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1.
A video plankton recorder (VPR) and a remotely operated vehicle (ROV) were utilized on three cruises in the oligotrophic North Pacific Subtropical Gyre (NPSG) between 1995 and 2002 to quantify the size and abundance of marine snow and Rhizosolenia diatom mats within the upper 305 m of the water column. Quantitative image analysis of video collected by the VPR and an ROV-mounted particle imaging system provides the first transect of marine snow size and abundance across the central North Pacific Gyre extending from 920 km NW of Oahu to 555 km off Southern California. Snow abundance in the upper 55 m was surprisingly high for this oligotrophic region, with peak values of 6.0–13.0×10 3 aggregates m −3 at the western- and easternmost stations. At stations located in the middle of the transect (farthest from HI and CA), upper water column snow abundance displayed values of 0.5–1.0×10 3 aggregates m −3. VPR and ROV imagery also provided in situ documentation of the presence of nitrogen-transporting, vertically migrating Rhizosolenia mats from the surface to >300 m with mat abundances ranging from 0 to 10 mats m −3. There was clear evidence that Rhizosolenia mats commonly reach sub-nutricline depths. The mats were noted to be a common feature in the North Pacific Gyre, with the lower salinity edge of the California Current appearing to be the easternmost extent of their oceanic distribution. Based on ROV observations at depth, flux by large (1.5 cm) mats is revised upward 4.5-fold, yielding an average value of 40 μmol N m −2 d −1, a value equaling previous estimates that included much smaller mats visible only to towed optical systems. Our results suggest that the occurrence across a broad region of the NPSG of particulate organic matter production events represented by high concentrations of Rhizosolenia mats, associated mesozooplankton, and abundant detrital marine aggregates may represent significant stochastic components in the overall carbon, nitrogen, and silica budgets of the oligotrophic subtropical gyre. Likewise, their presence has important implications for the proposed climate-driven, ecosystem reorganization or domain shift occurring in the NPSG. 相似文献
2.
The western subarctic gyre (WSG) and the eastern Alaska Grye (AG) on each side of the subarctic North Pacific, have many similarities.
In both gyres, macronutrients are generally high and chl is low, and hence both gyres are High Nitrate, Low Chlorophyll (HNLC)
regions. Despite the general similarities between these two gyres, there are many important differences. The time series station
established at Stn KNOT on the southwest edge of the WSG and two in situ mesoscale iron enrichment experiments at each of
the gyres has provided more information on iron concentrations, the dual role of iron and silicate limitation and seasonal
cycles in the gyres. There is more seasonality in many parameters at Stn KNOT than at Stn P. There is an increase in Chl and
primary productivity at Stn KNOT in May followed by increased iron limitation in summer. Low DIC:NO 3 ratios and high Si:NO 3 ratios in the WSG, indicate lower calcification and higher diatom production than at Stn P. The sources of iron for these
areas are still not clear, but horizontal transport of iron rich coastal water and vertical transport could be important sources
at certain times of the year in addition to dust input. Satellite images show that chl-rich coastal waters occasionally extend
to the vicinity of Stn KNOT and therefore Stn KNOT may not always represent conditions in the main part of the WSG. This review
focuses mainly on a comparison of Stn KNOT and Stn P, two time series stations on the edge of two very large gyres. At present,
we have a limited understanding of the temporal and spatial variability within each of these large gyres.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
3.
Concentrations and sinking rates of particulate biogenic silica (BSi), chlorophyll a (chl a) and phaeopigments (phae) (< 3 μm, 3–10 μm, > 10 μm and total), as well as the abundances of the major phytoplankton species, were studied during September 1991 in the Eastern Laptev Sea and the lower Lena River (Siberian Arctic). The highest chl a concentrations were found in two major “new” production regimes of the study area: (1) a deep chl a maximum (5.8 mg chl a m −3) (formed by the diatom Chaetoceros socialis) at 30 m depth on the outer shelf of the northern Laptev Sea, and (2) in the Lena River, where the phytoplankton community was dominated by fresh water diatoms (1.5 to 4.5 mg chl a m −3). Elevated chl a concentrations were also found in the river plume phytoplankton community (dominated by brackish water diatoms), NE of the Lena delta. In the Laptev Sea, the low chl a (0.1 to 3 mg chl a m −3) and high phae concentrations (0.5 to 14 mg phae m −3) indicated that the phytoplankton community (dominated by picoplanktic algae and nanoflagellates) was already senescent and affected by grazing losses. Biogenic silica values were highest in the Lena River (4 to 17 μM) as compared to the low values found in the Laptev Sea (0.3 to 4 μM). The large chl a size fraction, phae and BSi in the Lena River samples revealed the highest measured sinking rates (1.4, 2.3, and 1.5 m d −1, respectively). The formation of a strong halocline, decreasing turbulence, and possible nutrient deficiency resulted in death, disintegration and rapid sedimentation of fresh water diatoms. This was accompanied by a decrease in the BSi concentration and growth of the picoplanktic size fraction (< 3 μm) in the estuarine mixing zone (Gulf of Buorkhaya). Only a minor part of BSi was bound to intact diatom cells (< 3%) in the surface layer, most of which being apparently associated with detrital particles. In the Lena River, approximately 12% of the total silica was bound to BSi fraction, yet elsewhere in the Laptev Sea and in the estuarine mixing zone the BSi:total silica ratio was ≤ 5%. Thus, the results reflected the successional stage of a late summer phytoplankton community, characterized by dominance of small autotrophs and patchy distribution of senescent diatoms no longer able to affect the relative high levels of dissolved silica supplied by the Lena River. 相似文献
4.
