Impact of a spring phytoplankton bloom on the CO[sub 2] system in the mixed layer of the northwestern North Pacific.

To elucidate modulation of the CO[sub 2] system in the mixed layer of the northwestern North Pacific by a spring phytoplankton bloom, we conducted extensive oceanographic observations for two periods (May 13-15 and 24-28, 1999) during the M R99-K02 cruise by the R/V Mirai. During the first survey period, we found a patch of typical bloom conditions around lat 44.7°N, long 155.8°E (Stn 1), where the surface seawater partial pressure of CO[sub 2] (pCO[sub 2]) dropped to a level lower than 200 µatm, accompanied by high fluorescence reaching 10mgm[sup -3] (arbitrary unit). By comparing the vertical profiles of salinity-normalized total CO[sub 2] (nTCO[sub 2]) and nitrate + nitrite (nNO[sub x]) between Stn 1 and Stn 2 (a pre-bloom station), we found that drawdown of nTCO[sub 2] (approx. 109 µmolkg[sup -1]) and uNO[sub x] (approx. 20 µmol kg[sup -1]) occurred in the mixed layer. During the second survey period, spatial variations in the chemical properties at the sea surface in the survey area were not as conspicuous as found in the first survey period, but the drawdown of nTCO[sub 2] and nNO[sub x] extended to deeper layers at the selected stations (Stns 3-5) than at Stn 1, in association with deepening of the mixed layers. The air-sea CO[sub 2] fluxes calculated from the formula of Liss and Merlivat (In: Buat-Menard, P. (Ed.), The Role of Air Sea Exchange in Geochemical Cycling. NATO ASI Series C: Mathematical and Physical Science, Vol. 185, 1986, pp. 113-127) were approximately -12 to -16mmol m[sup -2]d[sup -1] (i.e. into the ocean) around Stns 1, 3, and 4, while the CO[sub 2] flux was approximately 3 mmol m[sup -2] d[sup -1] around Stn 2. For the nTCO[sub 2] drawdown, the influence of CaCO[sub 3] production/ dissolution was estimated to be negligible, because salinity-normalized total alkalinity+nNO[sub x] was almost constant against changes in nTCO[sub 2]. ΔC/ΔN (mol/mol) ratios were approximately 5.6 at Stns 1, 3, and 4,... [ABSTRACT FROM AUTHOR]