Sudden enhancement of sedimentation flux of <Superscript>210</Superscript>Pb<Subscript>ex</Subscript> as an indicator of lake productivity as exemplified by Lake Chenghai

The fundamental assumption of ²¹⁰Pb sediment dating is the stable flux of ²¹⁰Pb_ex, which was derived from atmosphere and then transferred into sediments via lake water. When the sedimentation rate is relatively constant, the ²¹⁰Pb_ex activity in sediments will be exponentially reduced with sedimentation age. ²¹⁰Pb_ex in lake water is incorporated into sediments mainly via organic particulates. If the sedimentation flux of organic matter in lake water is suddenly increased, ²¹⁰Pb_ex will be significantly deposited and then transferred into sediments. On the one hand such sudden purification effect is obviously unfit for the fundamental assumption of ²¹⁰Pb dating; on the other hand, the sudden enhancement of ²¹⁰Pb_ex flux would be indicative of the conspicuous variation of primary productivity of lake water. This problem will be discussed in accordance with the variation trend of ²¹⁰Pb_ex in the vertical profile of recent sediments of Lake Chenghai, Yunnan Province. The sediment core was collected from the deep-water area of Lake Chenghai in June 1997. The vertical profile of ¹³⁷Cs activity is characterized by a tree-peak pattern. This profile gave reliable ages, and also showed the stability of sediment accumulation in the recent ten years. The vertical profile of ²¹⁰Pb_ex activity displays a specific distribution of peaks, and is similar to the vertical profile of C_org. This phenomenon seems to be related to the mechanism of constraining the transfer of ²¹⁰Pb_ex into lake sediments. The average atomic ratios of H_org/C_org and C_org/N_org in Lake Chenghai sediments are 5.51 and 7.04, respectively, indicating that the organic matter was predominantly derived from the remains of endogenic algae. In terms of the three-stage evolutionary characteristics of organic matter in sediments, i.e., “deposition-de-composition-accumulation”, the sedimentation fluxes (F(C_org)) of organic carbon (C_org) since 1970 were calculated by modeling. The sedimentation fluxes of ²¹⁰Pb_ex (F(²¹⁰Pb_ex)) in different years display good synchronous relations with the sedimentation flux of organic carbon (F(C_org)), especially in the years of 1972–1974 and 1986–1989. The variation of F(C_org) led to the variation of F(²¹⁰Pb_ex); the variation of F(²¹⁰Pb_ex) reflects, to some extent, the historical variation of lake productivity.     

近百年来新疆博斯腾湖初级生产力的变化

本研究选择新疆博斯腾湖不同区域进行沉积岩芯采集.在210Pb、137Cs定年的基础上,利用漫反射光谱分析了湖泊沉积物中叶绿素a的含量,结合沉积速率、烧失量以及开都河的径流量记录等,探讨近百年来湖泊初级生产力的变化.结果表明:开都河的径流量变化对博斯腾湖沉积速率与生产力演化有一定影响,而人类活动干扰则是影响湖泊初级生产力演化的主要原因.1950s开始的新疆第一次大规模开垦活动导致了湖泊的沉积速率及初级生产力开始上升.沉积物的沉积速率以及叶绿素a的沉积通量在1970年左右出现最高值后下降,这与新疆的第二次大规模垦荒有关,同时,周边地区盐碱地除盐方式的改变对湖泊的生态环境变化也有很大影响.从1980s至今,由于流域内人类活动干扰增强,沉积物中的叶绿素a浓度逐渐增加,湖泊初级生产力呈上升的趋势.     

富营养湖泊冰封期物理因子对初级生产力的驱动作用：以辽宁含章湖为例

与非冰封期水体相比,冰封期湖泊初级生产力的研究较为薄弱,一方面在于完整冰封期的调查观测数据仍然较少,而完整的冰下初级生产力变化过程对于理解冰下生态系统对环境因子的响应至关重要,另一方面物理过程与冰下生态的联系仍然有待明确。本研究于2021 2022年冬季期间在大辽河口沿岸的含章湖开展野外调查,通过垂向归纳模型（vertically generalized production model,VGPM）计算了冰下初级生产力,分析了冰封期中初级生产力完整的变化过程,并探讨了冰封期初级生产力的关键物理驱动因素。结果表明：冰封期初级生产力呈现波动爬升的趋势,平均值为0.20 g C/（m2·d）;整个冰封期可以划分为3个时期,即结冰期、缓慢融冰期和快速融冰期,不同时期初级生产力的关键驱动因子不同,在结冰期水温是控制初级生产力的关键因素,在缓慢融冰期冰水界面光合有效辐射强度（photosynthetically active radiation,PAR）是控制初级生产力的关键因素,在快速融冰期水温和冰水界面PAR同时控制初级生产力。在结冰期冰下水体富营养化程度逐渐增加,在融冰期初级生产力随着升温和...     

Evidence from isotopic geochemistry as an indicator of eutrophication of Meiliang Bay in Lake Taihu, China

In this paper, Lake Taihu, a large shallow freshwater lake in China, is chosen as an example of reconstruction of eutrophication through the comparison between stable isotopes from dissolved nutrients and plants and water column nutrient parameters and integration of multiple proxies in a sediment core from Meiliang Bay including TN, TP, TOC, C/N,δ15N,δ13C, etc. Differences in aquatic plant species and trophic status between East Taihu Bay and Meiliang Bay are indicated by their variations inδ13C andδ15N of aquatic plants andδ15N of NH4 . A significant influence of external nutrient inputs on Meiliang Bay is reflected in temporal changes inδ15N of NH4 and hydro-environmental parameters. The synchronous change betweenδ13C andδ15N values of sedi-mented organic matter (OM) has been attributed to elevated primary production at the beginning of eutrophication between 1950 and 1990, then recent inverse correlation between them has been caused by the uptake of 15N-enriched inorganic nitrogen by phytoplankton grown under eutrophication and subsequent OM decomposition and denitrification in surface sediments, indicating that the lake has suffered from progressive eutrophication since 1990. Based on the use of a combination of stable isotopes and elemental geochemistry, the eutrophication of Meiliang Bay in Lake Taihu could be better traced. These transitions of the lake eutrophication respectively occurring in the 1950s and 1990s have been suggested as a reflection of growing impacts of human activities, which is coincident with the instrumental data.