首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
Net community biological production in the euphotic zone of the ocean fuels organic matter and oxygen export from the upper ocean, which has a large influence on the atmospheric pressure of carbon dioxide and is the driving force for metabolite distributions in the sea. We determine the net annual biological oxygen production in the mixed layer of the northeast subarctic Pacific Ocean from in situ O2 and N2 measurements. Temperature, salinity, total gas pressure and O2 were measured every 3 h for 9 months in 2007 at about 3 m depth on a surface mooring at Station P (50°N, 145°W). The concentration of nitrogen gas, N2, determined from separate total gas pressure and pO2 measurements, was used as an inert tracer of the physical processes that induce gas departure from thermodynamic equilibrium with the atmosphere. We use a simple model of the ocean’s mixed layer along with the nitrogen concentration to constrain the importance of bubbles, gas exchange and horizontal advection, which are then used in the oxygen mass balance to derive net biological oxygen production. The mixed-layer oxygen mass balance is dominated by exchange with the atmosphere, and we determine a mean summertime oxygen production of 24 mmol O2 m?2 d?1. The annual pattern in the difference between the supersaturation of oxygen and nitrogen in the surface waters reveals very little net oxygen production during the winter at this location. The calculated annual net community production (NCP) of carbon from this new method, 2.5 mol m?2 yr?1, agrees to within its error of about×40% with previous determinations at this location from oxygen mass balance, NO3? draw down and 234Th measurements. This value is either indistinguishable from or lower than annual NCP measurements in the subtropical North Pacific, indicating that there is no experimental evidence for differences in annual NCP between the subarctic and subtropical North Pacific Ocean.  相似文献   

2.
We discuss nitrous oxide (N2O) and methane (CH4) distributions in 49 vertical profiles covering the upper ∼300 m of the water column along two ∼13,500 km transects between ∼50°N and ∼52°S during the Atlantic Meridional Transect (AMT) programme (AMT cruises 12 and 13). Vertical N2O profiles were amenable to analysis on the basis of common features coincident with Longhurst provinces. In contrast, CH4 showed no such pattern. The most striking feature of the latitudinal depth distributions was a well-defined “plume” of exceptionally high N2O concentrations coincident with very low levels of CH4, located between ∼23.5°N and ∼23.5°S; this feature reflects the upwelling of deep waters containing N2O derived from nitrification, as identified by an analysis of N2O, apparent oxygen utilization (AOU) and NO3−, and presumably depleted in CH4 by bacterial oxidation. Sea-to-air emissions fluxes for a region equivalent to ∼42% of the Atlantic Ocean surface area were in the range 0.40–0.68 Tg N2O yr−1 and 0.81–1.43 Tg CH4 yr−1. Based on contemporary estimates of the global ocean source strengths of atmospheric N2O and CH4, the Atlantic Ocean could account for ∼6–15% and 4–13%, respectively, of these source totals. Given that the Atlantic Ocean accounts for around 20% of the global ocean surface, on unit area basis it appears that the Atlantic may be a slightly weaker source of atmospheric N2O than other ocean regions but it could make a somewhat larger contribution to marine-derived atmospheric CH4 than previously thought.  相似文献   

3.
Dissolved oxygen (DO) in the ocean is a tracer for most ocean biogeochemical processes including net community production and remineralization of organic matter which in turn constrains the biological carbon pump. Knowledge of oxygen dynamics in the North Atlantic Ocean is mainly derived from observations at the Bermuda Atlantic Time-series Study (BATS) site located in the western subtropical gyre which may skew our view of the biogeochemistry of the subtropical North Atlantic. This study presents and compares a 15 yr record of DO observations from ESTOC (European Station for Time-Series in the Ocean, Canary Islands) in the eastern subtropical North Atlantic with the 20 yr record at BATS. Our estimate for net community production of oxygen was 2.3±0.4 mol O2 m−2 yr−1 and of oxygen consumption was −2.3±0.5 mol O2 m−2 yr−1 at ESTOC, and 4 mol O2 m−2 yr−1 and −4.4±1 mol m−2 yr−1 at BATS, respectively. These values were determined by analyzing the time-series using the Discrete Wavelet Transform (DWT) method. These flux values agree with similar estimates from in-situ observational studies but are higher than those from modeling studies. The difference in net oxygen production rates supports previous observations of a lower carbon export in the eastern compared to the western subtropical Atlantic. The inter-annual analysis showed clear annual cycles at BATS whereas longer cycles of nearly 4 years were apparent at ESTOC. The DWT analysis showed trends in DO anomalies dominated by long-term perturbations at a basin scale for the consumption zones at both sites, whereas yearly cycles dominated the production zone at BATS. The long-term perturbations found are likely associated with ventilation of the main thermocline, affecting the consumption and production zones at ESTOC.  相似文献   

