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1.
Diatoms, dinoflagellates, coccolithophores, nanoflagellates, picophytoplankton and procaryote algae (Synechococcus spp. and prochlorophytes) were quantified by microscopy and flow cytometry, and their biomass determined, at 12 stations along a 1600 km transect across the Arabian Sea at the end of the SW monsoon in September, and during the inter-monsoon period of November/December 1994. The transect spanned contrasting oceanic conditions that varied from seasonally eutrophic, upwelling waters through mesotrophic, downwelling waters to permanently oligotrophic, stratified waters. The overall diversity of diatoms, dinoflagellates and coccolithophores along the transect was not significantly different between the SW monsoon and inter-monsoon. However, diatoms showed greatest diversity during the SW monsoon and coccolithophores were most diverse during the inter-monsoon. Integrated phytoplankton standing stocks during the SW monsoon ranged from 3 to 9 g C m-2 in the upwelling eutrophic waters, from 3 to 5 g C m-2 in downwelling waters, and from 1 to 2 g C m-2 in oligotrophic waters. Similar phytoplankton standing stocks were found in oligotrophic waters during the inter-monsoon, but were ca. 40% lower compared to the SW monsoon in the more physically dynamic waters. Phytoplankton abundance and biomass was dominated by procaryote taxa. Synechococcus spp. were abundant (often >108 cells l-1) during both the SW monsoon and inter-monsoon, where the nitrate concentration was ⩾0.1 μ mol l-1, and often dominated the phytoplankton standing stocks. Prochlorophytes were restricted to oligotrophic stratified waters during the SW monsoon period but were found at all stations along the transect during the inter-monsoon, dominating the phytoplankton standing stocks (>40%) in the oligotrophic region during this period. Of the nano- and micro-phytoplankton, only diatoms contributed significantly to phytoplankton standing stocks, and then only in near-shore upwelling waters during the SW monsoon. There were significant changes in the temporal composition of the phytoplankton community. In nearshore waters a mixed community of diatoms and Synechococcus spp. dominated during the SW monsoon. This gave way to a community dominated by Synechococcus spp. in the intermonsoon. In the downwelling zone, a Synechococcus spp. dominated community was replaced by a mixed procaryote community of Synechococcus spp. and prochlorophytes. In the oligotrophic stratified waters, the mix of procaryote algae was replaced by one dominated by prochlorophytes alone.  相似文献   

2.
Variations in the nutrient concentrations were studied during two cruises to the Arabian Sea. The situation towards the end of the southwest monsoon season (September/October 1994) was compared with the inter-monsoonal season during November and December 1994. Underway surface transects showed the influence of an upwelling system during the first cruise with deep, colder, nutrient-rich water being advected into the surface mixed layer. During the southwesterly monsoon there was an area of coastal Ekman upwelling, bringing colder water (24.2°C) into the surface waters of the coastal margin. Further offshore at about 350 km there was an area of Ekman upwelling, as a result of wind-stress curl, north of the Findlater Jet axis; this area also had cooler surface water (24.6°C). Further offshore (>1000 km) the average surface temperatures increased to >27°C. These waters were oligotrophic with no evidence of the upwelling effects observed further inshore. In the upwelling regions nutrient concentrations in the close inshore coastal zone were elevated (NO3=18 μmol l-1, PO4=1.48 μmol l-1); higher concentrations also were measured at the region of offshore upwelling off the shelf, with a maximum nitrate concentration of 12.5 μmol l-1 and a maximum phosphate concentration of 1.2 μmol l-1. Nitrate and phosphate concentrations decreased with increasing distance offshore to the oligotrophic waters beyond 1400 km, where typical nitrate concentrations were 35.0 nmol l-1 (0.035 μmol l-1) in the surface mixed layer. A CTD section from the coastal shelf, to 1650 km offshore to the oligotrophic waters, clearly showed that during the monsoon season, upwelling is one of the major influences upon the nutrient concentrations in the surface waters of the Arabian Sea off the coast of Oman. Productivity of the water column was enhanced to a distance of over 800 km offshore. During the intermonsoon period a stable surface mixed layer was established, with a well-defined thermocline and nitracline. Surface temperature was between 26.8 and 27.4°C for the entire transect from the coast to 1650 km offshore. Nitrate concentrations were typically between 2.0 and 0.4 μmol l-1 for the transect, to about 1200 km where the waters became oligotrophic, and nitrate concentrations were then typically 8–12 nmol l-1. Ammonia concentrations for the oligotrophic waters were typically 130 nmol l-1, and are reported for the first time in the Indian Ocean. The nitrogen/phosphorus (N/P) ratios suggest that phytoplankton production was potentially nitrogen-limited in all the surface waters of the Arabian Sea, with the greatest nitrogen limitation during the intermonsoon period.  相似文献   

