Joint Global Ocean Flux Study

The objectives of the U.S. Joint Global Ocean Flux Study (JGOFS) are to quantify and understand processes controlling the time-varying fluxes of carbon and associated biogenic elements and to predict the response of marine biogeochemical processes to climate change. The JGOFS Southern Ocean Program, a 3-year effort south of the antarctic Polar Front Zone, is aimed at

The southern oceans are critical in the global carbon cycle, as indicated by the region's size and the important physical processes that occur in it (e.g., deep and intermediate water formation), but its quantitative role in the contemporary global carbon cycle is uncertain. Because the broad continental shelf of the Ross Sea is characterized by relatively high biomass with large phytoplankton blooms in the austral spring and summer, this region has been selected for intensive process studies as part of the U.S. JGOFS comprehensive investigation of carbon and biogenic fluxes in the southern oceans.

Management and scientific services in support of the U.S. JGOFS Southern Ocean Program study: (1) Nutrients and (2) hydrography, coring, and site survey. Robert F. Anderson, Columbia University; Walker Smith, University of Tennessee . The JGOFS objectives for the southern oceans study include determining the response of the southern oceans to natural climate perturbations and predicting the response of the southern oceans to climate change.

To address these objectives successfully, a large field program has been designed to provide various investigators the opportunity to test specific hypotheses that relate to these broadly defined objectives. We expect the field test to last through March 1998 and use two U.S. research ships: the U.S. Antarctic Program's icebreaking research ship Nathaniel B. Palmer and a ship from the University National Oceanographic Laboratory System. Because most of the investigators will use hydrographic and nutrient data from these cruises, this project will support the analysis of nutrient concentrations during the 13 JGOFS cruises. A team of oceanographic experts representing various institutions has been assembled to complete these analyses. The data will be scrutinized for errors and provided in a timely fashion to all project investigators, as well as to the relevant oceanographic data-storage facilities. The hydrography and coring groups have been put together based on the successful model for the Arabian Sea JGOFS study. In conjunction with the nutrient data, the efforts of these groups will form a large portion of the southern oceans JGOFS database that both field investigators and modelers will use to clarify the role of the southern oceans in the global carbon cycle. (S-211 and S-255)

Mesoscale processes and primary productivity at the polar front. Mark R. Abbott, Oregon State University. The Antarctic Circumpolar Current (ACC) provides a pathway between the major basins of the Pacific, Atlantic, and Indian Oceans and has a critical role in the global redistribution of salt, heat, and other ocean properties between these basins. Although improved observations and more powerful numerical models have identified many critical processes, the scarcity of direct observations has greatly hampered our understanding of the physical environment. Scientific understanding of ocean biogeochemistry in this region is even worse. The challenge to understanding southern ocean dynamics (both physics and biogeochemistry) is exacerbated by the strong dynamic link between mesoscale processes and large-scale processes. The dynamic scales approach the scales typical of energetic coastal systems, yet the size of the southern oceans and the impacts on global biogeochemistry and climate make this an important region. Our JGOFS research focuses on the antarctic Polar Front Zone (APFZ) ecosystem, which behaves much like a coastal upwelling system and is characterized by episodic diatom blooms. These blooms result in high f-ratios, high preservation rates of biogenic silica, and relatively little vertical flux of particulate biogenic material. These processes are consistent with observations of bottom sediments. The lack of meridional barriers leads to a "recirculation" of diatoms associated with frontal meanders. Phytoplankton would alternately upwell and then downwell in these meanders, creating a helical circulation path. Although such bloom events may be unimportant in the total productivity of the system, they do have a disproportionate role in carbon fluxes. To obtain data, we will use a high-resolution sampling array of moorings with bio-optical sensors and current meters. During the high-resolution surveys, we will also deploy Lagrangian drifters (some equipped with bio-optical sensors) to characterize the temporal and spatial scales of variability when coupled with the Eulerian observations from the moorings. These data will be analyzed in conjunction with high-resolution SeaSoar surveys and high temporal resolution sediment traps that other JGOFS investigators will use to study the effects of mesoscale processes on productivity and horizontal and vertical fluxes of biological properties. (S-212)

Distribution, sources, and sinks of dissolved organic matter in the southern oceans. Edward T. Peltzer, Woods Hole Oceanographic Institution. The five principal objectives of our project are

