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1.
Biogeochemical ocean-atmosphere transfers in the Arabian Sea   总被引：2，自引：2，他引：2  
Transfers of some important biogenic atmospheric constituents, carbon dioxide (CO2), methane (CH4), molecular nitrogen (N2), nitrous oxide (N2O), nitrate , ammonia (NH3), methylamines (MAs) and dimethylsulphide (DMS), across the air–sea interface are investigated using published data generated mostly during the Arabian Sea Process Study (1992–1997) of the Joint Global Ocean Flux Study (JGOFS). The most important contribution of the region to biogeochemical fluxes is through the production of N2 and N2O facilitated by an acute, mid-water deficiency of dissolved oxygen (O2); emissions of these gases to the atmosphere from the Arabian Sea are globally significant. For the other constituents, especially CO2, even though the surface concentrations and atmospheric fluxes exhibit extremely large variations both in space and time, arising from the unique physical forcing and associated biogeochemical environment, the overall significance in terms of their global fluxes is not much because of the relatively small area of the Arabian Sea. Distribution and air–sea exchanges of some of these constituents are likely to be greatly influenced by alterations of the subsurface O2 field forced by human-induced eutrophication and/or modifications to the regional hydrography.  相似文献   

2.
ABSTRACT

Having a reliable ocean carbon flux (f(CO2)) retrieval model is essential to monitoring the global carbon cycle and to evaluating the climate change. Remote sensing techniques provide alternatives for f(CO2) retrieval with its advantages of wide area surveys and real-time monitoring. In the present study, a semianalytical f(CO2) estimation model was developed based on remote sensing data and in situ measurements in the Chinese Bohai Sea. The used model performed well (R2?=?0.84) in deriving f(CO2) based on the collected remotely sensed dataset, including sea surface temperature, estimated sea surface salinity, wind speed, Chl-a concentration. The results showed that the distribution of partial pressure of carbon dioxide (p(CO2)) and f(CO2) varied spatially and temporally during the 12 months in 2009. The spatial fluctuations of p(CO2) and f(CO2) in Bohai Sea in summer and autumn were more obvious than that in Spring and Winter. The highest values of p(CO2) and f(CO2) generally appeared in coastal regions. Moreover, the average f(CO2) value of the 12 months showed that the Bohai Sea performed as a weak carbon source in 2009. The results provided technical and data support for carbon management and climate negotiation in the Bohai Sea.  相似文献   

3.
A three-dimensional hydrodynamic-ecosystem model was used to examine the factors determining the spatio-temporal distribution of denitrification in the Arabian Sea. The ecosystem model includes carbon and nitrogen as currencies, cycling of organic matter via detritus and dissolved organic matter, and both remineralization and denitrification as sinks for material exported below the euphotic zone. Model results captured the marked seasonality in plankton dynamics of the region, with characteristic blooms of chlorophyll in the coastal upwelling regions and central Arabian Sea during the southwest monsoon, and also in the northern Arabian Sea during the northeast monsoon as the mixed layer shoals. Predicted denitrification was 26.2 Tg N yr−1,the greatest seasonal contribution being during the northeast monsoon when primary production is co-located with the zone of anoxia. Detritus was the primary organic substrate consumed in denitrification (97%), with a small (3%) contribution by dissolved organic matter. Denitrification in the oxygen minimum zone was predicted to be fuelled almost entirely by organic matter supplied by particles sinking vertically from the euphotic zone above (0.73 mmol N m−2 d−1) rather than from lateral transport of organic matter from elsewhere in the Arabian Sea (less than 0.01 mmol N m−2 d−1). Analysis of the carbon budget in the zone of denitrification (north of 10°N and east of 55°E) indicates that the modelled vertical export flux of detritus, which is similar in magnitude to estimates from field data based on the 234Th method, is sufficient to account for measured bacterial production below the euphotic zone in the Arabian Sea.  相似文献   

