US Joint Global Research Articles

Grazing control and iron limitation of primary production in the Arabian Sea: Implications for anticipated shifts in Southwest Monsoon intensity

The Arabian Sea is important in global biogeochemistry because of its high rates of primary production, its extensive zone of oxygen depletion and denitrification, and its expected strong response to global warming via ocean-atmosphere feedbacks to monsoon winds and upwelling intensity. Monsoon-driven upwelling leads to higher rates of primary production during the summer. However, the Arabian Sea paradoxically produces a weak and delayed phytoplankton response compared to other physically dynamic upwelling systems, and indeed has many of the characteristics of a high nutrient low chlorophyll (HNLC) regime. Thick blooms of large diatoms are uniquely not evident in the Arabian Sea despite apparently abundant nutrients (nitrogen, phosphorus and silicate), seed stocks and hydrographic conditions that appear conducive. The subdued bloom response shifts carbon export offshore of the coastal upwelling area and delays major flux events until the latter stages of the SW Monsoon.While the US Joint Global Ocean Flux Study in the Arabian Sea concluded that these observations can be explained by top-down control of primary production by grazers, recent data suggest that primary production may be limited by iron, consistent with other upwelling regimes. In this review, we examine the evidence for each hypothesis and offer some explanations that are consistent with both control mechanisms. We also assess how each hypothesis might explain how shifts in monsoon intensity and duration associated with climate change will affect Arabian Sea biogeochemistry. A strengthening monsoon will exacerbate iron limitation. This will lead to an eastward shift in the utilization of upwelled nutrients and intensify the oxygen minimum zone. A weakening monsoon will probably make iron limitation less important in the system.

Broad scale patterns in mesozooplankton biomass and grazing in the eastern equatorial Pacific

We investigated biomass distributions and grazing rates of mesozooplankton in the eastern equatorial Pacific between 110°-140°W and 4°S-4°N during cruises in December 2004 (EB04) and September 2005 (EB05). Median (±SE) euphotic zone estimates of zooplankton biomass, collected with a 200-μm mesh net, varied from 2.27±0.24 g dry weight m −2 during EB04 to 3.13±0.22 g dry weight m −2 for EB05 (however, when stations from overlapping regions were compared, no significant differences were found between years). Trends in gut fluorescence estimates of mesozooplankton grazing followed biomass, with significantly higher median rate estimates during EB05 (3.39±0.32 mg pigment m −2 d −1) than during EB04 (2.31±0.34 mg pigment m −2 d −1). Spatial gradients in mesozooplankton biomass and grazing on meridional transects sampled at 110°W in 2004 and 140°W in 2005 could be interpreted as either in situ growth/grazing responses or downstream advective flows relative to spatial patterns in phytoplankton. The present zooplankton biomass estimates for the equatorial Pacific are 80–90% higher than those from similar measurements made by the US Joint Global Ocean Flux Studies EqPac Program in 1992. Our grazing rates similarly exceed EqPac estimates by a factor of 2 or 3, in absolute terms and as percent of phytoplankton biomass consumed daily (11% - EB04; 14% - EB05). Although the equatorial region has not been regularly sampled between EqPac and the present study, both the magnitude and the direction of the observed changes are consistent with the documented decadal increase in mesozooplankton biomass in the adjacent North Pacific Subtropical Gyre based on monthly sampling at Stn. ALOHA, as well as an increase in the strength of the trade winds. These results may be indicative of a general shift up in productivity or community size structure and role of mesozooplankton in the open-ocean tropical/subtropical Pacific, and they provide important time points for validating the performance of ecosystem models of the region.