4.
A review of oceanographic properties in the vicinity of Ocean Station Papa (OSP) is presented, using data collected over the past 42 years. Average annual signals at OSP and seasonal characteristics along Line P represent variability on a large scale in the Gulf of Alaska. Between winter and summer, the upper ocean mixed layer varies between 120 and 40 m, monthly average winds decrease from 12 m/s in winter to 7 m/s in July, seawater temperatures warm from lows of 6°C to highs >12°C, waters freshen slightly in summer, and macronutrients are partially depleted by phytoplankton growth (removal of 7.8 μM NO3 in 1970s and 6.5 μM NO3 in 1990s). El Niño events influence this area by transporting heat northward. During the prolonged El Niño of the early 1990s, warming persisted at OSP through 1994, resulting in a reduced macronutrient supply during winter mixing. Changes in water properties over the four decades of observations are evident. There are trends towards warmer and less saline surface waters, lower winter nitrate and silicate levels, and less macronutrient utilisation in the 1990s compared to the 1970s. We speculate that these changes must be reducing the productivity of NE subarctic Pacific waters.  相似文献   

5.
The spatial and seasonal variability of nutrients and dissolved oxygen concentrations as well as the chemical characterization of the different water masses of the Southern Levantine Basin were determined in detail. In summer, the upper 150 m of the water body was stratified and the cross basin distribution of dissolved oxygen and nutrients was fairly constant. Surficial waters were saturated with dissolved oxygen, and a shallow oxygen maximum (oversaturated) was present at about 80 m depth. Oversaturation was attributed mainly to the physical process of rapid capping and trapping of oxygen in the Atlantic water (AW) mass, with only 28% of the excess oxygen originating from biological production. Nutrient concentrations were very low and showed an increase in the intermediate levels, coupled with a decrease in oxygen. The winter cross-section distribution showed an upper mixed layer of 100 m, with dissolved oxygen and nutrient concentrations fairly constant across the basin. The concentration of nitrate was higher than in summer, while phosphate was slightly lower and silicic acid similar. In winter, the influence of the physical features (gyres) could be detected up to the surface, and in summer they were detected by the chemical properties in the 150–600 m layer. In the transition layer between the Levantine intermediate water (LIW) and the deep water (DW) (400–700 m) there was a gradual decrease in dissolved oxygen and an increase in nutrient concentrations eastwards. The DW showed no seasonal variation, only spatial variability: dissolved oxygen decreased and silicic acid increased eastwards. No differences were found in nitrate and phosphate concentrations between the DW in the western and eastern provinces, indicating the oxidation of organic matter poor in N and P.N : P ratios in the upper water masses were seasonally dependent. The largest variation was found in the Levantine surface water (LSW), from an average of 52 in winter to 5 in summer. It is hypothesized that the gradual decrease from winter to summer values was due mainly to preferential atmospheric input of N in winter and P in summer, together with biological consumption and differential regeneration of N and P. In the DW, the N : P ratios were constant throughout the year (25.2±2.7, n=567), and higher than Redfield's ratio. It was speculated that the high N : P ratio in the DW was a result of oxidation of particulate organic matter deficient in P.The winter wet atmospheric input of N provided 12% of new N to the LSW. Average new production for the Southern Levantine Basin was estimated from the new N as 4.75 g C m−2 yr−1. The dry atmospheric contribution of P was estimated to significantly increase the P pool in the LSW. Dry deposition is not evenly distributed and occurs in episodic and localized events, which may have a large effect on productivity in the short periods when deposition occurs.There have been recently reported changes in the deep thermohaline circulation of the Eastern Mediterranean, with main contribution of the Aegean Sea as a source of DW. The data presented here can serve as a reference for assessing future changes in the chemical composition of the water masses in the Southern Levantine.  相似文献   