We will estimate the net rates of DOM production and consumption by a combination of in situ measurements and shipboard incubation experiments. Large-scale rates will be calculated from a combination of simple box and advection-diffusion models. We will routinely collect samples via small water samples drawn from 10-liter Niskin bottles on the conductivity-temperature-depth rosette. Our work emphasizes making large-scale surveys and repeating these surveys on a seasonal basis and will be conducted in collaboration with a complementary study that emphasizes process-oriented measurements of bacterial remineralization rates. Because the southern oceans constitute the only JGOFS study site where the vertical stability is low and where intermediate and deep water masses are being formed, we are able to observe directly the rate at which DOM is sequestered in the deep ocean. (S-213)

Measurements of carbon dioxide during the JGOFS Southern Ocean Program. Taro Takahashi, Columbia University. Water masses, such as Antarctic Bottom Water and Antarctic Intermediate Water, originate in the high-latitude southern ocean areas and spread through the interior of the major ocean basins, forming a major conduit for exchange of heat and such dissolved gases as carbon dioxide and oxygen between the atmosphere and the interior of the oceans. An improved understanding of the processes governing the physical and biogeochemical properties of the source waters in the southern oceans is important not only for gaining quantitative knowledge of the carbon-nutrient cycle in the global oceans but also for predicting the future course of atmospheric carbon dioxide and hence the climate of the Earth. Our research consists of three kinds of field observations. First, we will make continuous underway measurements of the total carbon dioxide concentration in surface waters throughout the JGOFS cruises to determine the seasonal and geographic variations and the causes of oceanic carbon dioxide sink/source conditions. Second, we will measure the partial pressure of gaseous carbon dioxide and total carbon dioxide in discrete sea water samples to observe depth profiles of the carbon chemistry (especially in the upper 500 meters) at hydrographic stations located at various latitudes during two seasonal cruises. Finally, high-resolution measurements of the hydrographic structure of the uppermost 250 meters and of nutrients will be conducted to document the mesoscale variability of biogeochemical properties within the antarctic Polar Front Zone. We will use a towed pumping system, designated as SeaSoar, on one of the cruises to the antarctic Polar Front Zone to make these measurements. (S-214)

Regulation of primary productivity in the southern oceans: Phytoplankton photosynthesis characteristics from individual cell measurements. Robert J. Olson and Heidi M. Sosik, Woods Hole Oceanographic Institution. Central to the mission of the U.S. JGOFS Southern Ocean Program is an understanding of the composition of the planktonic food web and the efficiency with which it transports carbon to the deep ocean. One of the specific questions being posed is the mechanism of regulation of phytoplankton growth and productivity in this area. Light limitation, micronutrient availability, and grazing pressure all have been proposed to explain the persistence of high macronutrient concentrations and the relatively low productivity often observed. To distinguish among these mechanisms, we will examine photosynthetic characteristics of phytoplankton in natural populations under a range of environmental conditions. We will use chlorophyll-fluorescence induction measurements of individual cells to estimate photochemical efficiency and absorption cross-sections of photosystem 2 (PS2), as well as relative pigment content per cell for different groups of phytoplankton that can be distinguished either microscopically or flow cytometrically. These measurements, carried out on samples from depth profiles and experimentally manipulated assemblages of phytoplankton, should allow us to distinguish among limiting factors for phytoplankton growth. For example, limitation by micronutrients should result in low photochemical efficiency (reflecting the loss of functioning PS2 reaction centers), whereas low-light limitation (due to deep mixing) would be indicated by high fluorescence (i.e., pigment content) in near-surface cells. Grazing control would be implied if photosynthetic characteristics indicate no limitation of intrinsic growth rates. The individual cell nature of the proposed measurements will allow us to evaluate the condition of different groups of phytoplankton (or different cells) in a given water sample and to examine unambiguously the effects of environmental gradients or experimental treatments on given species. In addition, as a result of these measurements, we will be able to characterize the composition of the phytoplankton community by microscopic and flow-cytometric analysis. By observing the physiological state of cells in natural assemblages and experiments and combining this information with information on community structure, we will be able to evaluate the relative importance of light, iron, and grazing as limiting factors for productivity in the southern oceans and interpret more accurately bulk measurements of phytoplankton properties that can be obtained in a high-resolution survey with fast-repetition-rate fluorometry. (S-215)