Vertical zonation and distributions of calanoid copepods through the lower oxycline of the Arabian Sea oxygen minimum zone

This paper provides the first comprehensive analysis of calanoid copepod vertical zonation and community structure at midwater depths (300–1000 m) through the lower oxygen gradient (oxycline) (0.02 to ∼0.3 ml/L) of an oxygen minimum zone (OMZ). Feeding ecology was also analyzed. Zooplankton were collected with a double 1 m 2 MOCNESS plankton net in day and night vertically-stratified oblique tows from 1000 m to the surface at six stations during four seasons as part of the 1995 US Joint Global Ocean Flux Study (JGOFS) Arabian Sea project. The geographic comparison between a eutrophic more oxygenated onshore station and an offshore station with a strong OMZ served as a natural experiment to elucidate the influence of depth, oxygen concentration, season, food resources, and predators on the copepod distributions. Copepod species and species assemblages of the Arabian Sea OMZ differed in their spatial and vertical distributions relative to environmental and ecological characteristics of the water column and region. The extent and intensity of the oxycline at the lower boundary of the OMZ, and its spatial and temporal variability over the year of sampling, was an important factor affecting distributional patterns. Calanoid copepod species showed vertical zonation through the lower OMZ oxycline. Clustering analyses defined sample groups with similar copepod assemblages and species groups with similar distributions. No apparent diel vertical migration for either calanoid or non-calanoid copepods at these midwater depths was observed, but some species had age-related differences in vertical distributions. Subzones of the OMZ, termed the OMZ Core, the Lower Oxycline, and the Sub-Oxycline, had different copepod communities and ecological interactions. Major distributional and ecological changes were associated with surprisingly small oxygen gradients at low oxygen concentrations. The calanoid copepod community was most diverse in the most oxygenated environments (oxygen >0.14 ml/L), but the rank order of abundance of species was similar in the Lower Oxycline and Sub-Oxycline. Some species were absent or much scarcer in the OMZ Core. Two copepod species common in the Lower Oxycline were primarily detritivorous but showed dietary differences suggesting feeding specialization. The copepod Spinocalanus antarcticus fed primarily on components of the vertical particulate flux and suspended material, a less versatile diet than the co-occurring copepod Lucicutia grandis. Vertical zonation of copepod species through the lower OMZ oxycline is probably a complex interplay between physiological limitation by low oxygen, potential predator control, and potential food resources. Pelagic OMZ and oxycline communities, and their ecological interactions in the water column and with the benthos, may become even more widespread and significant in the future ocean, if global warming increases the extent and intensity of OMZs as predicted.

A decade of synthesis and modeling in the US Joint Global Ocean Flux Study

Abstract A decade-long Synthesis and Modeling Project (SMP) was conducted as the final element of the US Joint Global Ocean Flux Study (JGOFS). The SMP goal was to synthesize knowledge gained from field studies into a set of models that reflect our current understanding of the oceanic carbon cycle. Specific, innovative aspects of the project included the close partnership among scientists conducting field, laboratory, remote sensing, and numerical research and the strong emphasis on data management and web-based, public release of models and data products. Several recurrent science themes arose across the SMP effort including: the development of a new generation of ocean ecosystem and biogeochemistry models that include iron limitation, flexible elemental composition, size structure, geochemical functional groups, and particle composition; the application of inverse models and data assimilation techniques to marine food-web data; the creation of whole-ocean synthesis products from the JGOFS global CO 2 survey and other studies; and the analysis and modeling of ecosystem and biogeochemical responses to climate and CO 2 system perturbations on time-scales ranging from seasonal and inter-annual variability to anthropogenic climate warming and longer.

The US JGOFS data management experience

Abstract The US Joint Global Ocean Flux Study (JGOFS) database management system is an online, oceanographic database assembled from the research activities of the US JGOFS field and Synthesis and Modeling Program (SMP). It is based on modern, object-oriented programming, with a web-based user interface ( http://usjgofs.whoi.edu/jg/dir/jgofs ) that gives all users, regardless of the computer platform being used, equal access to the data and metadata. It is populated with an extensive set of biogeochemical data from the US JGOFS community along with the attendant metadata. This article summarizes the lessons learned that may serve as a primer for future oceanographic and earth science research programs. Good data management requires devoted resources, about 5–10% of the total cost of the program. A data management office should be established at the initiation of the program, conventions for standard methods, names, and units need to be established before the field program begins, and an agreed-to list of metadata must be collected systematically along with the data. Open and accessible data management depends upon investigators agreeing to share their data with each other, leading to more rapid scientific discovery. Data management should support data distribution and archival; interactions between the data managers and the principal investigators make the database a living database. Innovative use of commercial products in information technology can save time and money in scientific database management. Technology allows access to the database to be transparent in location and intuitive in use. Finally, the most important investments in data management are the people hired.

