Effect of ocean acidification on coastal phytoplankton composition and accompanying organic nitrogen production
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The effect of ocean acidification, caused by the increase in pCO2 in seawater, on phytoplankton population and on related organic nitrogen production was experimentally examined by use of a natural coastal microbial population. pCO2 and pH were controlled by aeration with air in which pCO2 was at the current level (control), for which ambient air was used, and with air in which pCO2 was “800” and “1200” ppm, in 500-L culture vessels. The experiment was continued for 15 days after addition of the inorganic nutrients such as nitrate, phosphate, and silicate. During most of the experimental period, a minor increase in phytoplankton biomass was noted, probably because of low irradiance, an increase in phytoplankton biomass was observed at the end of the experiment. Flow cytometric and microscopic observations revealed that this increase was because of Chrysochromulina sp. (Haptophyceae). The growth of Chrysochromulina sp. was most obvious in the control vessel, and tended to be obscured by increasing pCO2 (decrease in pH), indicating the possibility that ocean acidification inhibits the growth of specific phytoplankton groups, for example Chrysochromulina sp. Production of particulate organic nitrogen (PON), determined by the 15N tracer method, also diminished under acidified conditions compared with that at the current level.
KeywordsOcean acidification pH pCO2 Phytoplankton composition Haptophyte Chrysochromulina Organic nitrogen production Flow cytometry
We wish to thank Associate Professor Y. Kyodo of the University of Tsukuba for fruitful discussions concerning the culture system. We are also indebted to staff members of the Shimoda Marine Research Center, University of Tsukuba, for their valuable assistance during the culture experiment. This work was financially supported by a grant from Global Environmental Research Fund of the Ministry of the Environment, Japan. This report is contribution no. 753 from the Shimoda Marine Research Center, University of Tsukuba.
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