Interplay of diverse environmental settings and their influence on the plankton community off Myanmar during the Spring Intermonsoon

The northern Andaman Sea including the Myanmar waters is one of the least studied regions of the northern Indian Ocean. The freshwater and suspended sediments carried by Ayeyawady/Irrawaddy, the peculiar surface circulation, coastline morphology and shallow bathymetry in the Gulf of Mottama facilitate several diverse environmental settings in the study region. In order to understand the environmental settings and their linkages to the plankton community in the study region, this paper combined in situ data of 'First India-Myanmar Joint Oceanographic Expedition' with satellite oceanography observations. The study period was the Spring Intermonsoon (March-May), which was characterized by high tidal activity in the Gulf of Mottama region (tidal height 6-8. m) causing strong tidal currents and re-suspension of sediments. The tidal currents and eastward advection of Ayeyawady influx caused the lowest salinity, highest concentration of nutrients, suspended sediments and chlorophyll a in the Gulf of Mottama region. Conversely, high salinity, highest temperature, lowest nutrients and suspended sediments prevalent in the offshore waters of the northern Andaman Sea induced a massive bloom of Trichodesmium erythraeum, which was mostly in the declining phase during the observation. The in situ and satellite remote sensing data clearly showed that the T. erythraeum bloom observed in the offshore waters was closely linked to a warm core eddy. The decomposition of the bloom favored swarms of siphonophores and hydromedusae through a trophic link involving copepods and appendicularians. Aided by satellite remote sensing data and multivariate statistical tools, five diverse environmental settings have been identified in the study domain. The analysis showed a close coupling between phytoplankton biomass and nutrients with their higher values in the Gulf of Mottama, off Rakhine, Ayeyawady and Thanintharyi region as compared to the offshore waters in the northern Andaman Sea. The zooplankton community dominated by copepods and chaetognaths preferred regions with high salinity, chlorophyll, deep mixed layer and low suspended sediments as existed off Rakhine, Ayeyawady and Thanintharyi regions. The study evidences, for the first time, the spatial segregation of environmental settings and its linkages to the plankton community off Myanmar during the Spring Intermonsoon.