U.S. JGOFS Southern Ocean Process Study Implementation Plan
Carbon fluxes in the Southern Ocean represent quantitatively significant components of the global carbon cycle; they are highly susceptible to perturbation; and they are less well understood than are fluxes in more readily accessible regions of the ocean. Individual subsystems within the Southern Ocean act both as sources of CO2 to the atmosphere and as sinks for atmospheric CO2 that, independently, represent quantitatively significant terms in the global budget for air-sea exchange of CO2. However, our observations in Southern Ocean waters are so restricted, both in space and in time, that we do not know whether the region as a whole acts as a net source or sink for atmospheric CO2.
Biological productivity, and the accompanying processes that transport fixed carbon and nutrients to the deep sea, are spatially and temporally variable in the Southern Ocean. Predictability of the rates of biological processes, and of the magnitude of biogenic fluxes, is low, largely because we still lack a fundamental understanding of the factors that limit biological productivity in waters that are replete with inorganic nutrients, of the coupling between grazers and primary producers, and of the efficiency of the microbial loop.
Air-sea exchange of CO2, biological productivity, the annual advance and retreat of sea ice, and ventilation of deep water masses are all influenced by the stability of the upper water column. Stratification of the upper water column is generally weak and is largely determined by salinity, which, in turn, could be upset by a reorganization of the freshwater balance of the region likely to be associated with global climate change. Our ability to predict the response of biological processes and of chemical fluxes to a perturbation of water column stability is hindered by our lack of a basic knowledge of the magnitude of the fluxes and of the factors which regulate biological processes. JGOFS-supported measurements will expand the data base describing the magnitude and variability of carbon fluxes in the Southern Ocean. Process studies, involving coordinated experimental, observational, and modeling efforts, will improve our ability to predict the response of these fluxes to perturbation by identifying the factors which regulate carbon fluxes and by gaining a mechanistic understanding of how these factors regulate carbon fluxes.
Broadly-defined objectives for JGOFS research in the Southern Ocean have been established following discussions at several planning meetings which have been held both at national and at international levels. These objectives support the overall goals of the JGOFS program, and include:
Siting the U.S. JGOFS Southern Ocean Process Study
The Southern Ocean is a large and heterogeneous system. Early in the planning of international JGOFS activities in the Southern Ocean, the decision was made to cover as much of the region as possible by various national JGOFS efforts to assess the magnitude and scales of spatial variability of carbon fluxes and of biogeochemical processes. Participants at a planning workshop (U.S. JGOFS Report No. 16) considered the relative merits of several options for siting a U.S. field program in the Southern Ocean. Consideration was given both to answering scientific questions that apply throughout the Southern Ocean (see Science Plan; Report No. 17) and toward providing as much coverage of the Southern Ocean as possible, through collaboration with other national JGOFS programs (Figure 1). The general undersampling of the Pacific sector of the Southern Ocean, combined with the opportunity to support cruise logistics out of New Zealand and McMurdo Station, along with the favorable feature that individual hydrographic and nutrient "fronts" of the region tend to be distinct and separated in space, contributed to the decision (U.S. JGOFS Report No. 16) to site the U.S. JGOFS Southern Ocean process study in the SW Pacific sector of the Southern Ocean (Figure 2). Portions of the field work will coincide with the NOAA repeat survey line at 170°W (see below), while the remainder will take place in the Ross Sea.
Spatial variability is most extreme in a meridional sense, whereas there is some degree of zonal regularity throughout the Southern Ocean, which can be divided into four principal zonal subsystems:
Continental shelves surrounding Antarctica contain some of the most highly productive (on a daily basis) waters of the world's oceans. Both the initiation and the termination of phytoplankton blooms in these regions are tied intimately to the seasonal advance and retreat of sea ice, although the detailed linkages between physical forcing and phytoplankton response remain largely undetermined. Species assemblages vary spatially, for phytoplankton (e.g., phaeocystis, diatoms) and for zooplankton (e.g., krill, copepods, salps). Physical conditions regulating spatial variability, that is, those factors favoring one assemblage over another, as well as the effect of each dominant assemblage on the fate of organic matter produced during intense blooms, represent important unknowns to be addressed by JGOFS. Furthermore, the susceptibility to perturbation by global change of the seasonal cycle of sea ice drives JGOFS to seek a better understanding of the linkages between physical forcing and biogeochemical response in developing reliable methods to monitor, and to predict, the response of the Southern Ocean to global change. Within the Ross Sea can be found regions of high, moderate, and low phytoplankton biomass and primary productivity, thereby enabling JGOFS to examine the factors regulating phytoplankton blooms, the fate of biogenic materials, and the scales of variability of these processes within a single study area of manageable scale.
Several features of the frontal systems of the Southern Ocean suggest that the APFZ would serve as a model system to assess the role of fronts in carbon dynamics. Recent studies by European JGOFS programs found high concentrations of biomass and high rates of primary production associated with frontal features, without fully characterizing the conditions responsible for the enhanced biomass and production. The APFZ is a region of active ventilation of intermediate and deep water. Fluxes of dissolved inorganic carbon (DIC) and of dissolved organic carbon (DOC) associated with this ventilation are likely to be large and, therefore, warrant quantitative evaluation. High accumulation rates of biogenic sediments in the APFZ seem to be decoupled from low annual-average rates of primary production, suggesting that remineralization processes are fundamentally different here compared to the rest of the ocean. Frontal systems are susceptible to perturbation by climate change. Paleoceanographic proxies suggest that the mean annual position of the APFZ migrated northward by roughly five degrees of latitude during glacial times, whereas physical principles suggest that such a large displacement of the fronts is unlikely. Glacial climate conditions induced either (a) a profound change in species assemblages and ecosystem structure of the APFZ, or (b) a radical reorganization of circumpolar circulation and, probably, of the ventilation of the deep sea. Studies of paleo indicators in the modern ocean are needed to better interpret the record of past responses of the frontal systems to forcing by climate change.