3.
Variations in the distribution of chemotaxonomic pigments were monitored in the Arabian Sea and the Gulf of Oman at the end of the SW monsoon in September 1994 and during the inter-monsoon period in November/December 1994 to determine the seasonal changes in phytoplankton composition. The Gulf of Oman was characterized by sub-surface chlorophyll maxima at 20-40 m during both seasons, and low levels of divinyl chlorophyll a indicated that prochlorophytes did not contribute significantly to the total chlorophyll a. Prymnesiophytes (19′-hexanoyloxyfucoxanthin), diatoms (fucoxanthin) and chlorophyll b containing organisms accounted for most of the phytoplankton biomass in September, while prymnesiophytes dominated in November/December. In the Arabian Sea in September, high total chlorophyll a concentrations up to 1742 ng l-1 were measured in the coastal upwelling region and a progressive decline was monitored along the 1670 km offshore transect to oligotrophic waters at 8°N. Divinyl chlorophyll a was not detected along this transect except at the two most southerly stations where prochlorophytes were estimated to contribute 25–30% to the total chlorophyll a. Inshore, the dominance of fucoxanthin and/or hexanoyloxyfucoxanthin indicated that diatoms and prymnesiophytes generally dominated the patchy phytoplankton community, with zeaxanthin-containing Synechococcus also being important, especially in surface waters. At the southern oligotrophic localities, Synechococcus and prochlorophytes dominated the upper 40 m and prymnesiophytes were the most prominent at the deep chlorophyll maximum. During the inter-monsoon season, total chlorophyll a concentrations were generally half those measured in September and highest levels were found on the shelf (1170 ng l-1). Divinyl chlorophyll a was detected at all stations along the Arabian Sea transect, and we estimated that prochlorophytes contributed between 3 and 28% to the total chlorophyll a, while at the two oligotrophic stations this proportion increased to 51–52%. While procaryotes were more important in November/December than September, eucaryotes still accounted for >50% of the total chlorophyll a. Pigment/total chlorophyll a ratios indicated that 19′-hexanoyloxyfucoxanthin-containing prymnesiophytes were the dominant group, although procaryotes accounted for 65% at the two southerly oligotrophic stations.  相似文献   

