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1.
A one-dimensional ecosystem model with two explicit size classes of phytoplankton was developed for the NE subarctic Pacific to investigate variations in the export of organic particles to the ocean interior due to potential changes in the environment. Specifically, the responses of the planktonic ecosystem to permanent removal of iron limitation and to warming (of 2 and 5 °C) were explored. The ecosystem model consists of five components (small and large phytoplankton, microzooplankton, detritus and nitrogen), and includes grazing by mesozooplankton that varies in time according to long-term observations at Ocean Station Papa (OSP). The model addresses the role of iron limitation on phytoplankton growth and includes temperature dependence of physiological rates. The ecosystem model was forced with annual wind and solar heating from OSP. The model best reproduced the low chlorophyll high nitrate conditions of the NE subarctic Pacific when both small and large phytoplankton were limited by iron such that their maximum specific growth rate was reduced by 10 and 70%, respectively. Sensitivity analysis showed that model results depended on the value of the iron limitation parameter of large phytoplankton (LFe-L) and the grazing parameters of micro- and mesozooplankton. To explore the effect of iron limitation, simulations were carried out varying the iron limitation parameters while maintaining the nitrogen flux at the base of the model constant and the grazing pressure by mesozooplankton unchanged. In the warming case, simulations were carried out increasing ocean temperatures by 2° and 5 °C applied only to the ecological components, the flux of nitrate at the base of the model was increased to obtain a steady annual cycle, and grazing by mesozooplankton remained constant. When compared with the standard case, model simulations indicated that both permanent removal of iron limitation and warming cause changes in food web structure and the carbon cycle. The response was more dramatic in the iron-replete case where the phytoplankton community structure in spring changed from one dominated by pico- and nanoplankton to one dominated by large phytoplankton, and primary production increased until it consumed all the external nutrient (N) supply to the upper layer. However, reducing iron deficiency actually led to lower annual primary production due to a decrease in the regeneration of nitrogen in the euphotic zone. These changes in food web structure influenced the magnitude, composition and seasonal cycle of sinking particles.  相似文献   

2.
High temporal resolution measurements of physical and bio-optical variables were made in the upper ocean using a mooring located at 0°, 140°W from 9 February 1992 to 15 March 1993 as part of the equatorial Pacific Ocean (EgPac) study. Chlorophyll and primary productivity time-series records were generated using the mooring data. Primary productivity varied by about 50% around the mean on time scales of weeks and by over a factor of four within our observational period. The mooring observations encompassed both El Niho and cool conditions. Kelvin waves were evident during the El Nifio phase, and tropical instability waves (TIWs) were dominant during the cool phase. The two extreme conditions also were observed concurrently with complementary ship-based measurements. In addition, bio-optical drifters provided simultaneous spatial data concerning net phytoplankton growth rates during passage of a TIW. The collective data sets have been used to examine the causes of the observed variability in phytoplankton biomass and productivity. Our joint results and analyses appear to support the hypothesis that the vertical transport of iron into the upper layer and primary production rates are modulated by variability of the depth of the Equatorial Undercurrent and by equatorial longwaves. In particular, our results are consonant with the suggestion of Barber et al. (1996) that passage of a TIW may be considered to be a natural analog of a small iron enrichment experiment. Predicting primary productivity and, thus, carbon flux in the equatorial Pacific requires continuous, long-term observations of a few physical, biological, and optical properties that can be used to parameterize the biological variability.  相似文献   