Regardless of whether past responses were primarily physical or ecological, we know that the APFZ experienced extensive changes in response to past climate forcing, from which knowledge we may infer that the APFZ is similarly susceptible to perturbations by future climate change. In light of these considerations, a study of the APFZ offers the opportunity to acquire new understandings of carbon fluxes, and of the susceptibility of biogeochemical cycles to perturbation by global change.
General- Shipboard experiments are required to evaluate carbon fluxes and to study most of the critical biological processes influencing the fluxes of carbon and related species, and to evaluate the rates of these processes. Similarly, shipboard studies are also required to assess standing stocks and compositions of communities, to measure the inventories and distributions of particulate phases and of dissolved organic matter, and to evaluate directly many of the biogeochemical fluxes of interest to JGOFS. Shipboard studies are, however, limited in space and time. Remote monitoring of key parameters, including wide-area coverage provided by satellites and time-series records provided by in situ sensors, will provide the larger context for evaluating carbon fluxes and the response of biogeochemical processes to physical forcing. Results of each type of measurement (process rates, time-series records, synoptic surveys) will, in turn, be used to test model algorithms, to improve the ability of models to predict the response of biogeochemical processes and fluxes to physical forcing, and, ultimately, to predict the sensitivity of these processes to global change.
The Southern Ocean field program will consist of a strategic balance of survey-mode and time-series shipboard observations. In previous JGOFS studies, time-series observations carried out over relevant scales of temporal variability have generated new understanding about the linkages between biological processes and physical forcing. In emphasizing a time-series approach for the U.S. JGOFS Southern Ocean process study, it is essential to consider the time scales of the dominant processes influencing primary production and carbon fluxes. While much remains to be learned about the processes and conditions influencing the magnitude and variability of primary production and of carbon fluxes in the Southern Ocean, some reasonable generalizations can be made. For example, within the seasonal ice zone, the dominant time scales are probably seasonal, reflecting ice melt and radiation balance, with secondary forcing on shorter time scales by ice movement and local weather. In contrast, whereas seasonal forcing by radiation balance is certainly important in the frontal zones, higher-frequency events (days to weeks) associated with meso scale circulation and the structure of the fronts may be the dominant feature forcing productivity and carbon fluxes.
Important features of the carbon cycle need to be addressed in all seasons. Spring cruises will examine the response of phytoplankton to increasing levels of light. Summer and fall cruises will test hypotheses about the delayed response of carbon consumption in the Southern Ocean. Cruises in all three seasons will examine the influence of water column stability on primary production, and will test hypotheses pertaining to the factors postulated to limit the utilization of nutrients in Southern Ocean surface waters. A winter cruise will examine biological processes which have, to date, been little studied, including ice-edge productivity under minimum-light conditions and carbon consumption under the ice. Air-sea exchange of CO2 and ventilation of deep water masses will be examined in all seasons, but particular emphasis will be given to a winter cruise when processes responsible for ventilating deep water masses may be most active. Collaboration with the NOAA CO2 program (see below) will produce a comprehensive time-series coverage of air-sea fluxes of CO2 within the frontal systems that will address variability on both seasonal and interannual time scales.
The frontal zone is a region of intense mesoscale variability, attributable both to the meandering of the frontal jets and to the generation of eddies from these jets. Such a complex physical oceanographic system will be difficult to characterize, and it will be impossible to employ a term-balance approach to evaluate carbon fluxes within this system. Nevertheless, it is precisely these complex physical conditions that influence nutrient transport and the ventilation of intermediate and deep waters, and which appear to provide the locus of enhanced biomass accumulation in surface waters as well as enhanced export of biogenic detritus to the deep sea. Study of this complex system requires a dual approach employing underway measurements (survey mode) to establish relationships between physical structure of the upper water column and the distributions of chemical (e.g., nutrients, CO2) and biological (e.g., biomass, pigments) parameters, combined with process-level studies at discrete stations where biological rates, relevant fluxes, and other parameters requiring on-station measurements will be evaluated. Used together, these approaches will resolve important scales of spatial and temporal variability while evaluating biological rates and chemical fluxes within the context of the physical and chemical conditions regulating these processes.
Two-Ship Field Program- Seagoing investigations will utilize two ships and two ports: a large UNOLS vessel working out of Lyttleton (the port of Christchurch, New Zealand) and the N.B. PALMER, working out of both McMurdo Station and Lyttleton. The Science Plan stresses the strong seasonal cycle expected in nearly all processes of interest. This means that the northern portions of the system should be studied from ships in all seasons, and the southern portions should be studied over the entire time that they are accessible to an ice breaker. This is not to imply that year-round data will be needed for every component study of the JGOFS program; for example, spring and summer data may be adequate for studies of primary production and closely related processes. To meet these needs, the UNOLS vessel must spend approximately one-half year in the study region, making a series of cruises that begin and end in Lyttleton. The PALMER will make a Lyttleton-McMurdo cruise as early in the spring as ice conditions permit (perhaps stopping along the way for an intensive study of the marginal ice zone in spring). It should end the season with a McMurdo-Lyttleton cruise late in autumn, and make a series of McMurdo-McMurdo cruises in between. Scheduling the cruises this way will make it possible to get several different science parties onto each ship by flying them commercially to Christchurch and using the U.S. Navy flights between Christchurch and McMurdo, as these typically begin in October and end in February. Thus the first PALMER cruise could leave Lyttleton in October with the last one returning in March or early April.