4.
Concentrations of dissolved Al and Fe in the surface mixed layer were measured during five cruises of the 1995 US JGOFS Arabian Sea Process Study, Concentrations of both Al and Fe were relatively uniform between January and April, the NE Monsoon and the Spring Intermonsoon period, ranging from 2 to 11 nM Al (mean 5.3 nM) and 0.5 to 2.4 nM Fe (mean 1.0 nM). In July/August, after the onset of the SW Monsoon, surface water Al and Fe concentrations increased significantly (Al range 4.5–20.1 nM; mean=10 nM, Fe range 0.57–2.4 nM; mean=1.3 nM), particularly in the NE part of the Arabian Sea, as the result of the input and partial dissolution of eolian dust. Using the enrichment of Al in the surface waters, we estimate this is the equivalent to the deposition of 2.2–7.4 g m−2 dust, which is comparable to values previously estimated for this region. Approximately one month later (August/September), surface water concentrations of both Al and Fe were found to have decreased significantly (mean Al 7.4 nM, mean Fe 0.90 nM) particularly in the same NE region, as the result of export of particulate material from the euphotic zone. Fe supply to the surface waters is also affected by upwelling of sub-surface waters in the coastal region of the Arabian Sea during the SW Monsoon. Despite the proximity of high concentrations of Fe in the shallow sub-oxic layer, freshly upwelled water is not drawn from this layer and the NO3/Fe ratio in the initially upwelled water is below the value at which Fe limitation is through to occur. Continued deposition of eolian Fe into the upwelled water as it advects offshore provides the Fe required to raise this ratio above the Fe limitation value.  相似文献   

5.
As a part of the US-JGOFS Arabian Sea Process Study (ASPS), we deployed a mooring array consisting of 16 Mark-7G time-series sediment traps on five moorings, each in the mesopelagic and interior depths in the western Arabian Sea set along a transect quasi-perpendicular to the Omani coast. The array was deployed for 410 days to cover all monsoon and inter-monsoon phases at 4.25-, 8.5- or 17-day open-close intervals, all of which were synchronized at 17-day periods. Total mass flux, fluxes of organic, inorganic carbon, biogenic Si and lithogenic Al (mg m−2 day−1) were obtained from samples representing 667 independent periods. The average total mass fluxes estimated in the interior depth along this sediment trap array at Mooring Stations 1–5 (MS-1–5) during 1994-5 ASPS were 147, 235, 221, 164 and 63 mg m−2 day−1, respectively. Mass fluxes during the southwest (SW) Monsoon were always larger than during the northeast (NE) Monsoon at all divergent zone stations, but the difference was insignificant at the oligotrophic station, MS-5. Four major pulses of export flux events, two each at NE Monsoon and SW Monsoon, were observed in the divergent zone; these events dominated in quantity production of the annual mass flux, but did not dominate temporally. Export pulses were produced by passing eddies and wind-curl events, but the direct processes to produce individual export blooms at each station were diversified and highly complex. The onset of these pulses was generally synchronous throughout the divergent zone. Export pulses associated with specific biogeochemical signatures such as the ratio of elevated biogenic Si to inorganic carbon indicate a supply of deep water to the euphotic layer in varying degrees. The variability of mass fluxes at the oligotrophic station, MS-5, also represented both monsoon events, but with far less amplitude and without notable export pulses.  相似文献   

6.
The ocean is an important sink for carbon and heat, yet high-resolution measurements of biogeochemical properties relevant to global climate change are being made only sporadically in the ocean at present. There is a growing need for automated, real-time, long-term measurements of CO2 in the ocean using a network of sensors, strategically placed on ships, moorings, free-drifting buoys and autonomous remotely operated vehicles. The ground-truthing of new sensor technologies is a vital component of present and future efforts to monitor changes in the ocean carbon cycle and air–sea exchange of CO2.A comparison of a moored Carbon Interface Ocean Atmosphere (CARIOCA) buoy and shipboard fugacity of CO2 (fCO2) measurements was conducted in the western North Atlantic during two extended periods (>1 month) in 1997. The CARIOCA buoy was deployed on the Bermuda Testbed Mooring (BTM), which is located 5 km north of the site of the US Joint Global Ocean Flux Study (JGOFS) Bermuda Atlantic Time-series Study (BATS). The high frequency of sampling revealed that temperature and fCO2 responded to physical forcing by the atmosphere on timescales from diurnal to 4–8 days. Concurrent with the deployments of the CARIOCA buoy, frequent measurements of surface fCO2 were made from the R/V Weatherbird II during opportunistic visits to the BTM and BATS sites, providing a direct calibration of the CARIOCA buoy fCO2 data. Although, the in situ ground-truthing of the CARIOCA buoy was complicated by diurnal processes, sub-mesoscale and fine-scale variability, the CARIOCA buoy fCO2 data was accurate within 3±6 μatm of shipboard fCO2 data for periods up to 50 days. Longer-term assessments were not possible due to the CARIOCA buoy breaking free of the BTM and drifting into waters with different fCO2-temperature properties. Strategies are put forward for future calibration of other in situ sensors.  相似文献   

