OceanObs 2009 (September 2009 in Venice, Italy) Session ... · OceanObs 2009 (September 2009 in...

Based on the time-series observation for the biogeochemistry at station KNOT (44N/155E) between 1998 and 2001, which was Japanesenational project under an umbrella of Joint Global Ocean Flux Study (JGOFS), it was verified that the North Pacific Western Subarctic Gyre(WSG) has large seasonal variability in nutrients, pCO2, primary productivity and particulate organic carbon flux, and that time-seriesobservation is very important in order to quantify carbon cycle in the ocean and air-sea exchange of CO2 by, especially, the biological activity(biological pump). Since 2001, time-series observation has been conducted at station K2 (47N/160E) (Fig. 1) by using the subsurface mooringsystems (Fig. 2) and research vessel. Our subsurface mooring system consists of various automatic sensor or samplers such as an opticalsensor package (BLOOMS), a water sampler (RAS) and sediment traps deployed at multiple layers. Time-series observation of optical field andnutrients at ~ 35 m by BLOOMS and RAS, respectively, revealed that phytoplankton increases and nutrients, especially silicate, decreaseslargely between late June and early July (Fig. 3). During this time, increase of fluxes of particulate organic carbon and biogenic opal at ~ 150 mwas observed by sediment trap. It is indicative of that primary produced or assimilated organic carbon is transported quickly to the oceaninterior. Multiple sediment traps from 150 m to 5000 m revealed that 1) biogenic materials are transported vertically without significant lateraltransport (Fig. 5), 2) sinking velocity of particles increases with depth, and 3) biogenic opal plays an important role in organic carbon transport(Fig. 4). Seasonal observation of primary productivity, nutrients and natural radionuclide (thorium 234) by research vessel has also revealedthat new production, export flux and export ratio are higher than those in other oceans, indicating that the biological pump at station K2 is veryefficient for uptake of atmospheric CO2. On the other hand, long-term increase of dissolved inorganic carbon following increase ofatmospheric CO2 has been observed at station K2 (Fig. 6). It is noted that increase rate of atmospheric pCO2 (pCO2(air)) in winter was higherthan that of sea surface pCO2 (pCO2(sea)) in winter. Though pCO2(sea) in winter has been higher than pCO2(air) in winter until now, it is predictedthat pCO2(sea) will be higher than pCO2(air) all year round after the middle 21 century. It is indicative of possibility that the ocean acidification willbe accelerated after that period and ocean ecosystem will change in the WSG. In order to predict change in the biological pump and itsfeedback to the global environment, time-series observation should be continued with a new mooring system (optical sensor packageincluding FRRF supported by underwater winch: Fig. 7) at not only station K2, but also a new station located in the Western Pacific SubtropicalGyre as a counterpart of station K2 (Station S1: 30N/145E).

Time-series observation for biogeochemistry in the Western Pacific Subarctic Gyre

Honda, M. C., S. Watanabe, K. Matsumoto, H. Kawakami, M. Wakita, T. Fujiki and T. Saino(Japan Agency for Marine-Earth Science and Technology)

hondam@jamstec.go.jp

OceanObs 2009 (September 2009 in Venice, Italy) Session 02c: Biochemistry and ecosystems 6

http://www.jamstec.go.jp/j/

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Fig. 3 (a) Seasonal variabilitiesin sum of normalized nitrateand nitrite (N-NOx) (upper) andnormalized silicate (N-Si(OH)4))(lower) between Mar 2005 andMay 2006. These were observedby automatic water sampler(RAS). It is suspected thatdecrease in nutrients fromMarch 2005 to October 2005was attributed to uptake ofnutrients by biological activityand increase in nutrients wasattributed to supply of nutrientsfrom subsurface by wintervertical mixing.Fig. 3(b) Seasonal variabilities inN-Si(OH)4 (circles) and biogenicopal flux at 150 m observed bytime-series sediment trap (bargraphs) between March 2005and September 2005. Increasein biogenic opal fluxsynchronized well with decreasein N-Si(OH)4.

Fig. 4 Observed particulateorganic carbon (POC) fluxobtained by time-seriessediment trap since 1998 andmodeled POC flux. ModeledPOC fluxes were estimatedunder assumption that POC fluxwere transported by ballasts(opal, CaCO3 and clay minerals)and with multiple regressionanalysis. It was suspected thatapproximately 70% of POC weretransported by opal.

Fig. 7 New mooring system (POPPS)that will be deployed at stations K2and S1. Sensor package consists ofCTD, gas tension device (GTD),fluorometer, DO, PAR and fastrepetition rate fluorometer (FRRF).Sensor package is usually locatedat around 100 m and ascends oncea day with observation. Aftertransmitting data via satellite,sensor package descends to thehome position. Communicationbetween mooring and land office isinteractive.

Fig. 6 (a) Time-series data ofwinter sea surface-pCO2estimated with carbonatechemistry in the watertemperature minimum layer (bluecircles) and winter atmosphericpCO2 (purple circles). Winter seasurface pCO2 is higher thanwinter atmospheric pCO2 and K2is a source of CO2 in winter. BothpCO2 increases gradually.However increase rate of wintersea surface pCO2 is smaller thanthat of winter atmospheric pCO2.Fig. 6 (b) Predicted winter surfacepCO2 and atmospheric CO2. Ifthese increase trends continue, atthe middle of this century, wintersea surface pCO2 will be lowerthan winter atmospheric pCO2. Itmeans K2 will be a sink of CO2even in winter.

Fig. 5 (a) Seasonal variabilityin primary productivityestimated with underwateroptical data obtained byBLOOMS between Mar 2005and July 2006. Circles are primaryproductivity observed onboard.Fig. 5 (b) ~ (e) Seasonalvariability in POC flux at 150m, 540 m (or 300 m), 1000 mand 4810 m obtained by time-series sediment traps betweenMar 2005 and July 2006.Increase of primaryproductivity in the euphoticlayer in June and July waspropagated to deeper depthsas increase of POC flux withtime lag.

Fig. 1 Time-series station K2(47°N / 160°E, ~5300 m). Thisstation is located in theWestern Pacific SubarcticGyre. Time-series observationfor biogeochemistry withmooring systems and R/V hasbeen conducted since 2001.From 2010, time-seriesobservation will be alsoconducted at comparativetime-series station S1 (30°N /145°E, ~ 5800m) in the WesternPacific Subtropical Gyre.Ocean color is monthlycomposite of SeaWiFschlorophyll-a data in October2001.

Fig. 2 Subsurface mooring system for the study of biogeochemistry. BLOOMS:optical sensor package, SID: subsurface incubation device, PPS: phytoplanktonsampler , RAS: automatic water sampler , ZPS: zooplankton sampler, ST: time-seriessediment trap. The depth of top buoy was ~ 30 m and BLOOMS, SID, PPS, RAS andZPS were located in the euphotic layer (upper 50 m).

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