Beyond the general strategy described above, some special needs with respect to scheduling cruises have been identified.
Ross Sea- Continental shelf waters around Antarctica are among the most highly productive waters in the world ocean. Furthermore, because productivity is linked to the seasonal advance and retreat of sea ice, the productivity of these waters is also sensitive to perturbation by climate change. Whereas mid-bloom conditions have been studied extensively, little is known about the onset or the termination of blooms on the shelf, thereby limiting the ability to predict the sensitivity of shelf blooms to climate change.
Phytoplankton blooms on Antarctic shelves often begin as soon as ice concentrations are reduced in spring, which often results from local movement of ice by winds, rather than the surface heat budget. Open waters (or reduced ice concentrations) on the Ross Sea continental shelf appear first near the ice shelf. However, because of the logistical difficulty in moving a ship through the expansive sea ice remaining at this time, these early-season blooms have received relatively little study. Similarly, relatively little work has been done on the termination of blooms in Antarctic continental shelf waters. It has been postulated that the return of ice may terminate blooms before nutrients are exhausted, but this hypothesis has not been tested. The U.S. JGOFS Southern Ocean process study will examine seasonal processes on the Ross Sea shelf, including conditions associated with the initiation and the termination of phytoplankton blooms, as well as the factors controlling the succession of species (e.g., prymnesiophytes, diatoms) during the course of a bloom, the fate of biogenic materials produced during the bloom, and the scales of variability of these processes. These activities will require the presence of the Palmer in the Ross Sea, at least intermittently, from mid-October through March.
Frontal Systems- The JGOFS Southern Ocean study will take place in a region characterized by strong density fronts and intense winds. Productivity seems to be enhanced within the frontal systems (see Science Plan), but the factors leading to enhanced productivity remain poorly understood. Satellite infra-red images of sea surface temperature often show small (20-30 km) eddies forming along strong fronts. Transport of nutrients and of phytoplankton by mesoscale features within the frontal zones as well as uncoupling of phytoplankton-zooplankton processes is likely to be an important aspect of the physical forcing of biological processes.
A control volume experiment to evaluate carbon fluxes is not feasible in the complex and dynamic region of the Antarctic Polar Front Zone. Nevertheless, JGOFS will be required to carry out a more comprehensive characterization of physical oceanographic conditions than has been the case in earlier process studies. Objectives of this work will be the qualitative constraint of advective fluxes, and a detailed description of physical oceanographic features (e.g., structure and motion of the water column associated with fronts and eddies) and processes influencing concentrations and fluxes of carbon species, nutrient distributions, phytoplankton biomass, rates of primary production, and the fate of biogenic production. Intensive frontal surveys will need to be made using a towed undulating recorder to collect physical and biological data (salinity, temperature, density, fluorescence, irradiance, and an optical and/or acoustic zooplankton counter). Development of chemical sensors, as well as systems capable of pumping water samples to the ship, are encouraged. Shipboard ADCP data will be collected continuously and combined with data collected by current meters deployed on mooring arrays (see below) to describe the current field. Drifter clusters (with bio-optical instrumentation) will be used, as well, to study sub-meso scale processes in the frontal zone.
Models play important roles (see below) in interpreting the regulation by ocean physics of carbon fluxes and of biological processes. This strategy requires that coupled physical-biogeochemical regional models be developed which are capable of resolving mesoscale features while retaining adequate computational capabilities to simulate multiple classes of primary production, grazing, bacterial remineralization, and processes influencing the export of biogenic matter from the euphotic zone. Models will be initiated with boundary conditions established by physical measurements described in the paragraph above, and will incorporate rate parameters evaluated by shipboard experiments. Experimentalists are encouraged to design research programs to supply parameters required by the models. Improvement of model capabilities to simulate biogeochemical processes under conditions of intense mesoscale variability will be made, in turn, through data assimilation techniques, including the use of physical oceanographic information obtained from other programs and satellite images capable of defining scales of variability (e.g., altimeter data). Model performance will be assessed by the ability to reproduce detailed relationships between biogeochemical parameters and prevailing physical conditions.
Ocean Color- Field studies are required to support several aspects of algorithm development and calibration of SeaWiFS before ocean-color data from satellites can be used to quantitatively assess spatial and temporal variability of phytoplankton abundance and, eventually, primary productivity. At the beginning of the program, it will be necessary to develop accurate algorithms for quantitative work in the Southern Ocean. This is also the most critical need, in part because we suspect that the Southern Ocean requires bio-optical algorithms for pigment and productivity that are fundamentally different from those which will be applicable at lower latitudes. It will be necessary to continue field work throughout the program to ensure that sensor calibration is stable through time, or that changes which occur in the sensor's responses are quantified. Southern Ocean waters experience seasonal and, possibly, interannual variability of dominant phytoplankton assemblages (e.g., phaeocystis, diatoms). Algorithm development must address the effect of species succession on the signal recovered by ocean color sensors.
It would be best to carry out SeaWiFS-related studies during a time of the year when a large range of pigments/productivity might be expected and when ice concentrations are low. Limited work should also be considered, as well, for early and late in the season (i.e., spring and/or late autumn) to determine the capability of SeaWiFS to retrieve accurate water-leaving radiances when solar zenith angles are are large and when water-leaving radiances are relatively small.