7.
Particulate manganese (Mn) fluxes measured with six time series sediment traps showed that the annual settling fluxes were 3–6 times higher in the west compared to those in the east and central Arabian Sea. Annual detrital Mn (Mndt) flux was nearly the same in the eastern and western Arabian Sea, but excess Mn (Mnex) fluxes were much higher (>4 times) in the western Arabian Sea. Atmospheric inputs cannot account for these high-Mn fluxes. Central and eastern Arabian Sea traps are overlain by a thick and intense denitrification layer, which may cause reductive dissolution of Mn oxides from settling particles and consequently low Mnex fluxes. As the exchange of intermediate waters between the Arabian Sea and the rest of the Indian Ocean is confined largely to the western Arabian Sea, relatively more oxic and dynamic conditions prevail in this region. Increased oxidizing conditions coupled with higher inputs of dissolved Mn through intermediate and surface advective processes might have led to in situ oxidation of Mn, thus resulting in higher vertical fluxes of Mnex. Mnex fluxes in traps at ∼1000 m depth exhibited seasonal variability with a minimum during the winter monsoon (January–February) and maximum during the pre- and early- south-west monsoon (March–June). This variation is correlated with water mass movements and bacterial abundance observed during the Joint Global Ocean Flux Study (JGOFS). The possible involvement of bacteria and the microbial loop is suggested for the concentration and vertical transport of excess Mn.  相似文献   

8.
During CREAMS expeditions, fCO2 for surface waters was measured continuously along the cruise tracks. The fCO2 in surface waters in summer varied in the range 320–440 μatm, showing moderate supersaturation with respect to atmospheric CO2. In winter, however, fCO2 showed under-saturation of CO2 in most of the area, while varying in a much wider range from 180 to 520 μatm. Some very high fCO2 values observed in the northern East Sea (Japan Sea) appeared to be associated with the intensive convection system developed in the area. A gas-exchange model was developed for describing the annual variation of fCO2 and for estimating the annual flux of CO2 at the air-sea interface. The model incorporated annual variations in SST, the thickness of the mixed layer, gas exchange associated with wind velocity, biological activity and atmospheric concentration of CO2. The model shows that the East Sea releases CO2 into the atmosphere from June to September, and absorbs CO2 during the rest of the year, from October through May. The net annual CO2 flux at the air-sea interface was estimated to be 0.032 (±0.012) Gt-C per year from the atmosphere into the East Sea. Water column chemistry shows penetration of CO2 into the whole water column, supporting a short turnover time for deep waters in the East Sea. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

9.
Strong seasonal patterns in upper ocean total carbon dioxide (TCO2), alkalinity (TA) and calculated pCO2 were observed in a time series of water column measurements collected at the US Joint Global Ocean Flux Study (JGOFS) BATS site (31 °50′N, 64 °10′W) in the Sargasso Sea. TA distribution was a conservative function of salinity. However, in February 1992, a non-conservative decrease in TA was observed, with maximum depletion of 25–30 μmoles kg−1 occuring in the surface layer and at the depth of the chlorophyll maximum (˜ 80–100 m). Mixed-layer TCO2 also decreased, while surface pCO2 increased by 25–30 μatm. We suggest these changes in carbon dioxide species resulted from open-ocean calcification by carbonate-secreting organisms rather than physical processes. Coccolithophore calcification is the most likely cause of this event although calcification by foraminifera or pteropods cannot be ruled out. Due to the transient increase in surface pCO2, the net annual transfer of CO2 into the ocean at BATS was reduced. These observations demonstrate the potential importance of open-ocean calcification and biological community structure in the biogeochemical cycling of carbon.  相似文献   