6.
The distributions of humic-like fluorescent dissolved organic matter (at excitation/emission wavelengths of 340 nm/440 nm, F(340/440)) and apparent oxygen utilization (AOU) are determined from water samples taken at 27 stations along 7.5°N, in the equatorial Atlantic Ocean. The relationship between F(340/440) and AOU is evaluated. The influence of water mass mixing is removed through multiple regressions of both F(340/440) and AOU with salinity and temperature for the ocean interior. A general and significant relationship between the residuals of F(340/440) and AOU is found for the entire water column deeper than 200 m (R2=0.79, n=360, p-value <0.001), endorsing the idea that changes in fluorescence intensity are directly related to in situ oxidation of organic matter by microbial activity in the dark equatorial Atlantic Ocean. In addition, we analyse and discuss the relationships between the residuals of F(340/440) and AOU for all individual water masses.  相似文献   

7.
JGOFS-KERFIX (KERguelen point FIXe) time-series station, located south of the polar front in the Indian sector of the Antarctic Ocean, was occupied monthly between January 1990 and March 1995. Annual cycles of dissolved inorganic carbon (DIC), total alkalinity (TALK), oxygen (O2) and nutrients (nitrate, silicate, phosphate and ammonia) in the upper ocean are presented for this site. From seasonal drawdown of nutrients and DIC, we estimate a spring–summer net community production of 3.2±0.5 mol m−2 and C/N/P ratios of 100/16/1. The Si/N ratio varies between 1.8 and 3, suggesting low iron concentrations. The spring–summer biogenic silicon export derived from silicate drawdown is 1.18 mol m−2, consistent with model estimates of silicate export at this site. Seasonal and interannual variations of oxygen, nitrate and DIC due to physical and biological processes are quantified using a simple month-to-month budget formulation. From these budgets, an annual net community production of 5.7±3.3 mol m−2 yr−1 is estimated, about twice the averaged spring–summer production, indicating that, at KERFIX, there is a positive net community production throughout the year. Air–sea CO2 fluxes show that KERFIX is a strong CO2 sink for the atmosphere of 2.4–5.1 mol m−2 yr−1 in 1993, depending on the gas exchange formulation used. A 2.1–3.3 mol m−2 yr−1 outgassing of O2 is observed at KERFIX except in 1993 and 1994 where a decreasing trend of temperature induces an increase of O2 solubility.  相似文献   

8.
A combination of 2-year-long mooring-based measurements and snapshot conductivity–temperature–depth (CTD) observations at the continental slope off Spitsbergen (81°30′N, 31°00′E) is used to demonstrate a significant hydrographic seasonal signal in Atlantic Water (AW) that propagates along the Eurasian continental slope in the Arctic Ocean. At the mooring position this seasonal signal dominates, contributing up to 50% of the total variance. Annual temperature maximum in the upper ocean (above 215 m) is reached in mid-November, when the ocean in the area is normally covered by ice. Distinct division into ‘summer’ (warmer and saltier) and ‘winter’ (colder and fresher) AW types is revealed there. Estimated temperature difference between the ‘summer’ and ‘winter’ waters is 1.2 °C, which implies that the range of seasonal heat content variations is of the same order of magnitude as the mean local AW heat content, suggesting an important role of seasonal changes in the intensity of the upward heat flux from AW. Although the current meter observations are only 1-year long, they hint at a persistent, highly barotropic current with little or no seasonal signal attached.  相似文献   