Temporal Variations in the Dissolved Nutrient Stocks in the Surface Water of the Western North Atlantic Ocean

Changes in patterns of undetectability and molar ratios of dissolved nutrients in the euphotic zone of the oligotrophic western North Atlantic Ocean were investigated utilizing the Bermuda Atlantic Time-series Study (BATS) data set of the US Joint Global Ocean Flux Study (JGOFS). Our aim was to examine the temporal dynamics of nutrient stocks over a decade (1989∼1998) and to gain insight into the interactions between the different biotic and abiotic factors underlying BATS. Patterns of nutrient undetectability clearly revealed the depleted nature of the nutrients in surface water at the BATS location, particularly phosphorous. The N:P ratio was consistently far above the nominal Redfield ratio (mean, 38.5) but was significantly lower during the 1993∼1994 period (22.1). Over the same period the proportion of samples depleted in N only increased while the proportion of samples depleted in P only decreased. This indicates an overall reduction of N relative to P in the surface water at BATS during the 1993∼1994 period, the reasons for this anomaly, though, are not clear. The correlation analysis between the biotic and abiotic variables at BATS has indicated some interesting relationships that can help understand some of the parameters affecting nutrient stocks in the euphotic zone and their consequent impacts on marine biota. Although nutrient stocks in the oligotrophic environment are limited, they might be subject to interannual variation that may become anomalous in some cases. These variations might underlay significant feedback mechanisms by affecting marine productivity, the prime factor controlling the sequestration of atmospheric CO2 by the oceans.

Mesopelagic microplankton of the Arabian Sea

The Arabian Sea is notable for its dramatic monsoonal effects on euphotic zone biogeochemical processes and the large spatial extent of its mesopelagic oxygen minimum zone. As part of the US Joint Global Ocean Flux Study Arabian Sea project, we sampled microplankton (organisms 20–200 μm including diatoms, dinoflagellates, ciliates, sarcodines and nauplii) at five depths from 250 to 1000 or 1100 m at six stations during four seasonal cruises in 1995. Abundances of groups of organisms at discrete depths averaged 1–2 l −1 seasonally. Mean seasonal integrated biomass of the assemblage was 29 mg C m −2 during the late Northeast Monsoon, 37 mg C m −2 during the Spring Intermonsoon, 47 mg C m −2 during the late Southwest Monsoon and 49 mg C m −2 during the early Northeast Monsoon. Overall, protozoans dominated the mesopelagic microplankton assemblage. Integrated biomass peaked during the late SW Monsoon at two stations as expected if microplankton responded to surface productivity and mesopelagic organic carbon fluxes. At three stations, microplankton biomass peaked during the early NE Monsoon; this may reflect a continuing response to SW Monsoon productivity signals by these larger, slow-growing organisms. Protozooplankton abundance did not appear to be negatively affected by low (<0.1 ml dissolved O 2 l −1) oxygen, whereas naupliar abundance and biomass were higher where oxygen concentration was higher. Total microplankton biomass was highest where oxygen concentrations and also mesozooplankton biomass were lowest, suggesting that predation also played a role in microplankton distributions. Calculations based on allometric relationships indicated that the mesopelagic heterotrophic microplankton assemblage could, on average, respire 9–38% of the particulate carbon flux that entered the system at 100 m and possibly 18–76% of the flux remaining at 250 m. Microplankton may therefore be significant carbon cyclers in the ocean's vast “twilight zone”.