10.
Upper-ocean fluxes of particulate organic carbon (POC) and biogenic silica (bSi) are calculated from four US JGOFS cruises along 170°W using a thorium-234 based approach. Both POC and bSi fluxes exhibit large variability vs. latitude during the seasonal progression of diatom dominated blooms. POC fluxes at 100 m of up to 50 mmol C m−2 d−1 are found late in the bloom, and farthest south near the Ross Sea Gyre. Biogenic Si fluxes also peak late in the bloom as high as 15 mmol Si m−2 d−1, but this flux peak occurs at a different latitude, just south of the Antarctic Polar Front (APF), which is centered around 60°S along this cruise track. The ratios of both POC and bSi export relative to their production rates are large, suggesting an efficient biological pump at these latitudes. The highest relative bSi/POC flux ratios at 100 m are found just south of the APF, coincident with a bSi/POC flux peak seen in 1000 m traps during this same program by Deep-Sea Research II (Honjo et al., Deep-Sea Research II 47, 3521–3548). These data suggest that efficient export at these latitudes can support the high accumulation rates of bSi found in the sediments under and south of the APF, despite the generally low biomass and productivity levels in this region.  相似文献   

11.
In the summers of 1999 and 2003, the 1st and 2nd Chinese National Arctic Research Expeditions measured the partial pressure of CO2 in the air and surface waters (pCO2) of the Bering Sea and the western Arctic Ocean. The lowest pCO2 values were found in continental shelf waters, increased values over the Bering Sea shelf slope, and the highest values in the waters of the Bering Abyssal Plain (BAP) and the Canadian Basin. These differences arise from a combination of various source waters, biological uptake, and seasonal warming. The Chukchi Sea was found to be a carbon dioxide sink, a result of the increased open water due to rapid sea-ice melting, high primary production over the shelf and in marginal ice zones (MIZ), and transport of low pCO2 waters from the Bering Sea. As a consequence of differences in inflow water masses, relatively low pCO2 concentrations occurred in the Anadyr waters that dominate the western Bering Strait, and relatively high values in the waters of the Alaskan Coastal Current (ACC) in the eastern strait. The generally lower pCO2 values found in mid-August compared to at the end of July in the Bering Strait region (66–69°N) are attributed to the presence of phytoplankton blooms. In August, higher pCO2 than in July between 68.5 and 69°N along 169°W was associated with higher sea-surface temperatures (SST), possibly as an influence of the ACC. In August in the MIZ, pCO2 was observed to increase along with the temperature, indicating that SST plays an important role when the pack ice melts and recedes.  相似文献   