Benthic Studies- The sea floor is the largest and the most important sediment trap of all. Closure of the organic carbon cycle cannot be obtained without an evaluation of sea bed fluxes. Furthermore, benthic studies provide prima facie evidence for a low efficiency of recycling of biogenic material within the Antarctic Polar Front Zone.
The Pacific sector of the Southern Ocean is a region of rugged sea floor topography, throughout which are vast areas without modern sediment accumulation. Sedimentary features of this region have not received a thorough survey, so we have little knowledge of those areas in which modern sediments are accumulating. Consequently, it will be necessary to conduct preliminary surveys of the region, prior to undertaking comprehensive benthic process study work. At a minimum, continuous seismic surveys and the collection of surface sediments should be made throughout each of the cruises early in the program (e.g., mooring deployments). Ideally, a dedicated benthic survey cruise would be carried out prior to the process study as was done, for example, in preparation for the U.S. JGOFS Equatorial Pacific process study. Should a pre-process study benthic survey cruise prove to be feasible, then the cruise should incorporate extensive coring and bottom photography in addition to seismic survey work.
Instrumentation and Sensors
Moorings- Even with cruises covering each season, abrupt changes in productivity and carbon fluxes will occur which will not be sampled by shipboard operations. Consequently, JGOFS must rely on moored instrumentation for the time-series measurement of key physical, chemical and biological variables. In addition to the physical (e.g., temperature, conductivity, currents), geochemical (e.g., sediment traps) and bio-optical (e.g., fluorometers, both, natural and stimulated fluorescence, PAR, beam transmissometer, surface and in situ downwelling irradiance, upwelling radiance) measurements which are now made routinely, there is a realistic expectation that nutrients and CO2 system parameters (e.g., PCO2, pH) can be monitored with moored instruments by the time the Southern Ocean moorings are deployed.
Taut moorings required for deployment of deep sediment traps are incompatible with surface moorings on which meteorological and bio-optical instruments will be placed. Consequently, separate moorings will be deployed for surface and for deep instruments. Deep moorings (e.g., sediment traps) will be deployed year-round. Considering weather and ice conditions, surface moorings (bio-optical instruments, etc.) will be deployed only during the spring-to-fall field season.
Compared to earlier studies, fundamentally-different strategies will be required for mooring-based observations in the Ross Sea and in the APFZ. For example, a single highly-instrumented mooring in the APFZ would be incapable of resolving time-varying processes under conditions of intense mesoscale variability. A preferred strategy to assess temporal and spatial variability of biogeochemical parameters will be to deploy an array of low-cost moorings with limited instrumentation and sensors.
Instrumented Drifters- Technological development of in situ sensors (e.g., spectroradiometers) has progressed to the point where expendable drifters can be used cost effectively, in certain circumstances. Drifters telemeter their position, along with data collected, via ARGOS. Furthermore, the use of instrumented drifters brings the added advantage of providing information about the physical circulation of the study area, which will be particularly useful in the study of mesoscale circulation in frontal zone. The optimum strategy for JGOFS work in the APFZ will be to combine the use of both moored instruments and multiple clusters of instrumented drifters.
Towed Instruments- Just as moored instruments offer the opportunity to assess the temporal variability of a number of parameters, underway measurements and towed-body instruments can be successfully employed to generate information about spatial variability. Towed instruments will form a centerpiece for mapping distributions of chemical and biological parameters in regions of fronts and eddies where strong spatial variability can be anticipated. Satellite data will be used to locate the ship and towed instrument with respect to frontal or eddy features. Towed instrument packages (e.g., SeaSoar) can be instrumented to carry a CTD, fluorometer, transmissometer, and radiometers. Measurements should include bio-optical parameters, spectral absorption of suspended particles, spectral irradiance that matches SeaWiFS sensors, natural fluorescence, strobe fluorescence, and beam transmission. Acoustic sampling systems can be added to assess distributions of fish and small zooplankton.
Satellites- The abundance and distribution of phytoplankton chlorophyll hopefully will be provided by SeaWiFS, an ocean color sensor that is planned to be launched in 1995. Sea surface temperature imagery will be collected by NOAA's AVHRR. Both sensors are restricted to cloud-free areas, thus severely limiting the amount of data available from the Southern Ocean. Measurements of sea surface topography made by TOPEX/Poseidon will be used to estimate ocean circulation. Scatterometer data can be used to estimate wind velocity over the ocean. The European Space Agency will continue such measurements in its ERS series of satellites. NASA plans to launch a scatterometer aboard the Japanese ADEOS satellite in 1996. Passive microwave data (from SSM/I) will be used to determine sea ice distributions as well as atmospheric water vapor. SSM/I can also be used to estimate wind speed. DMSP images will provide an independent view of ice distributions. Surface solar irradiance data, required for estimation of primary productivity, will be available from the International Satellite Cloud Climatology Program. JGOFS will support the collection and processing of high-resolution SeaWiFS data, as well as the assembly of processed data from other satellite sensors for near-real-time transmission to shipboard investigators during the field program.
The U.S. JGOFS Process Study in the Southern Ocean will take place over a period of approximately one and a half years (Figure 3), to encompass two Austral spring-summer periods of maximum biological productivity. Process study cruises will begin in Austral spring of 1996 (e.g., October), and terminate in late summer (e.g., March/ April) of 1998. Although work during the first field season will be concentrated on the Ross Sea continental shelf, and during the second year the focus will shift to the APFZ, the split field season design of the program will permit limited observations in both regions covering two years during periods of maximum anticipated biological productivity.