Abundances and biomasses of planktonic ciliates and copepod nauplii, major components of the microzooplankton community, were investigated in the subarctic North Pacific and the Bering Sea in summer of 1997. Their regional variation was illustrated by demarcating the entire area into five regions. Ciliates always predominated both in abundance (>94%) and biomass (>78%) over nauplii. Regional means of ciliates in the water column were higher in the Alaskan Gyre (120 × 10 6 cells/m 2) and the Western Subarctic Gyre (110 × 10 6 cells/m 2) in terms of abundance, and rich in the Bering Sea Gyre (360 mgC/m 2) and the Western Subarctic Gyre (340 mgC/m 2) in terms of biomass. By contrast, standing crops of ciliates were poor in the Oyashio Region (67 × 10 6 cells/m 2; 170 mgC/m 2) and the Transition Region (64 × 10 6 cells/m 2; 160 mgC/m 2). The values of biomass reported here are generally in agreement with the values reported previously from the Bering Sea Gyre and the Alaskan Gyre but are considerably higher than the previous value found in the Western Subarctic Gyre. No significant correlations could be found between chlorophyll a crop and standing crops of ciliates and copepod nauplii over the entire subarctic North Pacific and the Bering Sea during this summer. 相似文献
5.
Dynamics of transparent exopolymer particles (TEP) was studied during the first in situ iron-enrichment experiment conducted in the western subarctic Pacific in July–August 2001, with the goal of evaluating the contribution of TEP to vertical flux as a result of increased primary production following iron enrichment in open ocean ecosystems. Subsequent to the enhancement of phytoplankton production, we observed increase in TEP concentration in the surface layer and sedimentation of organic matter beneath it. Vertical profiles of TEP, chlorophyll a (Chl a) and particulate organic carbon (POC) were obtained from six depths between 5 and 70 m, from a station each located inside and outside the enriched patch. TEP and total mass flux were estimated from the floating sediment traps deployed at 200 m depth. Chl a and TEP concentrations outside the patch varied from 0.2 to 1.9 μg L −1 and 40–60 μg XG equiv. L −1, respectively. Inside the patch, Chl a increased drastically from day 7 reaching the peak of 19.2 μg L −1 on day 13, which coincided with the TEP peak of 189 μg XG equiv. L −1. TEP flux in the sediment trap increased from 41 to 88 mg XG equiv. m −2 d −1, with 8–14% contribution of TEP to total mass flux. This forms the basic data set on ambient concentrations of TEP in the western subarctic Pacific, and evaluation of the effect of iron enrichment on TEP. 相似文献
6.
Biweekly composite averages of the standing stock of sea-surface chlorophyll (SSC) were derived from SeaWiFS satellite ocean-color data at 44 benthic sampling stations occupied along the continental slope and rise by the Deep Gulf of Mexico Benthos (DGoMB) program. At the 22 DGoMB sites north of 26°N and west of 91°W in the NW Gulf of Mexico, annual average SSC was 0.19 mg m −3, ranging at most locations from annual highs of about 0.3 mg m −3 in November–February to lows of about 0.1 mg m −3 in May–August. Comparison of three years of SeaWiFS data (January 1998–December 2000) showed little inter-annual variation at these NW Gulf stations. In contrast, at the 22 NE Gulf sites north of 26°N and east of 91°W, SSC averaged 2.8 times higher than in the NW Gulf, showing also strong inter-annual variation. Maxima in the NE region occurred in November–February and also during summers. The summer maxima were associated with Mississippi River water transported offshore to the east and southward by anticyclonic eddies in the NE Gulf. The apparent increases in SSC in June–August at NE Gulf stations reached average monthly concentrations >50% greater than in November–February. Based on a primary productivity model and a vertical flux model, the calculated export of particulate organic carbon (POC flux reaching the seafloor) was estimated as 18 mg C m −2 day −1 at the 22 NE Gulf stations, and 9 mg C m −2 day −1 at the 22 NW Gulf stations. These estimates are comparable to fluxes measured by benthic lander by others in the DGoMB program, which may drive the differences in west versus east bathymetric zonation and community structure of macrobenthos that were sampled with large box corers by others in the DGoMB program. 相似文献
7.
