【作者】 赵真；

【导师】 陈中原；

【作者基本信息】 华东师范大学 ， 自然地理学， 2010， 硕士

【摘要】 近几十年,长江流域物质输入发生很大变化。建坝使输沙量减少从而降低河口溶解硅浓度,流域人类活动加剧导致了营养盐N、P的增加。物质输入的变化必然影响长江口表层生产力的变化。从而对河口生态环境造成影响。本文通过生物硅指标,揭示人类活动影响下长江口海区表层生产力的变化。本研究利用长江口杭州湾外泥质沉积区获得的两个浅孔(Cx21、Cx38),进行了210pb和137Cs、粒度、生物硅、有机碳和有机氮含量分析,旨在揭示近百年长江入海物质变化在沉积物中的纪录。依据两钻孔沉积物210pb和137Cs测试结果一平均沉积速率分别为Cx21孔0.94cm／yr和Cx38孔2.05cm／yr,结合沉积物粒度-长江输沙量变化对比,确定了两孔的年代模式。在此基础上,综合各指标,将两孔近百年沉积物从下向上分为4个阶段。Ⅰ、Cx21孔100-70cm和Cx38孔200-125cm,对应于上世纪50年代初以前的沉积物堆积。沉积物含砂量较高,生物硅和有机质含量均较低。生物硅平均含量分别为0.76%和0.64%,有机碳平均含量分别为0.49%和0.50%,有机氮平均含量分别为0.063%和0.066%；生物硅和有机碳、氮波动基本一致；Ⅱ、Cx21孔69-55cm和Cx38孔124-95cm,可对应于十九世纪50年代初-60年代初的堆积。两孔沉积物颗粒较细,生物硅含量较高,Cx21孔有机质较高,Cx38孔有机质略高。生物硅平均含量分别为0.87%和0.82%,有机碳平均含量分别为0.56%和0.54%,有机氮平均含量分别为0.074%和0.069%；生物硅和有机碳、氮波动在Cx21孔仍基本一致,但在Cx38孔有所分异；Ⅲ、Cx21孔54-26cm和Cx38孔94-55cm,可对应于十九世纪60年代初-80年代中期的堆积。两孔沉积物含砂量均较高,呈现两峰一谷：60年代早中期--峰值,70年代--谷值,80年代早期--峰值,生物硅和有机质的平均含量都处于较低水平。两孔生物硅平均含量分别为0.80%和0.65%,有机碳平均含量分别为0.51%和0.50%,有机氮平均含量分别为0.065%和0.066%；Cx21孔中生物硅和有机碳、氮波动基本一致,Cx38孔两者完全分异；Ⅳ、Cx21孔自25cm以上,Cx38孔自54cm以上,相当于十九世纪80年代中期以来长江入海泥沙显著减少时期的堆积。两孔沉积物中含砂量显著减少,生物硅含量于80年代后期到90年代前期迅速增加,自90年代中后期以来基本维持在低值。Cx21孔中有机碳、氮的分布和生物硅波动基本一致,而Cx38孔有机碳、氮的平均含量却持续增加,并在钻孔表层达到全孔的最高值。本段生物硅平均含量分别为0.82%和0.71%,有机碳平均含量分别为0.60%和0.56%,有机氮平均含量分别为0.068%和0.073%。以上4个阶段生物硅和有机碳、氮含量的变化可反映近百年来长江流域的自然和人类活动过程,尤其是流域人类活动包括：水库截砂—入海溶解硅降低和近几十年来工农业发展导致长江入海N、P营养盐物质输入的增加。上世纪初至中叶,长江输沙量和营养盐的输入变化受人类活动干扰较小,长江口浮游藻类以硅藻为主,两钻孔沉积物中生物硅和有机质含量基本一致,又由于表层海温较低,从而导致生物硅和有机质含量均较低。自上世纪50年代以来,随着新中国成立,人口恢复增长,生产活动加快,流域人类活动开始增强。但对河口生态环境开始出现明显影响的时期始于上世纪70年代末,即改革开放以后,入海溶解N、P开始增加,长江口外浮游藻类受到影响。1985年之后,随着葛洲坝的建成,入海输沙量急剧减少,导致溶解硅下降,相反在工农业活动下,溶解N、P浓度却急剧增加,所以本研究沉积物中生物硅自80年代后期先呈现短暂高峰后显著降低,而有机碳氮却持续上升,反映了营养盐增加时期,首先是硅藻的大量繁殖,但随着溶解硅的消耗,硅藻类生物受到硅限制,过多的氮磷导致甲藻等其他有毒藻类繁殖。不过上述现象出现在离长江口较近的Cx38孔,在离长江口较远的Cx21孔,上世纪80年代末、90年代初也出现了生物硅的短暂高峰,并在此后显著降低,但有机碳、氮基本也呈同步变化,并未在表层出现高含量现象,反映了该孔所在位置有毒藻类的繁殖尚不明显。Cx38孔位于低氧区,而Cx21孔在低氧区之外,前者水深较大,沉积物颗粒较细,后者水深较浅,沉积物颗粒较粗,可能是造成上述差异的主要原因。更多还原