N. B. Palmer- Field work will commence aboard the N. B. Palmer with a short cruise that encompasses a survey of the sea bed, to assess regions of modern sediment accumulation, and the deployment of deep moored sediment traps in both the APFZ (five moorings) and in the Ross Sea (two moorings). This work must be completed, and personnel exchanged via air transport through McMurdo, in time to permit the Palmer to be on site when the spring bloom commences (approximately 15 October). Initial observations will concentrate on the conditions leading to the development of the spring bloom, to be followed by studies of carbon transfer, flux and remineralization of particulate and dissolved organic and inorganic phases mediated by the biological community.
Shipboard work during summer months will continue to examine species succession, scales of variability of bloom conditions, and the factors regulating these conditions. The scale of study will expand from the relatively confined examination of the spring bloom originating in the polynya to encompass a much larger region of the Ross Sea. Summer experiments will examine grazing, aggregation, and other processes controlling the remineralization and export of organic matter from the euphotic zone, as well as the fate of exported biogenic debris (e.g., remineralization at the sea bed; burial; and advective transport away from the region in which it was produced).
The Fall cruise in the Ross Sea will examine conditions of advancing sea ice under which shelf blooms appear to be terminated, and follow up on summer studies of grazing, remineralization, and other loss terms in the organic carbon budget of the shelf ecosystem. Sediment trap moorings will be recovered, refurbished, and redeployed from the Palmer as the ship leaves the Ross Sea at the end of the season.
A winter cruise is scheduled to coincide roughly with the period of maximum extent of sea ice. This cruise will examine biological processes which have, to date, been little studied, including ice-edge productivity under winter conditions and carbon turn over under the ice. Air-sea exchange of CO2 and ventilation of deep water masses will be examined in all seasons, but particular emphasis will be placed on the winter cruise when processes responsible for ventilating deep water masses should be most active. The winter JGOFS cruise compliments well cruises of the NOAA CO2 program, which will have been carried out under summer and fall conditions. Collaboration with the NOAA CO2 program (see below) will produce a comprehensive time-series coverage of air-sea fluxes of CO2 within the frontal systems, as well as fluxes of DIC and of DOC associated with deep ventilation in the frontal zone, that will address variability on both seasonal and interannual time scales.
A final cruise aboard the Palmer is scheduled for the 1997-98 field season. JGOFS work during the 1996-97 field season will have provided an opportunity to examine biological processes and carbon fluxes in the Ross Sea on an unprecedented scale. These studies will undoubtedly give birth to new paradigms and algorithms which will require further testing. Field testing of these algorithms, to ensure that they are reliably transposed into model code, will provide an important component of the Synthesis of JGOFS findings from the Southern Ocean process study (see below) which, in turn, will improve our ability to extrapolate JGOFS findings from the Ross Sea study to other shelf regions surrounding Antarctica.
UNOLS Vessel- Work aboard the UNOLS vessel will be carried out during a continuous block covering roughly six months, beginning in October, 1997. During each transit to and from the APFZ, underway measurements will be made of surface-water samples (CO2, nutrients, pigments), while a towed system will be used to map the hydrography and variability of phytoplankton biomass in the upper water column. While entailing a minor commitment of resources, these measurements will advance our understanding of the scales of variability of primary production in the vast region of nutrient-rich waters situated between the Subtropical Front and the Antarctic Polar Front Zone (Figure 2).
Five moorings instrumented with time-series sediment traps and current meters will be deployed at 2-degree intervals through the APFZ, from ~56°S to ~64°S (Figure 4). Note that the general features evident in the transect in Figure 4, showing data from 160°W, will be shifted slightly to the south in the JGOFS study region at 170°W. Furthermore, detailed hydrographic features shown in Figure 4 exhibit random temporal variability. This transect should be used only for general planning purposes. Final decisions about station locations will be made following a site survey.
The field program in the APFZ will be launched with a Spring survey cruise to map the detailed structure of the frontal systems, and to define initial relationships between biogeochemical parameters (e.g., pigments, nutrients, CO2-system, DOC) and the hydrographic structure of the water column. Near real-time satellite images (e.g., AVHRR, color, sea surface topography) will be beneficial in locating the ship with respect to fronts and eddies throughout the survey cruise. Because the entire APFZ encompasses an area too broad for detailed repeat surveys, following an initial pass through the region, and return to the northern boundary, survey work will be reduced in scope to focus on one of the three fronts typically occurring within the APFZ (the Subantarctic Front, the Antarctic Polar Front, the South Antarctic Circumpolar Current Front; Figures 2 and 4), specifically, on the front wherein pigment distributions and/or air-sea delta pCO2 are most clearly related to hydrographic features. Underway surveys will then be carried out covering a gridded box with dimensions between 100- and 200-km2. Clusters of drifters will be deployed during the high-resolution survey to resolve dynamic processes affecting distributions of biomass, nutrients, and air-sea delta-pCO2. Following each high-resolution survey, discrete stations will be occupied through the surveyed area to conduct process-oriented measurements.
The second cruise in the APFZ will begin, like the first, with a survey of underway measurements through the APFZ to define any changes that may have occurred since the initial survey cruise. Following the underway survey, the cruise will undertake process-oriented studies (e.g., primary production, grazing, bacterial production, and other loss terms) at discrete stations (of ~ 2-day duration each) corresponding to each of the sediment-trap moorings. These stations will be followed by a high-resolution survey of one of the fronts, like that described above, and this survey, in turn, followed by time-seies acquisition of process-level measurements within the survey box.