Benthic fluxes of dissolved inorganic nitrogen (NO 3− and NH 4+), dissolved organic nitrogen (DON), N 2 (denitrification), O 2 and TCO 2 were measured in the tidal reaches of the Bremer River, south east Queensland, Australia. Measurements were made at three sites during summer and winter. Fluxes of NO 3− were generally directed into the sediments at rates of up to −225 μmol N m −2 h −1. NH 4+ was mostly taken up by the sediments at rates of up to −52 μmol N m −2 h −1, its ultimate fate probably being denitrification. DON fluxes were not significant during winter. During summer, fluxes of DON were observed both into (−105 μmol m −2 h −1) and out of (39 μmol m −2 h −1) the sediments. Average N 2 fluxes at all sampling sites were similar during summer (162 μmol N m −2 h −1) and winter (153 μmol N m −2 h −1). Denitrification was fed both by nitrification within the sediment and NO 3− from the water column. Sediment respiration rates played an important role in the dynamics of nitrification and denitrification. NO 3− fluxes were significantly related to TCO 2 fluxes ( p<0.01), with a release of NO 3− from the sediment only occurring at respiration rates below 1000 μmol C m −2 h −1. Rates of denitrification increased with respiration up to TCO 2 fluxes of 1000 μmol C m −2 h −1. At sediment respiration rates above 1000 μmol C m −2 h −1, denitrification rates increased less rapidly with respiration in winter and declined during summer. On a monthly basis denitrification removed about 9% of the total nitrogen and 16% of NO 3− entering the Bremer River system from known point sources. This is a similar magnitude to that estimated in other tidal river systems and estuaries receiving similar nitrogen loads. During flood events the amount of NO 3− denitrified dropped to about 6% of the total river NO 3− load. 相似文献
8.
Based on the Simple Ocean Data Assimilation(SODA) products,we study the mean properties and variations of the Southern Hemisphere subpolar gyres(SHSGs) in this paper.The results show that the gyre strengths in the SODA estimates are(55.9±9.8)×10~6m~3/s for the Weddell Gyre(WG),(37.0±6.4) ×10~6m~3/s for the Ross Gyre(RG),and(27.5±8.2)×10~6m~3/s for the Australian-Antarctic Gyre(AG),respectively.There exists distinct connectivity between the adjacent gyres and then forms an oceanic super gyre structure in the southern subpolar oceans.And the interior exchanges are about(8.0±3.2)×10~6m~3/s at around 70°E and(4.3±3.1)×10~6m~3/s at around 140°E.The most pronounced variation for all three SHSGs occurs on the seasonal time scale,with generally stronger(weaker)SHSGs during austral winter(summer).And the seasonal changes of the gyre structures show that the eastern boundary of the WG and AG extends considerably further east during winter and the interior exchange in the super gyre structure increases accordingly.The WG and RG also show significant semi-annual changes.The correlation analyses confirm that the variations of the gyre strengths are strongly correlated with the changes in the local wind forcing on the semi-annual and seasonal time scales. 相似文献
9.
Fatty acids and hydrocarbons of sedimenting particles were investigated in the northeastern Adriatic Sea from November 1988 to December 1989. Particles were collected at approximately monthly intervals, using sediment traps deployed at 30 m depth (2 m above bottom). Seasonal changes in sedimentation of particulate matter were very pronounced. Hydrocarbon fluxes and concentrations were found to vary significantly depending on the season. They averaged 2.69 ± 1.44 mg m −2 day −1 and 232.4 ± 90.93 μg g −1 in winter, respectively. In late spring-early summer the corresponding values amounted to 0.045 ± 0.015 mg m −2 day −1 and 13.72 ± 5.56 μg g −1, and they increased towards autumn, when mean values of 0.517 ± 0.228 mg m −2 day −1 and 98.86 ± 48.72 μg g −1 were obtained. In contrast, fatty acid fluxes and concentrations were low during winter (0.26 ± 0.08 mg m −2 day −1 and 21.95 ± 3.35 μg g −1), increased slightly towards the summer (0.48 ± 0.12 mg m −2 day −1 and 139.9 ± 44.6 μ g −1) and reached maximum rate and concentration in autumn, when average values were 1.98 ± 1.30 mg m 2 day −1 and 489.1 ± 186.7 μg g −1, respectively. The differences in composition, concentrations and fluxes of the fatty acids and hydrocarbons were related to the sources of sedimenting material, reflecting the influence of resuspension of bottom sediments during winter and the appearance of mucus aggregates during summer and their subsequent deposition in autumn. 相似文献
10.
