Study on the long-term monitoring of the marine environments in the Seto Inland Sea and their predictions

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In this project, long-term monitoring of water quality including nutrients was conducted at the tip of Sada Peninsula by using the autonomous monitoring system. The long-term environmental variations of nearby areas were analysed by using the historical data and the sediment samples. Moreover, the structure of the ecosystem at present were surveyed.The major results relating to the nutrient environments of the Seto Inland Sea are as follow. (1) More than 60% of the nitrogen and phosphorous in the Seto Inland Sea are originated from the Pacific ocean. In case of Bungo Channel, supply of the nutrients from the Pacific Ocean is mainly carried out by the intrusion of cold, nutrient-rich water mass from the bottom layer of the shelf slope region south of the channel. (2)The intrusion of the nutrients into Bungo Channel varied at the decadal time scale. It generally decreased from 1980 to 2000, resulting in the decreases of the nutrient concentration and the spieces diversity of phytoplankton in Iyo-Nada.These are very important results showing the high possibility that the nutrient conditions and the ecosystems in the Seto Inland Sea can vary due to the variations of global ocean circulation, and supporting the significance of our long-term nutrient monitoring.The analyses of the sediment samples in the Bungo Channel revealed the state of recent pollutions by organic matter and organotin due to the fish culture. Surveys of the sand dune areas peculiar to the Seto Inaldn Sea were conducted, and some roles of the sand dunes in the enhancement of primary production and material cycling in the ecosystem. Moreover, the microbe surveys in Bungo Channel revealed some part of the variations of microbial loop due to the oceanic disturbances such as the kyucho and the bottom intrusion. These result will provide the important reference data in the future research verifying the long-term variations of the environments and ecosystems in the Seto Inland Sea.