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タイトル: Spatial distribution and its temporal variation of hypoxic water mass and phytoplankton bloom around Sumida river estuary, Odaiba, Tokyo Bay
著者: Biswas, Md. Masudur Rahman
著者(別言語): ビスワス, モハマド マスドゥ ラハマン
キーワード: Spatial
time series
hypoxia
phytoplankton bloom
発行日: 2012年9月27日
抄録: Water quality deterioration often causes hypoxia and phytoplankton bloom in estuarine and coastal system, but the mechanism that controls hypoxia and phytoplankton bloom vary among estuaries and often difficult to distinguish. A series of automated water column time series observation has been established around Sumida river estuary, Odaiba, Tokyo Bay. The time series data obtained from the three fixed observation stations at the outer and inner part of Odaiba during the period of June to September 2011 reveal the occurrence of episodic surface phytoplankton bloom followed by hypoxic events. The goal of the present study was to indentify and clarify the mechanism of hypoxic events and phytoplankton blooms in relation to hydrographic and environmental factors to mitigate the adverse effect on the estuary natural ecosystem. Five hypoxic events and phytoplankton blooms occurred during the summer observation period around the estuary in summer 2011. Spatial and temporal variations in dissolved oxygen and chlorophyll-a concentration and the current severity of phytoplankton blooms were higher in the eutrophicated and stratified outer part (station 1) of the estuary than that of inner part (station 2 and 3). In contrast hypoxia were pronounced at the inner part than that of outer part of the estuary. In the inner part of the estuary, the main factor explaining dissolved oxygen and chlorophyll-a concentration variations were stratification, a floating fence on the surface water with a dominant effect on oxygen consumption processes and transport of phytoplankton towards the enclosed area of the estuary. In contrast, in the outer part of the estuary, the main factor explaining dissolved oxygen and chlorophyll-a concentration variations were tidal circulation, input of river discharge and direct source of phytoplankton from downstream directed ebb tidal currents suggesting that the effect of oxygen production and accumulation of phytoplankton through transportation were stronger than that of inner part. The outer part of the estuary showed strong water-column stratification and very high tidal flushing, and ventilation processes through the water column were more important than oxygen production or consumption processes. Most hypoxic events were developed at the three stations with the increase of discharge followed by precipitation, north wind, ebb tide directed to downstream, light limitation coincided with low solar radiation and breakdown of each hypoxic event followed the opposite hydrographic and environmental phenomena. However, the most dominant factors to occur hypoxic events were summer stratification induced by north wind. Higher concentration of dissolved oxygen concentration in subsurface layer of the water column could be observed at the outer part (station 1) corresponding well with the distribution of chlorophyll-a concentration as because at the inner part (station 2 and 3) of estuary correspond with low dissolved oxygen concentration with low chlorophyll-a concentration compared to outer part of the estuary. Phytoplankton biomass was concentrated more at the station 1 than those of station 2 and 3 as station 1 was more influenced by river water as well as sea water which are the dominant of source phytoplankton accumulation around the estuary. In the other sense, there was a significant difference in chlorophyll-a concentration between station 2 and 3 as a floating fence was positioned on the surface water which may restrict/hinder the horizontal transport of chlorophyll-a from station 2 towards 3. Thus the highest concentration of chlorophyll-a was found at the outer part (station 1) and second highest concentration was observed at station 2 and the lowest concentration of chlorophyll-a was found at the station 3. The observed phytoplankton bloom exhibited with several environmental and hydrographic factors such as south wind forcing, light availability and river discharge structures, current direction and relatively stable water column. However, the most dominant factors to occur phytoplankton bloom in this area were summer solar radiation and transport of high chlorophyll-a water from the upstream to the downstream region around the mouth of the river. HADCP data result shows that downstream directed along river currents can force to accumulate high chlorophyll-a concentration water around the estuary that caused phytoplankton bloom more at station 1 than those of station 2 and 3. Conversely, intrusion of salt water from the bay by tidal circulation of upstream directed currents can force hypoxic water mass around this area rapidly with significant implications for water quality. The effects were intensified by the prevailing south wind that accumulated high concentration of chlorophyll-a water around the estuary and low dissolved oxygen water mass were developed at the onset of north wind associated with spring-neap tidal modulation that possibly enhanced the estuarine circulation. Peak along river velocity was noted at the end of ebb, when the river discharge and tidal discharge combined into one flow. Similarly weak velocity was observed at the end of flood phase. The observations from summer 2011 suggest that the hypoxic events and phytoplankton blooms have close correlation with predominant wind, seasonal water column stratification, fresh water discharge and tidal circulation. Based on these findings, it can be projected into future years the time when summer blooms and hypoxia are most likely to occur around this area which will initiate long-term estuarine monitoring to investigate the adverse effect of water quality deterioration on estuarine ecosystem.
内容記述: 報告番号: ; 学位授与年月日: 2012-09-27 ; 学位の種別: 修士 ; 学位の種類: 修士(環境学) ; 学位記番号: 修創域第4536号 ; 研究科・専攻: 新領域創成科学研究科環境学研究系社会文化環境学専攻
URI: http://hdl.handle.net/2261/53250
出現カテゴリ:025 修士論文
1223625 修士論文(環境学研究系社会文化環境学専攻)

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