3.
Primary production in the eastern tropical Pacific: A review   总被引：2，自引：12，他引：2  
The eastern tropical Pacific includes 28 million km2 of ocean between 23.5°N and S and Central/South America and 140°W, and contains the eastern and equatorial branches of the north and South Pacific subtropical gyres plus two equatorial and two coastal countercurrents. Spatial patterns of primary production are in general determined by supply of macronutrients (nitrate, phosphate) from below the thermocline. Where the thermocline is shallow and intersects the lighted euphotic zone, biological production is enhanced. In the eastern tropical Pacific thermocline depth is controlled by three interrelated processes: a basin-scale east/west thermocline tilt, a basin-scale thermocline shoaling at the gyre margins, and local wind-driven upwelling. These processes regulate supply of nutrient-rich subsurface waters to the euphotic zone, and on their basis we have divided the eastern tropical Pacific into seven main regions. Primary production and its physical and chemical controls are described for each.Enhanced rates of macronutrient supply maintains levels of primary production in the eastern tropical Pacific above those of the oligotrophic subtropical gyres to the north and south. On the other hand lack of the micronutrient iron limits phytoplankton growth (and nitrogen fixation) over large portions of the open-ocean eastern tropical Pacific, depressing rates of primary production and resulting in the so-called high nitrate-low chlorophyll condition. Very high rates of primary production can occur in those coastal areas where both macronutrients and iron are supplied in abundance to surface waters. In these eutrophic coastal areas large phytoplankton cells dominate; conversely, in the open-ocean small cells are dominant. In a ‘shadow zone’ between the subtropical gyres with limited subsurface ventilation, enough production sinks and decays to produce anoxic and denitrified waters which spread beneath very large parts of the eastern tropical Pacific.Seasonal cycles are weak over much of the open-ocean eastern tropical Pacific, although several eutrophic coastal areas do exhibit substantial seasonality. The ENSO fluctuation, however, is an exceedingly important source of interannual variability in this region. El Niño in general results in a depressed thermocline and thus reduced rates of macronutrient supply and primary production. The multi-decadal PDO is likely also an important source of variability, with the ‘El Viejo’ phase of the PDO resulting in warmer and lower nutrient and productivity conditions similar to El Niño.On average the eastern tropical Pacific is moderately productive and, relative to Pacific and global means, its productivity and area are roughly equivalent. For example, it occupies about 18% of the Pacific Ocean by area and accounts for 22–23% of its productivity. Similarly, it occupies about 9% of the global ocean and accounts for 10% of its productivity. While representative, these average values obscure very substantial spatial and temporal variability that characterizes the dynamics of this tropical ocean.  相似文献   

4.
A nutrient dynamic model coupled with a 3D physical model has been developed to study the annual cycle of phytoplankton production in the Yellow Sea. The biological model involves interactions between inorganic nitrogen (nitrate and ammonium), phosphate and phytoplankton biomass. The model successfully reproduces the main features of phytoplankton-nutrient variation and dynamics of production. 1. The well-mixed coastal water is characterized by high primary production, as well as high new production. 2. In summer, the convergence of tidal front is an important hydrodynamic process, which contributes to high biomass at frontal areas. 3. The evolution of phytoplankton blooms and thermocline in the central region demonstrate that mixing is a dominant factor to the production in the Yellow Sea. In this simulation, nitrate- and ammonium-based productions are estimated regionally and temporally. The northern Yellow Sea is one of the highly ranked regions in the Yellow Sea for the capability of fixing carbon and nitrogen. The annual averaged f-ratio of 0.37 indicates that regenerated production prevails over the Yellow Sea. The result also shows that phosphate is the major nutrient, limiting phytoplankton growth throughout the year and it can be an indicator to predict the bloom magnitude. Finally, the relative roles of external nutrient sources have been evaluated, and benthic fluxes might play a significant role in compensating 54.6% of new nitrogen for new production consumption.  相似文献   

5.
To test the iron hypothesis in the subarctic Pacific Ocean, an in situ iron-enrichment experiment (SEEDS) was performed in the western subarctic gyre in July–August 2001. About 350 kg of iron (as acidic iron sulfate) and 0.48 mol of the inert chemical tracer sulfur hexafluoride were introduced into a 10-m deep surface mixed layer over an 80 km2 area. This single iron infusion raised dissolved iron levels to 2.9 nM initially. Dissolved iron concentrations rapidly decreased after the infusion, but levels remained close to 0.15 nM even at the end of the 14-day experimental period. During SEEDS there were iron-mediated increases in chlorophyll a concentrations (up to 20 μg l−1), primary production rates, biomass and photosynthetic energy conversion efficiency relative to waters outside the iron-enriched patch. The rapid and very high accumulation of phytoplankton biomass in response to the iron addition appeared to be partly attributable to shallow mixed-layer depth and moderate water temperature in the western subarctic Pacific. However, the main reason was a floristic shift to fast-growing centric diatom Chaetoceros debilis, unlike the previous iron-enrichment experiments in the equatorial Pacific and the Southern Ocean, in both of which iron stimulated the growth of pennate diatoms. The iron-mediated blooming of diatoms resulted in a marked consumption of macronutrients and drawdown of pCO2. Biological and physiological measurements indicate that phytoplankton growth in the patch became both light- and iron-limited, making phytoplankton biomass relatively constant after day 9. The increase in microzooplankton grazing rate after day 9 also influenced the net growth rate of phytoplankton. There was no significant increase in the export flux of carbon to depth during the 14-day occupation of the experimental site. The export flux between day 4 and day 13 was estimated to be only 13% of the integrated primary production in the iron-enriched patch. The major part of the carbon fixed by the diatom bloom remained in the surface mixed layer as biogenic particulate matter. Our findings support the hypothesis that iron limits phytoplankton growth and biomass in a ‘bottom up’ manner in this area, but the fate of algal carbon remains unknown.  相似文献   

