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1.
Thirteen years (1998–2010) of satellite-measured chlorophyll a are used to establish spatial patterns in climatological phytoplankton biomass seasonality across the California Current System (CCS) and its interannual variability. Multivariate clustering based on the shape of the local climatological seasonal cycle divides the study area into four groups: two with spring-summer maxima representing the northern and southern coastal upwelling zones, one with a summer minimum offshore in mid-latitudes and a fourth with very weak seasonality in between. Multivariate clustering on the seasonal cycles from all 13 years produces the same four seasonal cycle types and provides a view of the interannual variability in seasonal biogeography. Over the study period these seasonal cycles generally appear in similar locations as the climatological clusters. However, considerable interannual variability in the geography of the seasonal cycles is evident across the CCS, the most spatially extensive of which are associated with the 1997–1999 El Niño-Southern Oscillation (ENSO) signal and the 2005 delayed spring transition off the Oregon and northern and central California coasts. We quantify linear trends over the study period in the seasonal timing of the two seasonal cycles that represent the biologically productive coastal upwelling zones using four different metrics of phenology. In the northern upwelling region, the date of the spring maximum is delaying (1.34 days yr−1) and the central tendency of the summer elevated chlorophyll period is advancing (0.63 days yr−1). In the southern coastal upwelling region, both the initiation and cessation of the spring maximum are delaying (1.78 days yr−1 and 2.44 days yr−1, respectively) and the peak is increasing in duration over the study period. Connections between observed interannual shifts in phytoplankton seasonality and physical forcing, expressed as either basin-scale climate signals or local forcing, show phytoplankton seasonality in the CCS to be influenced by changes in the seasonality of the wind mixing power offshore, coastal upwelling in the near-shore regions and basin-scale signals such as ENSO across the study area.  相似文献   

2.
Effective fisheries management needs to consider spatial behavior in addition to more traditional aspects of population dynamics. Acoustic telemetry has been extensively used to provide information on fish movements over different temporal and spatial scales. Here, we used a fixed-receiver array to examine the movement patterns of Labrus bergylta Ascanius 1767, a species highly targeted by the artisanal fleet of Galicia, NW Spain. Data from 25 individuals was assessed for a period of 71 days between September and November 2011 in the Galician Atlantic Islands Maritime-Terrestrial National Park. Fish were present within the monitored area more than 92% of the monitored time. The estimated size of individual home ranges, i.e. the area where fish spent most of their time, was small. The total minimum convex polygons area based on all the estimated positions was 0.133 ± 0.072 km2, whereas the home range size estimated using a 95% kernel distribution of the estimated positions was 0.091 ± 0.031 km2. The core area (50% kernel) was 0.019 ± 0.006 km2. L. bergylta exhibited different patterns of movement in the day versus the night, with 92% of the fish detected more frequently and traveling longer distances during the daytime. In addition, 76% of the fish displayed a larger home range during the day versus during the night. The linearity index was less than 0.005 for all fish suggesting random movements but within a relatively small area, and the volume of intersection index between consecutive daily home ranges was 0.75 ± 0.13, suggesting high site fidelity. The small home range and the sedentary behavior of L. bergylta highlight the potential use of small MPAs as a management tool to ensure a sustainable fishery for this important species.  相似文献   

