近海域环境变化的历史,为鸭绿江口乃至北黄海海域的生态环境保护和可持续发展提供科学依据。
鸭绿江口生源要素的垂直分布及对环境变化的响应
Vertical distribution of the biogenic elements and their response to the environmental change in the Yalu River estuary
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摘要: 限于鸭绿江口特殊的地缘环境,对该区域生源要素的历史演变进程不明确。在对鸭绿江主汊道A4柱状样210Pb和137CS测年的基础上,分析了总有机碳(TOC)、总氮(TN)、总磷(TP)等生源要素的垂向分布、特征值及埋藏通量,揭示了鸭绿江口历史时期的环境变化。结果表明,鸭绿江主汊道沉积物中TOC、TN、TP含量的变化范围分别为6.56—14.82 mg·g−1、0.59—1.65 mg·g−1、0.73—0.85 mg·g−1。除受到生物化学或早期成岩作用影响的26 cm外,生源要素垂向上都呈现先升后降的趋势,在56 cm左右达到了极值。TOC/TN显示沉积物中有机质是双源,自下而上陆源增加、海源减少。TN与TP在56 cm达到极值,都是河口沉积环境对流域营养盐输入变化的响应。根据生源要素沉积通量判断,鸭绿江河口环境演变大致分为3个阶段:上世纪80年代以前,以自然源输入为主;上世纪80年代至90年代初,人类活动带来了河口营养盐结构的变化;上世纪90年代以后,鸭绿江口的生态环境得到了很大改善。Abstract: The historical evolution process of biogenic elements in the Yalu River estuary has been unclear owing to the constraint imposed by the special geographical environment. Age determination was conducted based on 210Pb and 137Cs concentrations in the main branch of Yalu River; the vertical distribution of TOC, TN, and TP, characteristic value, and burial flux were analyzed to understand the environmental evolution of the Yalu River estuary. The range of content of TOC, TN, and TP in the sediments was 6.56—14.82 mg·g−1, 0.59—1.65 mg·g−1, and 0.73—0.85 mg·g−1, respectively, in the main branch. Except for effects of biochemical or early diagenesis at 26 cm, the vertical change in biogenic elements increased initially and then decreased, reaching an extreme value at approximately 56 cm. TOC/TN values revealed that the organic matters in the sediments had dual sources. The land sources increased and sea sources decreased with increase depth. The extreme values of TN and TP at 56 cm was caused due to the response of the estuarine depositional environment to changes in the nutrient salt input into the basin. The environmental evolution of the Yalu River estuary was divided into three stages: natural source input before the 1980s, structural changes in the nutrient input caused by human activities from the 1980s to the early 1990s, and great improvement in the ecological environment after the 1990s.
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Key words:
- Yalu River estuary /
- biogenic elements /
- environmental change
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表 1 鸭绿江口柱状样生源要素与含水率和黏土含量相关系数
Table 1. Correlation of TOC, TN, TP, water content and clay content
相关系数
CorrelationTOC含量
TOC contentTN含量
TN contentTP含量
TP content含水率
Water content黏土含量
Clay contentTOC含量 1 0.23 0.15 0.042 0.17 TN含量 1 0.43 0.0029 0.0093 TP含量 1 0.00004 0.43 含水率 1 0.51 黏土含量 1 表 2 生源要素特征值
Table 2. Characteristic values of the biogenic elements in the core of A4
特征值
Characteristic valuesTOC /(mg·g−1) TN /(mg·g−1) TP /( mg·g−1) 最小值 6.56 0.59 0.73 最大值 14.82 1.65 0.85 平均值 9.87 1.18 0.79 -
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