We estimated the extent of suspended solids (SS) and particulate organic matter (POM) discharged from a freshwater reservoir, called the Isahaya Reservoir, into a sea area by tracking the diatom frustules produced in the reservoir. The estimation method is based on the fact that Skeletonema subsalsum and S. costatum, are the predominant diatoms in the reservoir and the sea, respectively, and the discharged SS and POM contain the freshwater diatom, S. subsalsum, and that the diatom frustules remain undecomposed in the environment even after the plankton decays. The results of the sediment trap experiment and bottom sediment survey showed that the distribution of diatom frustules in the bottom sediment had good agreement with that in the water column in the sea, and the greatest amounts of the drained SS and POM were estimated to have reached and settled down on the bottom sediment in the sea area within approximately 2 km from the drainage gates of the reservoir.

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