3.
Recent hydrographic measurements within the eastern South Pacific (1999–2001) were combined with vertically high-resolution data from the World Ocean Circulation Experiment, high-resolution profiles and bottle casts from the World Ocean Database 2001, and the World Ocean Atlas 2001 in order to evaluate the vertical and horizontal extension of the oxygen minimum zone (<20 μmol kg−1). These new calculations estimate the total area and volume of the oxygen minimum zone to be 9.82±3.60×106 km2 and 2.18±0.66×106 km3, respectively. The oxygen minimum zone is thickest (>600 m) off Peru between 5 and 13°S and to about 1000 km offshore. Its upper boundary is shallowest (<150 m) off Peru, shoaling towards the coast and extending well into the euphotic zone in some places. Offshore, the thickness and meridional extent of the oxygen minimum zone decrease until it finally vanishes at 140°W between 2° and 8°S. Moving southward along the coast of South America, the zonal extension of the oxygen minimum zone gradually diminishes from 3000 km (15°S) to 1200 km (20°S) and then to 25 km (30°S); only a thin band is detected at ∼37°S off Concepción, Chile. Simultaneously, the oxygen minimum zone's maximum thickness decreases from 300 m (20°S) to less than 50 m (south of 30°S). The spatial distribution of Ekman suction velocity and oxygen minimum zone thickness correlate well, especially in the core. Off Chile, the eastern South Pacific Intermediate Water mass introduces increased vertical stability into the upper water column, complicating ventilation of the oxygen minimum zone from above. In addition, oxygen-enriched Antarctic Intermediate Water clashes with the oxygen minimum zone at around 30°S, causing a pronounced sub-surface oxygen front. The new estimates of vertical and horizontal oxygen minimum zone distribution in the eastern South Pacific complement the global quantification of naturally hypoxic continental margins by Helly and Levin [2004. Global distribution of naturally occurring marine hypoxia on continental margins. Deep-Sea Research I 51, 1159–1168] and provide new baseline data useful for studies on the role of oxygen in the degradation of organic matter in the water column and the related implications for biogeochemical cycles. Coastal upwelling zones along the eastern Pacific combine with general circulation to provide a mechanism that allows renewal of upper Pacific Deep Water, the most oxygen-poor and oldest water mass of the world oceans.  相似文献   

4.
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   

5.
We conducted a research cruise in late summer (July–August) 2000 to study the effect of mesoscale circulation features on zooplankton distributions in the coastal upwelling ecosystem of the northern California Current. Our study area was in a region of complex coastline and bottom topography between Newport, Oregon (44.7°N), and Crescent City, California (41.9°N). Winds were generally strong and equatorward for >6 weeks prior to the cruise, resulting in the upwelling of cold, nutrient-rich water along the coast and an alongshore upwelling jet. In the northern part of the study area, the jet followed the bottom topography, creating a broad, retentive area nearshore over a submarine shelf bank (Heceta Bank, 44–44.4°N). In the south, a meander of the jet extended seaward off of Cape Blanco (42.8°N), resulting in the displacement of coastal water and the associated coastal taxa to >100 km off the continental shelf. Zooplankton biomass was high both over the submarine bank and offshore in the meander of the upwelling jet. We used velocities and standing stocks of plankton in the upper 100 m to estimate that 1×106 m3 of water, containing an average zooplankton biomass of ～20 mg carbon m?3, was transported seaward across the 2000-m isobath in the meandering jet each second. That flux equated to offshore transport of >900 metric tons of carbon each day, and 4–5×104 tons over the 6–8 week lifetime of the circulation feature. Thus, mesoscale circulation can create disparate regions in which zooplankton populations are retained over the shelf and biomass can accumulate or, alternatively, in which high biomass is advected offshore to the oligotrophic deep sea.  相似文献   