A summer survey cruise is prescribed to examine rates and variability of primary production under conditions of maximum thermal stratification and irradiance supply, with emphasis on the identification of factors and conditions limiting the efficiency of nutrient utilization by phytoplankton. Studies of chemical limitation by micro nutrients (e.g., iron) will be afforded high priority during this cruise.
A multipurpose cruise involving mooring recovery, benthic studies, and a detailed examination of paleoceanographic proxies within the context of a comprehensive biogeochemical study is planned for February, 1998. This time period is selected for this cruise as it offers the only opportunity for the UNOLS vessel to recover moorings deployed in the Ross Sea. Benthic flux studies in the Ross Sea will be carried out at this time as well. Advantage will be taken of having a ship in this remote area to collect piston cores during this cruise. Piston cores will be returned to a major U.S. core repository for description and archival. Although collection of piston cores, and detailed studies of paleoceanographic proxies are important components of the U.S. JGOFS Southern Ocean process study, JGOFS is not expected to support paleoceanographic studies of the long sediment records. Rather, these cores will be made available to studies supported by other sources of funding. Given the wealth of flux data and results from proxy studies to be derived from the JGOFS program, piston cores from this region are expected to yield valuable new insights into response of the Southern Ocean to forcing by past climate change.
The final cruise of the program, a Fall process-oriented cruise, will examine primary production under conditions of waning light, as well as the fate of organic carbon produced during spring blooms and summer conditions of maximum light. The strategy for this cruise will follow that of the Spring-early Summer process-oriented cruise.
Management and Cruise Services
Travel and Shipping- Following precedents of earlier U.S. JGOFS process studies, support for personnel traveling to and from the cruises, as well as the shipping of equipment and supplies to the ship, will be covered by a separate arrangement between the National Science Foundation, the U.S. JGOFS Planning Office, and Antarctic Support Associates.
Nutrients and Hydrography- Similarly, following precedents of earlier U.S. JGOFS process studies, basic data required by most investigators will by provided through subcontracts arranged under the program management. These common data include conductivity, bottle salinity, temperature, Winkler oxygen, nutrients (nitrate, nitrite, ammonia, phosphate and silica), and ship-based meteorology.
The U.S. JGOFS Southern Ocean program will include a strong and active modeling component that allows for the development and implementation of many levels of modeling effort. The development of small scale models (e.g., mixed layer-biological models) is encouraged in order to better define the local processes that limit primary production in the Southern Ocean. In particular, there is a need to develop bio-optical models for application to Southern Ocean environments that can be used with mixed layer models. Mixed-layer models, in turn, should incorporate fairly sophisticated parameterizations of turbulence and boundary-layer conditions. Along with the development of small scale models there is a need for the development of models that are focused on processes in defined regions (e.g. regional models). Regional models are likely the ones to which the data sets obtained in a Southern Ocean JGOFS field study will be most applicable. For example, high-resolution regional models are likely to be the most useful for looking at the coupling between biological and physical processes in the upper ocean associated with meso scale circulation fields. Biological response to eddy activity appears to be important and asymmetric (compared, for example, to mixing effects on a passive tracer). Information gained about non-linearities among relationships (e.g., involving primary or new production and ocean color or optical properties) gained from mixed-layer and regional models will lead to improved parameterization of these processes in models covering larger scales, which may not be capable of resolving eddies, but which are consistent with questions that relate to climate change. Associated with larger-scale modeling efforts, application of inverse models will provide an independent means of evaluating fluxes of DIC, DOC, and of related species. Furthermore, given the dependence of the Antarctic ecosystem on ice, there is a need to incorporate sea ice models, where appropriate, in conjunction with other modeling activities identified above.
Modeling efforts should be undertaken before the Southern Ocean JGOFS field studies to ensure collection of optimal data sets. This early modeling would incorporate existing historical data sets and scientific knowledge, and would be directed toward planning the field program. Early modeling efforts should address the critical elements of variability, including those parameters most in need of experimental study. Early modeling will help define the frequency, spacing, accuracy and precision required of critical measurements. This need pertains to the full suite of measurements to be undertaken during the process study, including: the production and remineralization of carbon and related biogenic phases; the production, transport and melting of sea ice; the physical structure of the water column; and, larger-scale circulation. Because JGOFS will not have sufficient resources to undertake a comprehensive physical oceanographic study of its own accord, it is particularly important that early modeling efforts define the minimum amount, frequency, spacing and type of physical measurements (e.g., hydrography, drifters, current meters) required to determine whether or not reasonable physical models are being used as the basis for interpreting and modeling biogeochemical processes.
New techniques must be developed as well, including the nesting of models having variable degrees of resolution. This might, for example, take the form of a mixed-layer bio-optical model embedded in a high-resolution eddy-resolving three-dimensional model which, in turn, is embedded in a general circulation model of the Southern Ocean. Such a hierarchy of models would enable investigators to examine the influence on carbon fluxes of a large range of factors, including weather conditions, the structure and depth of the mixed layer, upwelling and the supply of both macro- and micronutrients, and the advective loss of biogenic products from surface waters. At a minimum, regional eddy-resolving models may need to rely on GCM's to supply boundary conditions. New data generated by the field program will be used to test and improve existing models, while working toward the eventual goal of models which can rely on data acquired by remote sensing to monitor changes in the biogeochemistry of the Southern Ocean.
