Abstract
This study of community structure of zooplankton in the Inner Gulf of Thailand was initiated in late 2011 in response to a major flood disaster event and continued intermittently to 2018. Six cruises with 13-20 replicate stations/cruise, provided data for contour maps of zooplankton, nutrients, water hydrography, pigments and algae; this manuscript utilized this diverse data set to focus on mesozooplankton. Immediately after the flood, November 2011, mesozooplankton communities were dominated by calanoid copepods with contributions of about 78% of the total. This changed in 2012 to 2018 which the proportion of calanoid copepods was decreased while the other zooplankton such as hydromedusae, polychaete larvae, barnacle nauplii, pelagic shrimp (Lucifer sp. and protozoea) and chaetognaths were increased. The density of mesozooplankton varied between 1.70 x 104 to 1.26 x 106 ind./100 m3 with noticeably high densities near the four river mouths that flow into Inner Gulf. Distribution patterns of mesozooplankton emerged that are likely similar to the long-term patterns of this region which effected by water circulation in the Inner Gulf of Thailand. However, high density of zooplankton in November 2012 and October 2017 appeared along the north-eastern coast and may be related to the availability of food (i.g., algae). Zooplankton abundances in the Inner Gulf of Thailand were significantly correlated with environmental parameters mainly temperature, salinity and food supplies as represented by chlorophyll a concentrations.
Highlights
The Inner Gulf of Thailand is the most important coastal ecosystem in Thailand receiving freshwater, nutrients and pollutants from four major rivers namely Bangpakong River, Chao Phraya River, Tha Chin River and Mae Klong River
Holoplankton composed 24 taxa of mesozooplankton including foraminiferans, radiolarians, hydromedusae, siphonophores, jelly fish, ctenophores, planktonic polychaetes, cladocerans, ostracods, copepods, cumaceans, isopods, amphipods, pelagic shrimps (e.g. Lucifer sp. and protozoea larvae of Lucifer sp., mysids and acetes), chaetognaths, pteropods, larvaceans and salps
All the zooplankton taxa found in this study have been reported as members of the general zooplankton found in coastal areas of the Inner Gulf of Thailand [4], [8]
Summary
The Inner Gulf of Thailand is the most important coastal ecosystem in Thailand receiving freshwater, nutrients and pollutants from four major rivers namely Bangpakong River, Chao Phraya River, Tha Chin River and Mae Klong River. In 2011, Thailand faced a flooding disaster, the worst within 50 years, affecting over 13 million people, causing the inundation of 11.2 million rai (18,000 km2) of land in 66 provinces [1]; flooding resulted in decreased salinity in coastal areas of the Inner Gulf of Thailand due in large part to approximately 16 billion cubic meters of discharge from Chao Phraya River [2]. Earlier studies in the Gulf of Thailand found that spatial and temporal variations in community structure of zooplankton are usually under the influence of environmental factors e.g. river runoff delivers freshwater and nutrients into the coastal sea, which affect the variability of phytoplankton and zooplankton abundance and population structure [7]. It is important to study variations in community structure of zooplankton during and/or immediately after a flood disaster and the long-term changes, if any, in the Inner Gulf of Thailand
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