300年来鄱阳湖营养盐演化重建与模拟

水体富营养化已经成为全球性的问题而受到广泛关注,然而其发生的过程和机制尚未完全明了。在湖泊营养演化过程中,水文和生态是两个最基本的制约因素。相对于短期的和试验性的研究,长时间尺度的营养盐变化过程能更全面地揭示营养盐的演化机制。以我国最大的淡水湖——鄱阳湖为例,采用湖泊水体交换周期模型和湖泊生态-营养盐动力耦合模型,重建鄱阳湖营养盐的长期变化,并利用沉积钻孔代用指标加以验证。在此基础上探讨其演化机制,模拟的时间序列中营养盐变化对气候水文与生态系统存在两种不同的响应模式。敏感因子分析显示:典型同步响应期中(1812—1828 AD),气候水文因子的贡献率达79.1%,生态因子为20.9%;典型异步响应期中(1844—1860 AD),两者贡献率分别为36.4%和63.6%。在模拟的营养盐变化时间序列中同步期占62.5%,说明气候因子在营养盐演化过程中起重要的作用;异步期虽只占12.5%,但对湖泊营养盐作用、营养盐反馈生物量同样至关重要。相关分析结果显示,生物量增长与TP含量基本呈线性关系,但存在一个阈值。在没有超过阈值前,生物量对TP具有较好的调节作用;当超过阈值之后,生物量的调节作用减弱。

Reconstructions and simulations of nutrient evolution in Poyang Lake over the past 300 years

Lake eutrophication has become a global environmental problem and severely affects water resource quality. Although studies have addressed eutrophication, the mechanisms are not fully understood to date because of the water-biomass-sediment complexity in lakes, and limited observation data makes analysis of the entire nutrient evolution process difficult. Therefore, to reveal eutrophication mechanisms more comprehensively, analysis should be based on long-term processes of nutrient changes. In lake ecosystems, all stresses and forces interact during the eutrophication, among which, hydrology and ecosystem play key roles. Hydraulic residence time impacted nutrient supply, lake productivity, and nutrient accumulation processes profoundly. With water cycles and energy exchanges in lake ecosystems, primary productivity is a critical process contributing to lake trophic states. Poyang Lake, the largest fresh water lake in China, is located on the southern bank of the middle-reach of the Yangtze River. Compared with most other eutrophic lakes in the region, water quality of Poyang Lake was mesotrophic during the 20th century. However, water quality has declined since the beginning of the 21th century. To discern the trophic evolution of Poyang Lake, hydraulic residence time and lake ecosystem-nutrient dynamic models, were used in the present study to determine long-term changes of nutrients and biomass in the lake. Furthermore, sediment records, including total organic carbon(TOC), Pollen concentration and DI-TP series derived by using diatom-TP transfer function, compared the simulation results of the past 300 years. The simulation revealed that nutrient TP changed differentially in response to changes in climatic-hydrological and climatic-ecosystem conditions. Control runs for 1955-2008 proved that TP increase was consistent with the observed growth of primary producer biomass. On this basis, we ran the models for a long-term period from 1700 to 1899. The time-series analysis for the simulations showed that the period 1812-1828 was synchronized between hydrology-forced and ecosystem-feedback nutrient changes. Ratio of hydrology and aquatic biomass accounted for 79.1% and 20.9% of this simulated annual series. The period 1844-1860 AD showed an asynchronous match in which hydrology-forced and ecosystem-feedback nutrient changes occurred. The ratio of the two were 36.4% and 63.6%, respectively. In the simulated time series, the synchronous period was 62.5% of the entire time, suggesting climatic-hydrological factors played a major role in the process of nutrient evolution. Although the asynchronous period only accounted for 12.5%, it also had a critical effect on TP changes in the lake. Furthermore, simulations of nutrient and biomass were done by correlation analysis. The result revealed a quasilinear relationship between nutrient TP and biomass. However, a bifurcation appeared when TP > 40%, indicating a threshold existed in this pattern, suggesting that TP changed significantly differently when it increased above the threshold and decreased below the threshold.