6.
Despite much research on Euphausia superba, estimates of their total biomass and production are still very uncertain. Recently, circumpolar krill databases, combined with growth models and revisions in acoustics have made it possible to refine previous estimates. Net-based databases of density and length frequency (KRILLBASE) yield a summer distributional range of ～19×106 km2 and a mean total abundance of 8×1014 post-larvae with biomass of 379 million tonnes (Mt). These values are based on a standardised net sampling method but they average over the period 1926–2004, during which krill abundance has fluctuated. To estimate krill biomass at the end of last century we combined the KRILLBASE map of relative krill density around Antarctica with an acoustics-derived biomass estimate of 37.3 Mt derived for the Scotia Sea area in 2000 by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). Thus the CCAMLR 2000 survey area contains 28% of the total stock, with total biomass of ～133 Mt in January–February 2000. Gross postlarval production is estimated conservatively at 342–536 Mt yr?1, based on three independent methods. These are high values, within the upper range of recent estimates, but consistent with the concept of high energy throughput for a species of this size. The similarity between the three production estimates reflects a broad agreement between the three growth models used, plus the fact that, for a given population size, production is relatively insensitive to the size distribution of krill at the start of the growth season. These production values lie within the envelope of what can be supported from the Southern Ocean primary production system and what is required to support an estimated predator consumption of 128–470 Mt yr?1. Given the range of recent acoustics estimates, plus the need for precautionary management of the developing krill fishery, our net-based data provide an alternative estimate of total krill biomass.  相似文献   

7.
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   

8.
pH and alkalinity measurements from a coastal upwelling area located near 30°S (Coquimbo, Chile), are used to describe the short-term variations of CO2 air–sea exchanges over a period of one week in summer 1996. A 180 km ocean–coastal transect, together with two almost-synoptic grid surveys off Coquimbo covering approximate 2500 km2 each, showed that during and immediately after a 4 day long southwesterly wind event (24–28 January) a large area of cold surface water (≈14°C), highly supersaturated in CO2 (fCO2 up to 900 μatm), was located near the coast. Three days after the end of the event, the second grid survey showed that in most of the study area the surface temperature and pH had increased significantly (by 1–3°C and 0.05–0.2, respectively), and that the surface water was no longer supersaturated in CO2. The CO2-supersaturated water observed in the first grid survey was identified as upwelled subsurface equatorial water, a water mass with its core at about 200 m depth: the depth from which the water upwells is a major determinant of the surface water fCO2. Integrated C fluxes within a 20 km wide coastal strip (1900 km2) indicate a strong outgassing of CO2 from the ocean under upwelling conditions (Grid 1; 121 t C day-1), while the net C exchange was directed to the ocean during the relaxation period (Grid 2; 19 t C day-1). Estimates of CO2 fluxes in upwelling areas based on surface water fCO2 measurements must therefore take into account these short-term variations: reliance on longer-term averages and interpolation will lead to erroneous results.  相似文献   

9.
Satellite remote sensing offers new means of quantifying particulate organic carbon, POC, concentration over large oceanic areas. From SeaWiFS ocean color, we derived 10-year data of POC concentration in the surface waters of the global ocean. The 10-year time series of the global and basin scale average surface POC concentration do not display any significant long-term trends. The annual mean surface POC concentration and its seasonal amplitude are highest in the North Atlantic and lowest in the South Pacific, when compared to other ocean basins. POC anomalies in the North Atlantic, North Pacific, and global concentrations seem to be inversely correlated with El Niño index, but longer time series are needed to confirm this relationship. Quantitative estimates of POC reservoir in the oceanic surface layer depend on the choice of what should represent this layer. Global average POC biomass is 1.34 g m?2 if integrated over one optical depth, 3.62 g m?2 if integrated over mixed layer depth, and up to 6.41 g m?2 if integrated over 200-m layer depth (when assumed POC concentration below MLD is 20 mg m?3). The global estimate of total POC reservoir in the surface 200-m layer of the ocean is 228.61×1013 g. We expect that future estimates of POC reservoir may be even larger, when more precise calculations account for deep-water organic-matter maxima in oligotrophic regions, and POC biomass located just below the seasonal mixed layer in spring and summer in the temperate regions.  相似文献   

