Simulation of Seasonal Circulations and Thermohaline Variabilities in the Gulf of Thailand

Based on the Princeton Ocean Model （POM）, the seasonal thermohaline feature and the ocean circulation in the Gulf of Thailand （GOT）, situated between 6°N to 14°N latitude and 99°E to 105°E longitude, were studied numerically with 37 × 97 orthogonal curvilinear grid and 10 vertical sigma levels conforming to a realistic bottom topography. A spin-up phase of the first model run was executed using wind stress calculated from climatological monthly mean wind, restoring-type surface heat and salt, and climatological monthly mean fresh water flux data. In this paper, the temperature and salinity fields taken from Levitus94 data sets and the calculated temperature and salinity from the model run for 12-month mean and for each season are presented where the winter, summer, rainy, and end of the rainy seasons of Thailand are represented by the months January, April, July, and October, respectively. The simulated circulations are also described. The results show that the temperature in the GoT is warmer than the temperature of the other parts connected to the South China Sea （SCS）. At any depth of inflow from SCS into the GoT, the salinity is high, but in the outflow from the GoT at the surface, the salinity is low. The strong circulations are clockwise during summer and the rainy seasons of Thailand, which are the East Asian monsoon periods, northeasterly and southwesterly during summer. They occur near Pattani and Narathiwat provinces during summer and in the central GoT during the rainy seasons. Sensitivity experiments were designed to investigate the effects of wind forcing and open boundary conditions. Wind forcing is shown to be the important factor for generating the circulation in the GoT. The lateral velocity at the open boundaries is of considerable importance to current circulation for the rainy and end of the rainy seasons, with insignificant effect for the winter and summer seasons of Thailand.

Distribution and composition of authigenic minerals in surface sediments of the western Gulf of Thailand

Generation, morphology, and distribution of authigenic minerals directly reflect sedimentary environment and material sources. Surface sediments were collected from the western Gulf of Thailand during 2011–2012, and 159 samples were analyzed to determine detrital minerals. Authigenic minerals, including siderite, pyrite, and glauconite, are abundant whereas secondary minerals, such as chlorite and limonite, are distributed widely in the study area. Siderite has a maximum content of 19.98 g/kg and appears in three types from nearshore to continental shelf, showing the process of forming-maturity-oxidation. In this process, the Mn O content in siderite decreases, but FeOand Mg O content increase. Colorless or transparent siderite pellets are fresh grains generated within a short time and widely distributed throughout the region; high content appears in coastal area where river inputs are discharged. Translucent cemented double pellets appearing light yellow to red are mature grains; high content is observed in the central shelf. Red-brown opaque granular pellets are oxidized grains,which are concentrated in the eastern gulf. Pyrite is mostly distributed in the central continental shelf with an approximately north–south strip. Pyrite are mainly observed in foraminifera shell and distributed in clayey silt sediments, which is similar to that in the Yangtze River mouth and the Yellow Sea. The pyrite in the gulf is deduced from genetic types associated with sulfate reduction and organic matter decomposition. Majority of glauconite are granular with few laminar. Glauconite is concentrated in the northern and southern parts within the boundary of 9.5° to 10.5°N and is affected by river input diffusion. The distribution of glauconite is closely correlated with that of chlorite and plagioclase, indicating that glauconite is possibly derived from altered products of chlorite and plagioclase. The KO content of glauconite is low or absent, indicating its short formation time.

Embryonic development of nudibranch species(Mollusca:Opisthobranchia)in the Gulf of Thailand

Objective:To find the possible way to predict the mode of embryonic development of nudibranchs,focusing on egg mass characteristics,embryonic development,and shell patterns of the veligers.Methods:Eight species of nudibranch were collected in the eastern part of the Gulf of Thailand.The specimens were allowed to copulate and lay their egg masses under laboratory conditions.Embryonic development was monitored under a light microscope with a digital camera every day until hatching.Results:Most of the species of nudibranch collected had a single larva in each egg capsule that developed,except for Jorunna funebris,which had 1-4 larvae in each egg capsule.All the specimens had the same pattern of cell division and hatching into the water column during the veliger form.However,the species developed at different rates in each stage.Conclusions:All species in the current study had planktotrophic development except Doriprismatica atromarginata,which showed lecithotrophic development.Based on embryonic development among the nudibranchs that showed planktotrophic development,Jorunna funebris appeared to be the most advantageous species for culture development with regard to utilization and conservation in the future.

Managing the Chao Phraya River and Delta in Bangkok, Thailand: Flood Control, Navigation and Land Subsidence Mitigation

Land subsidence as a result of the groundwater withdrawals in the Chao Phraya Delta in Thailand will be studied along with any successful remediation efforts. Most of the deltas of South East Asia are sinking for a variety of reasons. Thailand has national concerns related to land subsidence as a result of sea level rise, climate change, flooding, storm surges, skyscrapers, compaction, and groundwater extraction for rice paddies, shrimp ponds and the drinking water and household needs of approximately 15 million people living on the Chao Phraya Delta. The Chao Phraya River shoreline line is eroding and significant land areas and wetlands are being lost and becoming open water. Urban areas are periodically flooded and require earthen levees or floodwalls. The objective is to assess and mitigate land subsidence as a result of groundwater withdrawals for rice paddies, aquaculture, shrimp ponds, industry, drinking water and household needs of 15 million people living on Chao Phraya Delta in Bangkok. The impact of groundwater extraction and reduced sedimentation on land subsidence in the Chao Phraya Delta will be assessed and mitigation methods recommended. Lessons learned and successful remediation efforts in one Southeast Asia delta may or may not be applicable to other South East Asia deltas. There need to be mitigation methods identified to add sediment to existing Chao Phraya Delta wetlands. River water also needs to be injected deep into the underlying alluvial sediments in the delta. Navigation in the canal systems could be adversely affected by proposed massive Chao Phraya River water injections into the Chao Phraya Delta alluvial sediments underlying Bangkok.

Comparative Analysis of Three Types of Fishing Gear Marking for Anchored Fish Aggregating Devices in Purse Seine Fishery in Thai Waters

Anchored fish aggregating devices(AFADs)have been widely used for fishing in Thai waters.However,abandoned,lost,and discarded fishing gears(ALDFGs),including lost AFADs,may cause environmental impacts.Fishing gear marking(FGM)is considered as a tool to help identification of ALDFGs.The main objective of this study is to compare the durability represented by the percentage of remaining condition(R-value)of three material types of FGM applied for AFADs,i.e.,stainless steel(SS),colored acrylic(CA),and polypropylene(PP).This study was carried out using 50 AFADs deployed in the Gulf of Thailand(GOT)and the Andaman Sea(ANS)between July and October 2020 in cooperation with 10 fishers.The AFADs were deployed in similar habitat(bottom depth and type)between the GOT and the ANS.The three material types of FGM were assumed to be sufficiently durable to last for the lifespan of the AFADs in both the GOT and the ANS(within 2 months and 3.5 months,respectively)though some FGMs in the ANS were detached from cable ties or broken before AFADs were lost.The loss of AFADs and FGMs was mainly caused by adverse weather condition(rough sea).Only data from the ANS was included in comparative analysis due to the insufficient variance data obtained from the GOT.The analysis revealed that SS had the higher durability than CA and PP when the AFADs lasted for less than 3.5 months.As a result of our study,some recommendations were made.For example,the cable ties can be replaced by ropes or threads to improve the installation method.This study serves as a basis to develop FGM and to support responsible fisheries.Beneficiaries of the study include fisheries policy makers,managers,and fishers.