Development of nutrient circulation model for ecological system control in estuary

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Title Development of nutrient circulation model for ecological system control in estuaryIn order to develop nutrient circulation model in estuary, the behavior of nutrient is investigated in river basin, river beds, rivermouth and estuary. The main results are as follows1.The mechanism of nutrient origin in river basin was surveied and a numerical model was developed in the Natori River basin, The difference between ordinal flow condition and floods was clearly observed. The nutrient load was evaluated in various mode of transport.2.The feature of nutrient transport in river was measured and modeled. Nutrient exchange between flow and river bed was anlysed and inportance of river bed was confirmed in circulation process.3.Behavior of saline wedge was observed at the Abukuma rivermouth. The mixing processes of water and nutrient were analyzed. The spread of nutrient in the sea was also studied.4.The equilibrium model for humic-iron complex in estuarine condition was developed on the basis of Dubye-Huckel theory taking account of the electrostatic interaction between iron and humic substance. The dissociation of humic-iron complex with increasing tonic strengh in estuaries was well explained by the developed model.5.Behavior of ecosystem at the river mouth was estimated by the tidal flat model. It was found that microalgae on sediment decrease the accumulation of organic matters in the model. Furthermore, the model showed flow of organic matter in bivalve-dominant system become more active than that in control system. Therefore, bivalves might be useful to inhibit the accumulation of organic matters.6.A physical model of staratification and mixing in closed bay was developed. The heat balance and water exchange at bay mouth were taken in consideration. The results show good agreement with observation.