10.
A review of oceanographic properties in the vicinity of Ocean Station Papa (OSP) is presented, using data collected over the past 42 years. Average annual signals at OSP and seasonal characteristics along Line P represent variability on a large scale in the Gulf of Alaska. Between winter and summer, the upper ocean mixed layer varies between 120 and 40 m, monthly average winds decrease from 12 m/s in winter to 7 m/s in July, seawater temperatures warm from lows of 6°C to highs >12°C, waters freshen slightly in summer, and macronutrients are partially depleted by phytoplankton growth (removal of 7.8 μM NO3 in 1970s and 6.5 μM NO3 in 1990s). El Niño events influence this area by transporting heat northward. During the prolonged El Niño of the early 1990s, warming persisted at OSP through 1994, resulting in a reduced macronutrient supply during winter mixing. Changes in water properties over the four decades of observations are evident. There are trends towards warmer and less saline surface waters, lower winter nitrate and silicate levels, and less macronutrient utilisation in the 1990s compared to the 1970s. We speculate that these changes must be reducing the productivity of NE subarctic Pacific waters.  相似文献   

11.
An extended time series of particle fluxes at 3800 m was recorded using automated sediment traps moored at Ocean Station Papa (OSP, 50°N, 145°W) in the northeast Pacific Ocean for more than a decade (1982–1993). Time-series observations at 200 and 1000 m, and short-term measurements using surface-tethered free-drifting sediment traps also were made intermittently. We present data for fluxes of total mass (dry weight), particulate organic carbon (POC), particulate organic nitrogen (PON), biogenic Si (BSi), and particulate inorganic carbon (PIC) in calcium carbonate. Mean monthly fluxes at 3800 m showed distinct seasonality with an annual minimum during winter months (December–March), and maximum during summer and fall (April–November). Fluxes of total mass, POC, PIC and BSi showed 4-, 10-, 7- and 5-fold increases between extreme months, respectively. Mean monthly fluxes of PIC often showed two plateaus, one in May–August dominated by <63 μm particles and one in October–November, which was mainly >63 μm particles. Dominant components of the mass flux throughout the year were CaCO3 and opal in equal amounts. The mean annual fluxes at 3800 m were 32±9 g dry weight g m−2 yr−1, 1.1±0.5 g POC m−2 yr−1, 0.15±0.07 g PON m−2 yr−1, 5.9±2.0 g BSi m−2 yr−1 and 1.7±0.6 g PIC m−2 yr−1. These biogenic fluxes clearly decreased with depth, and increased during “warm” years (1983 and 1987) of the El Niño, Southern Oscillation cycle (ENSO). Enhancement of annual mass flux rates to 3800 m was 49% in 1983 and 36% in 1987 above the decadal average, and was especially rich in biogenic Si. Biological events allowed estimates of sinking rates of detritus that range from 175 to 300 m d−1, and demonstrate that, during periods of high productivity, particles sink quickly to deep ocean with less loss of organic components. Average POC flux into the deep ocean approximated the “canonical” 1% of the surface primary production.  相似文献   

12.
Phytoplankton and bacterial abundance, size-fractionated phytoplankton chlorophyll-a (Chl-a) and production together with bacterial production, microbial oxygen production and respiration rates were measured along a transect that crossed the Equatorial Atlantic Ocean (10°N–10°S) in September 2000, as part of the Atlantic Meridional Transect 11 (AMT 11) cruise. From 2°N to 5°S, the equatorial divergence resulted in a shallowing of the pycnocline and the presence of relatively high nitrate (>1 μM) concentrations in surface waters. In contrast, a typical tropical structure (TTS) was found near the ends of the transect. Photic zone integrated 14C primary production ranged from ∼200 mg C m−2 d−1 in the TTS region to ∼1300 mg C m−2 d−1 in the equatorial divergence area. In spite of the relatively high primary production rates measured in the equatorial upwelling region, only a moderate rise in phytoplankton biomass was observed as compared to nearby nutrient-depleted areas (22 vs. 18 mg Chl-a m−2, respectively). Picophytoplankton were the main contributors (>60%) to both Chl-a biomass and primary production throughout the region. The equatorial upwelling did not alter the phytoplankton size structure typically found in the tropical open ocean, which suggests a strong top-down control of primary producers by zooplankton. However, the impact of nutrient supply on net microbial community metabolism, integrated over the euphotic layer, was evidenced by an average net microbial community production within the equatorial divergence (1130 mg C m−2 d−1) three-fold larger than net production measured in the TTS region (370 mg C m−2 d−1). The entire region under study showed net autotrophic community metabolism, since respiration accounted on average for 51% of gross primary production integrated over the euphotic layer.  相似文献   