12.
This investigation focused on the weaker and less well understood of the two Arabian Sea monsoonal wind phases, the NE Monsoon, which persists for 3–4 months in the October to February period. Historically, this period has been characterized as a time of very low nutrient availability and low biological production. As part of the US JGOFS Arabian Sea Process Study, 17 stations were sampled on a cruise in January 1995 (late NE Monsoon) and, 15 stations were sampled on a cruise in November 1995 (early NE Monsoon). Only the southern most stations (10° and 12°N) and one shallow coastal station were as nutrient-depleted as had been expected from the few relevant prior studies in this region. Experiments were conducted to ascertain the relative importance of different nitrogenous nutrients and the sufficiency of local regeneration processes in supplying nitrogenous nutrients utilized in primary production. Except for the southern oligotrophic stations, the euphotic zone concentrations of NO3− were typically 5–10-fold greater than those of NO2− and NH4+. There was considerable variation (20–40-fold) in nutrient concentration both within and between the two sections on each cruise. All nitrogenous nutrients were more abundant (2–4-fold) later in the NE Monsoon. Strong vertical gradients in euphotic zone NH4+ concentration, with higher concentrations at depth, were common. This was in contrast to the nearly uniform euphotic zone concentrations for both NO3− and NO2−. Half-saturation constants for uptake were higher for NO3− (1.7 μmol kg−1 (s.d.=0.88, n=8)) than for NH4+ (0.47 μmol kg−1 (s.d.=0.33, n=5)). Evidence for the suppressing effect of NH4+ on NO3− uptake was widespread, although not as severe as has been noted for some other regions. Both the degree of sensitivity of NO3− uptake to NH4+ concentration and the half-saturation constant for NO3− uptake were correlated with ambient NO3− concentration. The combined effect of high affinity for low concentrations of NH4+ and the effect of NH4+ concentration on NO3− uptake resulted in similarly low f-ratios, 0.15 (s.d.=0.07, n=15) and 0.13 (s.d.=0.08, n=17), for early and late observations in the NE Monsoon, respectively. Stations with high f-ratios had the lowest euphotic zone NH4+ concentrations, and these stations were either very near shore or far from shore in the most oligotrophic waters. At several stations, particularly early in the NE Monsoon, the utilization rates for NO2− were equal to or greater than 50% the utilization rates for NO3−. When converted with a Redfield C : N value of 6.7, the total N uptake rates measured in this study were commensurate with measurements of C productivity. While nutrient concentrations at some stations approached levels low enough to limit phytoplankton growth, light was shown to be very important in regulating N uptake at all stations in this study. Diel periodicity was observed for uptake of all nitrogenous nutrients at all stations. The amplitude of this periodicity was positively correlated with nutrient concentration. The strongest of these relationships occurred with NO3−. Ammonium concentration strongly influenced the vertical profiles for NO3− uptake as well as for NH4+ uptake. Both NO2− and NH4+ were regenerated within the euphotic zone at rates comparable to rates of uptake of these nutrients, and thus maintenance of mixed layer concentrations did not require diffusive or advective fluxes from other sources. Observed turnover times for NH4+ were typically less than one day. Rapid turnover and the strong light regulation of NH4+ uptake allowed the development and maintenance of vertical structure in NH4+ concentration within the euphotic zone. In spite of the strong positive effect of light on NO2− uptake and its strong negative effect on NO2− production, the combined effects of much longer turnover times for this nutrient and mixed layer dynamics resulted in nearly uniform NO2− concentrations within the euphotic zone. Responses of the NE Monsoon planktonic community to light and nutrients, in conjunction with mixed layer dynamics, allowed for efficient recycling of N within the mixed layer. As the NE Monsoon evolved and the mixed layer deepened convectively, NO2− and NO3− concentrations increased correspondingly with the entrainment of deeper water. Planktonic N productivity increased 2-fold, but without a significant change the new vs. recycled N proportionality. Consequently, NO3− turnover time increased from about 1 month to greater than 3 months. This reflected the overriding importance of recycling processes in supplying nitrogenous nutrients for primary production throughout the duration of the NE Monsoon. As a result, NO3− supplied to the euphotic zone during the NE Monsoon is, for the most part, conserved for utilization during the subsequent intermonsoon period.  相似文献   

13.
We report here the non-conservative behaviour of DOC in the northwestern Indian Ocean by studying this parameter together with other carbon and nitrogen components. This contrasts with earlier reports of conservative behaviour. Concentrations of DOC, 3–4 times higher than those reported earlier, were found to decrease northward from the equator. Total carbon dioxide (TCO2) increases in proportion of the oxygen utilized, thus revealing the dominant biological role in the carbon turnover. The CO2 added through dissolution of biogenic debris is found to decrease southward, in general. Decomposition of organic material contributes at least 64% to the CO2 addition that increases southward, the rest being from dissolution of skeletal material. Evidence is provided for the utilization of oxygen and nitrate for DOC oxidative decomposition. Accumulation of DOC without its complete oxidation to CO2 could be the main reason for the TCO2 decrease in southern Arabian Sea. Relationships of DOC with nitrification and denitrification processes show that the microbial population plays a major role in regulating the DOC contents in the seawater of this region. Consumption/decomposition by denitrifying bacteria and other micro-organisms responsible for nitrogen cycling in the sea are found to be intimately related to the DOC dynamics and are responsible for decreased DOC concentrations in the north. DOC accumulation in the southern Arabian Sea seems to facilitate bacterio-particulate aggregate formation and consequent nitrification, which results in excess nitrate. Application of a one-dimensional advection-diffusion model to the present data set provides evidence for the non-conservative nature of DOC in the Arabian Sea.  相似文献   