Benthic cycling and accumulation of organic matter, biogenic opal, and CaCO3 in the JGOFS southern ocean study areas. Frederick Sayles, Woods Hole Oceanographic Institution. Our JGOFS study focuses on benthic remineralization and burial and how these processes contribute to the cycling of the major biogenic components (organic matter, CaCO3, and biogenic opal) in the southern oceans. (S-217)

Oxygen dynamics during the JGOFS southern ocean process study. Michael L. Bender and Mary Lynn Dickson, University of Rhode Island. Our research focuses on using oxygen to constrain carbon fluxes on three scales-large, meso, and local. For the large scale, we will collect air samples and measure the oxygen/nitrogen ratio. Because oxygen concentrations in the atmosphere vary on seasonal time scales as a result of primary production in the spring and summer months and ventilation of the ocean in the fall and winter, knowledge of the amplitude of the oxygen/nitrogen signal will provide a measure of the magnitude and timing of net production for the southern oceans. We collect air samples daily from ships that will participate in the JGOFS experiment to have high-resolution records of zonal and temporal variations of the atmospheric oxygen field. Similarly, we will determine net primary productivity from the mesoscale oxygen field in two ways: first by measuring oxygen/argon ratios and isotopic oxygen-18 from oxygen in sea water, which will be collected from the mixed layer, and second by continually measuring subsurface oxygen concentrations along the cruise track. These two approaches, together with the air-sampling program, will provide a temporally and spatially integrated estimate of net production. Finally, we will measure daily gross and net oxygen production and nitrate assimilation to constrain carbon flow. Gross oxygen production is calculated by spiking and incubating sea-water samples with oxygen-8 labeled water and measuring the isotopic oxygen-18 of the photosynthetically produced oxygen. Net oxygen production is measured by comparing the relative change in the oxygen concentration between initial and incubated samples by automated high-precision Winkler titrations. These measurements will provide daily depth-integrated gross and net production rates in addition to respiration rates. Furthermore, we will conduct photosynthetic irradiance experiments to measure gross and net production during cruises to extrapolate local measurements up to meso- and large-scale using bio-optical data from moorings and satellites. (S-218)

Iron in antarctic polar front zones. Kenneth H. Coale, Moss Landing Marine Laboratory. Our JGOFS project concerns the effect of iron on the productivity of the upper layers of the ocean. Dissolved iron concentrations in surface waters of the open ocean are often extremely low and have been shown to limit primary production in high-nitrate and low-chlorophyll regions of the global ocean. As a result of an earlier JGOFS study in the equatorial Pacific Ocean, models describing the spatial and temporal variability in production and export now include the role of iron as a limiting micronutrient. The observed patterns of production in the equatorial Pacific were consistent with the supply and distribution of iron in this region. For the southern oceans, however, few data are available on iron concentrations and distribution of iron that could be used for such interpretations. For example, we do not know if iron concentrations are enhanced or depleted in southern ocean upwelled waters. Also, little is known concerning the concentrations, distributions, or sources of iron in the southern oceans and, as a result, correlating patterns of production in these waters with the distribution of iron is difficult. As part of our study, we will make systematic measurements of trace metal distributions in the Ross Sea and in the Antarctic Polar Front Zone with an emphasis on iron. These data will provide a base that will allow us to evaluate southern ocean biological processes in terms of a possible iron limitation. The results of this work will contribute significantly to our understanding of iron biogeochemistry in the southern oceans and of factors that control rates of new production in these regions, and they will have a direct bearing on our understanding of the global carbon cycle. (S-219)

Does copepod grazing control large phytoplankton in the HNLC region of the southern oceans? Michael Dagg, Louisiana Universities Marine Consortium. Large phytoplankton, especially diatoms, typically grow slowly in the permanently ice-free regions of the southern oceans, including the Antarctic Polar Front Zone (APFZ), because of limitations of micronutrients (Fe) or light or other limitations associated with deep mixing. Marine scientists believe that under these conditions the growth of large phytoplankton is balanced by grazing from the copepod community. When the growth of large phytoplankton is enhanced by increasing the amount of iron in the ecosystem or by improving light conditions, we believe that the additional growth is mostly absorbed by the functional response of the copepod community. The increased growth of phytoplankton, however, will saturate the grazing diatoms and increases the silicon-to-carbon ratio of sinking matter. This occurs because about 70 percent of ingested carbon is assimilated across the copepod gut wall but silicon is digestively inert. These relationships will be addressed by shipboard experiments designed to measure the feeding rates of large copepods on large phytoplankton of the APFZ and by measuring the ratio of carbon to biogenic silica in copepod fecal pellets and in sediment trap materials. (S-220)

