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The floating-leaved plant Trapa japonica, one of the abundant aquatic plants in Lake Izunuma, was used for nutrient uptake and release tests. By comparing the nutrient elution rates from the sediment and the nutrient uptake rates by Trapa japonica, it was estimated that in Lake Izunuma the Trapa japonica communities could take up most of the nutrients eluted from the sediment during summer. It was also confirmed that NH 4 -N released from Trapa japonica changed to NO 3 -N via NO 2 -N. This result was quite similar to the NH 4 -N, NO 2 -N and NO 3 -N concentration changes in Lake Izunuma from autumn to winter. In Lake Izunuma, the dissolved inorganic nitrogen concentration was high in autumn and winter. On the basis of the results of the discharge experiment to determine the effect of the reduction in the water level by tilting the water gate of Lake Izunuma, we consider that the nutrients released by the aquatic plant from autumn to winter can be removed effectively by this water level reduction.

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