【Abstract】 In recent decades, material inputs of the Yangtze River Basin have changed dramatically. Dam construction reduces sediment load and the DSi concentration of the estuary. Human activities have contributed to the increase of nutrient N and P. Changes of the material input can inevitably affect the sea surface productivity and the ecological environment of the Yangtze Estuary. In this paper, biogenic silica is used to reveal sea surface productivity changes affected by human activities in the Yangtze estuary.In this paper, two vibrocores (Cx21 and Cx38) were obtained from the muddy sediments off the Yangtze estuary to examine changes in sediment inputs from the river basin. Core sediment samples were analysized for 210Pb and 137Cs, grain size, biogenic silica (BSi), organic carbon (TOC) and nitrogen (TN) content. Chronological sediment patterns of about one-hundred years of the two cores were achieved on the basis of sedimentation rates derived from 210Pb and 137Cs test and associated sediment lithology. Four sediment stages of the past century can be identified as shown below (bottom upward):StageⅠcan correspond to the core depth 100-70cm of Cx21 and 200-125cm of Cx38. This sediment stage is characterized by the high content of sand and the general lows of BSi, TOC and TN. Sedimentation rates suggest a time period of late 19th century to 1950s. The average BSi contents of the two vibrocores are 0.76% and 0.64%, the average TOC contents are 0.49% and 0.50%, the average TN contents are 0.063% and 0.066%; BSi fluctuates synchronously with TOC and TN.StageⅡcan correspond to the core depth 70-55cm of Cx21 and 125-95cm of Cx38. This sediment stage is denoted by the markedly decrease of the sand content, and the general highs of BSi, TOC and TN. Sedimentation rates of vibrocores suggest the time period of 1950s-1960s. The average BSi contents are 0.87% and 0.82%, the average TOC contents are 0.56% and 0.54%, the average TN contents are 0.074% and 0.069%; In Cx21, BSi fluctuates synchronously with TOC and TN, but in Cx38, they begin to fluctuate unsynchronously.StageⅢcan correspond to the core depth 55-25cm of Cx21 and 95-55cm of Cx38. This sediment stage occurs the high content of sand and the general lows of BSi, TOC and TN. Sedimentation rates of the two vibrocores infer to the time period of 1960s-1980s. The average BSi contents are 0.80% and 0.65%, the average TOC contents are 0.51% and 0.50%, the average TN contents are 0.065% and 0.066%; In Cx21, BSi fluctuates still synchronously with TOC and TN, but in Cx38, they fluctuate unsynchronously.Stage IV can be linked to the core depth 25-0cm of Cx21 and 55-0cm of Cx38. This sediment stage corresponds to 1980s-2005, in which sand content has significantly reduced, but, BSi increased in the late 1980s-1990s and lowed after the mid-1990s. In Cx21, TOC and TN still fluctuate synchronously with BSi. But the average TOC and TN contents of the vibrocore Cx38 reached the highest in the sediment profile. The average BSi contents of the two vibrocores are 0.82% and 0.71%, the average TOC contents are 0.60% and 0.56%, the average contents of TN are 0.068% and 0.073%.Four phases of BSi, TOC and TN in the Yangtze River basin over the past century may reflect the natural-human activities processes, especially the human activities of watershed: DSi declined to the sea after the constructions of reservoir, and the nutrients N and P increased to the estuary with the industrial and agricultural development recent years.From the early 20th century to 1950s, human activities made less impact on sediment. load and nutrient inputs, and diatoms were main planktonic algae in the Yangtze estuary. In the two vibrocores, BSi were consistent with organic matter. Otherwise, lower SST (the sea surface temperature) associated with lower nutrients, may result in lower content of BSi and organic matter.After 1950s, with the growth of population and production, human activities of watershed began to increase, and the obvious changes of estuarine environment began from the late 1970s, when the DIN, DIP began to increase. Especially after 1985, with the completion of Gezhouba reservoir, DSi decreased because of drastically reduction of sediment load into the sea. On the contrary, with the strengthening of industrial and agricultural activities, the DIN, DIP increased dramatically. In this study, BSi presents high content at late 1980s then significantly reduces, while TOC and TN continue to increase, reflecting that with the increase of nutrients during the first period, the large population of diatoms present, but with the consumption of DSi, diatoms are silicon-limited. Too much nitrogen and phosphorus lead to toxic algae such as Heterocapsa circularisquama.This phenomenon appears in Cx38 which is closer with the Yangtze estuary. In Cx21, which is farther away from the Yangtze estuary, BSi presents a peak at the late 80s, and then decreases. TOC and TN fluctuate synchronously with BSi, reflecting that toxic algal bloom have not been serious at the location area. Vibrocore Cx38 is in the hypoxic zone with larger water depth and finer grain size, while Cx21 is not in the hypoxic zone with shallower water depth and coarser sediments, which may be the main reason of the difference.更多还原