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 (<em>Lucifer</em> sp. and protozoea) and chaetognaths were increased. The density of mesozooplankton varied between 1.70 x 10⁴ to 1.26 x 10⁶ ind./100 m³with 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.