Increasing atmospheric CO2 over the Anthropocene due to human activities has reduced seawater pH and it continues as long as burning fossil fuels. In this review, we summarize and discuss the methodology of ocean acidification study and the effects of ocean acidifica tion on phytoplankton and biogeochemical cycle of nutrients. In addition, we compare the pH variability and phytoplankton dynamics over the past 60 million years with the recent phゃnomenon of ocean acidification. We found that: (1) there is a significant lack of knowledge on the effect of ocean acidification because previous studies were not examined using subpolar to polar phytoplankton species and also not considered their habitat and life cycle. (2) We should study the effect of pH on dissolved inorganic Fe(II) bioavailability relative to dissolved inorganic Fe(III) species and the availability of Fe(III)-ligand complex. The role of Fe(II) and chemical structureof the ligand are also important issues. (3) Particulate organic carbon to phosphorus ratio increased in response to the decrease in pH whereas particulate organic carbon to nitrogen ratio was unaffected by pH. However, there is a lack of knowledge with regard to the effect of ocean acidification on dissolved organic matter and silicic acid dynamics. (4) We should study further the interactive effects of pH and trace metals especially for iron on N2 fixation rate by cyanobacteria to improve our knowledge about nitrogen cycle.
Water characteristics and vertical structures of warm core rings that are located off Sanriku (hereinafter WCR1) and off the southeast of Hokkaido (hereinafter WCR2) in Japan were investigated for the period from January to July 2009 using Argo profiling floats and satellite observations. The two WCRs are detached from the Kuroshio Extension almost at the same time around February 2007. The WCR1 and WCR2 have both vertical structures of anticyclonic eddy, containing warmer and saltier water of the Kuroshio within the eddy than ambient water in the surrounding area, on isopycnal surfaces. Calculation of the mixing ratio of Kuroshio and Oyashio waters shows that the mixing ratios of WCR1 and WCR2 are about 20-50% and more than 80%, respectively, indicating that the mixing between the eddy and the ambient occurs more notably for the WCR2, where the ambient water has characteristics closer to the Oyashio water. The analysis of two Argo profiling floats with a high temporal resolution with 2 day observation cycle, and ship observation data reveals that the water characteristics of the WCR1 has a marked temporal change, in which cold and fresh Oyashio water around the eddy intrudes into the eddy intermittently every several days with short time intervals in narrow density ranges. The water characteristics of the eddy gradually change to the Oyashio water, suggesting that the small-scale intrusion of the Oyashio water into the eddy is responsible for the observed change in water characteristics.