US Southern Ocean JGOFS Program (AESOPS): Part III

Abstract This is the third and final volume of manuscripts comprising the results from a series of process studies conducted as part of the US Joint Global Ocean Flux Study. The objectives of this study were to provide insights into the quantities and controls of fluxes in the Southern Ocean, to elucidate the processes that control primary production in both space and time, and to use these results to model the processes that were dominant in past environments. The results demonstrate the importance of the Southern Ocean with regard to both the present and past productivity patterns and vertical fluxes from the surface layer.

Physical control of biological processes in the central equatorial Pacific Ocean

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

Meanders in the Antarctic Polar Frontal Zone and their impact on phytoplankton

The Antarctic Polar Front is a complex set of meandering jets, which appear to support enhanced primary productivity. The US Joint Global Ocean Flux Study conducted a series of survey and process studies in part to study the processes regulating primary productivity in this high nutrient, low chlorophyll region. We deployed a set of surface velocity drifters, some of which were equipped with bio-optical sensors, to study the temporal and spatial scales of biological and physical processes in the Antarctic Polar Frontal Zone. There were two primary sets of deployments: November 1997 before the spring bloom and January 1998 after the spring bloom. The November deployment revealed a strong spring bloom that lasted about 10 days. In late spring, when incoming solar radiation began to increase, the vertical motions associated with the meanders strongly affected the accumulation of phytoplankton biomass, primarily through their impact on light availability. Weaker meandering was observed in the January deployment, and chlorophyll values remained relatively constant. As the bloom began to decay, it appears that nutrient availability became more important in regulating phytoplankton photosynthesis. Some of the drifters in the November deployment were deployed in coherent clusters, thus allowing us to calculate vertical velocities associated with the meanders. Estimates of fluorescence/chlorophyll suggest that areas of upwelling and downwelling alternately decrease and increase photosynthetic stress, perhaps as a result of changes in the availability of iron or light during the formation of the bloom.

The US Southern Ocean Joint Global Ocean Flux Study: Volume Two

This is the second volume of manuscripts comprising results from the process studies in the Southern Ocean conducted by the US Joint Global Ocean Flux Study (US JGOFS). The overall objectives of the study were to better constrain the fluxes of carbon in the Southern Ocean, to identify the factors and processes that regulate the magnitude and variability of primary productivity (as well as the fate of biogenic material), and to gain a sufficient understanding to model past and present carbon fluxes with sufficient accuracy to predict their response to future global changes. Field work was conducted both on the continental shelf of the Ross Sea, and within the Antarctic Circumpolar Current (ACC) near 170°W. Results from both regions described in this volume significantly address the goals and objectives of the entire program.

Overview of the US JGOFS Bermuda Atlantic Time-series Study (BATS): a decade-scale look at ocean biology and biogeochemistry

The Bermuda Atlantic Time-series Study (BATS) commenced monthly sampling in October 1988 as part of the US Joint Global Ocean Flux Study (JGOFS) program. The goals of the US JGOFS time-series research are to better understand the basic processes that control ocean biogeochemistry on seasonal to decadal time-scales, determine the role of the oceans in the global carbon budget, and ultimately improve our ability to predict the effects of climate change on ecosystems. The BATS program samples the ocean on a biweekly to monthly basis, a strategy that resolves major seasonal patterns and interannual variability. The core cruises last 4-5 d during which hydrography, nutrients, particle flux, pigments and primary production, bacterioplankton abundance and production, and often complementary ancillary measurements are made. This overview focuses on patterns in ocean biology and biogeochemistry over a decade at the BATS site, concentrating on seasonal and interannual changes in community structure, and the physical forcing and other factors controlling the temporal dynamics. Significant seasonal and interannual variability in phytoplankton and bacterioplankton production, biomass, and community structure exists at BATS. No strong relationship exists between primary production and particle flux during the 10 yr record, with the relationship slightly improved by applying an artificial lag of 1 week between production and flux. The prokaryotic picoplankton regularly dominate the phytoplankton community; diatom blooms are rare but occur periodically in the BATS time series. The increase in Chi a concentrations during bloom periods is due to increases by most of the taxa present, rather than by any single group, and there is seasonal succession of phytoplankton. The bacterioplankton often dominate the living biomass, indicating the potential to consume large amounts of carbon and play a major ecological role within the microbial food web. Bacterial biomass, production, and specific growth rates are highest during summer. Size structure and composition of the plankton community may be an important factor controlling the quality of dissolved organic matter produced and could affect production of bacterioplankton biomass. Larger heterotrophic plankton play an integral role in the flux of material out of the euphotic zone at BATS. Protozoans are abundant and can constitute a sizable component of sinking flux. Zooplankton contribute significantly to flux via production of rapidly sinking fecal pellets, and vertically migrating zooplankton can actively transport a significant amount of dissolved organic and inorganic carbon and nitrogen to deep water. An important question that remains to be further addressed at BATS is how larger climatological events drive some of the interannual variability in the biogeochemistry. (C) 2001 Published by Elsevier Science Ltd.