14.
Marginal seas play important roles in regulating the global carbon budget, but there are great uncertainties in estimating carbon sources and sinks in the continental margins. A Pacific basin-wide physical-biogeochemical model is used to estimate primary productivity and air-sea CO_2 flux in the South China Sea(SCS), the East China Sea(ECS), and the Yellow Sea(YS). The model is forced with daily air-sea fluxes which are derived from the NCEP2 reanalysis from 1982 to 2005. During the period of time, the modeled monthly-mean air-sea CO_2 fluxes in these three marginal seas altered from an atmospheric carbon sink in winter to a source in summer. On annualmean basis, the SCS acts as a source of carbon to the atmosphere(16 Tg/a, calculated by carbon, released to the atmosphere), and the ECS and the YS are sinks for atmospheric carbon(–6.73 Tg/a and –5.23 Tg/a, respectively,absorbed by the ocean). The model results suggest that the sea surface temperature(SST) controls the spatial and temporal variations of the oceanic pCO_2 in the SCS and ECS, and biological removal of carbon plays a compensating role in modulating the variability of the oceanic pCO_2 and determining its strength in each sea,especially in the ECS and the SCS. However, the biological activity is the dominating factor for controlling the oceanic pCO_2 in the YS. The modeled depth-integrated primary production(IPP) over the euphotic zone shows seasonal variation features with annual-mean values of 293, 297, and 315 mg/(m~2·d) in the SCS, the ECS, and the YS, respectively. The model-integrated annual-mean new production(uptake of nitrate) values, as in carbon units, are 103, 109, and 139 mg/(m~2·d), which yield the f-ratios of 0.35, 0.37, and 0.45 for the SCS, the ECS, and the YS, respectively. Compared to the productivity in the ECS and the YS, the seasonal variation of biological productivity in the SCS is rather weak. The atmospheric pCO_2 increases from 1982 to 2005, which is consistent with the anthropogenic CO_2 input to the atmosphere. The oceanic pCO_2 increases in responses to the atmospheric pCO_2 that drives air-sea CO_2 flux in the model. The modeled increase rate of oceanic pCO_2 is0.91 μatm/a in the YS, 1.04 μatm/a in the ECS, and 1.66 μatm/a in the SCS, respectively.  相似文献   

15.
The South China Sea (SCS) exhibits strong variations on seasonal to interannual time scale, and the changing Southeast Asian Monsoon has direct impacts on the nutrients and phytoplankton dynamics, as well as the carbon cycle. A Pacific basin-wide physical-biogeochemical model has been developed and used to investigate the physical variations, ecosystem responses, and carbon cycle consequences. The Pacific basin-wide circulation model, based on the Regional Ocean Model Systems (ROMS) with a 50-km spatial resolution, is driven with daily air-sea fluxes derived from the National Centers for Environmental Prediction (NCEP) reanalysis between 1990 and 2004. The biogeochemical processes are simulated with the Carbon, Si(OH)4, Nitrogen Ecosystem (CoSINE) model consisting of multiple nutrients and plankton functional groups and detailed carbon cycle dynamics. The ROMS-CoSINE model is capable of reproducing many observed features and their variability over the same period at the SouthEast Asian Time-series Study (SEATS) station in the SCS. The integrated air-sea CO2 flux over the entire SCS reveals a strong seasonal cycle, serving as a source of CO2 to the atmosphere in spring, summer and autumn, but acting as a sink of CO2 for the atmosphere in winter. The annual mean sea-to-air CO2 flux averaged over the entire SCS is +0.33 moles CO2 m−2year−1, which indicates that the SCS is a weak source of CO2 to the atmosphere. Temperature has a stronger influence on the seasonal variation of pCO2 than biological activity, and is thus the dominant factor controlling the oceanic pCO2 in the SCS. The water temperature, seasonal upwelling and Kuroshio intrusion determine the pCO2 differences at coast of Vietnam and the northwestern region of the Luzon Island. The inverse relationship between the interannual variability of Chl-a in summer near the coast of Vietnam and NINO3 SST (Sea Surface Temperature) index in January implies that the carbon cycle and primary productivity in the SCS is teleconnected to the Pacific-East Asian large-scale climatic variability.  相似文献   