13.
The three-dimensional structure of two potential mesoscale upwelling areas that are located in the external waters of the Philippine archipelago (i.e. northwest of Luzon and east of Mindanao) were constructed by analysis of historical data. A unique characteristic of both upwelling sites is that they can be identified by their anomalously cold subsurface temperatures rather than sea surface temperature distributions. As such, they cannot be observed in sea surface temperature fields derived by satellite imagery. The data used in the analysis were obtained from the National Oceanographic Data Center hydrographic database. Objective analysis was performed to produce monthly temperature fields at several standard depths within the upper 500 m of the region 0–30°N and 100–140°E with a horizontal grid resolution of 0.5°. The extent and timing of these upwelling areas are described. A review of existing hypothesis on the mechanisms for their evolution and seasonal modulation are presented. The change in heat content during the upwelling season is greater than 300 W m−2 in both areas. Based on the excursion of isotherms, vertical velocities of 83 cm day−1 and 26 cm day−1 were obtained for upwelling northwest of Luzon and east of Mindanao, respectively.  相似文献   

14.
Cold filaments associated with Eastern Boundary Currents are typically narrower than 100 km but can be several hundred kilometers long, extending from the coast to the open ocean in upwelling areas. One such structure, observed off Penı&#x0301;nsula de Mejillones (23°S, Chile), was studied with both satellite images and two 5-days hydrographic cruises carried out during January 1997. The study used a coastal grid of 31 stations in an area of 165 ×155 km2, approximately. The spatial distribution of the filament and its change between cruises are described from the horizontal distributions of dynamic height, temperature, salinity and dissolved oxygen. The filament was a shallow feature (thickness <100 m) and extended at least 165 km toward the open ocean. A meandering northward current flowed at the borders of the filament, separating oceanic and coastal waters of different physical properties. Comparisons of cross sections of the filament near the coast and in the oceanic zone show the ascent of the shallow salinity minimum (SSM), and its extension toward the ocean, bound to the filament. It is concluded that Subantarctic Water ((SAAW) distinguish by low salinity, high dissolved oxygen) and Equatorial Subsurface Water ((ESSW) high salinity, low dissolved oxygen, high nutrient content) form this filament, and that their relative proportions depend on the strength of the coastal upwelling. Thus, the knowledge of the dynamics of these structures is fundamental to better understanding of the spatial distribution of important biological variables, such as nutrients and chlorophyll, in the coastal ecosystem.  相似文献   

15.
The coastal waters of Peru and Chile are among the most productive of the world's oceans. A striking source of interannual variability in this upwelling ecosystem, El Niño, results in large population and community variations. During El Niño the seasonal upwelling ceases and warm, clear oceanic waters occur close inshore, setting a unique oceanographic scenario in which the performance of populations and communities can be studied. While most attention has been focused on the pelagic components of such systems, numerous changes occur in inshore, benthic populations. Likewise, little attention has been paid to the critical role of humans as predators or active users of inshore, benthic resources. Humans as components of the ecosystem can impose significant alterations on population and community structure. In this paper the fishery statistics of three economically important inshore, benthic resources (the gastropod Concholepas concholepas, the cephalopod Octopus vulgaris and the kelp Lessonia nigrescens) are analysed in relation to the strong 1982/83 El Niño event. In particular, trends are described for landings in the far northern regions of Chile, where the marked effect of El Niño was concurrent with high levels of exploitation of C. concholepas and L. nigrescens.  相似文献   