Collaboration with Other Programs
International JGOFS- Because the various national JGOFS programs are distributed throughout the Southern Ocean (Figure 1), international coordination and collaboration will, for the most part, take the form of data synthesis and modeling. Subsystem-specific coupled biogeochemical-physical models will be constructed for each regional study. During integration and synthesis of the results, the various model outputs will be compared, both as a measure of the spatial and temporal variability of carbon fluxes throughout the Southern Ocean and as a test of the models themselves.
Investigators in New Zealand, Australia and Italy have active JGOFS programs, and have expressed interest in collaborating with the U.S. in Southern Ocean work. The New Zealand JGOFS program presently involves approximately ten scientists studying fluxes of carbon in different water masses surrounding New Zealand, with a special emphasis on the region of the Subtropical Convergence. The New Zealand program aims to identify the relationships between physical and chemical variability and the marine food web, with the goal of developing a predictive understanding of the influence of the physical and chemical environment on the transfer of nutrients through the food web. A further goal of the program is to determine the importance of the Subtropical Convergence as a major Southern Hemisphere sink for atmospheric carbon dioxide. The New Zealand JGOFS program has completed two cruises in 1993. Forthcoming plans include a cruise (Feb 25 - March 10, 1995) off the east coast of the South Island in the STC region in the area 42-46°S, 174-177°E. The focus of this cruise will be to assess the scales of variability of primary production and ocean optics in the area. There are tentative plans for a cruise in February-March 1996 to assess carbon fluxes in the STC on both the east and west coast of New Zealand. This cruise would encompass most, if not all, JGOFS standard protocols and would complement the cruises to date. Collaboration with JGOFS investigators in New Zealand will greatly enhance our understanding of biogeochemical processes at the northern terminus of the U.S. JGOFS Southern Ocean study area.
The Australian Southern Ocean JGOFS program involves a mixture of survey cruises, process studies, remote sensing and modelling over a period from 1990 to 1998. A key part of the Australian program depends on piggyback JGOFS measurements on a series of repeat WOCE transects along SR3 which runs SW from Tasmania, then south to the Antarctic shelf. Aside from the standard WOCE data, including nutrients, JGOFS investigators are measuring carbon (CO2, HCO3- and CO32-) and carbon isotopes, HPLC pigments, and P vs. I parameters. There are plans to add DOC and optics. Additional underway PCO2 measurements are being made on Aurora Australis resupply voyages.
The Australian Antarctic Division is conducting a program of marine research in the shelf and sea ice zones, concentrated near Prydz Bay. The research involves plankton production and grazing and sediment trap studies in collaboration with Japanese researchers. There are plans to establish a limited JGOFS time series station off Prydz Bay in collaboration with Japanese and French researchers.
Additional Australian JGOFS-related research is being conducted in the Sub-tropical Convergence Zone (40°S-50°S). Four seasonal shelf/slope process cruises involving measurements of primary production parameters, nutrients, pigments, zooplankton, and sediment fluxes using free-floating and deep moored PARFLUX traps, have been completed in the 1991/93 time frame. An offshore process/optics cruise is planned for January 1995. There are preliminary plans to conduct additional process cruises in the Subantarctic/Antarctic Circumpolar Current zone, and the sea-ice zone, in 1996/98.
There are plans to receive AVHRR and SeaWiFS data at the CSIRO Marine Labs remote sensing facility, with coverage of the Southern Ocean between Western Australia and New Zealand. An X-band reception facility being completed in Hobart will allow collection of SAR and other remotely-sensed data sets over the Southern Ocean.
Investigators in Italy have proposed hydrographic, biogeochemical, sea-ice, and geological studies in the Ross Sea, operating out of the Italian research base at Terra Nova Bay. A sediment trap mooring was deployed early in 1994 at ~76° 30' S, ~169°E as one of the initial efforts of the Italian JGOFS program. Members of the U.S. JGOFS Southern Ocean process study planning group are in contact with Italian investigators, and some tentative proposals for collaboration have been initiated. While no specific details of the collaboration have been confirmed, as yet, it is hoped that collaboration with Italian investigators will enhance our opportunities for process study work near the southern terminus of the U.S. JGOFS Southern Ocean study area.
U. S. Programs Sharing Common Interests with JGOFS
The Global Ocean Ecosystems Dynamics program (GLOBEC) is undertaking a series of studies to understand the effects of physical processes on predator-prey interactions and population dynamics of zooplankton, and their relation to ocean ecosystems in the context of the global climate system and anthropogenic change. GLOBEC plans to carry out a Southern Ocean study near the Antarctic Peninsula. This region is thought to have substantially different food web structures compared to the JGOFS study area near 170°W, with krill being more predominant near the Antarctic Peninsula. One mutually-beneficial collaborative effort between JGOFS and GLOBEC would be to examine the influence of the different trophic structures of the two regions on the export and recycling of biogenic materials. Another area in which JGOFS and GLOBEC have common interests is in advancing the capability to model biological responses to physical forcing. JGOFS and GLOBEC are planning a jointly-supported modeling endeavor.
The International Global Atmospheric Chemistry Program is planning a Southern Hemisphere Marine Aerosol Characterization Experiment (ACE-I) to take place south of Australia in November-December, 1995. The experiment will define the role of chemical and physical processes controlling the evolution and properties of atmospheric aerosols relevant to radiative forcing and climate. JGOFS investigators should consider making use of IGAC aerosol data to evaluate the atmospheric supply of trace substances which may influence biological productivity (e.g., iron).