6.
A five-component (phytoplankton, zooplankton, ammonium, nitrate, detritus) physical–biological model was developed to investigate the effects of physical processes on daily to interannual time scales, on the lower trophic levels of the central equatorial Pacific. Many of the biological processes included in the ecosystem model respond to environmental fluctuations with time scales between 1 and 10 d, which are not typically resolved by basin- to global-scale circulation models. Therefore, the equatorial Pacific ecosystem model is forced using daily information (solar radiation, velocity, temperature) from the Tropical Atmosphere Ocean (TAO) mooring array. The ecosystem model also requires vertical velocity information which necessitated the development of a method for computing daily vertical velocities from the TAO array. Much of the variability in primary production, plankton and nutrient concentrations observed in 1992 during the US Joint Global Ocean Flux Study Equatorial Pacific Process Study time-series cruises (TS1 and TS2), is well reproduced in the model simulations. Simulations demonstrate that lower primary productivities during TS1 as compared to TS2 result from the deeper thermocline that persisted during TS1 as a result of El Niño conditions; however, because of the simultaneous reduction in grazing pressure, simulated chlorophyll levels are similar for these two time periods. Simulations of this single-species ecosystem model successfully reproduce data collected both during and after the El Niño, suggesting that species composition changes are not of first-order importance when examining the effects of the 1991–92 El Niño on the equatorial Pacific ecosystem. A 60–70% increase in chlorophyll concentration and a 400% increase in the chlorophyll contribution by diatoms was associated with the passage of a tropical instability wave (20-d period) across the study site during TS2. This period of high chlorophyll concentration and diatom abundance coincided temporally with strong northward velocities and strong downwelling velocities in the upper euphotic zone. Observations and simulations suggest that this increase in chlorophyll concentration and change in species composition not only results from in situ diatom growth stimulated by increased iron concentrations, but also results from the advection of diatoms toward the convergent front located along the leading (western) edge of the instability wave. Equatorially trapped internal gravity waves can also stimulate in situ phytoplankton growth as high-frequency vertical motions introduce limiting micronutrients, such as iron, into the euphotic zone. Because iron can be taken up by the picoplankton on time scales much shorter than the wave period (6–8 days), these waves may provide a mechanism for effecting a large flux of iron into the euphotic zone. Exclusion of these high-frequency motions results in an iron flux to the euphotic zone that may be underestimated by more than 30%.  相似文献   

7.
To fill temporal gaps in iron-enrichment experimental data and gain further understanding of marine ecosystem responses to iron enrichments, we apply a fifteen-compartment ecosystem model to three iron-enrichment sites, namely SEEDS (the Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study; 48.5°N, 165°E) in the western North Pacific, SOIREE (the Southern Ocean Iron RElease Experiment; 61°S, 140°E) in the Southern Ocean, and IronExII (the second mesoscale iron enrichment experiment; 3.5°S, 104°W) in the Equatorial Pacific. The ecological effects of iron in the model are represented by changing two photosynthetic parameters during the iron-enrichment period. The model results successfully reproduce the observed biogeochemical responses inside and outside the iron patch at each site, such as rapid increases in plankton biomass and biological productivity, and decreases in surface nutrients and pCO2, inside the patch. However, the modeled timing and magnitude of changes differ among the sites because of differences in both physical environments and plankton species. After the iron enrichment, the diatom productivity is strongly controlled by light at SOIREE and by silicate at IronExII and SEEDS. Light limitation due to self-shading by the phytoplankton is significant during the bloom at all sites. Sensitivity analysis of the model results to duration of the iron enrichment reveals that long-term multiple infusions over more than a week would not be effective at SEEDS because of strong silicate limitation on diatom growth. Sensitivity of the model to water temperature shows that export production is higher at lower temperatures, because of slower recycling of particulate organic carbon. Therefore, the e-ratio (the ratio of export production to primary production) is inversely correlated with temperature, and the relationship can be described with a linear function. Through this study, we conclude that ecosystem modeling is a powerful tool to help design future iron-enrichment experiments and observational plans.  相似文献   