The seasonal dynamics of inorganic nutrients and phytoplankton biomass (chlorophyll a), and its relation with hydrological features, was studied in the NW Alboran Sea during four cruises conducted in February, April, July and October 2002. In the upper layers, the seasonal pattern of nutrient concentrations and their molar ratios (N:Si:P) was greatly influenced by hydrological conditions. The higher nutrient concentrations were observed during the spring cruise (2.54 μM NO 3−, 0.21 μM PO 43− and 1.55 μM Si(OH) 4, on average), coinciding with the increase of salinity due to upwelling induced by westerlies. The lowest nutrient concentrations were observed during summer (<0.54 μM NO 3−, 0.13 μM PO 43− and 0.75 μM Si(OH) 4, on average), when the lower salinities were detected. Nutrient molar ratios (N:Si:P) followed the same seasonal pattern as nutrient distribution. During all the cruises, the ratio N:P in the top 20 m was lower than 16:1, indicating a NO 3− deficiency relative to PO 43−. The N:P ratio increased with depth, reaching values higher than 16:1 in the deeper layers (200–300 m). The N:Si ratio in the top 20 m was lower than 1:1, excepting during spring when N:Si ratios higher than 1:1 were observed in some stations due to the upwelling event. The N:Si ratio increased with depth, showing a maximum at 50–100 m (>1.5:1), which indicates a shift towards Si-deficiency in these layers. The Si:P ratio was much lower than 16:1 throughout the water column during the four cruises. In general, the spatial and seasonal variation of phytoplankton biomass showed a strong coupling with hydrological and chemical fields. The higher chlorophyll a concentrations at the depth of the chlorophyll maximum were found in April (2.57 mg m −3 on average), while the lowest phytoplankton biomass corresponded to the winter cruise (0.74 mg m −3 on average). The low nitrate concentrations together with the low N:P ratios found in the upper layers (top 20 m) during the winter, summer and autumn cruises suggest that N-limitation could occur in these layers during great part of the year. However, N-limitation during the spring cruise was temporally overcome by nutrient enrichment caused by an intense wind-driven upwelling event. 相似文献
11.
Seasonal change in the downward carbon transport due to respiration and mortality through diel vertical migration (DVM) of the calanoid copepods Metridia pacifica and Metridia okhotensis was estimated in the Oyashio region, western subarctic Pacific during six cruises from June 2001 to June 2002. M. pacifica (C4, C5 and adult females) was an active migratory species throughout the year though its DVM amplitude varied among seasons and stages. The mean distribution depths of adult females during the daytime were positively related with the illumination level in the water column, being shallowest in April and deepest in January. M. okhotensis generally showed less-extensive migrations than M. pacifica. Therefore, together with their lower abundance, this species is considered to be a less-important mechanism of downward transport of carbon except for April when their DVM was more active and descended deeper than M. pacifica, which remained in the upper 150 m even during the daytime. The mean migrating biomass of the two Metridia species was 558 mg C m −2 d −1 and was high during summer to winter (263–1676 mg C m −2 d −1) and low during spring (59–63 mg C m −2 d −1). Total downward flux through DVM fluctuated between 1.0 and 20.0 mg C m −2 d −1 with an annual mean of 8.0 mg C m −2 d −1. Contribution of the respiratory flux was greater than the mortality flux and accounted for 64–98% of total migratory flux throughout the year except for January when contribution of both fluxes was equal. Overall the annual carbon transport by DVM of Metridia spp. was estimated as 3.0 g C m −2 year −1, corresponding to 15% of the annual total POC flux at 150 m at the study site, suggesting that DVM is a significant process for carbon export in the subarctic region as well as that in tropical and subtropical oceanic regions. Since DVM in M. pacifica is more active during the non-bloom season when the gravitational flux of particulate matter is low, this species plays an important role in driving the biological pump in the subarctic Pacific during summer to winter. 相似文献
12.
