Ocean acidification (OA) and global warming-induced water column stratification can significantly alter phytoplankton-related biological activity in the marine ecosystem. Yet how these changes may play out in the tropical Indian Ocean remains unclear. This study investigated the ecological and metabolic responses of the different phytoplankton functional groups to elevated CO 2 partial pressure and nitrate deficiency in two different environments of the eastern Indian Ocean (EIO). It is revealed that phytoplankton growth and metabolic rates are more sensitive to inorganic nutrients rather than CO 2 . The combined interactive effects of OA and N-limitation on phytoplankton populations are functional group-specific. In particular, the abundance and calcification rate of calcifying coccolithophores are expected to be enhanced in the future EIO. The underlying mechanisms for this enhancement may be ascribed to coccolithophore's lower carbon concentrating mechanisms (CCMs) efficiency and OA-induced [HCO 3ˉ ] increase. In comparison, the abundance of non-calcifying microphytoplankton (e.g., diatoms and dinoflagellates) and primary productivity would be inhibited under those conditions. Different from previous laboratory experiments, interspecific competition for resources would be an important consideration in the natural phytoplankton populations. These combined factors would roughly determine calcifying coccolithophores as "winners" and non-calcifying microphytoplankton as "losers" in the future ocean scenario. Due to the large species-specific differences in phytoplankton sensitivity to OA, comprehensive investigations on oceanic phytoplankton communities are essential to precisely predict phytoplankton eco-physiological response to ocean acidification.

Haijiao LIU;Yuying ZHAO;Chao WU;Wenzhe XU;Xiaodong ZHANG;Guicheng ZHANG;Satheeswaran THANGARAJ;Jun SUN

Research Centre for Indian Ocean Ecosystem,Tianjin University of Science and Technology,Tianjin 300457,China;College of Marine Science and Technology,China University of Geosciences(Wuhan),Wuhan 430074,China;Department of Marine Science,Incheon National University,Incheon 22012,Republic of Korea

海洋湖沼学报（英文版）

[1]Haijiao LIU;Yuying ZHAO;Chao WU;Wenzhe XU;Xiaodong ZHANG;Guicheng ZHANG;Satheeswaran THANGARAJ;Jun SUN-.The ecological response of natural phytoplankton population and related metabolic rates to future ocean acidification)[J].海洋湖沼学报（英文版）,2022(03):999-1011

coccolithophores,coccolithophore,microphytoplankton,losers

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