Center for Environmental Biology and Ecosystem Studies, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan.
Center for Environmental Measurement and Analysis, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan.
Sci Total Environ. 2021 Mar 20;761:143257. doi: 10.1016/j.scitotenv.2020.143257. Epub 2020 Nov 24.
Remobilization of radiocesium from anoxic sediments can be an important mechanism responsible for long-term contaminations of lakes. However, it is unclear whether such remobilization occurs in shallow lakes, where concentrations of dissolved oxygen in the hypolimnion (bottom DO) change temporally in response to meteorological conditions, and whether remobilized radiocesium influences the activity in fish. We examined the seasonal dynamics of the activities of dissolved Cs and Cs in fish (pond smelt and crucian carp) from Lake Kasumigaura, a shallow, hypereutrophic lake, five years after the Fukushima Daiichi Nuclear Power Plant accident. The activities of both dissolved Cs and Cs in fish declined during that time, but the declines showed a clear seasonal pattern that included a summer peak of Cs activity. The activity of dissolved Cs increased when the bottom DO concentration decreased, and a nonlinear causality test revealed significant causal forcing of dissolved Cs activity by bottom DO. The fact that NH-N concentrations in bottom waters were higher in the summer suggested that remobilization of Cs from sediments could result from highly selective ion-exchange with NH-N. Despite the shallow depth of Lake Kasumigaura, winds had little influence bottom DO concentrations or dissolved Cs activities. The fact that seasonal means of Cs activities in pond smelt and crucian carp were positively correlated with the seasonal means of dissolved Cs activities suggested that remobilized Cs may have influenced the seasonal dynamics of radiocesium in fish through food-chain transfer, but higher feeding rates in warm water could may have also contributed to the seasonal dynamics of Cs activity in fish. Our findings suggest that in shallow lakes, intermittent but repeated hypoxic events may enhance remobilization of radiocesium from sediments, and remobilized radiocesium may contributed to long-term retention of radiocesium in aquatic organisms.
再悬浮作用是导致湖泊放射性铯长期污染的重要机制之一，这种作用可能发生在缺氧沉积物中。然而，在浅水湖泊中是否存在这种再悬浮作用尚不清楚，因为在浅水湖泊中，底层水中的溶解氧浓度会随气象条件而发生时间变化。此外，我们也不清楚被再悬浮的放射性铯是否会影响鱼类的放射性活度。本研究以日本霞浦湖为研究对象，该湖泊为浅水、富营养化湖泊，在福岛第一核电站事故发生五年后，我们调查了湖泊鱼类（日本七鳃鳗和鲫鱼）体内溶解态铯和铯的活性的季节性变化。结果表明，尽管时间推移，鱼类体内溶解态铯和铯的活性均呈现下降趋势，但下降趋势存在明显季节性变化，其中夏季溶解态铯的活性最高。此外，当底层水中的溶解氧浓度降低时，溶解态铯的活性增加，非线性因果关系检验结果表明，底层水中的溶解氧浓度对溶解态铯的活性具有显著的因果驱动作用。夏季底层水中的 NH4-N 浓度较高，表明铯可能是通过与 NH4-N 进行高度选择性离子交换而从沉积物中再悬浮出来的。尽管霞浦湖较浅，但风对底层水中的溶解氧浓度或溶解态铯的活性影响较小。日本七鳃鳗和鲫鱼体内的铯活性季节性均值与溶解态铯活性的季节性均值呈正相关，这表明被再悬浮的铯可能通过食物链转移对鱼类中放射性铯的季节性动态产生影响，但鱼类体内铯活性的季节性变化也可能与温水时较高的摄食率有关。本研究结果表明，在浅水湖泊中，间歇性但反复发生的缺氧事件可能会增强放射性铯从沉积物中的再悬浮作用，而被再悬浮的放射性铯可能会导致水生生物中放射性铯的长期滞留。
