汞在新建水库食物网中生物累积与风险评价研究进展

A review of the studies related to mercury bioaccumulation and risk assessment in the food web of newly constructed reservoirs

汞是唯一参与全球循环的液态重金属。1974年,自美国学者Smith首次报道水库中鱼类总汞含量高于邻近自然湖泊以来,水库中鱼类汞升高的风险成为新建水库环境影响评价中的重要内容之一。汞在水库生态系统生物组分和非生物组分中含量升高的现象先后在世界各国报道,包括加拿大、美国、芬兰、泰国和巴西等。通过对系列的野外研究进行回顾,表明了水库形成后生态系统中汞的甲基化过程发生了变化。水库形成对汞在食物网中的鱼类、底栖生物、浮游生物的累积产生影响。水库中汞的生物累积、迁移转化主要与被淹没土壤和植物腐解过程有着直接或间接的关系。水库形成后,总汞、甲基汞和甲基汞比例在生态系统食物网各组分中的变化并不一致。蓄水后,水体中总汞变化较小,甲基汞和甲基汞比例上升明显;浮游生物尤其是浮游动物中总汞升高,但甲基汞和甲基汞比例升高更为明显;与浮游动物类似,底栖水生昆虫中总汞升高,甲基汞和甲基汞比例升高也更为明显;鱼类作为食物网顶级消费者,甲基汞比例一般在80%以上,在水库形成后鱼类总汞和甲基汞均明显升高,但甲基汞比例变化已经不大。这些变化揭示了水库形成后甲基汞在食物网传递的两个主要可能途径,一是微型生物食物网。通过悬浮颗粒物、浮游植物、浮游动物这一环节,甲基汞和甲基汞比例有明显的增加。第二个途径是底层生物食物网。通过悬浮颗粒物、细菌、碎屑食性底栖水生昆虫、肉食型底栖水生昆虫环节,甲基汞和甲基汞比例明显增加。这两种途径均能导致以水生昆虫、小鱼、甲壳类等为食的肉食性鱼类汞含量增加。水库形成后,生态系统中汞的甲基化发生了明显的"加速"过程。这种"加速"过程最直接的因素是成库后大量土壤淹没使得汞的甲基化平衡被打破。这个过程主要有两方面的影响。一方面是直接影响,被淹没土壤和植被在腐解过程中主动或被动地将甲基汞释放到水库生态系统中;另一方面是间接影响,被淹没土壤和植被的腐解使水库底部形成厌氧环境,有利于无机汞从被淹没土壤和植被中溶出,为甲基化反应提供充裕的、可供甲基化的无机汞,同时腐解产生的大量营养物质为微生物提供丰富食物来源,使硫酸盐还原菌大量繁殖,促进无机汞的甲基化。在我国,有关汞在新建水库食物网中生物累积和风险评价的研究有待进一步加强。

Mercury is the only liquid metal in the global biogeochemical cycles. Since American scholar Smith first reported higher mercury concentrations in the muscle tissue of fish from reservoirs than in adjacent natural lakes in the USA,the risk of increased mercury levels in fish became a major issue in the assessment of an environmental impact of newly constructed reservoirs. Increased mercury concentrations in abiotic and biotic components of a reservoir ecosystem were reported in different countries,such as Canada,USA,Finland,Thailand,and Brazil. On the basis of the review of a series of fieldstudies,it appears that the mercury methylation rate in an aquatic ecosystem changes after impoundment of the reservoir.Reservoir formation affects mercury bioaccumulation in fish,in benthic organisms,and plankton in the food web. Mercury bioaccumulation,migration,and transformation in a reservoir ecosystem are mainly related to the decomposition process in the flooded soil and vegetation directly or indirectly. After reservoir formation,total mercury concentrations,methylmercury concentrations,and the ratio of methylmercury to total mercury in the components of the food web of an aquatic ecosystem vary widely. After impoundment of the reservoir,methylmercury concentrations and the ratio of methylmercury to total mercury increase significantly while total mercury varies slightly in water. Total mercury concentrations increase while methylmercury concentrations and the ratio of methylmercury to total mercury increase more strongly in plankton（ especially zooplankton）. Similar results were also reported for benthic aquatic insects. Because fish are at the top of the food chain,the ratio of methylmercury to total mercury in fish has generally been above 80%. After reservoir formation,total mercury and methylmercury concentrations increase while the ratio of methylmercury to total mercury shows little or no change in fish. These results are indicative of two possible pathways of accumulation of methylmercury in the food web after reservoir formation. One is the plankton food web. Methylmercury bioaccumulates up from the suspended particulate matter,phytoplankton to zooplankton,and methylmercury concentrations and the ratio of methylmercury to total mercury increase.The other pathway is the benthic food web. Methylmercury bioaccumulates up from suspended particulate matter,from bacterial and detrivorous benthic aquatic insects to carnivorous benthic aquatic insects, whereas methylmercury concentration and the ratio of methylmercury to total mercury increase. Through these two pathways,methylmercury and the ratio of methylmercury to total mercury increase significantly in predatory fish that eat aquatic insects,small fish,and crustacean plankton. Mercury methylation in an aquatic ecosystem accelerates after reservoir formation. The balance of mercury methylation and demethylation changes and is affected by flooded soil and vegetation directly and indirectly.Methylmercury is released directly into a reservoir ecosystem during the process of microbial degradation of a flooded soil and vegetation. An anaerobic environment is formed via the degradation of the flooded soil and vegetation at the bottom of the water column of the reservoir. This process helps to dissolve inorganic mercury from the flooded soil and vegetation. The increased level of inorganic mercury is ready for methylation. Meanwhile,nutrients from the degradation serve as a food resource for microorganisms. Sulfate-reducing bacteria reproduce massively and contribute to methylation of inorganic mercury. In China,there is a need for more research on bioaccumulation and risk assessment of mercury in the food web of newly constructed reservoirs.