4.
The latitudinal distributions of phytoplankton biomass, composition and production in the Atlantic Ocean were determined along a 10,000-km transect from 50°N to 50°S in October 1995, May 1996 and October 1996. Highest levels of euphotic layer-integrated chlorophyll a (Chl a) concentration (75–125 mg Chl m−2) were found in North Atlantic temperate waters and in the upwelling region off NW Africa, whereas typical Chl a concentrations in oligotrophic waters ranged from 20 to 40 mg Chl m−2. The estimated concentration of surface phytoplankton carbon (C) biomass was 5–15 mg C m−2 in the oligotrophic regions and increased over 40 mg C m−2 in richer areas. The deep chlorophyll maximum did not seem to constitute a biomass or productivity maximum, but resulted mainly from an increase in the Chl a to C ratio and represented a relatively small contribution to total integrated productivity. Primary production rates varied from 50 mg C m−2 d−1 at the central gyres to 500–1000 mg C m−2 d−1 in upwelling and higher latitude regions, where faster growth rates (μ) of phytoplankton (>0.5 d−1) were also measured. In oligotrophic waters, microalgal growth was consistently slow [surface μ averaged 0.21±0.02 d−1 (mean±SE)], representing <20% of maximum expected growth. These results argue against the view that the subtropical gyres are characterized by high phytoplankton turnover rates. The latitudinal variations in μ were inversely correlated to the changes in the depth of the nitracline and positively correlated to those of the integrated nitrate concentration, supporting the case for the role of nutrients in controlling the large-scale distribution of phytoplankton growth rates. We observed a large degree of temporal variability in the phytoplankton dynamics in the oligotrophic regions: productivity and growth rates varied in excess of 8-fold, whereas microalgal biomass remained relatively constant. The observed spatial and temporal variability in the biomass specific rate of photosynthesis is at least three times larger than currently assumed in most satellite-based models of global productivity.  相似文献   

5.
Despite the fact that marine viruses have been increasingly investigated in the last decade, knowledge on virus abundance, biomass and distribution in mesopelagic and bathypelagic waters is limited. We report here the results of a large-spatial-scale study (covering more than 3000 km) on the virioplankton distribution in epi-, meso- and bathypelagic waters in 19 areas of the Mediterranean Sea, from the Alboran Sea and Western Mediterranean, to the Tyrrhenian Sea, Sicily Channel and Ionian Sea. Integrated viral abundance in epipelagic waters was significantly higher than in deep-sea waters (on average, 2.4 vs. 0.5×1012 viruses m−3). However, abundance of viruses in the deep-Mediterranean waters was the highest reported so far for deep seas worldwide (7.0 and 3.1×1011 viruses m−3 in mesopelagic and bathypelagic waters, respectively) and their biomass accounted for 13–18% of total prokaryotic C biomass. The significant relationship between viral abundance and prokaryotic abundance and production in deep waters suggests that also deep-sea viruses are closely dependent on the abundance and metabolism of their hosts. Moreover, virus to prokaryote (and nucleoid-containing cell (NuCC)) abundance ratio increased with increasing depths suggesting that deep waters may represent optimal environments for viral survival or proliferation. Overall, our results indicate that deep waters may represent a significant reservoir of viruses and open new perspectives for future investigations of viral impact on the functioning of meso-bathypelagic ecosystems.  相似文献   

6.
A one-dimensional quasi-Eulerian model, describing the biological and chemical interactions of autotrophic and heterotrophic plankton populations within the upper 100 m of the water column, was used to explain the relative impact of iron fertilization from Saharan dust on phosphorus cycling by the cyanobacterium, Trichodesmium spp. We examined the Caribbean surface waters off the west coast of Barbados using seasonal cases of dust delivery (summer peak) in relation to periods of elevated phosphorus stocks found in episodic pulses of Amazon River water advected past Barbados by the Guiana Current. The additional iron and phosphorus supplied by fluvial sources, available after biological depletion of nitrogen, could alleviate the growth limitation of Trichodesmium far downstream of the river plumes. The model results were compared with measurements made of Trichodesmium stocks 8 km off the west coast of Barbados. Modeled concentrations within the low-salinity Amazon plumes reached ∼7.4 mg chl m−2, as seen at Barbados. In the absence of the low-salinity signal, the Trichodesmium biomass shifted to phosphorus limitation with little response to iron fertilization. This indicates that Trichodesmium stocks may indeed be mainly a function of phosphorus availability within the upper water column. Therefore, the supply of phosphorus from subtropical/tropical rivers, such as the Amazon and Orinoco, limits the potential cyanophyte response, despite adequate atmospheric iron delivery. This alters our view not only of the western North Atlantic nitrogen budget, but potentially several regions downstream of low N:P river systems.  相似文献   