9.
Remineralization ratios (–O2:P, Corg.:P, N:P) in the ocean are estimated from ocean tracer data using a new approach, which takes into account the effects of local exchange across neutral surfaces. This approach is applied to temperature, salinity, phosphate, nitrate, dissolved oxygen, alkalinity, and dissolved inorganic carbon data from the low- and mid-latitude Pacific, Indian, and South Atlantic Oceans. The consideration of local exchange effects tends to reduce the –O2:P and Corg.:P remineralization estimates above 1500 m compared to earlier estimates. Below 1500 m, exchange effects can be neglected (except in the South Atlantic) and earlier estimates appear robust. In the deep South Atlantic, the consideration of these effects leads to increased –O2:P and Corg.:P remineralization ratio estimates, bringing them more in line with the robust deep ocean estimates. For reasonable, open ocean mixing coefficient values and several choices for phosphate remineralization rate profiles, –O2:P (Corg.:P) remineralization ratios in the ocean increase from about 140 (100) at 750 m depth to about 170 (130) at 1500 m and remain so deeper down. Such an increase down through the upper ocean thermocline implies significant fractionation during remineralization of organic matter—nutrients are released higher in the water column than inorganic carbon. These results also argue for a –O2:P (Corg.:P) uptake ratio in new production of about 140–150 (100–110). N:P remineralization ratios decrease from about 15 at 750 m to about 12 at 1500–2000 m. This may reflect a “true” N:P remineralization (and uptake) ratio of about 16, modified by denitrification.These results imply that applications of derived, quasi-conservative tracers, based on the assumption of constant remineralization ratios, may be subject to significant error for depths less than 1500 m. In addition, present Ocean General Circulation Models of the natural carbon cycle in the ocean–atmosphere system assume remineralization to occur without fractionation but have problems simulating observed, pre-industrial levels of atmospheric pCO2, given observed ocean inventories of alkalinity and dissolved inorganic carbon. Implementation of uptake and (depth-dependent) remineralization ratios estimated here would likely reduce this problem considerably. Furthermore, calculations with a simple global carbon cycle model show that fractionation in the modern ocean, as estimated in the present work, has reduced atmospheric pCO2 by more than 20 ppm below the level it would have had without fractionation.  相似文献   

10.
Satellite remote sensing offers new means of quantifying particulate organic carbon, POC, concentration over large oceanic areas. From SeaWiFS ocean color, we derived 10-year data of POC concentration in the surface waters of the global ocean. The 10-year time series of the global and basin scale average surface POC concentration do not display any significant long-term trends. The annual mean surface POC concentration and its seasonal amplitude are highest in the North Atlantic and lowest in the South Pacific, when compared to other ocean basins. POC anomalies in the North Atlantic, North Pacific, and global concentrations seem to be inversely correlated with El Niño index, but longer time series are needed to confirm this relationship. Quantitative estimates of POC reservoir in the oceanic surface layer depend on the choice of what should represent this layer. Global average POC biomass is 1.34 g m?2 if integrated over one optical depth, 3.62 g m?2 if integrated over mixed layer depth, and up to 6.41 g m?2 if integrated over 200-m layer depth (when assumed POC concentration below MLD is 20 mg m?3). The global estimate of total POC reservoir in the surface 200-m layer of the ocean is 228.61×1013 g. We expect that future estimates of POC reservoir may be even larger, when more precise calculations account for deep-water organic-matter maxima in oligotrophic regions, and POC biomass located just below the seasonal mixed layer in spring and summer in the temperate regions.  相似文献   