Physical and biological controls of carbon dioxide levels in the southern oceans: A multitracer approach. Paul D. Quay, University of Washington. The southern oceans have been implicated, by ocean models, as an important region of oceanic uptake of anthropogenically produced carbon dioxide. Few data, however, are available to corroborate these model predictions. The three most likely characteristics of the southern oceans to have a major impact on global carbon-dioxide distribution are

The interplay between these three processes controls the surface carbon-dioxide concentrations and, thus, the direction and magnitude of air-sea gas exchange. In our JGOFS project, we will use a combination of chemical measurements and a wind-driven circulation model to determine how deep water upwelling in this region exchanges carbon dioxide with the surface layer and how the air-sea exchange and biological productivity offset the effects of upwelling on surface carbon-dioxide levels. The measurement and modeling activities will yield a quantitative explanation of how the air-sea flux of carbon dioxide in the southern oceans is affected by circulation, gas exchange, and biological productivity. We will use the model results to predict how the surface carbon-dioxide levels and the ratio of carbon isotopes in the dissolved inorganic carbon pool of the ocean respond to changes in circulation rates and pathways caused by different atmospheric conditions. These results will help us to reconstruct past conditions of carbon-dioxide levels in the southern oceans, based on proxy measurements in marine sediments and continental ice cores. (S-221)

The role of particulate organic carbon, small particles, and aggregates in biogeochemical cycles in southern ocean fronts. W.D. Gardner, Texas A&M University. Our component of JGOFS concerns the role of particulate organic carbon (POC), small particles, and aggregates in biogeochemical cycling, and it makes use of optical techniques to quantify the flux of organic carbon. Specifically, the results of our investigation will help us to identify the factors and quantify the processes that regulate the magnitude and variability of primary productivity and the fate of biogenic materials. We will use a suite of optical instruments to measure the in situ distribution of particles over the size range from microns to millimeters. To measure beam attenuation and light scattering, we will attach a transmissometer to over-the-side instruments at profiling stations. We will also make underway light transmission observations with the shipboard in-line sea-water system and will use an underwater video camera to define the in situ particle size distribution. The optical data will be correlated with measurements of particulate matter and particulate organic carbon and with discrete microplankton, chlorophyll, and primary productivity measurements made by others in the program. We will integrate these data with measurements of fluorescence, carbon dioxide, microplankton, mixed-layer dynamics, diel variations, frontal dynamics, nutrients, oxygen, light levels, wind mixing, and dissolved/particulate radioisotope distributions. The objective is to evaluate the role of particles in the packaging and export of carbon and particulate matter from the euphotic zone and the exchange between the large- and small-particle pools. (S-222)

Carbon and nitrogen in dissolved organics: A contribution to the U.S. JGOFS Southern Ocean Program. Dennis Hansell, Bermuda Biological Station for Research. We will collaborate with other researchers to evaluate microbial dynamics, carbon cycling, and the nitrogen cycle, particularly dissolved organic nitrogen (DON). Our goal is to determine the importance and role of dissolved organic carbon (DOC) as an intermediate in the oceanic carbon cycle. Nearly 1,000 gigatons of reduced carbon are estimated to reside in the DOC pool, which has a turnover time of 6,000 years. The average net oceanic carbon dioxide uptake is 2.1 gigatons, only a small fraction of the DOC pool. Although a perturbation in DOC production or sink could affect the balance between oceanic and atmospheric carbon, the dynamics of this large pool remain enigmatic. Our two specific objectives are to determine the concentrations of DOC and DON throughout the water column through an annual cycle and to measure the rates of DOC production by autotrophs and mineralization heterotrophs. (S-223)