16.
Data from array for real-time geostrophic oceanography (ARGO) profiling floats, oil tanker thermosalinographs, shipboard ADCP and towed-CTD surveys, and satellite altimetry are used to examine properties of two ∼200 km diameter, anticyclonic Yakutat Eddies that propagated westward at ∼1.5 km day−1 along the continental slope of the northern Gulf of Alaska (GOA) in 2001 and 2003. The eddies had lifetimes of up to 5 years, remained close to the shelfbreak, and had relatively constant size and strength until they encountered the Alaskan Stream where they appeared to spawn smaller, shorter-lived anticyclones. The azimuthal velocity field was vigorous (25–40 cm s−1) and in gradient wind balance with Rossby numbers of ∼0.05. Conservation of salt and azimuthal mass transports (between 20 m and the depth of the 32.2 isohaline) from shipboard surveys in May and August 2003 suggest little mass exchange occurred between the surface layers of the eddy and ambient waters. Chlorophyll concentrations were greater in the eddy than in ambient waters in both May and August. In May, the chlorophyll was patchily distributed, while in August dense chlorophyll concentrations occurred in and beneath the seasonal thermocline within 50 km of the eddy center. The high August chlorophyll concentrations might have been fostered by a broad and shallow (<∼150 m) upwelling of the eddy center between May and August.It appears likely that as Yakutat Eddies approach the shelfbreak non-linear processes will modify the slope flow field (and the stability and structure of the shelfbreak front), leading to cross-slope flows and flow reversals. This interaction may induce ∼30 km-wide streamers of shelf water to flow around the trailing edge of the anticyclone. The role of streamers in the freshwater and nutrient budgets of the GOA shelf and basin is unknown, but their contribution to these budgets will depend on the trajectory of a Yakutat Eddy, especially its proximity to the shelfbreak as the eddy propagates along the GOA continental slope.  相似文献   

17.
We examine the diatom flux collected between November 1996 and April 1998, and between January and October 1999 at the time-series study site in the Cariaco Basin, off Venezuela. The temporal dynamics of the total diatom flux mainly reflect seasonal, trade wind-driven changes in surface hydrographic conditions, including changes associated with the El Niño/Southern Oscillation (ENSO). Highest diatom fluxes (>1.8×107 valves m?2 d?1) coincided with the upwelling season in boreal winters 1997 and 1999. Changes in the composition of the diverse diatom community reflect variations in hydrographic and atmospheric conditions, as well as nutrient availability. Cyclotella litoralis, a neritic diatom typical of nutrient-rich waters, along with resting spores of several Chaetoceros spp., dominate during periods of high diatom flux, following trade wind-driven upwelling. During the boreal summers of 1997 and 1999, nutrient-depleted surface waters resulted in low diatom fluxes (<5.2×106 valves m?2 d?1). The seasonal pattern of high diatom production was altered from July 1997 through April 1998, when the ENSO affected the Caribbean Sea. The occurrence of ENSO during boreal winter 1997–1998 caused a major change in the qualitative composition of the diatom assemblage: the highly diverse diatom assemblage was composed of a mixture of pelagic (Nitzschia bicapitata, Thalassionema nitzschioides var. inflata, T. nitzschioides var. parva, Azpeitia tabularis) and coastal species (C. litoralis, resting spores of Chaetoceros, T. nitzschioides var. nitzschioides). The simultaneous occurrence of neritic and open-ocean diatoms during boreal summers reflects the fact that the Cariaco Basin is influenced by both offshore and coastal waters, with considerable short-term variability in hydrographic conditions and nutrient availability.  相似文献   