16.
The annual cycle of dissolved nutrients and the fugacity of CO2 (fCO2), calculated from the concentration of dissolved inorganic carbon (DIC) and pH, was studied over a 14-month long period (December 1993 to February 1995) at a site in Prydz Bay near Davis Station, Vestfold Hills, East Antarctica. Significant spring decreases in fCO2 began under the sea-ice in mid-October, when both water column and sea-ice algal activity resulted in the removal of nutrients and DIC and increased pH. Minimum fCO2 (<100 μatm) and lowest nutrient and DIC concentrations occurred in December and January. The low summer fCO2 values were clearly the result of biological activity. The seasonal depletion of dissolved nitrate reached 85% in mid-summer when chlorophyll-a concentrations exceeded 15 mg m−3. Oceanic uptake of carbon dioxide from the atmosphere, calculated from the fugacity difference and daily wind speeds, averaged more than 30 mmol m−2 day−1 during the summer ice-free period. This exchange replaced approximately half of the DIC consumed by biological activity. Apparent nutrient utilisation ratios (C/N/P) were close to Redfield values. In autumn fCO2 began to rise, continuing slowly well into winter, and reaching a maximum close to modern atmospheric values between July and September. This increase can be attributed to a combination of local remineralisation of organic carbon in the water column and the steady increase in the mixing depth of the water column. At first glance, this suggests that air–sea equilibration occurred in winter despite the sea-ice cover, perhaps by horizontal circulation from regions outside the pack ice, or through openings in the ice. However, the persistent 15 to 20% undersaturation of dissolved oxygen throughout the winter suggests an alternate explanation. The late winter fCO2 level may represent a characteristic established by global circulation, so that as a result of increasing atmospheric CO2 concentrations, these Antarctic waters are in transition from being a winter-time source of CO2 to the atmosphere to becoming a sink. Our fCO2 observations emphasize the need to address seasonal variations in assessing Antarctic contributions to the oceanic control of atmospheric CO2.  相似文献   

17.
The biological pump is a central process in the ocean carbon cycle, and is a key factor controlling atmospheric carbon dioxide (CO2). However, whether the Arctic biological pump is enhanced or reduced by the recent loss of sea ice is still unclear. We examined if the effect was dependent on ocean circulation. Melting of sea ice can both enhance and reduce the biological pump in the Arctic Ocean, depending on ocean circulation. The biological pump is reduced within the Beaufort Gyre in the Canada Basin because freshwater accumulation within the gyre limits nutrient supply from deep layers and shelves hence inhibits the growth of large-bodied phytoplankton. Conversely, the biological pump is enhanced outside the Beaufort Gyre in the western Arctic Ocean because of nutrient supply from shelves and greater light penetration, enhancing photosynthesis, caused by the sea ice loss. The biological pump could also be enhanced by sea ice loss in the Eurasian Basin, where uplifted isohaline surfaces associated with the Transpolar Drift supply nutrients upwards from deep layers. New data on nitrate uptake rates are consistent with the pattern of enhancement and reduction of the Arctic biological pump. Our estimates indicate that the enhanced biological pump can be as large as that in other oceans when the sea ice disappears. Contrary to a recent conclusion based on data from the Canada Basin alone, our study suggests that the biological CO2 drawdown is important for the Arctic Ocean carbon sink under ice-free conditions.  相似文献   