8.
Primary production, nutrient concentrations, phytoplankton biomass (incl. chlorophyll a) and water transparency (Secchi depth), are important indicators of eutrophication. Earlier basin-wide primary production estimates for the Baltic Sea, a shallow shelf sea, were based mainly on open-sea data, neglecting the fundamentally different conditions in the large river plumes, which might have substantially higher production. Mean values of the period 1993–1997 of nutrient concentrations (phosphate, nitrate, ammonium and silicate), phytoplankton biomass, chlorophyll a (chl a) concentration, turbidity and primary production were calculated in the plumes of the rivers Oder, Vistula and Daugava and Klaipeda Strait as well as the open waters of the Arkona Sea, Bornholm Sea, eastern Gotland Sea and the Gulf of Riga. In the plumes, these values, except for primary production, were significantly higher than in the open waters. N:P ratios in the plumes were >16 (with some exceptions in summer and autumn), indicating potential P-limitation of phytoplankton growth, whereas they were <16 in the open Baltic Proper, indicating potential N-limitation. On the basis of in situ phytoplankton primary production, phytoplankton biomass and nutrient concentrations, the large river plumes and the Gulf of Riga could be characterized as eutrophic and the outer parts of the coastal waters and the open sea as mesotrophic. Using salinity to define the border of the plumes, their mean extension was calculated by means of a circulation model. Taking into account the contribution of coastal waters, the primary production in the Baltic Proper and the Gulf of Riga was 42·6 and 4·3×106 t C yr−1, respectively. Hence, an annual phytoplankton primary production in the whole Baltic Sea was estimated at 62×106 t C yr−1. The separate consideration of the plumes had only a minor effect on the estimation of total primary production in comparison with an estimate based on open sea data only. There is evidence for a doubling of primary production in the last two decades. Moreover, a replacement of diatoms by dinoflagellates during the spring bloom was noticed in the open sea but not in the coastal waters. A scheme for trophic classification of the Baltic Sea, based on phytoplankton primary production and biomass, chl a and nutrient concentrations, is proposed.  相似文献   

9.
We use inverse analysis to model carbon and nitrogen flows in the upper ocean food web at Ocean Station Papa (OSP; 50°N, 145°W) for winter, spring, and late summer. The seasonal variability in basic physical, chemical, and biological characteristics is low, and the particulate carbon and nitrogen flux at 200 m is remarkably constant. Despite this apparent uniformity, the food web structure undergoes significant seasonal changes. The diversity of trophic pathways is higher during late summer than during the other two periods. The spring ecosystem is not in steady state and undergoes net phytoplankton growth and macronutrient consumption. The microbial loop is well developed only during late summer. Nevertheless, ammonium regeneration by the food web seems insufficient to meet demand by the primary producers. The difference may be due to recycling of dissolved organic nitrogen (urea+free amino acids), a process not represented in the model. The winter food web is the closest to steady state, with nitrate utilisation approximately in balance with export of particulate nitrogen. The inverse analysis suggests two main seasonally invariant features of the NE Pacific ecosystem. First, the major trophic pathway is always from picophytoplankton (0.2–5 μm) to microzooplankton (heterotrophic dinoflagellates and ciliates) to mesozooplankton. This supports the idea of a strong coupling between the microbial and metazoan food webs. Second, much of the primary production (and bacterial production in late summer) is not grazed and is recycled through the detrital pool. Both these features seem to arise from the requirement to conserve nitrogen as well as carbon in the food web. More complete measurements on the microzooplankton 20–200 μm in size, including the small metazoans like nauplii larvae, are required to improve the models presented here.  相似文献   