Seasonal and spatial variations in the flux of particulate organic carbon derived from thorium-234 in the U.S. JGOFS southern ocean process study. Ken O. Buesseler, Woods Hole Oceanographic Institution. As part of the U.S. JGOFS Southern Ocean Program, we provide a quantitative estimate of upper ocean export fluxes of particulate organic carbon on spatial and temporal scales relevant to understanding biological and physical processes. We will measure the concentration of thorium-234 (a particle-reactive tracer with a half-life of 24.1 days) and its ratio to particulate organic carbon (POC) and particulate organic nitrogen (PON) to derive the appropriate export data. Our research includes collecting thorium-234 and POC samples on cruises spanning the seasonal production cycle in the antarctic Polar Front Zone. The results will be used to constrain the upper ocean sinking fluxes and to derive relationships between production and export across physical gradients (i.e., fronts) during seasonal transitions between low and high productivity. We will also examine the sensitivity of these rate estimates to model assumptions and data quality. (S-224)

The distribution of iron and other reactive trace metals in contrasting productivity zones of the Antarctic Polar Front during the U.S. JGOFS southern oceans study. Christopher Measures, University of Hawaii. Antarctic waters are generally characterized by low standing stocks of phytoplankton and low rates of primary production, despite the abundance of inorganic nutrients in the water column. Many researchers have speculated that the lack of iron in these waters explains this paradox but no one has presented conclusive evidence of the importance of iron or its mechanisms in stimulating productivity. The major goals of the JGOFS Southern Ocean Process Study are, in part, to unravel the factors and processes that regulate the magnitude and variability of primary productivity and use this knowledge to determine how the southern oceans respond to naturally occurring climate changes both in the past and in the future. In this study, we will measure iron and other reactive trace metals in the water column of various productivity regimes of the Antarctic Polar Front Zone. These measurements will be used to further our understanding of the supply and distribution of iron in antarctic waters and, in concert with the measurements taken by other investigators, the importance of iron in affecting the structure of the microbial food web and the dynamics of the various biogeochemical cycles in the water column. As the complex role that iron plays in the magnitude and sequence of biological processes becomes better understood, these measurements will gain importance in modeling the biological effects of iron on marine ecosystems and, indirectly, its impact in global biogeochemical cycles. (S-225)

Physics and recycling efficiency as factors controlling new (nitrate) production in the southern oceans. Raymond Sambrotto, Columbia University, Lamont-Doherty Earth Observatory. The vast size of the southern ocean region together with its abundance of nutrients has led many scientists to speculate about its role in past and present exchanges of carbon with the atmosphere, as well as how this interaction may change in response to future climactic perturbation. To better understand the interaction between ocean and atmosphere, we will measure nitrate uptake and the uptake and regeneration of nitrogen from ammonium, urea, and dissolved amino acid sources in surface water. We also will measure how light affects nitrate uptake and analyze the size distribution to determine how vertical mixing can affect local rates of new production. Although we will sample all of the major physiographic regions of the southern oceans during this project, we will focus on the frontal systems at the northern portion of the study area, as well as the permanently ice-free region immediately to the south. This extensive region encompasses the largest, uninterrupted biogeochemical province in the world ocean. Thus, the work addresses the JGOFS goal of identifying the factors that regulate the magnitude and variability of biological production and its ultimate export from surface water and will be critical to the success of future models used to predict the response of the southern oceans to climate change. (S-226)

Latitudinal variations of particle fluxes in the southern oceans: A bottom-tethered sediment trap array experiment. Jack Dymond, Oregon State University. We will directly measure the downward flux of particulate matter through an array of bottom-mounted sediment traps. These traps will be maintained for 18 months and have a sampling resolution of 17 days with higher resolution during the intense summer phytoplankton bloom. The total export fluxes of organic carbon, nitrogen, biogenic silicates, and calcium carbonate that can be obtained from these measurements represent the basic core variables of the JGOFS program. (S-227)

Latitudinal variations of particle fluxes at the southern ocean: A bottom-tethered sediment trap array (U.S. JGOFS). Susumu Honjo, Woods Hole Oceanographic Institution. This project will make direct measurements of the downward flux of particulate matter through an array of bottom-mounted sediment traps. These traps will be maintained for an 18-month period and will have a sampling resolution of 17 days; during the intense summer phytoplankton bloom the resolution will be higher. The total export fluxes of organic carbon, nitrogen, biogenic silicates, and calcium carbonate that can be obtained from these measurements represent the basic core variables of the Joint Global Ocean Flux Study program. (S-228)

