In this study, we investigated the mechanism of eutrophication and hypoxia in the upper Gulf of Thailand from August 2014 to June 2015 based on field observation data, box model analysis, and the unscaled trophic status index (UNTRIX). Fresh water residence time derived from a simple box model was long (38.61 days) during the transition period between the southwest to northeast monsoon in September 2014. In contrast, fresh water residence time was short (2.63 days) during the late northeast monsoon in February 2015. Long residence time was related to the development of widespread strong hypoxia in near-bottom waters in over half of the gulf during the transition between the southwest and the northeast monsoon, when river discharge was also very large. UNTRIX is used to assess water trophic levels, and is based on water qualities including concentrations of chlorophyll-a (Chl-a), dissolved oxygen (DO), dissolved inorganic nitrogen (DIN), and dissolved inorganic phosphorus (DIP). Hypertrophic and eutrophic conditions were observed at river mouths; their seasonal eutrophication was related to river discharge and circulation. Nutrients were mainly increased by river discharge. Water column stratification and long residence time were required for the development of severe hypoxia in the study area.

本研究基于野外观测数据、箱型模型分析和无标度营养状态指数（UNTRIX），研究了2014年8月至2015年6月泰国湾上游富营养化和缺氧的机制。2014 年 9 月西南季风向东北季风过渡期，简单箱式模型得出的淡水滞留时间较长（38.61 天），而 2 月晚东北季风的淡水滞留时间较短（2.63 天） 2015. 滞留时间长与西南季风和东北季风过渡期间海湾近底海域大面积强缺氧发展有关，此时河流流量也很大。UNTRIX 用于评估水的营养水平，并基于水质，包括叶绿素 a (Chl-a)、溶解氧 (DO)、溶解无机氮 (DIN) 和溶解无机磷 (DIP) 的浓度。在河口观察到肥厚和富营养状况；它们的季节性富营养化与河流流量和循环有关。养分主要通过河流排放增加。研究区发生严重缺氧需要水柱分层和较长的停留时间。