10.
A coupled physical–biological model was developed to simulate the low-silicate, high-nitrate, and low-chlorophyll (LSHNLC) conditions in the equatorial Pacific Ocean and used to compute a detailed budget in the Wyrtki box (5°N–5°S, 180–90°W) for the major sources and cycling of nitrogen and silicon in the equatorial Pacific. With the incorporation of biogenic silicon dissolution, NH4 regeneration from organic nitrogen and nitrification of ammonia in the model, we show that silicon recycling in the upper ocean is less efficient than nitrogen. As the major source of nutrients to the equatorial Pacific, the Equatorial Undercurrent provides slightly less Si(OH)4 than NO3 to the upwelling zone, which is defined as 2.5°N–2.5°S. As a result, the equatorial upwelling supplies less Si(OH)4 than NO3 into the euphotic zone in the Wyrtki box, having a Si/N supply ratio of about 0.85 (2.5 vs. 2.96 mmolm−2 day−1). More Si(OH)4 than NO3 is taken up with a Si/N ratio of 1.17 (2.72 vs. 2.33 mmolm−2 day−1) within the euphotic zone. The difference between upwelling supply and biological uptake is balanced by nutrient regeneration and horizontal advection. Excluding regeneration, the net silicate and nitrate uptakes are nearly equal (1.76 vs. 1.84 mmolm−2 day−1). However, biogenic silica export production is slightly higher than organic nitrogen (1.74 vs. 1.59 mmolm−2 day−1) following a 1.1 Si/N ratio. In the central equatorial Pacific, low silicate concentrations limit diatom growth; therefore non-diatom new production accounts for most of the new production. Higher silicate supply in the east maintains elevated diatom growth rates and new production associated with diatoms dominate upwelling zone. In contrast, the new production associated with small phytoplankton is nearly constant or decreases eastward along the equator. The total new production has a higher rate in the east than in the west, following the pattern of surface silicate. This suggests that silicate regulates the diatom production, total new production, and thereby carbon cycle in this area. The modeled mean primary production is 48.4 mmolCm−2 day−1, representing the lower end of direct field measurements, while new production is 15.0 mmolCm−2 day−1, which compares well with previous estimates.  相似文献   

11.
A three-dimensional ocean biogeochemical model of the tropical Atlantic Ocean was run for more than half a century (1949–2000) in order to characterize the ocean biogeochemical response to variable forcing over this period. The seasonal cycle in the equatorial upwelling zone agrees reasonably well with observations and other published simulations but underestimates phytoplankton biomass under strong upwelling conditions. Away from the equator, modelled nutrient flux and biological production are maximal in each hemisphere's winter season, and appear to be proximately forced by evaporative cooling and wind stirring rather than by Ekman upwelling. The fraction of the total variance associated with the seasonal cycle is considerably smaller for modelled biogeochemical fields than for sea-surface temperature over this long simulation, and much of the biogeochemical variance is associated with interdecadal changes. The model results suggest that the tropical Atlantic became more productive following the Pacific climate shift of 1976 and remained so until about 1989. Summer surface nitrate concentrations during the 1990s were lower than those in the 1980s. The relationship between the equatorial and off-equatorial regimes may have changed following the 1976 event, with equatorial variability dominating the basin-wide variance patterns after 1976.  相似文献   

12.
The response of phytoplankton growth to iron supply and its modulation by large-scale circulation and tropical instability waves (TIWs) in the eastern equatorial Pacific has been investigated with an ocean biogeochemical model. This process study shows that iron can be efficiently advected from the New Guinea shelf through the Equatorial Undercurrent (EUC) to the eastern Pacific. The presence of a continental iron source is necessary for the maintenance of the observed subsurface iron maximum in the EUC core. In the eastern Pacific region, phytoplankton production is enhanced when additional iron is available in the EUC. Simulated phytoplankton variability is linked to TIWs activity, as revealed by a wavelet analysis of the total autotrophic carbon. The net local effect of the waves on phytoplankton can be either positive or negative depending on several factors. When the iron nutricline is sufficiently shallow to be reached by the wave vertical scale, the effect of the waves is to enhance iron availability in the euphotic zone leading to a net local increase of phytoplankton biomass. We therefore suggest that the local maxima of phytoplankton observed in moorings off the Equator in the eastern Pacific might be not only the result of concentration mechanisms, but also the result of an increase in local production sustained by advected iron.  相似文献   