11.
Seasonal depth stratified plankton tows, sediment traps and core tops taken from the same stations along a transect at 29°N off NW Africa are used to describe the seasonal succession, the depth habitats and the oxygen isotope ratios (δ18Oshell) of five planktic foraminiferal species. Both the δ18Oshell and shell concentration profiles show variations in seasonal depth habitats of individual species. None of the species maintain a specific habitat depth exclusively within the surface mixed layer (SML), within the thermocline, or beneath the thermocline. Globigerinoides ruber (white) and (pink) occur with moderate abundance throughout the year along the transect, with highest abundances in the winter and summer/fall season, respectively. The average δ18Oshell of G. ruber (w) from surface sediments is similar to the δ18Oshell values measured from the sediment-trap samples during winter. However, the δ18Oshell of G. ruber (w) underestimates sea surface temperature (SST) by 2 °C in winter and by 4 °C during summer/fall indicating an extension of the calcification/depth habitat into colder thermocline waters. Globigerinoides ruber (p) continues to calcify below the SML as well, particularly in summer/fall when the chlorophyll maximum is found within the thermocline. Its vertical distribution results in δ18Oshell values that underestimate SST by 2 °C. Shell fluxes of Globigerina bulloides are highest in summer/fall, where it lives and calcifies in association with the deep chlorophyll maximum found within the thermocline. Pulleniatina obliquiloculata and Globorotalia truncatulinoides, dwelling and calcifying a part of their lives in the winter SML, record winter thermocline (～180 m) and deep surface water (～350 m) temperatures, respectively. Our observations define the seasonal and vertical distribution of multiple species of foraminifera and the acquisition of their δ18Oshell.  相似文献   

12.
Data from the first systematic survey of inorganic carbon parameters on a global scale, the GEOSECS program, are compared with those collected during WOCE/JGOFS to study the changes in carbon and other geochemical properties, and anthropogenic CO2 increase in the Atlantic Ocean from the 1970s to the early 1990s. This first data-based estimate of CO2 increase over this period was accomplished by adjusting the GEOSECS data set to be consistent with recent high-quality carbon data. Multiple Linear Regression (MLR) and extended Multiple Linear Regression (eMLR) analyses to these carbon data are applied by regressing DIC with potential temperature, salinity, AOU, silica, and PO4 in three latitudinal regions for the western and eastern basins in the Atlantic Ocean. The results from MLR (and eMLR provided in parentheses) indicate that the mean anthropogenic CO2 uptake rate in the western basin is 0.70 (0.53) mol m?2 yr?1 for the region north of 15°N; 0.53 (0.36) mol m?2 yr?1 for the equatorial region between 15°N and 15°S; and 0.83 (0.35) mol m?2 yr?1 in the South Atlantic south of 15°S. For the eastern basin an estimate of 0.57 (0.45) mol m?2 yr?1 is obtained for the equatorial region, and 0.28 (0.34) mol m?2 yr?1 for the South Atlantic south of 15°S. The results of using eMLR are systematically lower than those from MLR method in the western basin. The anthropogenic CO2 increase is also estimated in the upper thermocline from salinity normalized DIC after correction for AOU along the isopycnal surfaces. For these depths the results are consistent with the CO2 uptake rates derived from both MLR and eMLR methods.  相似文献   

13.
Highly sensitive STOX O2 sensors were used for determination of in situ O2 distribution in the eastern tropical north and south Pacific oxygen minimum zones (ETN/SP OMZs), as well as for laboratory determination of O2 uptake rates of water masses at various depths within these OMZs. Oxygen was generally below the detection limit (few nmol L−1) in the core of both OMZs, suggesting the presence of vast volumes of functionally anoxic waters in the eastern Pacific Ocean. Oxygen was often not detectable in the deep secondary chlorophyll maximum found at some locations, but other secondary maxima contained up to ~0.4 µmol L−1. Directly measured respiration rates were high in surface and subsurface oxic layers of the coastal waters, reaching values up to 85 nmol L−1 O2 h−1. Substantially lower values were found at the depths of the upper oxycline, where values varied from 2 to 33 nmol L−1 O2 h−1. Where secondary chlorophyll maxima were found the rates were higher than in the oxic water just above. Incubation times longer than 20 h, in the all-glass containers, resulted in highly increased respiration rates. Addition of amino acids to the water from the upper oxycline did not lead to a significant initial rise in respiration rate within the first 20 h, indicating that the measurement of respiration rates in oligotrophic Ocean water may not be severely affected by low levels of organic contamination during sampling. Our measurements indicate that aerobic metabolism proceeds efficiently at extremely low oxygen concentrations with apparent half-saturation concentrations (Km values) ranging from about 10 to about 200 nmol L−1.  相似文献   