7.
Zooplankton communities, studied in the surface mixed layer on a 1600 m transect across the Arabian Sea, were found to differ in their temporal and spatial response to seasonal forcing. The transect studied, spanned seasonally eutrophic upwelling, mesotrophic downwelling and aseasonal oligotrophic waters. The nano- and microzooplankton communities constituted a relatively constant compartment in the tropical monsoon ecosystem, whilst the mesozooplankton showed a clear response to both upwelling and season. The heterotrophic nanoflagellates were concentrated in the surface mixed layer, except in the eutrophic upwelling waters of the SW monsoon. They reached maximum cell concentrations of 855 ml-1 during the SW monsoon and a maximum biomass of 8.4 mg C m-3 during the intermonsoon. Nanozooplankton standing stocks, in the surface mixed layer, ranged between 7 and 333 mg C m-2, with highest stocks found during the intermonsoon. The microzooplankton community was dominated by Protozoa, particularly aloricate ciliates and heterotrophic dinoflagellates, which accounted for up to 99% in terms of numbers and up to 71% of the biomass. Sarcodines and metazoan nauplii were recorded in lower numbers (<400 l-1). The microzooplankton were also concentrated in the surface mixed layer during both periods, except in the eutrophic coastal waters during the SW monsoon, when relatively high biomass values were found below the mixed layer depth. Their standing stocks, in the surface mixed layer, ranged between 50 and 182 mg C m-2, with the highest concentration found in the mesotrophic offshore waters during the late monsoon period. Total mesozooplankton standing stocks, in the surface 100 m, decreased with distance from the coastal to offshore waters and between seasons, decreasing from 1248 to 238 mg C m-2 during the late SW monsoon and 656–89 mg C m-2 during the following intermonsoon. The largest size class, of 1000–2000 μm sized organisms, dominated throughout except at the oligotrophic station during the intermonsoon period, when the smallest class, of 200–500 μm, were more important. The shift in size structure from large to small zooplankton occurred in response to a shift in dominance from large to small phytoplankton cells both spatially, along a eutrophic–oligotrophic gradient, and seasonally. These responses are a result of the physical forcing associated with the monsoon seasons in the Arabian Sea.  相似文献   

8.
Microzooplankton herbivory in the Arabian Sea was measured using dilution experiments towards the end of the SW monsoon in September and during the intermonsoon to NE monsoon period in November–December 1994. Microzooplankton grazing resulted in a turnover of phytoplankton stocks that ranged from 11 to 49% per day. This was equivalent to grazing fluxes of between 1 and 17 mg C m-3 d-1. Depth-integrated microzooplankton herbivory ranged between 161 and 415 mg C m-2 d-1 during the SW monsoon cruise, and between 110 and 407 mg C m-2 d-1 during the intermonsoon period. Microzooplankton grazed between 4 and 60% of daily primary production, with higher percentages found during the intermonsoon season. Phytoplankton growth rates during the SW monsoon ranged from 0.3 to 1.8 d-1, with lower values in upwelling waters and higher values in downwelling and oligotrophic areas. During the intermonsoon period, phytoplankton growth was more uniform across the basin and averaged 0.68±0.15 d-1. Microzooplankton abundance in experimental samples varied between 2800 and 16 162 cells l-1, equivalent to a biomass of between 1.1 and 7.2 mg C m-3. The mean cell carbon content of microzooplankton was similar in both periods and ranged from 0.33 to 0.55 ng C cell-1. Microzooplankton were smallest in downwelling waters and largest in oligotrophic waters. Average clearance rates in those taxa that took up fluorescently-labelled algae ranged from 0.2 to 14 μl ind-1 hr-1. Average mesozooplankton grazing rates, derived from biomass data, varied from 19 to 92 mg C m-2 d-1; these rates accounted for removal of between 4 and 12% of the daily primary production. Mesozooplankton herbivory was most pronounced in upwelling and downwelling waters and reduced in stratified oligotrophic waters during the SW monsoon period. Microzooplankton herbivory was greater than the average mesozooplankton herbivory at all stations, during both the SW monsoon and intermonsoon periods.  相似文献   