Primary production in the southern oceans. Richard Barber, Duke University; John Marra, Columbia University; Walker Smith, University of Tennessee. The southern oceans are critical to the global carbon cycle-as suggested by its size and the physical process that occurs there (e.g., deep and intermediate water formation)-but its present quantitative role is uncertain. To address the objectives of U.S. JGOFS effort successfully, measuring primary production is required for all process cruises planned for the southern oceans study. Three methods will be used: in situ incubations, deck incubations, and the photosynthetic irradiance response. The areas of study will be the continental shelf of the Ross Sea and the Polar Front region to the north of the Ross Sea. The controls on photosynthesis will also be investigated. We hypothesize that, on the continental shelf, irradiance limitation is the major factor controlling phytoplankton productivity, whereas in the Polar Front region, the availability of iron limits phytoplankton growth and influences the size distribution of the populations. The productivity data, in conjunction with hydrographic data, will form a large part of the southern oceans JGOFS database that at-sea investigators and modelers will use to clarify the role of the southern oceans in the global carbon cycle. (S-231, S-232, and S-233)

Silica cycling and the role of diatoms in the biological pump of the southern oceans (U.S. JGOFS). Mark Brzezinski, University of California at Santa Barbara, and David Nelson, Oregon State University. Our project will make use of the R/V Nathaniel B. Palmer and the R/V Thompson. This component of the Joint Global Ocean Flux Study (JGOFS) concerns the general topic of removal of organic carbon from the surface layers of the ocean and deals specifically with diatoms, a form of siliceous phytoplankton, as a major component of the biological pump of the southern oceans. Diatoms are known to be responsible for the majority of primary production in the Polar Front Zone (PFZ) and along the retreating ice edge in the Ross Sea.

Strong correlation between the fluxes of diatomaceous silica and organic carbon indicate that diatoms are also the autotrophic source of much of the organic matter exported from the surface waters of the southern oceans. The role of diatoms in the biological pump may be especially important in the PFZ, since nearly all of the silica flux in the southern oceans occurs beneath the PFZ. As a result, this region and the waters to the south constitute the largest area of modern siliceous sediment accumulation in the world. Surprisingly, that immense opal accumulation is supported by very low rates of primary production near the surface. Within the water column and upper sediments, the regional cycles of carbon and silica are decoupled to a degree that does not occur elsewhere, and thus, opal-rich, but organic-poor, sediments are forming throughout much of the abyssal southern ocean.

The observational program is designed to reveal the fate of the silica produced and to help constrain the contribution of diatoms to carbon fixation and export in the region. The insights gained regarding the magnitude, fate of biogenic silica, and the factors controlling silica cycling and diatom productivity will help evaluate the role of diatoms in the biogeochemical cycling of elements in the region and explain the mechanisms producing high accumulation rates of diatom silica in the southern oceans. (S-234 and S-235)

Nitrogen and carbon isotope paleo-proxy verification and calibration in the southern oceans (U.S. JGOFS). Mark Altabet, University of Massachusetts; Roger Francois, Woods Hole Oceanographic Institution. Over the last decade, researchers have proposed and developed, to varying degrees, several new deep-sea sediment proxies for important oceanic properties. An urgent need has been recognized for verification and calibration studies, however, particularly in the southern oceans. The southern oceans have long been known as a region important to paleo-oceanography, not only because the region responds strongly to and contributes to the forcing of climate cycles but also because its remoteness makes it one of the least studied regions with regard to paleoproxy verification and calibration. In this respect, the Joint Global Oceans Flux Study (JGOFS) Southern Oceans Program provides paleo-oceanographers with a unique opportunity for seasonal access to a region that extends from the subtropical convergence to the Ross Sea.