Configuring an ecosystem model using data from the Bermuda Atlantic Time Series (BATS)

Abstract The results of an assimilative approach to guide the configuration of an ecosystem model for the mixed layer of an oligotrophic environment are presented. The time series data from the US Joint Global Ocean Flux Study (JGOFS) Bermuda Atlantic Time Series (BATS), in conjunction with a data assimilation scheme, were used to estimate the model parameters and modify the Fasham et al. (J. Mar. Res. 48 (1990) 591–639) (FDM) model. The evolution of the model from initial to final configuration was driven by: (a) the comparison of the time series data to the model results; (b) analysis of the estimated parameters; (c) observations of the BATS ecosystem from the literature; and (d) corrections of the model pathways. The data assimilation technique was crucial to estimate the optimal parameter set for each of the tested model configurations. The model presented in this paper includes several critical modifications to the FDM model. First, a variable chlorophyll-to-nitrogen ratio is introduced by solving a full equation for chlorophyll a. Second, zooplankton are split into two functional groups: nano/microzooplankton and mesozooplankton. Third, a new formulation is introduced for the microbial loop that is capable of resolving and simulating many of the processes observed in natural environments as well as in laboratory experiments, but had, until now, not been combined in a model. These modifications lead to solving an equation for the temporal evolution of the bio-active dissolved organic-carbon pool. This modified model, in conjunction with data assimilation, allowed us to estimate the model parameters and replicate the annual nitrogen cycle in the upper mixed layer at BATS. Bacteria were found to be a key player in controlling the size of the dissolved organic matter pool and in the amount of regenerated production.

Microbial community structure and biomass in surface waters during a Polar Front summer bloom along 170°W

Abstract As part of the US Joint Global Ocean Flux Study (US JGOFS) Southern Ocean Program, flow cytometry and epifluorescent microscopy were utilized to determine abundance, distribution and size structure of the microbial community in the Polar Front region during the summer biomass maximum. Surface samples were collected approximately every 10 km along 170°W during two N–S transects, separated in time by two weeks. Phytoplankton abundance and size structure varied with distinct latitudinal trends. Autotrophic biomass was lowest north of the Polar Front reflecting the dominance of small cells. The highest biomass (170 μg C l −1 ) occurred at 65°S where the composition was strongly influenced by large centric diatoms. Farther south, the diatom community shifted to the dominance of smaller pennate diatoms. Total grazer biomass and size distributions followed similar patterns, ranging from 4 μg C l −1 in the north to 52 μg C l −1 in the south where larger (>20 μm) grazers were more abundant. Heterotrophic bacteria varied over an order of magnitude in abundance across the study site, with size generally increasing from north to south. In the second transect, phytoplankton biomass at 65°S was 50% lower, and grazer biomass and bacterial populations were slightly greater, indicating the decline of the bloom. The changes in biomass and community structure along 170°W and the reduction of phytoplankton standing stock at 65°S over time suggests adjacent, yet different, microbial systems in terms of carbon flux, spanning from primarily recycling to export-dominated.

