Effect of ocean acidification on coastal phytoplankton composition and accompanying organic nitrogen production

The effect of ocean acidification, caused by the increase in p CO 2 in seawater, on phytoplankton population and on related organic nitrogen production was experimentally examined by use of a natural coastal microbial population. p CO 2 and pH were controlled by aeration with air in which p CO 2 was at the current level (control), for which ambient air was used, and with air in which p CO 2 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 p CO 2 (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 15 N tracer method, also diminished under acidified conditions compared with that at the current level.