Seasonal dynamics of the activities of dissolved Cs and the Cs of fish in a shallow, hypereutrophic lake: Links to bottom-water oxygen concentrations.

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.