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The role of microzooplankton (MZP) in the pelagic trophodynamics is highly significant, but the responses of marine MZP to increasing CO 2 levels are rather poorly understood. Hence the present study was undertaken to understand the responses of marine plankton to increasing CO 2 concentrations. Natural water samples from the coastal Bay of Bengal were incubated under the ambient condition and high CO 2 levels (703-711 latm) for 5 days in May and June 2010. A significant negative correlation was obtained between phytoplankton and MZP abundance which indicated that phytoplankton community structure can considerably be controlled by MZP in this region. The average relative abundances of tintinnids under elevated CO 2 levels were found to be significantly higher (68.65 ± 5.63% in May; 85.46 ± 9.56% in June) than observed in the ambient condition (35.68 ± 6.83% in May; 79 ± 5.36% in June). The observed dominance of small chain forming diatom species probably played a crucial role as they can be potentially grazed by tintinnids. This fact was strengthened by the observed high negative correlations between the relative abundance of major phytoplankton and tintinnids. Moreover, particulate organic carbon and total bacterial counts were also enhanced under elevated CO 2 level and can serve as additional food source for ciliates. The observed responses of tintinnids to increasing CO 2 might have multiple impacts on the energy transfer, nutrient and carbon cycling in the coastal water. The duration of the present study was relatively short and therefore further investigation on longer time scale needs to be done and might give us a better insight about phytoplankton and MZP species succession under elevated CO 2 level.

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