Seasonal variation in water column conditions in the upper Gulf of Thailand (UGoT) was analyzed by considering four major factors including surface heat flux, freshwater discharge, tidal and wind stirrings. The coincidence of surface heat loss, low river discharge and strong wind resulted in vertical well-mixing in December. Strong stratification developed in September and October due to large river discharge and moderate heat flux. Strong surface heating in April and May has a potential to generate strong stratification, although not as large as that in September and October due to low river discharge. Although no factors are prominent during January and March, and June and August, weak to moderate stratification results, because the influences of river discharge and surface heating are still larger than those of tidal and wind stirrings. The results of water column analysis based on monthly average data agree well with analyses derived from cruise data in the same months. Most analytical results correspond to the distributions of temperature and salinity from field observations. Disagreement, however, was found in December 2003 (cruise CU-2) when stratification in some small regions occurs in the distribution of water properties, but the water column analysis suggests vertical well-mixing. This phenomenon is triggered by non-uniform distribution of freshwater over UGoT, which is related to river discharge, monsoonal wind and current. Compared to a previous study regarding surface chlorophyll dynamics, water column conditions may be used to explain the occurrence of phytoplankton bloom in this region. 相似文献
Seasonal variations in freshwater, salt, dissolved inorganic phosphorus (DIP) and dissolved inorganic nitrogen (DIN) in the
Bangpakong estuary, Thailand were investigated by employing the database obtained in the National Research Council of Thailand
(NRCT)-Japan Society for the Promotion of Science (JSPS) cooperative project from 1994 to 1997. The results showed that variation
in interaction between coastal sea and river discharge played an important role in controlling the characteristics of the
water in the estuary. Residence time of fresh water was short in wet season and dry season, but it was long in the transition
period from season to season. DIP and DIN load depended on river discharge, while high peak concentrations were related to
loading and the long residence time in the transition period between dry and wet seasons. A strong eutrophic condition could
possibly occur when the concentration of DIP and DIN were high during the onset of the wet season from April to July. The
annual average of inorganic nutrient budgets indicated that the Bangpakong estuary is the internal source of 38.2 tons/month
DIP and the internal sink of 4.9 tons/month DIN.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
Although plankton bloom incidents in the upper Gulf of Thailand (UGoT) have been reported, no dynamic investigation of the
phenomenon has been conducted. To address this need, a simple pelagic ecosystem model coupled with the Princeton Ocean Model
(POM) was employed to investigate seasonal variations in surface chlorophyll-a (chl-a) distributions to clarify phytoplankton
dynamics in this area. The results revealed patterns of seasonal chl-a distribution that correspond to local wind, water movement
and river discharge. High chl-a patchiness was found to be concentrated near the western coast following westward circulation
near the northern coast developed during the northeast monsoon. During the southwest monsoon high concentrations were observed
around the northeastern coast due to eastward flow. The simulated results could explain the seasonal shifting of phytoplankton
blooms, which typically arise along the western and eastern coasts during the northeast and the southwest monsoons, respectively.
Sensitivity analyses of simulated chl-a distributions demonstrate that water stability, including wind-induced vertical currents
and mixing, plays significant roles in controlling phytoplankton growth. Nutrients in the water column will not stimulate
strong plankton blooms unless upwelling develops or vertical diffusivity is low. This finding suggests an alternative aspect
of the mechanism of phytoplankton bloom in this region. 相似文献
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.
We conducted hydrographic observations throughout the year to investigate seasonal variations of the hypoxic water mass distribution in the Upper Gulf of Thailand (UGoT). Hypoxic water masses were observed from June to November, with half of the UGoT occupied by hypoxic water in September. A hypoxic water mass appeared in the northeastern part of the UGoT in June and August, and moved westward over time. Low-salinity surface water moved from east to west as the rotational direction of surface circulation shifted with the reversal of monsoon winds. Westward movement of low-salinity water causes strong stratification in the northwestern part of the UGoT, leading to severe hypoxia. Numerical experiments showed high dissolved oxygen consumption rates around and offshore of river mouths, where hypoxic water is generated. This finding suggests that hypoxic water masses are transported to the south by physical processes. We examined how flooding affects hypoxic water mass formation. The volume of hypoxia in a flood year was approximately 2.5 times greater than in a normal year. In addition, hypoxia occurred in the dry season and extensive hypoxia was observed in the year after flooding. These results suggest that the hypoxic water mass persists for a long time after flooding.