18.
Phototrophic and heterotrophic nanoplankton (PNAN, HNAN; 2–20 μm protists) and microplankton (PMIC, HMIC; 20–200 μm protists and micrometazoa) are major components of the producer and consumer assemblages in oceanic plankton communities. Abundances and biomasses of these microorganisms were determined from samples collected along two transects during the Northeast Monsoon and Spring Intermonsoon process cruises of the US JGOFS Arabian Sea Program in 1995. Vertical profiles of these assemblages were strongly affected by the presence of a subsurface oxygen minimum layer. Abundances of all four assemblages decreased dramatically below the top of this layer. Depth-integrated (0–160 m) abundances and biomasses of nanoplankton and microplankton were of similar magnitude for most samples. Exceptions to this rule were primarily due to PMIC (mostly diatom) species which dominated phytoplankton assemblages at a few stations during each season. Depth-integrated biomasses for the combined nano- and microplankton averaged over all stations for each cruise were surprisingly similar for the Northeast Monsoon and Spring Intermonsoon seasons in this ecosystem (2.0 and 1.8 g C m−2 [170 and 150 m moles C m−2] for the two seasons, respectively). Nano- and microplankton biomass for these two time periods constituted a signficant portion of the total amount of the particulate organic carbon (POC) in the water column. Summed over all stations, these assemblages constituted approximately 25–35% of the POC in the top 160 m of the northern Arabian Sea.  相似文献   

19.
Fugacity of CO2 (fCO2), temperature, salinity, nutrients, and chlorophyll-a were measured in the surface waters of southwestern East Sea/Japan Sea in July 2005. Surface waters were divided into three waters based on hydrographic characteristics: the water with moderate sea surface temperature (SST) and high sea surface salinity (SSS) located east of the front (East water); the water with high SST and moderate SSS located west of the front (West water); and the water with low SST and SSS located in the middle part of the study area (Middle water). High fCO2 larger than 420 μatm were found in the West water. In the Middle water, CO2 was undersaturated with respect to the atmosphere, with values between 246 and 380 μatm. Moderate fCO2 values ranging from 370 to 420 μatm were observed in the East water. For the East and West waters, estimates of temperature dependency of fCO2 (12.6 and 15.1 μatm °C−1, respectively) were rather similar to a theoretical value, indicating that SST is likely to be a major factor controlling the surface fCO2 distribution in these two regions. In the Middle water, however, the estimated temperature dependence was somewhat lower than the theoretical value, and relatively high concentrations of surface chlorophyll-a coincided with the low surface fCO2, implying that biological uptake may considerably affect the fCO2 distribution. The net sea-to-air CO2 flux of the study area was estimated to be 0.30±4.81 mmol m−2 day−1 in summer, 2005.  相似文献   

20.
《Marine Chemistry》2001,74(1):1-13
Measurements of methane (CH4) made during two surveys in the eastern and central Arabian Sea in April–May, 1996, and August–September, 1997, corresponding to late Spring Intermonsoon (SI) and Southwest Monsoon (SWM) seasons, respectively, revealed high spatial and temporal variability in surface saturation (110–2521%). The highest values were observed during the SWM in the inner shelf where coastal upwelling combined with freshwater runoff to produce very strong near-surface stratification. These values might result to a large extent from CH4 inputs from coastal wetlands through seasonal runoff as abnormally high saturations (up to ∼13,000%) were recorded in the estuarine surface water. In situ production of CH4, favoured by very high biological production in conjunction with the prevalence of suboxic conditions in the upwelled water, could be the other major CH4 source. In comparison, sedimentary inputs of CH4 seemed to be of lesser importance in spite of previously-reported occurrence of gas-charged sediments in this region.Methane profiles in the open central Arabian Sea showed two maxima. The more pronounced deeper maximum, occurring at 150–200 m depth, was similar to the feature seen elsewhere in the oceans, but was probably intensified here due to an acute oxygen deficiency. It showed some correlation with the subsurface particle maximum characteristic of the denitrifying layer. The dominant mechanism of its formation might be in situ production within particles rather than advection from the continental shelf as concluded by previous workers. The less pronounced and previously unreported shallower maximum, occurring in the well-oxygenated upper 50 m of the water column, was more dynamic probably as a result of variability of the balance between CH4 production due to biological activity and its losses through microbial oxidation and air–sea exchange.  相似文献   

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