13.
The equatorial Pacific is an HNLC (High-Nitrate Low-Chlorophyll) region. Modeling and in-situ process studies have confirmed the importance of microzooplankton grazing in this ecosystem. Unfortunately, both the parameters and functions representing microzooplankton grazing within current ecosystem models are poorly constrained. We used a simple 4-component food web model to test the assumption that a lower grazing threshold, which is common in many models, is necessary to achieve the HNLC condition. Without the grazing threshold, the model did not reproduce the HNLC condition. However, by raising the half-saturation constant within the microzooplankton functional response with no threshold, it was possible to reproduce the critical dynamics of the HNLC condition under both steady and moderate seasonal variability in nutrient input. It was also possible to reproduce the HNLC system using a sigmoidal functional response for the microzooplankton, with results somewhere between the other two forms of the model, although this version had the highest sensitivity to changes in its parameters. The three models predicted similar phytoplankton biomass and primary productivity under steady nutrient input, but diverge in these metrics as the amplitude of nutrient input variability increases. These three functional responses also imply certain important differences in the microzooplankton community. Whereas the threshold model had the least sensitivity to parameter choice, the high half-saturation constant, no-threshold model may actually be a better approximation when modeling a community of grazers. Ecosystem models that predict carbon production and export in HNLC regions can be very sensitive to assumptions concerning microzooplankton grazing; future studies need to concentrate on the functional responses of microzooplankton before these models can be used for predicting fluxes in times or regions where forcing is beyond that used to constrain the original model.  相似文献   

14.
This study documents the results of a multi-sensor satellite investigation aimed at comparing the seasonality and interannual variability of phytoplankton biomass and primary productivity (PP) in the western and eastern gyres of the subarctic Pacific. Satellite data helped discern several features, most importantly the existence of significant east-west gradients in the supply of nitrate in winter, in the consumption of nitrate by phytoplankton and in phytoplankton production and biomass accumulation over the growth season. In the western subarctic gyre many of these features appear to be regulated by the strength of sea surface winds through increased iron and nitrate inputs. Multiple regression analysis of data extracted from 12 boxes spanning different hydrographic regimes in the subarctic Pacific, showed that over 65% of the variations in PP in the subarctic Pacific could be explained solely on the basis of changes in the strength of sea surface winds and the intensity of incident irradiance (PAR). The dependence of PP on sea surface wind stress was far greater in the western subarctic Pacific Gyre (WSG), than in the Alaskan Gyre (ALG) due to diminishing impact of surface winds towards the east. Spring accumulation of phytoplankton biomass was greater in the WSG than in the ALG despite the higher rates of PP in the latter. This study assumes particular significance because it helps ascertain the existence of several sub-regions within the two broader domains of the WSG and the ALG. In addition, large interannual variations in phytoplankton biomass and PP were observed in the subarctic Pacific following the onset of the El-Niño event of 1997 and the transition to La-Niña conditions in 1999. These variations were largely the result of differences in meteorological and oceanographic conditions across the subarctic Pacific following the development of the El-Niño.  相似文献   

15.
Phytoplankton community composition, productivity and biomass characteristics of the mesohaline lower Neuse River estuary were assessed monthly from May 1988 to February 1990. An incubation method which considered water-column mixing and variable light exposure was used to determine phytoplankton primary productivity. The summer productivity peaks in this shallow estuary were stimulated by increases in irradiance and temperature. However, dissolved inorganic nitrogen loading was the major factor controlling ultimate yearly production. Dynamic, unpredictable rainfall events determined magnitudes of seasonal production pulses through nitrogen loading, and helped determine phytoplankton species composition. Dinoflagellates occasionally bloomed but were otherwise present in moderate numbers; rainfall events produced large pulses of cryptomonads, and dry seasons and subsequent higher salinity led to dominance by small centric diatoms. Daily production was strongly correlated (r = 0·82) with nitrate concentration and inversely correlated (r = −0·73) with salinity, while nitrate and salinity were inversely correlated (r = −0·71), emphasizing the importance of freshwater input as a nutrient-loading source to the lower estuary. During 1989 mean daily areal phytoplankton production was 938 mgC m−2, mean chlorophyll a was 11·8 mg m−3, and mean phytoplankton density was 1·56 × 103 cells ml−1. Estimated 1989 annual areal phytoplankton production for the lower estuary was 343 gC m−2.  相似文献   

