Impact of a spring phytoplankton bloom on the CO2 system in the mixed layer of the northwestern North Pacific

To elucidate modulation of the CO2 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 MR99-K02 cruise by the R/V Mirai. During the first survey period, we found a patch of typical bloom conditions around lat 44.7degreesN, long 155.8degreesE (Stn 1), where the surface seawater partial pressure Of CO2 (pCO(2)) dropped to a level lower than 200 muatm, accompanied by high fluorescence reaching 10 mg m(-3) (arbitrary unit). By comparing the vertical profiles of salinity-normalized total CO2 (nTCO(2)) and nitrate+nitrite (nNO(x)) between Stn 1 and Stn 2 (a pre-bloom station), we found that drawdown of nTCO(2) (approx. 109 mumol kg(-1)) and nNO(x) (approx. 20 mumol kg(-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(2) and nNO, 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 CO2 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 -16 mmol m(-2) d(-1) (i.e. into the ocean) around Stns 1, 3, and 4, while the CO2 flux was approximately 3 mmol m(-2) d(-1) around Stn 2. For the nTC02 drawdown, the influence of CaCO3 production/dissolution was estimated to be negligible, because salinity-normalized total alkalinity + nNO, was almost constant against changes in nTCO(2). AC/AN (mol/mol) ratios were approximately 5.6 at Stns 1, 3, and 4, which is significantly smaller than the classical Redfield ratio of 6.6 and close to the particulate organic carbon to nitrogen ratio (5.8) obtained at Stn 3. From significant ratios against DeltaSi, biological activity by diatoms was inferred to have caused the observed nTC02 drawdown. Observed DeltaSi/DeltaN ratios of about 2.0 were higher than the reported biomass ratio of diatoms (1.0), implying a more dominant role for diatom-related activity, and close to the reported ratios for iron-limited diatoms. Analyses of covariations in nTCO(2) and nutrients revealed that nTCO(2) would be higher when nSiO(4), not nNO(x) or nPO(4), was completely depleted, implying regulation by silicate.