Autotrophic and microheterotrophic plankton populations were monitored in the euphotic zone of the eastern subarctic Pacific during 6 one-month cruises in spring and summer, 1984, 1987 and 1988. Transmitted light, epifluorescence, and electron microscopy were used to identify, enumerate and estimate the biomass of size-populations of species. The 2–10μm size class dominated the biomass of both autotrophs and heterotrophs. The autotrophic flagellate, Phaeocystis pouchetii, was frequently observed in its non-colonial phase. Temporal variation in all the stocks was evident and could be explained only partially by the physical, chemical or biological factors investigated here. The general structure of the autotrophic community was similar to that in the North Atlantic, but major, unexplained variations between cruises occurred. Variation in mixed-layer depth and day length (but not variation in daily insolation) explained 25% of the variation in autotrophic doubling rate. Heterotrophic biomass comprised, in decreasing order of importance, non-pigmented flagellates, dinoflagellates, and ciliates. Ciliates rarely contributed more than 40% to the total. Microheterotrophic biomass rarely exceeded 30μg C 1 −1 (avg 15μg C 1 −1, 0–60m) whereas autotrophic biomass averaged 20μg C 1 −1, 0–60m, and reached 74μg C 1 −1 on one occasion, yet the grazing capacity of these microheterotrophs averaged 100% of primary production. 相似文献
13.
Physical forcing plays a major role in determining biological processes in the ocean across the full spectrum of spatial and temporal scales. Variability of biological production in the Bay of Bengal (BoB) based on basin-scale and mesoscale physical processes is presented using hydrographic data collected during the peak summer monsoon in July–August, 2003. Three different and spatially varying physical processes were identified in the upper 300 m: (I) anticyclonic warm gyre offshore in the southern Bay; (II) a cyclonic eddy in the northern Bay; and (III) an upwelling region adjacent to the southern coast. In the warm gyre (>28.8 °C), the low salinity (33.5) surface waters contained low concentrations of nutrients. These warm surface waters extended below the euphotic zone, which resulted in an oligotrophic environment with low surface chlorophyll a (0.12 mg m −3), low surface primary production (2.55 mg C m −3 day −1) and low zooplankton biovolume (0.14 ml m −3). In the cyclonic eddy, the elevated isopycnals raised the nutricline upto the surface (NO 3–N > 8.2 μM, PO 4–P > 0.8 μM, SiO 4–Si > 3.5 μM). Despite the system being highly eutrophic, response in the biological activity was low. In the upwelling zone, although the nutrient concentrations were lower compared to the cyclonic eddy, the surface phytoplankton biomass and production were high (Chl a – 0.25 mg m −3, PP – 9.23 mg C m −3 day −1), and mesozooplankton biovolume (1.12 ml m −3) was rich. Normally in oligotrophic, open ocean ecosystems, primary production is based on ‘regenerated’ nutrients, but during episodic events like eddies the ‘production’ switches over to ‘new production’. The switching over from ‘regenerated production’ to ‘new production’ in the open ocean (cyclonic eddy) and establishment of a new phytoplankton community will take longer than in the coastal system (upwelling). Despite the functioning of a cyclonic eddy and upwelling being divergent (transporting of nutrients from deeper waters to surface), the utilization of nutrients leading to enhanced biological production and its transfer to upper trophic levels in the upwelling region imply that the energy transfer from primary production to secondary production (mesozooplankton) is more efficient than in the cyclonic eddy of the open ocean. The results suggest that basin-scale and mesoscale processes influence the abundance and spatial heterogeneity of plankton populations across a wide spatial scale in the BoB. The multifaceted effects of these physical processes on primary productivity thus play a prominent role in structuring of zooplankton communities and could consecutively affect the recruitment of pelagic fisheries. 相似文献
14.
Observations of wintertime nutrient concentrations in surface waters are scarce in the temperate and subarctic North Atlantic Ocean. Three new methods of their estimation from spring or early summer observations are described and evaluated. The methods make use of a priori knowledge of the vertical distribution of oxygen saturation and empirical relationships between nutrient concentrations and oxygen saturation. A south–north increase in surface water winter nutrient concentration is observed. Winter nitrate concentrations range from very low levels of about 0.5 μmol dm −3 at 33°N to about 13.5 μmol dm −3 at 60°N. Previous estimates of winter nitrate concentrations have been overestimates by up to 50%. At the Biotrans Site (47°N, 20°W), a typical station in the temperate Northeast Atlantic, a mean winter nitrate concentration of 8 μmol dm −3 is estimated, compared to recently published values between 11 and 12.5 μmol dm −3. It is shown that most of the difference is due to a contribution of remineralised nitrate that had not been recognized in previous winter nutrient estimates. Mesoscale variation of wintertime nitrate concentrations at Biotrans are moderate (less than ±15% of the regional mean value of about 8 μmol dm −3). Interannual variation of the regional mean is small, too. In the available dataset, there was only 1 year with a significantly lower regional mean winter nitrate concentration (7 μmol dm −3), presumably due to restricted deep mixing during an atypically warm winter. The significance of winter nitrate estimates for the assessment of spring-bloom new production and the interpretation of bloom dynamics is evaluated. Applying estimates of wintertime nitrate concentrations of this study, it is found that pre-bloom new production (0.275 mol N m −2) at Biotrans almost equals spring-bloom new production (0.3 mol N m −2). Using previous estimates of wintertime nitrate yields unrealistically high estimates of pre-bloom new production (1.21–1.79 mol N m −2) which are inconsistent with observed levels of primary production and the seasonal development of biomass. 相似文献
15.