16.
Along with meteorological observations, complementary and systematic oceanographic observations of various physical, biological and chemical parameters have been made at Ocean Station P (OSP) (50°N, 145°W) since the early 1950s. These decadal time scale data have contributed to a better understanding of the physical, biological and chemical processes in the surface layer of the northeastern subarctic region of the Pacific Ocean. These data have demonstrated the importance of the North Pacific in the global carbon cycle and, in particular, the role of biological/chemical processes in the net exchange of CO2 across the air–sea interface. Although we do not fully comprehend how climatic variations influence marine communities or marine biogeochemistry, previous studies have provided some basic understanding of the mechanisms controlling the seasonal and inter-annual variations of biological and chemical parameters (such as phytoplankton, bacteria, nitrate/ammonium concentration) at OSP, and how they affect the carbon cycling in the subarctic North Pacific. In this study, we investigate how these mechanisms might alter the seasonal variations of these parameters at OSP under a 2XCO2 condition. We examine these influences using a new biological model calibrated by the climatological data from OSP. For the 2XCO2 simulation, the biological model is driven off line (i.e., no feedback to the ocean/atmospheric model components) by the climatology plus 2XCO2−1XCO2 outputs from a global surface ocean model and the Canadian GCM. Under the 2XCO2 condition, the upper layer ocean shows an increase in the entrainment rate at the bottom of the mixed layer for OSP during the late autumn and winter seasons, resulting in an increase in the f-ratio. Although there is an overall increase in the primary production (PP) by 3–18%, a decrease in the biomass of small phytoplankton and microzooplankton (due to mesozooplankton grazing) lowers the concentration of dissolved organic matter (DOM) by 4–25%. The model also predicts a significant increase in the concentrations of nitrate and ammonium, and in bacterial production during July and August. Doubling of the atmospheric CO2 from 330 to 660 ppm forces the marine pCO2 to increase by about 63%, much of which is driven by an increased flux of CO2 from the atmosphere to the oceans.  相似文献   

17.
A biogeochemical model of the tropical Pacific has been used to assess the impact of interannual variability in a western Pacific iron source on the iron-limited ecosystem of the central and eastern Pacific during the 1997–1998 El Niño. A reference simulation and two simulations with an iron source in the western Pacific have been performed. The two “source” simulations differed only in the temporal variability of the iron source. In the variable source simulation, the iron concentration in the source region was proportional to the velocity of the New Guinea Coastal Undercurrent (NGCUC). In the constant source simulation, the same time-averaged concentration of iron was imposed with no temporal variability. The variable source was designed to mimic variations of iron flux from the northeast slope of New Guinea to the NGCUC due to modulation of sedimentary iron resuspension as previously hypothesized. Through the comparison of these simulations, it appeared that: (i) an iron source in the NGCUC, regardless of its source variability, increases biomass in the eastern equatorial Pacific because of the greater eastward iron flux by the Equatorial Undercurrent and (ii) a variable NGCUC iron source does not change the temporal variability of eastern Pacific chlorophyll, and in particular the timing and intensity of the June 1998 bloom. To explain eastern Pacific biological variability, local rather than remote processes are needed, such as wind-driven upwelling, the local depth of the thermocline, tropical instability waves and biological processes such as high grazing pressure. Therefore, while the western Pacific sources of dissolved iron are important in our model to sustain annually integrated equatorial Pacific production, they are unlikely to strongly constrain the timing of blooms in the central and eastern Pacific such as during the 1998 La Niña.  相似文献   

18.
北太平洋副极地海区作为全球海洋三个高营养盐低叶绿素(high nutrient and low chlorophyll, HNLC)海区之一, 其浮游植物生长受到微量元素铁的限制。对于开阔大洋, 大气沉降是海洋表层铁的一个重要来源, 铁元素沉降进入海洋后能够促进浮游植物生长, 进而引起海洋初级生产力和生物泵的响应。本文利用SPRINTARS(Spectral Radiation-Transport Model for Aerosol Species)模式的时长为20a的日均大气沉降数据, 对北太平洋海区大气沉降的时空特征进行了分析。结果表明, 进入北太平洋海区的大气沉降量为26.81Tg·a-1, 并且存在显著的季节变化: 春季最高, 冬季最低, 5月份进入海洋的沉降量达到峰值。大气沉降主要来源于陆地区域, 在风场的驱动下向海洋传输, 因此大气沉降量的空间分布呈现出西高东低的特征。本文以2010年8月中旬卫星观测到的一次强沙尘(即高大气沉降量)事件为例, 研究了大气沙尘的传播路径。进一步结合2001年4月9—12日及2008年4月20—22日的沙尘事件, 分析了西北太平洋K2站位(47°N, 160°E)附近海域海洋初级生产力对大气沉降——沙尘事件的响应。结果表明, 三次沙尘事件后, K2站位的颗粒有机碳通量、叶绿素浓度均有明显增加, 即沙尘事件对北太平洋副极区海洋初级生产力存在促进作用。  相似文献   