Our effort focuses on sedimentary nitrogen and carbon isotopes, both of which, researchers have shown, exhibit large north/south gradients in the upper and surface portions of sediment cores during Quaternary climate cycles. The sedimentary record of these isotopes also indicates a systematic downcore shift associated with late Quaternary climate change. Our project has three objectives:

To accomplish our objectives, we will sample water seasonally in ocean areas between the subtropical convergence and the Ross Sea. Samples representing various aspects of surface-water chemistry, including nitrate-related compounds and dissolved isotopic carbon, will be collected at 1° and 2° latitude resolution along the cruise track. At each mooring, we will also take vertical profiles of the upper-water column. (S-236 and S-237)

New and regenerated production in the southern oceans: Ross Sea study (U.S. JGOFS). William P. Cochlan, University of Southern California; Deborah Bronk, University of Georgia. The broad continental shelf of the Ross Sea is characterized by relatively high biomass with large phytoplankton blooms in the austral spring-summer. This study is designed to assess the rates of new and regenerated production in the Ross Sea continental shelf regime during the productive growing season in Antarctica (spring, summer, autumn). The overall objectives of this study are to obtain accurate and quantitative estimates of the nitrogenous nutrition of the planktonic assemblages in this dynamic system and to understand the factors that control the magnitude and variability of primary production and the vertical flux of biogenic material from the euphotic zone (i.e., export production). A suite of core measurements will be made to estimate new and regenerated production over relevant spatial and temporal scales. New production in the Ross Sea is hypothesized as a function of the evolution of the ecosystems' development and thus regulated by

Core measurements include determining the uptake rates of four nitrogen substrates (nitrate, nitrite, ammonium, and urea) using the 15N-tracer technique at stations distributed along transects from the ice edge (marginal ice zone), across the shelf, slope and into the open ocean. Measurements will be taken at seven depths ranging from 100 percent to 1 percent of surface irradiance during morning, midday, and night. The degree of isotopic dilution will be quantified during nitrogen uptake experiments to determine more accurately the rates of uptake and to estimate rates of microheterotrophic nitrogen regeneration. In addition to the core measurements, additional experiments will answer specific questions about the system, and will refine our estimates of new and regenerated production. (S-238 and S-239)

Thorium isotopes as indicators of export flux and particle dynamics in the southern oceans: Joint Global Ocean Flux Study. Michael P. Bacon, Woods Hole Oceanographic Institution; J. Kirk Cochran, State University of New York at Stony Brook. We will use the U.S. Antarctic Program's icebreaking research ship Nathaniel B. Palmer and a ship from the University National Oceanographic Laboratory System on six process cruises between October to November 1996 and March 1998. Our effort focuses on the spatial and temporal variations in the distribution and particulate fluxes of thorium isotopes (thorium-234 and thorium-228). These isotopes serve as ocean tracers and yield fundamental information about the rates of biogeochemical processes that govern the production and fate of biogenic particles in the oceanic water column. While at sea, we will measure thorium-234 by beta and gamma-counting. Ashore, we will use alpha-spectrometry to measure thorium-228. The data will be interpreted within the context of biological rate measurements and other data to be collected during the study. We will use these data to estimate export fluxes of organic matter, carbonate, and silica from the euphotic zone; to quantify transformation processes such as aggregation and disaggregation in the water column; and to test biogeochemical models. Thorium is a particularly useful tracer in this context because its mode of production is well known; it is taken up quickly by biogenic particles; and its concentration can be measured quite precisely. The removal of thorium by falling particles can therefore be determined, and the inverse problem of inferring the integrated mass of falling particles (the particulate carbon flux) may be solved. (S-240 and S-241)

Bacterial production uncoupled from primary production: Implications for dissolved organic matter fluxes in the southern oceans. Farooq Azam, University of California at Davis; Hugh W. Ducklow, Virginia Institute of Marine Sciences, The College of William and Mary; David L. Kirchman, College of Marine Studies. Our investigation focuses on the role and quantitative importance of bacteria in transforming organic matter. Researchers hypothesize that extreme seasonal and spatial variation in the southern oceans leads to transient uncoupling between bacteria and primary producers, as mediated through the dissolved organic matter pool. The principal goal of our research will be comprehensive spatial and temporal coverage of core measurements (bacterial abundance, biomass, and production). This will allow an integrated quantitative assessment of the fraction of primary production consumed by bacteria. Equally important is the study of the mechanisms controlling the rate and time frame in which bacteria process primary production. This is critical to complement the core measurements and address these issues. Specifically, total bacteria counts will be complemented with counts of nucleoid-containing cells and respiring cells to determine the fraction of the bacterial assemblage that is active. In collaboration with other researchers, we will use incubation experiments to examine variation in dissolved organic matter lability and the growth yield of bacteria. This project will provide a quantitative and mechanistic understanding of variability in a major pathway of carbon flow, the microbial loop, in the southern ocean. (S-243, S-244, and S-245)