14.
Community metabolism (respiration and production) and bacterial activity were assessed in the upper water column of the central Arctic Ocean during the SHEBA/JOIS ice camp experiment, October 1997–September 1998. In the upper 50 m, decrease in integrated dissolved oxygen (DO) stocks over a period of 124 d in mid-winter suggested a respiration rate of ∼3.3 nM O2 h−1 and a carbon demand of ∼4.5 gC m−2. Increase in 0–50 m integrated stocks of DO during summer implied a net community production of ∼20 gC m−2. Community respiration rates were directly measured via rate of decrease in DO in whole seawater during 72-h dark incubation experiments. Incubation-based respiration rates were on average 3-fold lower during winter (11.0±10.6 nM O2 h−1) compared to summer (35.3±24.8 nM O2 h−1). Bacterial heterotrophic activity responded strongly, without noticeable lag, to phytoplankton growth. Rate of leucine incorporation by bacteria (a proxy for protein synthesis and cell growth) increased ∼10-fold, and the cell-specific rate of leucine incorporation ∼5-fold, from winter to summer. Rates of production of bacterial biomass in the upper 50 m were, however, low compared to other oceanic regions, averaging 0.52±0.47 ngC l−1 h−1 during winter and 5.1±3.1 ngC l−1 h−1 during summer. Total carbon demand based on respiration experiments averaged 2.4±2.3 mgC m−3 d−1 in winter and 7.8±5.5 mgC m−3 d−1 in summer. Estimated bacterial carbon demand based on bacterial productivity and an assumed 10% gross growth efficiency was much lower, averaging about 0.12±0.12 mgC m−3 d−1 in winter and 1.3±0.7 mgC m−3 d−1 in summer. Our estimates of bacterial activity during summer were an order of magnitude less than rates reported from a summer 1994 study in the central Arctic Ocean, implying significant inter-annual variability of microbial processes in this region.  相似文献   

15.
We examine seasonal variations in the stable carbon and oxygen isotopic composition of individual shells of the pteropods Limacina inflata and Styliola subula, collected from Oceanic Flux Program sediment traps (at 500 m depth) near Bermuda in the western Sargasso Sea. Calcification depths estimated from L. inflata δ18O vary between 200 and 650 m in late winter and spring, and between 50 and 250 m in late summer and fall. S. subula shows similar seasonal variability with calcification depths between 250 and 600 m in late winter and spring and 50–400 m in late summer and fall. These results suggest that both species calcify across a greater range of depths than indicated by previous geochemical studies. Furthermore, the data indicate that these species change their calcification depth in conjunction with changes in thermal stratification of the water column. Pteropod shell δ13C values vary inversely with δ13CDIC but show a positive correlation with seawater [CO32−] and temperature after depth differences in δ13CDIC are accounted for. We hypothesize that either the influence of temperature on metabolic CO2 incorporation during shell growth and/or the influence of ambient [CO32−] on shell geochemistry can explain these relationships. Taken together, the individual shell δ18O and δ13C data suggest that shell calcification, and by inference the life cycle, of these pteropods is several months or less. Individual pteropod shell analyses have potential for contributing to our understanding of the environmental parameters that play a role in seasonal calcification depth shifts, as well as to our knowledge of past upper ocean thermal structure.  相似文献   

16.
Climate models with biogeochemical components predict declines in oceanic dissolved oxygen with global warming. In coastal regimes oxygen deficits represent acute ecosystem perturbations. Here, we estimate dissolved oxygen differences across the global tropical and subtropical oceans within the oxygen minimum zone (200–700-dbar depth) between 1960–1974 (an early period with reliable data) and 1990–2008 (a recent period capturing ocean response to planetary warming). In most regions of the tropical Pacific, Atlantic, and Indian Oceans the oxygen content in the 200–700-dbar layer has declined. Furthermore, at 200 dbar, the area with O2 <70 μmol kg?1, where some large mobile macro-organisms are unable to abide, has increased by 4.5 million km2. The tropical low oxygen zones have expanded horizontally and vertically. Subsurface oxygen has decreased adjacent to most continental shelves. However, oxygen has increased in some regions in the subtropical gyres at the depths analyzed. According to literature discussed below, fishing pressure is strong in the open ocean, which may make it difficult to isolate the impact of declining oxygen on fisheries. At shallower depths we predict habitat compression will occur for hypoxia-intolerant taxa, with eventual loss of biodiversity. Should past trends in observed oxygen differences continue into the future, shifts in animal distributions and changes in ecosystem structure could accelerate.  相似文献   