9.
Thermocline ventilation rates for the subtropical North Pacific are determined using a 1-dimensional (meridional) along-isopycnal advective–diffusive model tuned to chlorofluorocarbon (CFC) concentrations measured along 152°W in 1991 during WOCE P16. Mean southward advection rates in the subtropics range from 1.03 to 0.56 cm s-1 between σθ=25.5 and 26.6. Model-derived ventilation times for the subtropical gyre increase from about 10 to 27 years for that isopycnal range. Oxygen utilization rates (OURs) determined using the advective-diffusive model decrease with depth from 6.6 to 3.2 μmol kg-1 yr-1 between σθ=25.5 and 26.6. Extrapolation of the OUR versus depth trend to the base of the euphotic zone with the 1/Z power function of Martin et al. (1987) and integration from 500 to 100 m depth implies a carbon export rate from the overlying euphotic zone of 2.2±0.5 moles C m-2 yr-1 at 30°N, 152°W. Analysis of the WOCE radiocarbon and salinity distributions indicates that zonal and cross-isopycnal transport terms would have to be considered in modeling these tracers in the subtropical North Pacific.  相似文献   

10.
The water mass structure and circulation of the continental shelf waters west of the Antarctic Peninsula are described from hydrographic observations made in March–May 1993. The observations cover an area that extends 900 km alongshore and 200 km offshore and represent the most extensive hydrographic data set currently available for this region. Waters above 100–150 m are composed of Antarctic Surface Water and its end member Winter Water. Below the permanent pycnocline is a modified version of Circumpolar Deep Water, which is a cooled and freshened version of Upper Circumpolar Deep Water. The distinctive signature of cold and salty water from the Bransfield Strait is found at some inshore locations, but there is little indication of significant exchange between Bransfield Strait and the west Antarctic Peninsula shelf. Dynamic topography at 200 m relative to 400 m indicates that the baroclinic circulation on the shelf is composed of a large, weak, cyclonic gyre, with sub-gyres at the northeastern and southwestern ends of the shelf. The total transport of the shelf gyre is 0.15 Sv, with geostrophic currents of order 0.01 m s-1. A simple model that balances across-shelf diffusion of heat and salt from offshore Upper Circumpolar Deep Water with vertical diffusion of heat and salt across the permanent pycnocline into Winter Water is used to explain the formation of the modified Circumpolar Deep Water that is found on the shelf. Model results show that the observed thermohaline distributions across the shelf can be maintained with a coefficient of vertical diffusion of 10-4 m2 s-1 and horizontal diffusion coefficients for heat and salt of 200 and 1200 m2 s-1, respectively. When the effects of double diffusion are included in the model, the required horizontal diffusion coefficients for heat and salt are 200 and 400 m2 s-1, respectively.  相似文献   

11.
The capacity of filter feeders to reduce seston and phytoplankton concentrations in the water column has important implications for restoration and management of coastal ecosystems. We directly measured changes in chlorophyll a concentration on commercially stocked intertidal oyster beds (Crassostrea gigas) in Willapa Bay, Washington, USA by recording water properties near small drifters as they tracked parcels of water across tide flats. Chlorophyll declined 9.6% per half hour in water passing on-bottom adult oysters and 41% for longline adult oysters, whereas chlorophyll concentrations increased as water flowed across tide flats without adult oysters. Field filtration rates, which were fit to exponential declines in chlorophyll and accounted for oyster density and water depth, averaged 0.35 L g− 1 h− 1 (shucked dry weight) for on-bottom aquaculture and 0.73 L g− 1 h− 1 for longline culture, compared to values of 2.5–12 L g− 1 h− 1 reported from laboratory studies of C. gigas. Field filtration rates may be lower than laboratory rates due to unfavorable field conditions (e.g., low initial chlorophyll concentrations) or masked by resuspension of benthic microalgae. In addition to distinctions among on-bottom, longline, and no-oyster habitats, Akaike's Information Criterion analysis showed temperature, initial chlorophyll concentration, and depth related to chlorophyll decline. This research corroborates mathematical models suggesting that benthic suspension feeders are exerting top-down control of pelagic production in this estuary, with strong patterns in chlorophyll emerging across extensive tideflats populated by C. gigas despite low field filtration rates.  相似文献   

