Study on structure of environmental problems in an inner bay from viewpoint of long-term change of watershed, estuary and ocean. -Arase study in Hamana Lake

(i)通过柑橘果园地区的河水中观察到高氮和磷酸盐浓度。磷肥和铵肥的浓度随施肥的增加而增加。降雨增加了土壤中磷的负荷，特别是颗粒磷的负荷。估计每年的养分负荷。Inohana湖和Hamana湖之间的水交换取决于风力。通道内的流动和磷的垂直分布都具有明显的两层结构，这对水和物质（营养物）的交换有很大的促进作用。通过建立盒子模型，定量地讨论了水和磷的流动和收支。（iii）根据50年的长期野外资料和模拟结果，提出了发生熵化和缺氧的几个原因。1980年以前，码头的建设使水的交换变得更好，从而改善了水质。然而，自1980年以来，水质一直在恶化。这项研究表明，造成这种现象的原因之一是水温升高。(4)采用自适应网格细化方法建立了一种数值模拟方法，并将其应用于哈曼湖浅水区。验证了该方法的有效性。(v)估计了伊诺哈纳湖和哈马纳湖周围柑橘果园施肥带来的养分负荷的长期变化。20世纪70年代中期以来，因依诺哈那湖周边施肥量的减少和哈玛那湖周边果园面积的减少，氮素负荷量呈下降趋势。自1993年以来，在哈马纳湖北部地区经常观察到蛤蜊的口角。这被认为是由于建造内部码头增加了水的交换率。它还导致了以蛤为食的贝类的增加

(i) High nitrogen and phosphate concentrations were observed in the river water through the citrus orchard area. The phosphate and ammonium concentrations have increasing trends with the application of fertilizer. The phosphorus loads, especially particulate phosphorus, increase with rain. Annual nutrient loads are estimated.(ii) Water exchange between Inohana and Hamana Lakes is dependent upon wind forcing. Both of the flow in the channel and vertical distribution of phosphorus have distinct two-layer structures, which contribute to the exchanges of water and material(nutrient)strongly. By developing a box model, the flows and budgets of waters and phosphorus are quantitatively discussed.(iii) From the long-term field data for 50 years and simulation results, several causes of entrophication and hypoxia occurrances were suggested. Prior to 1980, jetties construction made the water exchange better, as a result the water quality was improved. However, the water quality has been worse since 1980. This study suggests that one of the cause for it is the increase in water temperature.(iv)A numerical simulation method was developed using the adaptive mesh refinement method and applied to the shallow water region in Hamana Lake. The effectiveness of the method was confirmed.(v) Long-term changes of nutrient loads from the fertilization in citrus orchards around Inohana and Hamana lakes were estimated. Since mid-1970s, nitrogen load has been decreasing due to decrease in fertilization quantity around Inohana Lake and due to decrease in area of orchard around Hamana Lake.(vi) Since 1993, the clam spat has been frequently observed in northern area of Hamana Lake. This is considered due to increase in water exchange rate by constructing the inner jetties. It also induced increase of clam-eating shellfish