The seasonal variability of sea surface height (SSH) and currents are defined by analysis of altimeter data in the NE Pacific Ocean over the region from Central America to the Alaska Gyre. The results help to clarify questions about the timing of seasonal maxima in the boundary currents. As explained below, the long-term temporal mean of the SSH values must be removed at each spatial point to remove the temporally invariant (and large) signal caused by the marine geoid. We refer to the resulting SSH values, which contain all of the temporal variations, as the ‘residual’ SSH. Our main findings are: - 1. The maximum surface velocities around the boundaries of the cyclonic Alaska Gyre (the Alaska Current and the Alaska Stream) occur in winter, at the same time that the equatorward California Current is weakest or reversed (forming the poleward Davidson Current); the maximum surface velocities in the California Current occur in summer. These seasonal maxima are coincident with the large-scale atmospheric wind forcing over each region.
- 2. Most of the seasonal variability occurs as strong residuals in alongshore surface currents around the boundaries of the NE Pacific basin, directly connecting the boundaries of the subpolar gyre, the subtropical gyre and the Equatorial Current System.
- 3. Seasonal variability in the surface velocities of the eastward North Pacific Current (West Wind Drift) is weak in comparison to seasonal changes in the surface currents along the boundaries.
- 4. There is an initial appearance next to the coast and offshore migration of seasonal highs and lows in SSH, alongshore velocity and eddy kinetic energy (EKE) in the Alaska Gyre, similar to the previously-described seasonal offshore migration in the California Current.
- 5. The seasonal development of high SSH and poleward current residuals next to the coast appear first off Central America and mainland Mexico in May–June, prior to their appearance in the southern part of the California Current in July–August and their eventual spread around the entire basin in November–December. Similarly, low SSH and equatorward transport residuals appear first off Central America and Mexico in January–February before spreading farther north in spring and summer.
- 6. The maximum values of EKE occur when each of the boundary currents are maximum.
Article Outline- 1. Introduction and background
- 2. Data and methods
- 2.1. Altimeter and tide gauge data
- 2.2. Atmospheric forcing—sea level pressure
- 2.3. Statistical gridding
- 3. Results
- 4. Summary and discussion
- 4.1. Alaska Gyre
- 4.2. Connections around the boundaries of the subarctic and subtropical gyres
- 4.3. Connections to the North Pacific Current
- 4.4. Offshore ‘propagation’ of the seasonal height and transport signals
- 4.5. Connections to the equatorial current systems along the boundaries
- Acknowledgements
- References
1. Introduction and backgroundThis is the first of a two-part analysis of temporal variability of the NE Pacific Ocean’s surface circulation, as measured by satellite altimeters. Here we examine the seasonal variability. In Part 2 (Strub & James, 2002) we analyze the non-seasonal anomalies of the surface circulation over the 1993–1998 period, during which the 1997–1998 El Niño creates the largest signal. Formation of the seasonal cycles discussed here is the first step in creating the non-seasonal anomalies. The seasonal cycles themselves, however, provide new information on the response of the NE Pacific to strong seasonal forcing, on scales not previously addressed. This analysis quantifies the degree of connection, on seasonal time scales, between the boundary currents in the eastern subarctic and subtropical gyres, as well as the connection between the boundaries and the interior NE Pacific. It further shows a connection to the equatorial current system.Numerous papers describe aspects of the seasonal cycles for certain parameters in subregions of our larger domain. Chapters in Robinson and Brink (1998) review some of the past results from the coastal ocean in the regions between the Equator and the Alaska Gyre ( Badan; Hickey and Royer). Fig. 1 presents the climatological surface dynamic height field (relative to 500 m) in the NE Pacific, calculated from the long-term mean climatological temperature and salinity data of Levitus and Gelfeld (1992). The 500 m reference level is used to concentrate on the surface flow seen by altimeters. Although this climatology is overly smooth, it shows the major currents in the area. The broad, eastward North Pacific Current (also called the West Wind Drift) splits into the counterclockwise Alaska Gyre and the equatorward California Current. South of 20°N in summer, the California Current turns westward and flows into the North Equatorial Current, while in winter–spring, part of it continues along the Mexican mainland before turning westward ( Badan; Fiedler and Fiedler). The long-term climatology shows both paths. The North Equatorial Countercurrent (NECC) flows eastward between 5° –10°N to approximately 120°W, but is only weakly seen in the annual climatology from there to the cyclonic flow around the Costa Rica Dome near 8°N, 92°W. The NECC is a shallow current (found in the upper 200 m) and might appear more strongly if a shallower reference were used, but it is also seasonally intermittent. When the Intertropical Convergence Zone (ITCZ) is in its northern location near 10°N (summer), surface divergences and upwelling create a zonal trough in surface height, driving the NECC along the southern side of the trough. When the ITCZ moves south in winter, the NECC weakens or reverses. 相似文献
16.