12.
The mesoscale dynamics of the Scottish side of the Faroe–Shetland Channel have been investigated using synoptic in situ and remote sensing observations. A cold core cyclonic eddy, identified from an AVHRR image, had a diameter of about 50 km and surface current speeds of up to 50 cm s-1; it appeared to be attached to the 800 m isobath as it moved north-eastward along the edge of the channel at about 8 cm s-1. Speeds in the slope current were about 50 cm s-1 but increased to 70 cm s-1 where the current was compressed by the eddy. Offshore, over the 1000 m isobath in the cooler water, speeds in the current were slower (ca. 20 cm s-1). North-west of the Shetlands the offshore edge of the slope current was deflected across the channel for a distance of about 70 km from the shelf edge. The speed of drifters in the slope current increased to over 60 cm s-1 as they moved anti-cyclonically around this deflection. CTD profiles suggest that the movement of the surface waters was mirrored in the deep water of the channel. The deflection carried a very large quantity of North Atlantic Water into the central part of the channel; its cause and ultimate fate are not known, although it is likely to have had a significant impact on the dynamics of the channel.  相似文献   

13.
We determined the distributions and fluxes of methyl chloride and methyl bromide in the East China Sea (ECS) and the Southern Yellow Sea (SYS) in November 2007. Methyl chloride and methyl bromide concentrations in the surface waters ranged from 47.1 to 163 pmol L?1 and from 0.70 to 9.82 pmol L? 1, with average values of 87.6 and 2.97 pmol L? 1, respectively. The distributions of the two methyl halides were clearly influenced by the Yangtze (Changjiang) River effluent and Kuroshio water, with high concentrations appearing in the coastal zone and low values occurring in the open waters. A positive linear correlation was observed between methyl chloride and methyl bromide concentration anomalies in the surface waters, suggesting that they may share some origins in this coastal area. However, no correlation was found between the two methyl halide concentration anomalies and chlorophyll a in the surface waters. The vertical profiles of the two methyl halides were characterized by the maxima in the upper mixed layer. Both gases were generally supersaturated in the surface seawater, with mean sea-to-air fluxes of methyl chloride and methyl bromide of 391 and 20.0 nmol m?2 d? 1, respectively.  相似文献   

14.
Chlorophyll a (chl a) concentrations and primary production by the 0.2–2, 2–18 and >18 μm phytoplankton size-fractions were estimated along a transect in the NW Indian Ocean extending from the coast of Oman to 8°N 68°E during the late SW monsoon and autumn intermonsoonal seasons in 1994. Primary production was estimated using the 14C technique with either in situ or simulated in situ incubations. During the late monsoon season, maximal chl a and production values were recorded in the coastal upwelling zone with values of 69 mg m-2 and 3800 mg C m-2 d-1, respectively. The maxima, which were distributed patchily in this region, were dominated by the >18 μm size-fraction. Over the remainder of the transect chl a concentrations and production averaged 30 mg m-2 and 1500 mg C m-2 d-1, respectively, with approximately equal contributions by the three size-fractions in the case of chl a at the majority of stations, but in general, with a maximum in production in the 0.2–2 μm fraction. Immediately following cessation of the SW monsoon wind, chl a and production values over the northern part of the transect decreased to values similar to those over the southern part of the transect at the time of the SW monsoon, with the contributions by the three size-fractions being approximately equal. During the following intermonsoonal season, both chl a concentrations and production across the section were dominated by the 0.2–2 μm size-fraction, with average chl a and production values of the order of 20 mg m-2 and 750 mg C m-2 d-1, respectively. Considerable variation in production values, however, was exhibited across the transect. A clearly defined subsurface chl a maximum was only recorded at the southernmost stations of the transect in oligotrophic waters: the feature did not develop universally across the transect during the intermonsoon.  相似文献   