Seasonal evolution of hydrographic properties in the Antarctic circumpolar current at 170°W during 1997–1998

Abstract This paper discusses the seasonal evolution of the hydrographic and biogeochemical properties in the Antarctic Circumpolar Current (ACC) during the US Joint Global Ocean Flux (JGOFS) Antarctic Environment and Southern Ocean Process Study (AESOPS) in 1997–1998. The location of the study region south of New Zealand along ∼170°W was selected based on the zonal orientation and meridional separation of the physical and chemical fronts found in that region. Here we endeavor to describe the seasonal changes of the macronutrients, fluorescence chlorophyll, particulate organic carbon (POC), and carbon dioxide (CO2) in the upper 400 m of the ACC during the evolution of the seasonal phytoplankton bloom found in this area. While the ACC has extreme variability in the meridional sense (due to fronts, etc.), it appears to be actually quite uniform in the zonal sense. This is reflected by the fact that a good deal of the seasonal zonal changes in nutrients distributions at 170°W follow a pattern that reflects what would be expected if the changes are associated with seasonal biological productivity. Also at 170°W, the productivity of the upper waters does not appear to be limited by availability of phosphate or nitrate. While there is a significant decrease (or uptake) of inorganic nitrogen, phosphate and silicate associated with the seasonal phytoplankton bloom, none of the nutrients, except perhaps silicate (north of the silicate front) are actually depleted within the euphotic zone. At the end of the growing season, nutrient concentrations rapidly approached their pre-bloom levels. Inspection of the ratios of apparent nutrient drawdown near 64°S suggests N/P apparent drawdowns to have a ratio of ∼10 and N/Si apparent drawdowns to have a ratio of >4. These ratios suggest a bloom that was dominated by Fe limited diatoms. In addition, the surface water in the Polar Front (PF) and the Antarctic Zone (AZ) just to the south of the PF take up atmospheric CO2 at a rate 2–3 times as fast as the mean global ocean rate during the summer season but nearly zero during the rest of year. This represents an important process for the transport of atmospheric CO2 into the deep ocean interior. Finally, the net CO2 utilization or the net community production during the 2.5 growing months between the initiation of phytoplankton blooms and mid-January increase southward from 1.5 mol C m−2 at 55°S to 2.2 mol C m−2 to 65°S across the Polar Frontal Zone (PFZ) into the AZ.

Assimilation of JGOFS EqPac and SeaWiFS data into a marine ecosystem model of the Central Equatorial Pacific Ocean

Abstract A five-component (phytoplankton, zooplankton, ammonium, nitrate and detritus) ecosystem model developed for the central equatorial Pacific is reformulated in a data assimilative mode, using the variational adjoint technique. This method minimizes model/data misfits by adjusting six model parameters that were selected by assessing parameter co-dependencies and model sensitivity to parameter variations. Through the assimilation of cruise data from the US Joint Global Ocean Flux Study (JGOFS) Equatorial Pacific Process Study (EqPac), and ocean color data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), it is possible to reduce model/data misfit by estimating optimal parameters governing processes such as phytoplankton and zooplankton mortality, zooplankton grazing, phytoplankton growth, and the recycling of nutrients from detritus remineralization. The success of this approach is evident in that similar parameter sets are obtained even when independent data sets are assimilated. For example, the assimilation of in situ EqPac (depth-resolved) data from the 1991–1992 El Nino produces a parameter set that is nearly identical to that estimated via the assimilation of remotely sensed (surface) SeaWiFS data collected during the 1997–1998 El Nino. The assimilation of biological data also allows objective determination of whether or not a given model structure is consistent with a specific set of observations. For example, the assimilation process demonstrates that data collected during and after the 1991–1992 El Nino are consistent with the same single-species ecosystem model, thereby suggesting that El Nino conditions may not necessarily be associated with shifts in species composition. In contrast, the increased abundance of diatoms associated with the passage of a tropical instability wave in October 1992 as well as a brief period of macronutrient limitation observed from November 1997 through January 1998 violate key assumptions of the model. Assimilation of data that include these dynamics results in unrealistic simulations of the lower trophic levels. The successful simulation of these particular data sets will require that the model dynamics allow for species composition changes and alternation between macro- and micronutrient limitation. In this way, assimilation of biological data into marine ecosystem models cannot necessarily overcome inappropriate model dynamics and structure; rather, it can serve to guide model reformulation.