18.
Phytoplankton samples were collected from the equatorial Pacific (10°S to 10°N along 155°E) in June 1992 as part of the Australian contribution to the JGOFS program. Chlorophyll and carotenoid pigments were measured by HPLC, and a PC-based computer program (CHEMTAX) was used to estimate the contribution of 9 algal classes to the total chlorophyll a concentration in 9 separate depth bands at each location. This cruise occurred in the middle of the prolonged 1991/1993 El Niño, and the results are compared with similar data from a cruise in October 1990 which occurred before this El Niño but after the 1988/1989 La Niña.Changes in the pigment : chlorophyll a ratios appeared consistent across algal classes and, apart from some minor exceptions, consistent between cruises. Pigments involved in light-harvesting generally increased relative to chlorophyll a with increasing depth, whereas the ratio for photoprotective pigments (e.g. diadinoxanthin) usually decreased with depth. The zeaxanthin concentration per cell for cyanobacteria decreased with depth in the surface 75 m during 1992 as would be expected for a photoprotective pigment.Based on their contribution to the total chlorophyll a concentration, haptophytes, prochlorophytes, cyanobacteria (Synechococcus) and chlorophytes were the dominant algal classes in 1992. The chlorophyte contribution to chlorophyll a in 1992 (14.8%) was almost double that in 1990 (7.8%). This increase was largely at the expense of the cyanobacteria and haptophytes, which both decreased significantly. The increase in chlorophytes in 1992 was particularly noticeable in the surface waters south of the equator at about 4°S, where there was evidence of upwelling.  相似文献   

19.
Plankton community net and gross production and dark respiration were determined from in vitro changes in dissolved inorganic carbon and dissolved oxygen during September 1994 along a southeast offshore transect in the Arabian Sea. Surface rates of gross production decreased from 17±0.7 mmol C m-3 d-1 at a coastal upwelling station to 3±0.8 mmol C m-3 d-1 at the most offshore station. The euphotic zone at the time of sampling was predominantly heterotrophic, with integrated net community production values ranging from 15±7 mmol C m-2 d-1 inshore to −253±32 mmol C m-2 d-1 offshore. Calculations of the respiration attributable to the major plankton groups could account for 61–87% of the dark community respiration measured at the inshore stations, but only 15–26% of the community respiration determined offshore. Comparison of the fluxes of dissolved inorganic carbon and oxygen revealed a tendency for higher respiratory quotients than those calculated for organic metabolism prevailing at the offshore stations.  相似文献   

20.
It is demonstrated that weakened wind mixing and strengthened water column stratification resulted in the anomalously low sea surface chlorophyll in the northern South China Sea during the 1997–1998 El Niño event. Remotely sensed sea surface temperature, wind and chlorophyll, which were validated by shipboard observations at the SouthEast Asian Time-series Study (SEATS) station (18°N, 116°E) in the northern South China Sea (SCS) provided the basis for this study. During the 1997–1998 winter at the SEATS station, the sea surface temperature was elevated by about 2 °C above the climatological mean, while the wind speed of the northeast monsoon was reduced from a climatological mean of 9.4 to 6.8 m/s. The concentration of surface chlorophyll-a dropped from 0.2 to 0.1 mg/m3. The monthly area-averaged integrated primary production estimated for the northern SCS area (112–119°E, 15–21°N) was reduced by about 40% of the normal winter value. Under the anomalously high sea surface temperature and weak monsoon, the mixed-layer depth would have been reduced from an average of 65 to 45 m and the nutrients in the mixed layer would have been reduced by half, according to observations at the SEATS station in more recent years. During the 1997–1998 El Niño event, the onset of warming in the northern SCS lagged behind that in the eastern equatorial Pacific by about 5 months and lingered for 11 months. This course of change resembled that of the western Pacific warm pool region. However, contrary to the northern SCS, the sea surface chlorophyll was enhanced in the warm pool region during the event, probably mainly because of the uplifted nutricline. Unlike the eastern equatorial Pacific, the dramatic recovery of biological production did not happen in the SCS in the summer of 1998. These distinctive biogeochemical responses reflect fundamental differences between the SCS and the equatorial Pacific in terms of upper water column dynamics.  相似文献   

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