15.
We tested the idea that bacterial cells with high nucleic acid content (HNA cells) are the active component of marine bacterioplankton assemblages, while bacteria with low nucleic acid content (LNA cells) are inactive, with a large data set (>1700 discrete samples) based on flow cytometric analysis of bacterioplankton in the Northeast Pacific Ocean off the coast of Oregon and northern California, USA. Samples were collected in the upper 150 m of the water column from the coast to 250 km offshore during 14 cruises from March 2001 to September 2003. During this period, a wide range of trophic states was encountered, from dense diatom blooms (chlorophyll-a concentrations up to 43 μg l−1) at shelf stations during upwelling season (March–September) to lower chlorophyll-a concentrations (0.1–5 μg l−1) during winter (November–February) and at basin stations (>1700 m depth). We found only weakly positive relations of log total bacterial abundance to log chlorophyll-a concentration (as a proxy for availability of organic substrate), and of HNA bacteria as a fraction of total bacteria to log chlorophyll-a. Abundance of HNA and LNA bacteria co-varied positively in all regions, although HNA bacteria were more responsive to high phytoplankton biomass in shelf waters than in slope and basin waters. Since LNA cell abundance in general showed responses similar to those of HNA cell abundance to changes in phytoplankton biomass, our data do not support the hypothesis that HNA cells are the sole active component of marine bacterioplankton.  相似文献   

16.
Between November 2001 and March 2002 an Australian/Japanese collaborative study completed six passes of a transect line in the Seasonal-Ice Zone (south of 62°S) along 140°E. Zooplankton samples were collected with a NORPAC net on 22–28 November, and a Continuous Plankton Recorder on 10–15 January, 11–12 February, 19–22 February, 25–26 February, and 10–11 March. Zooplankton densities were lowest on 22–28 November (ave=61 individuals (ind) m−3), when almost the entire transect was covered by sea ice. By 10–15 January sea surface temperature had increased by ∼2 °C across the transect line, and the study area was ice-free. Total zooplankton abundance had increased to maximum levels for the season (ave=1301 ind m−3; max=1979 ind m−3), dominated by a “Peak Community” comprising Oithona similis, Ctenocalanus citer, Clausocalanus laticeps, foraminiferans, Limacina spp., appendicularians, Rhincalanus gigas and large calanoid copepodites (C1–3). Total densities declined on each subsequent transect, returning to an average of 169 ind m−3 on 10–11 March. The seasonal density decline was due to the decline in densities of “Peak Community” taxa, but coincided with the rise of Euphausia superba larvae into the surface waters, increased densities of Salpa thompsoni, and an increased contribution of C4 to adult stages to the populations of Calanoides acutus, Calanus propinquus and Calanus simillimus. The seasonal community succession appeared to be influenced by the low sea ice extent and southward projection of the ACC in this region. The relatively warm ACC waters, together with low krill biomass, favoured high densities of small grazers during the January/February bloom conditions. The persistence of relatively warm surface waters in March and the seasonal decrease in chlorophyll a biomass provided favorable conditions for salps, which were able to penetrate south of the Southern Boundary.  相似文献   