The Palmer Long-Term Ecological Research (LTER) Project, which is based at the Palmer Station on Anvers Island on the western coast of the Antarctic Peninsula, aims to define ecological processes linking the extent of annual pack ice with the biological dynamics of different trophic levels within antarctic marine communities. Program objectives are: (1) to document interannual variability in annual pack ice, primary producers and populations of key species from different trophic levels in the antarctic marine food web; and (2) to quantify and understand the processes that underlie natural variation in these representative populations. To provide an observational basis for addressing the Palmer LTER research objectives, an intensive sampling program has been undertaken. This consists of a series of core measurements made every austral spring and summer in the near vicinity of Palmer Station at approximately weekly intervals. Additional information is obtained from an annual research cruise that covers an area about 400 km by 200 km around Palmer Station, and from occasional longer duration process cruises.
Many of the core measurements made as part of the Palmer LTER field program are the same as those made during JGOFS process cruises and at the existing JGOFS time series stations. The Palmer LTER program also includes a modeling effort designed to provide linkages on multiple spatial and temporal scales between biological and environmental components of the ecosystem. An additional goal of this program that is compatible with U.S. JGOFS is to gain a knowledge of the ecosystem adequate to detect the difference between short term variability and long-term trends on decadal time scales. Comparisons between measurements of carbon cycling in the Antarctic Peninsula with those from the Ross Sea will provide a basis for testing hypotheses of zonal regularity within the Southern Ocean.
Plans are well underway for a joint NOAA-WOCE cruise in the region of the proposed JGOFS transect. Investigators at NOAA-PMEL have been involved in a long-term program to document the entry of anthropogenic CFCs, carbon dioxide species, and other physical and biogeochemical tracers into the intermediate and deep waters of Pacific Ocean. As part of this program, plans have been made to re-occupy a series of hydrographic sections in the Pacific at intervals of 5-10 years. This includes a variety of physical and chemical measurements on these sections. One of the key NOAA-PMEL repeat meridional sections is along 170°W (from about 10°S to 67°S) in the southwest Pacific Ocean (WOCE line P15S). Reoccupation of this section is planned for January, 1996 (Austral summer), with station spacing and tracer measurements which meet WOCE Hydrographic Program guidelines. It is anticipated that that both NOAA and NSF supported investigators will be involved in this work, as well as Australian physical oceanographers interested in this region.
An additional request has been submitted to NOAA for ship time to do the work along P15S in November, 1996, to study gas exchange and ventilation at a time when residual winter water remains in the area. It may be some time before any final decisions are made on funding for this section, and on the availability of a suitable research vessel. If funded as planned, this NOAA cruise would coincide with the initial cruise of the JGOFS process study and would, therefore, offer opportunities for multi-ship coordinated research.
NOAA representatives regularly participate in U.S. JGOFS Steering Committee meetings. These meetings, and others as necessary, will be used to co-ordinate planning of NOAA and JGOFS work in this region. By collaborating with the NOAA program, it will be possible to obtain good seasonal coverage of upper-ocean physical, chemical and biological parameters during the 1996-1997 time period. Combining this information with data collected during earlier NOAA cruises, and during WOCE cruises, will further provide a good assessment of interannual variability of physical processes and of CO2 fluxes in the region.
Synthesis of JGOFS Findings
Process measurements are but the first step toward JGOFS goals defined at the beginning of this document. Individual findings must be interpreted in the context of complementary data sets. This will be achieved through a series of data and synthesis workshops. Initially, small groups of investigators working on related topics will gather to assemble composite features built from their individual results. Subsequently, the full contingent of investigators involved in the Southern Ocean process study will come together in a coordinated workshop format to assemble a complete picture of carbon fluxes, and of the biogeochemical cycling of carbon and related elements. Finally, a series of overview papers will be written to synthesize topical findings (e.g., gas exchange, primary production, new production, export flux, remineralization processes) in the context of a complete carbon balance of each region. Synthesis activities are not restricted to interpretation of results generated by the process study. Investigators will build upon observations made during the field program to create a more complete understanding of carbon fluxes and of biogeochemical cycles over broader areas of the Southern Ocean. Strategies to achieve this goal will include exploitation of historical data sets, ranging from archived oceanographic data to processed satellite images, and the use of data generated by contemporary programs, to establish the mean and variability of relevant parameter fields (e.g., pigments, ice, fronts, eddies). Regular interaction between experimentalists and modelers will be required, as well, to improve model capabilities to simulate carbon fluxes and biological processes. Travel support for participating in data workshops and for synthesis working groups will be provided through the U.S. JGOFS Planning Office and, therefore, need not be incorporated into the budgets of individual investigators.
U.S. JGOFS process studies are designed to produce an improved mechanistic understanding of the processes regulating fluxes of carbon within the ocean, as well as fluxes of carbon between the ocean and the atmosphere, and of the physical conditions that influence the temporal and spatial variability of these processes. Long-term goals of JGOFS include the assimilation of these findings into algorithms that incorporate mechanistic understanding and experimentally-constrained parameters from process studies, and assimilate remote data from satellites, moorings, etc. Applications of these algorithms will include:
Investigators supported to participate in U. S. JGOFS programs are obliged to comply with the program's policy of data submission to the U. S. JGOFS Data Management Office at the Woods Hole Oceanographic Institution. Specific requirements for data submission vary depending on the nature of the data. A detailed description of the data submission policy is available separately from the U.S. JGOFS Planning Office at the Woods Hole Oceanographic Institution.