19.
The recent NE subarctic Pacific study of the Canadian JGOFS project was designed primarily to address why phytoplankton biomass and production at Ocean Station Papa (OSP: 50°N, 145°W) are not as high as the nitrate concentrations could potentially support. To examine the possible role of iron (Fe) limitation in concert with microzooplankton grazing and physical supply of nitrate, we have coupled a four-compartment Nitrogen–Phytoplankton–Zooplankton–Detritus planktonic ecosystem model with a 60-layer (each 2 m thick) one-dimensional mixed-layer model (Mellor–Yamada level 2.5), driven by annual forcing characteristic of OSP. Both the physical and ecological models are forced with the same annual heat budget, mean phytoplankton concentration was tuned with the equilibrium solution of the model, and the zooplankton parameter values were chosen to be representative of microzooplankton. Modelled sea surface temperature ranged between 6 (fixed – late winter) and 13–14°C, depending on the distribution and amount of phytoplankton and detritus calculated by the model. Simulations with Fe limitation reducing the maximum specific growth rate of phytoplankton (for Fe-replete conditions) by a factor of ∼3 best reproduced the annual cycle of surface layer nitrate, although the resulting annual f-ratio calculated from the fluxes into and out of the nitrogen compartment was marginally higher than recent estimates of f-ratio based on observations at OSP. The best simulations with Fe limitation agreed with observations of the annual cycle of surface nitrate concentration, the f-ratio, particulate nitrogen concentration in the euphotic layer, the export production, and the remineralization depth scale for sinking detritus, to within ∼50%, probably within the range of observational uncertainty and/or seasonal and interannual variability. Possible modifications include separating the detrital pool into suspended and sinking organic matter, decreasing the rate of remineralization with increasing depth, and examining the supply of nitrate to the surface layer by means of horizontal advection. The observational basis required to formulate these processes is marginal at present.  相似文献   

20.
The seasonality of primary productivity plays an important role in nutrient and carbon cycling. We quantify the seasonality of satellite-derived, oceanic net primary production (NPP) and its interannual variability during the first decade of the SeaWiFS mission (1998 to 2007) using a normalized seasonality index (NSI). The NSI, which is based upon production half-time, t(1/2), generally becomes progressively more episodic with increasing latitude in open ocean waters, spanning from a relatively constant rate of primary productivity throughout the year (mean t(1/2) ~5 months) in subtropical waters to more pulsed events (mean t(1/2) ~3 months) in subpolar waters. This relatively gradual, poleward pattern in NSI differs from recent estimates of phytoplankton bloom duration, another measure of seasonality, at lower latitudes (~40°S–40°N). These differences likely reflect the temporal component of production assessed by each metric, with NSI able to more fully capture the irregular nature of production characteristic of waters in this zonal band. The interannual variability in NSI was generally low, with higher variability observed primarily in frontal and seasonal upwelling zones. The influence of the El Niño–Southern Oscillation on this variability was clearly evident, particularly in the equatorial Pacific, where primary productivity was anomalously episodic from the date line east to the coast of South America in 1998. Yearly seasonality and the magnitude of annual production were generally positively correlated at mid-latitudes and negatively correlated at tropical latitudes, particularly in a region bordering the Pacific equatorial divergence. This implies that increases of annual production in the former region are attained over the course of a year by shorter duration but higher magnitude NPP events, while in the latter areas it results from an increased frequency or duration of similar magnitude events. Statistically significant trends in the seasonality, both positive and negative, were detected in various patches. We suggest that NSI be used together with other phenomenological characteristics of phytoplankton biomass and productivity, such as the timing of bloom initiation and duration, as a means to remotely quantify phytoplankton seasonality and monitor the response of the oceanic ecosystem to environmental variability and climate change.  相似文献   

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