17.
Particulate organic carbon (POC) concentrations from 0 to 1000 m were quantified in size-fractionated particulate matter samples obtained by the multiple unit large volume in situ filtration system (MULVFS) in 1996 and 1997 along the 1600 km long “line P” transect from continental slope waters near southern Vancouver Island to Ocean Station PAPA (OSP, 50°N, 145°W). Regression of in situ POC vs. beam attenuation coefficient, c, from a simultaneously deployed 1-m pathlength SeaTech transmissometer gave slope, intercept and r2 values of 6.15±0.19×10−5 m−1 (nmol C l−1)−1, 0.363±0.003 m−1, and 0.951 (n=145), respectively. This result agreed within several percent of calibrations obtained from two 2600-km-long transects of the equatorial Pacific in 1992 (Bishop, 1999). Data from other, more frequently deployed transmissometers were standardized against the 1-m instrument, and the combined optical data set was used to document POC variability at finer spatial and temporal scales than could be sampled directly using either conventional water bottle casts or MULVFS. Published bottle POC vs. c relationships show much more variability and remain problematic. Along the line P transect in the salinity-stratified upper 100 m, POC isolines shoaled from winter to summer in concert with seasonal stratification. At the same time, POC was progressively enriched in subeuphotic zone waters to depths greater than 500 m. Near-surface POC fields sampled in the winter time showed strong temporal POC variability over time scales of days as well as between years. POC concentrations at OSP in February 1996 were higher than those found at any other time of year. Less variability was found along line P in other seasons. In May 1996, kilometer-scale spatial variability of POC at OSP was small; dawn vs. dusk variations of c were used to calculate 0–100 m POC turnover times shorter than 6 d. Calculations also suggest that 25–50% of primary productivity was expressed as dissolved organic carbon at OSP in May 1996.  相似文献   

18.
Between 2 and 3 km depth, North Pacific deep waters contain a plume of water with high silicic acid concentrations. The plume extends outward from Cascadia Basin (the Washington Margin), where waters can contain in excess of 200 μM off the coast of Oregon and Washington. To identify the source of the high Si concentrations in Cascadia Basin, we measured silicic acid and germanium concentrations in deep waters, and their fluxes from sediments using incubated cores. The mean flux of silicic acid into bottom waters is 0.81±0.05 mmol/m2-day, and the Ge/Si ratio of this flux is 0.7±0.1 μmol/mol. A box model, incorporating these results with hydrographic data, indicates that (1) no more than 5% of the silicic acid added to Basin deep waters can have a hydrothermal source (either hot or warm seeps), and (2) the total input of silicic acid to Basin deep waters is 0.06±0.02 Tmol/y. This input is nearly all from remineralized biogenic debris and should contribute about 0.5% of the 14 Tmol/y that are estimated to be necessary to maintain the North Pacific plume.  相似文献   

19.
A time-series sediment trap was deployed from October 2007 to May 2011 in the western subtropical Pacific with the aim of understanding the seasonal and inter-annual variability on particle flux in response to El Niño-Southern Oscillation (ENSO) events. Total mass fluxes varied from 3.04 mg m−2 day−1 to 31.1 mg m−2 day−1, with high fluxes during February–April and low fluxes during other months. This seasonal variation was also characterized by a distinct change in the CaCO3 flux between the two periods. The marked increase in particle flux during February–April may be attributed to enhanced biological productivity in surface waters caused by strong wind-driven mixing in response to the western North Pacific monsoon system. The 2009/10 strong El Niño was accompanied by a significant reduction in particle flux, whereas the La Niña had no recognizable effect on particle flux in the subtropical Pacific. In particular, in the mature phase of the 2009/10 strong El Niño, the fluxes of organic carbon and biogenic silica decreased by 70–80% compared with those during the normal period, implying that the El Niño acted to suppress biological productivity in surface waters. The suppression of biological productivity during the 2009/10 strong El Niño is attributed to the decrease in precipitation due to the shift in the western Pacific warm pool. This finding is opposite that of other studies of the western equatorial Pacific, where El Niño events were observed to result in an increase in biological productivity and particle flux. The difference in particle flux between the western equatorial and subtropical Pacific is attributed to the regional differences in oceanic and atmospheric circulation systems generated by the strong El Niño.  相似文献   

20.
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   

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