17.
Apparent oxygen utilization (AOU), dissolved organic carbon (DOC), coloured dissolved organic matter (CDOM) absorption spectra, and CDOM fluorescence characteristic of aromatic amino acids (excitation/emission 280 nm/320 nm; F(280/320)) and marine-humic like substances (320 nm/410 nm; F(320/410)) were measured in full depth during a cruise in the temperate Eastern North Atlantic (ENA). An optimum multi-parameter (OMP) inverse method was run to calculate water mass proportion-weighted average (archetypal) concentrations of these chemical parameters for all water masses and samples. Archetypal concentrations retain the variability due to water mass mixing and basin scale mineralization from the water mass formation sites to the study area. Conversely, the difference between measured and archetypal concentrations, retain the variability due to dissimilarities in mineralization processes within the study area. Our analysis indicates that DOC supported 26±3% of the AOU in the dark temperate ENA and that basin scale processes occurring at and from the formation area of the water masses explained 63% of the total DOC variability. Our data also suggests that DOC remineralized at the basin scale was of lower molecular weight, and with a lower proportion of fluorescent aromatic amino acids than found within the study area. The relationship between the absorption coefficient at 254 nm (aCDOM(254)) and AOU indicates that aCDOM(254) was consumed during organic matter remineralization in the dark ocean, with 55% of the variability being explained by basin scale processes. The relationships of F(320/410) with AOU and DOC confirmed that marine humic-like substances are produced by microbial degradation processes, at a rate of 6.1±0.9×10−3 mg equivalents of QS mol AOU−1. Our results also indicate that basin-scale remineralization processes account for 85% of the total variability of F(320/410), emphasizing that large scale processes control the formation of humic-like substance in the dark ENA.  相似文献   

18.
A 1-D coupled physical-biogeochemical model is used to study the seasonal cycles of silicon and nitrogen in two High Nutrient Low Chlorophyll (HNLC) systems, the Antarctic Circumpolar Current (ACC) and the North Pacific Ocean, and a mesotrophic system, the North Atlantic Ocean. The biological model consists of nine compartments (diatoms, nano-flagellates, microzooplankton, mesozooplankton, two types of detritus, nitrate, ammonium and silicic acid) forced by irradiance, temperature, mixing and deep nitrate and silicic acid concentrations. At all sites, nanophytoplankton standing crop variations are low, in spite of variations in primary production, because of a “top–down” control by microzooplankton. Although nanophytoplankton sustain more than 60% of the annual primary production in these areas, their contribution to the export production does not exceed 1% of the total. The differences in the seasonal plankton cycle among these regions come mainly from differences in the dynamics of large phytoplankton (here diatoms). In the ACC, the chlorophyll maximum remains <1.5 mg m−3, as an unfavourable light/mixing regime and a likely trace-metal limitation keep diatoms from blooming. In the northeast Pacific, trace-metal limitation seems to keep diatoms from blooming throughout the year. In both these systems, light or iron limitations induce high Si/N uptake ratios. Incidentally these high Si/N uptake ratios lead to a net excess of silicic acid utilization over nitrate, and to a subsequent silicic acid limitation during the summertime. In the North Atlantic, under favourable light/mixing regime and nutrient-replete conditions at the onset of the growing period, diatoms outburst and sustain a bloom >3.5 mg Chl-a m−3. Thereafter, mesozooplankton grazing pressure and silicic acid limitation induce the collapse of the chlorophyll maximum and the persistence of lower chlorophyll concentrations in summer. Although the ACC and the North Pacific show HNLC features, they support a high biogenic silica production (1.9 and 1.07 mol Si m−2 yr−1) and export flux (0.79 and 0.61 mol Si m−2 yr−1), compared to the North Atlantic (production: 0.23 mol Si m−2 yr−1, export: 0.12 mol Si m−2 yr−1). The differences in Si production and export between the HNLC systems and the mesotrophic North Atlantic come from both higher Si concentrations and Si/N uptake ratios in the HNLC areas compared to the North Atlantic. Also, the low dissolution rate of biogenic silica compared to nitrogen degradation rate, and the inhibition of nitrate uptake by ammonium, reinforce the net excess of silicic acid utilization over nitrate. As a result, the model also illustrates the efficiency of the silica pump for the three sites: about 50% of the biogenic silica synthesized in the euphotic layer is exported out of the first 100 m, while only 4–11% of the particulate organic nitrogen escapes recycling in the surface layer.  相似文献   