Export flux in the western and central equatorial Pacific: zonal and temporal variability

Particulate organic carbon export fluxes were measured along the equator to resolve the zonal extent of high productivity in the equatorial Pacific during two cruises: the French JGOFS FLUPAC study aboard the R/V l'Atalante in October 1994 and the Zonal Flux study aboard the R/V Thomas G. Thompson in April 1996. Both cruise tracks went along the equator from 165°E to 150°W. The cruises took place under different seasonal and El Niño-Southern Oscillation (ENSO) conditions: FLUPAC during a strong El Niño in the boreal fall and Zonal Flux during a mild La Niña in the boreal spring. Drifting sediment traps were deployed at the base of the euphotic zone and calibrated using 234 Th . These traps showed over-trapping by 2.7±1.5 times during FLUPAC and 1.5±0.7 times during Zonal Flux. During the FLUPAC time-series at 167°E, the upper euphotic zone was devoid of nitrate, and particulate organic carbon export was low (6±1 mmol m −2 d −1). The FLUPAC time-series at 150°W had abundant nitrate and much higher particulate organic carbon export (12±1 mmol m −2 d −1). Similarly high levels of particulate organic carbon export were observed all along the equator during the Zonal Flux cruise (10±2 mmol m −2 d −1), when cold tongue, high nitrate conditions extended west of 165°E. Synthesis of this data with results from the US Joint Global Ocean Flux Study (JGOFS) equatorial Pacific (EqPac) program allowed a detailed evaluation of equatorial production variability. Data from the TOGA-TAO array illustrated that both Kelvin Waves and tropical instability waves (TIW) were present during the FLUPAC cruise, while neither wave type was present during Zonal Flux. Comparison with results from the US JGOFS EqPac cruises suggested that the ubiquity of super-μM nitrate was the major forcing for new production and particle export near the equator, accounting for a doubling of production over areas with only subsurface nitrate. Within the high nitrate zone, new production and particle export were both found to be enhanced during TIW activity and diminished during Kelvin Wave activity. While the geographical extent of surface nutrients and associated enhanced production is clearly a strong function of season and ENSO, we suggest that equatorially trapped waves — rather than long-term variability in upwelling velocity — are the dominant sources of variability within the equatorial upwelling zone. Comparison of new production and particle export and regressions between nitrate and total organic carbon (TOC) suggest that accumulation and transport of TOC accounts for 17–27% of new production.