The whole core squeezing method was used to simultaneously obtain profiles of nitrous oxide (N 2O), nitrogenous nutrients, and dissolved oxygen in sediments of Koaziro Bay, Japan (coastal water), the East China Sea (marginal sea), and the central Pacific Ocean (open ocean). In the spring of Koaziro Bay, subsurface peaks of interstitial N 2O (0.5–3.5 cm depth) were observed, at which concentrations were higher than in the overlying water. This was also true for nitrate (NO 3−) and nitrite (NO 2−) profiles, suggesting that the transport of oxic overlying water to the depth through faunal burrows induced in situ N 2O production depending on nitrification. In the summer of Koaziro Bay, sediment concentrations of N 2O, NO 3− and NO 2− were lower than in the overlying water. In most East China Sea sediments, both N 2O and NO 3− decreased sharply in the top 0.5–2 cm oxic layer (oxygen: 15–130 μM), which may have indicated N 2O and NO 3− consumption by denitrification at anoxic microsites. N 2O peaks at subsurface depth (0.5–6.5 cm) implied in situ production of N 2O and/or its supply from the overlying water through faunal burrows. However, the occurrence of the latter process was not confirmed by the profiles of other constituents. In the central Pacific Ocean, the accumulation of N 2O and NO 3− in the sediments likely resulted from nitrification. Nitrous oxide fluxes from the sediments, calculated using its gradient at the sediment–water interface and the molecular diffusion coefficient, were −45 to 6.9 nmolN m −2 h −1 in Koaziro Bay in the spring, −29 to −21 nmolN m −2 h −1 in the summer, −46 to 37 nmolN m −2 h −1 in the East China Sea, 0.17 to 0.23 nmolN m −2 h −1 in the equatorial Pacific, and <±0.2 nmolN m −2 h −1 in the subtropical North Pacific, respectively. 相似文献
17.
The Alaskan Stream is the westward boundary current of the North Pacific subarctic gyre. In the central region of the North Pacific, the Alaskan Stream serves as a connection between the Alaskan gyre, Western subarctic gyre and Bering Sea gyre. Its volume transport is very important in estimating the magnitude of the subarctic circulation in the North Pacific. In order to clarify its seasonal and interannual variation, we conducted observations along a north-south section at 180° during June from 1990 to 1997. Moorings were deployed from 1995 to 1997. Hydrographic casts were made at intervals of 37 km to a depth of 3000 m. Moorings were set between CTD stations, with Moor1 (Moor2) at the center (southern edge) of the Alaskan Stream. Geostrophic volume transport (referred to 3000 m) revealed large interannual variability in the Alaskan Stream. Average volume transport over the 8 years was 27.5 × 10 6 m 3s -1 with a standard deviation of 6.5 × 10 6 m 3s -1. Maximum transport was 41.0 × 10 6 m 3s -1 (1997) and minimum was 21.7 × 10 6 m 3s -1 (1995). Stable westward flows were observed at Moor1 1500 m (259°, 11.7 cm s -1) and 3000 m (240°, 3.7 cm s -1, 1996–1997 year average). The ratio of eddy to mean kinetic energy ( KE/
) was very small (<0.6) throughout the year. A relatively weak and unstable westward flow was observed at Moor2 at 3000 m depth. Conversely, the average flow direction at Moor2 5000 m was eastward. 相似文献
18.