19.
A time series of a standard hydrographic section in the northern Rockall Trough spanning 23 yr is examined for changes in water mass properties and transport levels. The Rockall Trough is situated west of the British Isles and separated from the Iceland Basin by the Hatton and Rockall Banks and from the Nordic Seas by the shallow (500 m) Wyville–Thompson ridge. It is one pathway by which warm North Atlantic upper water reaches the Norwegian Sea and is converted into cold dense overflow water as part of the thermohaline overturning in the northern North Atlantic and Nordic Seas. The upper water column is characterised by poleward moving Eastern North Atlantic Water (ENAW), which is warmer and saltier than the subpolar mode waters of the Iceland Basin, which also contribute to the Nordic Sea inflow. Below 1200 m the deep Labrador Sea Water (LSW) is trapped by the shallowing topography to the north, which prevents through flow but allows recirculation within the basin. The Rockall Trough experiences a strong seasonal signal in temperature and salinity with deep convective winter mixing to typically 600 m or more and the formation of a warm fresh summer surface layer. The time series reveals interannual changes in salinity of ±0.05 in the ENAW and ±0.04 in the LSW. The deep water freshening events are of a magnitude greater than that expected from changes in source characteristics of the LSW, and are shown to represent periodic pulses of newer LSW into a recirculating reservior. The mean poleward transport of ENAW is 3.7 Sv above 1200 dbar (of which 3.0 Sv is carried by the shelf edge current) but shows a high-level interannual variability, ranging from 0 to 8 Sv over the 23 yr period. The shelf edge current is shown to have a changing thermohaline structure and a baroclinic transport that varies from 0 to 8 Sv. The interannual signal in the total transport dominates the observations, and no evidence is found of a seasonal signal.  相似文献   

20.
For the investigation of organic carbon fluxes reaching the seafloor, oxygen microprofiles were measured at 145 sites in different sub-regions of the Southern Ocean. At 11 sites, an in situ oxygen microprofiler was deployed for the measurement of oxygen profiles and the calculation of organic carbon fluxes. At four sites, both in situ and ex situ data were determined for high latitudes. Based on this data set as well as on previous published data, a relationship was established for the estimation of fluxes derived by ex situ measured O2 profiles. The fluxes of labile organic matter range from 0.5 to 37.1 mg C m?2 d?1. The high values determined by in situ measurements were observed in the Polar Front region (water depth of more than 4290 m) and are comparable to organic matter fluxes observed for high-productivity, upwelling areas like off West Africa. The oxygen penetration depth, which reflects the long-term organic matter flux to the sediment, was correlated with assemblages of key diatom species. In the Scotia Sea (～3000 m water depth), oxygen penetration depths of less than 15 cm were observed, indicating high benthic organic carbon fluxes. In contrast, the oxic zone extends down to several decimeters in abyssal sediments of the Weddell Sea and the southeastern South Atlantic. The regional pattern of organic carbon fluxes derived from microsensor data suggests that episodic and seasonal sedimentation pulses are important for the carbon supply to the seafloor of the deep Southern Ocean.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号