Nobuo Tsurushima

Decadal change of dissolved inorganic carbon in the subarctic western North Pacific Ocean

Dissolved inorganic carbon (DIC) was measured from 1992 to 2008 at two time-series sites in the subarctic western North Pacific; this region is a source of atmospheric CO2 in winter due to vertical mixing of deep waters rich in DIC. To estimate the decadal DIC increase resulting from CO2 uptake from the atmosphere, we corrected DIC for the contribution of biological activity below the temperature minimum (Tmin) layer (¡«100 m), which is the remnant of the mixed layer from the preceding winter. Decadal DIC increases in the Tmin layer and upper intermediate water (1.3¨C1.5 ¦Ìmol kg-1 yr-1; 100¨C200 m) were higher than those expected from oceanic equilibration with increasing atmospheric CO2 and those previously reported in the open North Pacific. The increase in water column CO2 was estimated to be 0.40 ¡À 0.08 mol m-2 yr-1. The decadal DIC change in the Tmin layer affects winter CO2 emission. The increase of atmospheric xCO2 in winter (2.1 ± 0.0 ppm yr-1) is higher than that of oceanic CO2 (0.7 ¡À 0.5 ppm yr-1) that calculated from DIC and total alkalinity in the Tmin layer. This difference suggests reduction of CO2 emission in winter is possibly controlled by the increase of total alkalinity.

Seasonal variability of picophytoplankton and bacteria in the western subarctic Pacific Ocean at station KNOT

Picoplankton community structure in the upper 200 m at Station KNOT in the western subarctic North Pacific was studied by flow cytometric analysis of the time-series samples collected from June 1998 through June 2000. Phototrophic picoplankton, Synechococcus and picoeukaryotes, displayed a clear seasonal cycle in population abundance during the 2-yr period, where chlorophyll a concentration remained relatively low and constant throughout the year, except in spring when the diatom bloom occurred. High picoplankton abundance occurred after the spring diatom bloom when the water column became more stratified. The maximum abundance of Synechococcus occurred in late June to early August, whereas the peak in picoeukaryotes abundance appeared to take place earlier in May and remained high through the summer. Heterotrophic bacteria abundance also displayed a seasonal variation pattern that was closely related to the picoplankton biomass, but not total chlorophyll biomass.

Impact on bacterial activities of ocean sequestration of carbon dioxide into bathypelagic layers

The ocean sequestration of carbon dioxide (CO2), direct injection of CO2 into bathypelagic layers, is one of the climate change mitigation options. It is essential to assess the potential environmental impacts on the marine ecosystem. In bathypelagic layers, bacteria are dominant organisms and play significant roles in oceanic carbon cycling through utilization and transformation of organic matter. We performed laboratory experiments by acidifying bathypelagic seawater with CO2 gas or buffer solutions to examine the impact on bacterial activities (abundance, production rate, and proportion of viable cells). In the laboratory experiments, we observed some potential effects by artificial changes in CO2 concentration, pH, or both, on bacterial activities. It was suggested that trophic conditions of bacterial assemblage strongly influence the magnitude of the impacts on bacterial activities and metabolisms by CO2 sequestration.

Dissolution rate of calcium carbonate in high pCO 2 seawater under high pressure

To understand how the dissolution rate of calcium carbonate changes, laboratory experiments were conducted under high pressure and high pCO2 condition. As a result, rapid dissolution was observed at above 5000 ppm of pCO2. The initial dissolution rates of the well correlated with initial concentration of dissolved inorganic carbon. Dissolution rate were generally decreased with time. Dissolution of calcium carbonate increased dissolved inorganic carbon and total alkalinity in the seawater. Then, degrees of undersaturation and dissolution rate of calcite in the seawater were decreased with time. Dissolution rates normalized with apparent surface area were well fit to the rate law in the empirical kinetic showed in previous studies. Using the function of dissolution rate of calcite, we constructed 3-D map of Omega and dissolution rate of calcite in the western North Pacific.