The Oman upwelling zone during 1993, 1994 and 1995

Satellite-derived sea-surface temperature, TOPEX/POSEIDON (T/P) sea-level anomalies (SLAs), model wind data, and hydrographic data are used to characterize the upwelling along the Oman coast during the US Joint Global Ocean Flux Study (US JGOFS) Arabian Sea Process Study (ASPS) in 1995 as well as to look at interannual variability in the upwelling over the period 1993–1995. Empirical orthogonal function (EOF) analysis of the satellite-derived sea-surface temperature (SST) at the locations of the US JGOFS standard stations shows the first mode, which represents a biannual variability, contributes 67% of the total variance. In addition, the SST shows the upwelling “front” moving offshore with the development of Southwest (SW) Monsoon in early June 1995, reaching a maximum distance of approximately 120 km by late August 1995. Finally, SST shows the persistence of cold upwelling waters for nearly a month after the end of the SW Monsoon within the bays along the Oman coast. TOPEX/POSEIDON SLAs indicate that with the onset of the SW Monsoon, a 30-cm drop in steric height is observed along the Oman coast associated the presence of the cool upwelled waters. This drop in steric height sets up a horizontal pressure gradient and results in a compensating along-shore, northeastward-flowing, geostrophic current (East Arabian Current; EAC) during the SW Monsoon. Similarly, the altimeter data slow an offshore decrease in steric height during the Northeast (NE) Monsoon, indicating a seasonal reversal in direction of the EAC with flow to the southwest. Subsurface temperature data indicate that the actual uplifting of isotherms associated with the upwelling can be found to a distance of approximately 260 km from the shore and to a depth of 150–200 m. Using along-track altimetry data, we estimate that, for a region 260 km in offshore distance and 600 km alongshore, 2.2×10 6, 1.4×10 6 and 0.55×10 6 m 3 s −1 were upwelled through the 100 m level with upwelling velocities O (2.0×10 −5 m s −1), during the SW Monsoons of 1993, 1994 and 1995, respectively. The reduced upwelling in the summer of 1995 is attributed to a reduction in wind-stress curl along the Arabian coast when compared to 1993 and 1994.

The spring bloom in the Antarctic Polar Frontal Zone as observed from a mesoscale array of bio-optical sensors

The US Joint Global Ocean Flux Study (JGOFS) conducted a series of survey and process studies in part to understand the processes regulating primary productivity and carbon flux in the APFZ, which is a high-nutrient, low-chlorophyll (HNLC) region. We deployed a high-resolution array of 12 moorings (average horizontal spacing 30 km) equipped with bio-optical and physical sensors to study the temporal and spatial scales of biological and physical processes in the APFZ. The moorings collected data from November 1997 to March 1998, effectively observing the growing season. Estimates of chlorophyll and sun-stimulated fluorescence/chlorophyll (F/C) were derived from the bio-optical measurements. Each mooring showed a strong spring bloom beginning in early December as the upper ocean began to stratify, with chlorophyll levels nearly quadrupling. The time series, along with ship studies, suggest that phytoplankton were initially light-limited as a result of deep, late spring mixing, followed by intense zooplankton grazing or silicate limitation, which controlled the maximum chlorophyll concentration, and finally by iron limitation, which led to increasing photoadaptive stress. These results suggest that phytoplankton in the APFZ are regulated by a confluence of processes involving light, grazing, silicate, and iron, and that models comprising a single mechanism may not be sufficient. The spring bloom in the APFZ is a transient event, persisting for only a few weeks, and therefore it is difficult to draw conclusions from sporadic ship cruises. Moreover, its spatial scales are also small so that widely spaced hydrographic stations can easily overlook critical processes.

Carbon cycling in primary production bottle incubations: inferences from grazing experiments and photosynthetic studies using [formula omitted] and [formula omitted] in the Arabian Sea

Estimates of photosynthesis based on the incorporation of 14 C -labeled inorganic carbon into particulate carbon were compared to estimates of gross photosynthesis based on net O 2 production and the production of 18 O 2 from H 2 18 O during the US Joint Global Ocean Flux Study (US JGOFS) Arabian Sea process cruises. For samples incubated below the surface and at optical depths<3, the 14 C uptake : gross photosynthesis ratio averaged 0.45±0.1. This result is in accord with theoretical considerations of the combined effects of the Mehler reaction, photorespiration, dark respiration, excretion, and grazing effects on the two estimates of photosynthesis. The 14 C uptake : gross photosynthesis ratio was distinctly higher (0.62) for samples incubated at the surface. This result is likely due to UV light effects, since the O 2 and 14 C incubations were done in quartz and polysulfone bottles, respectively. The 14 C uptake : gross photosynthesis ratio was lower (0.31) for bottles incubated at optical depths>3. This result probably reflects an increase in the ratio of dark respiration to net photosynthesis in the vicinity of the compensation light level.