Uptake of inorganic carbon and ammonium by the plankton community of three North Carolina estuaries was measured using 14C and 15N isotope methods. At 0% light, C appeared to be lost via respiration, and at increasing light levels uptake of inorganic carbon increased linearly, saturated (mean Ik = 358±30 μEin m −2 s −1), and frequently showed inhibition at the highest light intensities. At 0% light NH 4+ uptake was significantly greater than zero and was frequently equivalent to uptake in the light (light independent); at increasing light levels NH 4+ uptake saturated (mean Ik = 172±44 μEin m −2 s −1) and frequently indicated strong inhibition. Light-saturated uptake rates of inorganic carbon and NH 4+ were a function of chlorophyll a ( r2 = 0·7−0·9); average assimilation numbers were 625 nmol CO 2 (μg chl. a) −1 h −1 and 12·9 nmol NH 4+ (μg chl. a) −1 h −1 and were positively correlated with temperature ( r2 = 0·3−0·7). The ratio of dark to light-saturated NH 4+ uptake tended to be near 1·0 for large algal populations at low NH 4+ concentrations, indicating near light independence of uptake; whereas the ratio was lower for the opposite conditions. These data are interpreted as indicative of nitrogen stress, and it is suggested that uptake of NH 4+ deep in the euphotic zone and at night are mechanisms for balancing the C:N of cellular pools. A 24-h study using summed short-term incubations confirmed this; the cumulative C:N of CO 2 and NH 4+ uptake during the daylight period was 10–20, whereas over the 24-h period the ratio was 6 due to dark NH 4+ uptake. Annual carbon and nitrogen primary productivity were respectively estimated as 24 and 4·0 mol m −2 year −1 for the South River estuary, 42 and 7·3 mol m −2 year −1 for the Neuse River estuary, and 9·6 and 1·6 mol m −2 year −1 for the Newport River estuary. 相似文献
19.
Seasonal and diurnal reduced sulfur gas emissions were measured along a salinity gradient in Louisiana Gulf Coast salt, brackish and freshwater marshes. Reduced sulfur gas emission was strongly associated with habitat and salinity gradient. The dominant emission component was dimethyl sulfide (average: 57·3 μg S m −2 h −1) in saltmarsh with considerable seasonal (max: 144·03 μg S m −2 h −1; min: 1·47 μg S m −2 h −1) and diurnal (max: 83·58 μg S m −2 h −1; min: 69·59 μg S m −2 h −1) changes in flux rates. Hydrogen sulfide was dominant (average: 21·2 μg S m −2 h −1, max: 79·2 μg S m −2 h −1; min: 5·29 μg S m −2 h −1) form in brackishmarsh and carbonyl sulfide (average: 1·09 μg S m −2 h −1; max: 3·42 μg S m −2 h −1; min: 0·32 μg S m −2 h −1) was dominant form in freshwater marsh. A greater amount of H 2S was evolved from brackishmarsh (21·22 μg S m −2 h −1) as compared to the saltmarsh (2·46 μg S m −2 h −1) and freshwater marsh (0·30 μg S m −2 h −1). Emission of total reduced sulfur gases decreased with decrease in salinity and distance inland from the coast. Emission of total reduced sulfur gases over the study averaged 73·3 μg S m −2 h −1 for the saltmarsh, 32·1 μg S m −2 h −1 for brackishmarsh and 2·76 μg S m −2 h −1 for the freshwater marsh. 相似文献
20.
The zooplankton community of the subarctic Pacific is relatively simple, and contains a similar set of major species in all deep water areas of the subarctic Pacific. Their role in the food web varies considerably between coastal and offshore locations. In the oceanic gyres, microzooplankton and other mesozooplankton taxa replace phytoplankton as the primary food source for the dominant mesozooplankton species. Micronekton and larger zooplankton probably replace pelagic fish as major direct predators. Productivity and upper ocean biomass concentrations are intensely seasonal, in part because of seasonality of the physical environment and food supply, but also because of life history patterns involving seasonal vertical migrations (400–2000 m range) and winter dormancy. During the spring–summer season of upper ocean growth, small scale horizontal and vertical patchiness is intense. This can create local zones of high prey availability for predators such as planktivorous fish, birds, and marine mammals. On average, the cores of the subarctic gyres have lower biomass and productivity than the margins of the gyres. There is also some evidence that the Western Gyre is more productive than the Alaska Gyre, but more research is needed to confirm whether this east–west gradient is permanent. There is increasing evidence that the pattern of zooplankton productivity is changing over time, probably in response to interdecadal ocean climate variability. These changes include 2–3 fold shifts in total biomass, 30–60 day shifts in seasonal timing, and 10–25% changes in average body length. 相似文献
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