现代湖泊水体中浮游植物生物标志物在营养盐(磷、氮和硅)驱动下响应规律的建立及其在古湖沼研究中的应用

Eutrophication has become a major environmental problem in Chinese lakes. The past variation of trophic status in lakes is crucial to environmental evaluation and setting up strategies for lake restoration. Given the short time span and limited data from instrumental monitoring, such variation in most lakes can be acquired only from geochemical and biological records in sediment. In particular, the proxies based on biomarkers in sediment (e.g., biomarker abundance and ratio) were considerably valued by scientists as the biomarker is chemical molecule that is derived from organisms in lake water, and thus these proxies bear the great advantage from both biological and chemical indicators. However, the biomarker proxies were generally empirical in reconstructing the trophic status of lakes, because the fidelity and application scope of these proxies were not assessed by direct evidences. As “the present is the key to the past”, response of biomarker proxies to nutrient (phosphorus, nitrogen and silicon) forcing found in modern lakes had an important implication for using these proxies in paleolimnology studies. Here, the response rules were established as follow: (1) a extensive field investigation was undertaken in modern lakes and the response rules were statistically established between biomarker proxies and nutrient abundances (or ratios) in the water; (2) the statistical rules in (1) was verified by nutrient enrichment incubation experiment; (3) variation of trophic status in some specific lakes (with monitoring data lasting >30 years) was established based on the response rules and biomarker proxies in the sediment, and the putative results were then compared with the monitoring records to test the fidelity of the response rules. The proxies examined by the measures above were finally used to reconstruct long-term nutrient variation in lakes without (or with sporadic) nutrient monitoring but suffering from severe eutrophication.

中国湖泊富营养化问题突出，但稀少的监测资料无法展现湖泊营养状态的历史变化，这不利于湖泊环境评估和修复方案制定。沉积物中的生物标志物指标具有生物和化学指标的双重优势成为反演湖泊营养状态变化的重要手段，但该类指标(生物标志物丰度、比值等)属于经验指标，缺乏直接的使用依据和适用范围，导致实用性降低。依照“将今论古”的原理，现代湖泊水体中该类指标在营养盐(磷、氮和硅)驱动下的响应规律是其应用于古湖沼研究的依据。因此，本次研究首先在大范围湖泊调查的基础上从统计学上建立现代湖泊水体中生物标志物指标与营养盐的关系；其次通过原位营养盐添加实验模拟上述关系确定其真实性；最后利用以上关系以及特殊湖泊(具有30年以上连续监测)沉积物柱中的相关生物标志物重建过去几十年水体营养状态的变化并与监测资料比较以验证指标实际应用的可靠性。通过验证的生物标志物指标最终应用于长尺度下监测数据少而富营养化严重湖泊的营养状态重建。

为构建湖泊体系中生物标志物与水体营养盐的关系，本项目在四年的执行过程中，既研究了深水和浅水湖泊水体生物标志物浓度、组成对水体营养盐变化的响应，也通过抚仙湖和东太湖的原位营养盐添加实验对湖泊调查中的规律进行了验证。此外，本项目还研究了太湖和其他30个长江中下游湖泊表层沉积物中生物标志物丰度和组成的变化及其对水体营养盐的响应。在此基础上，以前期研究中发掘的生物标志物反演近百年来太白湖，长荡湖和洱海水体营养盐的转变并与已有研究结果和监测资料对比，证实指标应用时的可靠性。研究结果表明，磷是控制湖泊水体生物标志物变化的关键营养盐。深水湖泊中C25:1烃、胆甾醇和C16:4w3脂肪酸的浓度与水体总磷浓度有很好的正相关性，而在浅水湖泊中，C17正构烷烃和C32烷基二醇与水体总磷浓度有很好的正相关性。在长江中下游地区30个浅水湖泊的表层沉积物中，C17正构烷烃和C32烷基二醇的丰度也与上覆水体中的磷浓度具有相关性，虽然相关指数只处于0.5至0.6之间。以C17正构烷烃为主要依据重建太白湖和长荡湖近百年的营养盐变化。太白湖1970–1990期间营养盐小幅增加，可能与渔业和农业的发展有关，而在1990年之后该湖泊快速富营养化，达到重度富营养阶段，这与直接将化学肥料应用于水产养殖有关。长荡湖在1950–1982期间营养盐水平有所增加，而在1982年之后营养盐大幅增加，但是低于太白湖的水平。两阶段营养盐的变化分别与流域内农业活动的发展以及工业化和城市化的影响有关。以C25:1烃为主要依据，发现洱海在1995年以后水体营养盐水平显著增加。基于以上生物标志物所得到的三个湖泊水体营养盐的变化与已有的研究结论和湖泊监测数据一致，表明该类生物标志物具有反演湖泊水体营养盐变化的潜力。