Organic geochemical studies in the southern oceans. John I. Hedges, University of Washington; Cindy Lee, State University of New York at Stony Brook; Stuart G. Wakeham, Skidaway Institute of Oceanography. Our JGOFS project is a cooperative study of organic-matter cycling in the water column and in the surface sediments of the southern oceans. The main goals of the effort are

While onboard ship, we will collect suspended and raining particulate material within the water column and from the underlying sediment at several sites along the proposed study transect. We will use both plankton net tows and high-volume filtration of surface water to collect suspended particles. Modular sediments traps will be deployed in arrays of four to collect sinking particles. Specially designed individual traps will selectively collect sinking particles and reject swimming zooplankton without loss of sample or biocide. Two of the four traps on each array will be fitted with 12-tube subsampling carousels to study shorter term variations. We will analyze the collected trap- and sediment-core samples for their basic biochemical building blocks, such as carbohydrates, amino acids, lipids, lignins, and pigments. These biochemical components make up the bulk of the organic matter produced in the upper-water column and are an important fraction of the material buried in the sediment. At the molecular level, they can provide unequivocal information on the sources of organic matter and its decomposition at depth and on the pathways by which biogenic material is transferred to the sediment. We will combine these data with information obtained during prior JGOFS experiments to produce a global picture of the organic chemistry of particulate matter in the ocean. (S-246, S-247, and S-248)

Structure and dynamics of plankton communities in the antarctic front zone: Interactions of physical forcing, iron limitation, and microzooplankton grazing (U.S. JGOFS). Michael Landry, University of Hawaii. The southern oceans are a vast and variable environment with a potentially large role in global carbon cycling. The Joint Global Ocean Flux Study Southern Ocean Program seeks to advance the understanding of this region by investigating seasonal and spatial dynamics in two important subsystems: the Ross Sea shelf and the Antarctic Polar Front Zone. This project will contribute to this effort by determining microbial community structure and by assessing rates of phytoplankton growth and microzooplankton grazing on four cruises covering the spring, summer, and autumn seasons in the open-ocean front zone.

The overall goal of this research is to understand how physical processes (advection, mixing, and light), iron-limitation, and grazing interact to determine plankton community structure and production in the open oceans. Population abundances and biomasses of the plankton community will be assessed by analytical flow cytometry (bacteria and phytoplankton) and microscopy (nano- and microplankton) to determine their temporal and spatial patterns relative to physical features of the polar front. Growth rates of phytoplankton and bacteria and of microzooplankton herbivory and bacterivory will be measured with a suite of complementary methods including dilution, fluorescently labeled prey, size fractionation, and an Isozyme assay for bacterivory.

Results of this project will contribute to the general understanding of phytoplankton control mechanisms and trophic interactions in high-nutrient, low-chlorophyll regions of the world's oceans. They will also help to identify how potential global change affects water-column stability and how physical forcing in the southern oceans may alter food web structure, carbon storage, and export from the euphotic zone. (S-249)

U.S. JGOFS Southern Ocean Process Study: Zooplankton processes (U.S. JGOFS). Mark Huntley, University of California at San Diego. Making use of the R/V Nathaniel B. Palmer and the R/V Roger Revelle, this project will be a seasonal study of the mesoscale spatial distribution of the carbon utilization by zooplankton in the Antarctic Polar Front Zone and the Ross Sea. Evidence strongly indicates that the egestion of pellets by zooplankton can contribute significantly to a highly variable and episodic biogenic carbon flux in the southern oceans. The research approach used will quantify the rate of total fecal production by the meso- and macrozooplankton community at scales that dominate the variability-eddy-resolving spatial scales and seasonal time scales-in both the Antarctic Polar Front Zone and the Ross Sea study regions.

The process of quantification will depend upon measurements of zooplankton abundance and distribution using a SeaSoar-mounted Optical Plankton Counter (OPC) in the Polar Front Zone, a net-mounted OPC in the Ross Sea study area, and underway acoustic current measurements in both regions. Experimental measurements of ingestion rate will be directed at developing empirical relationships to body weight, food availability, and diel periodicity for the principal species of antarctic zooplankton grazers. These relationships will then be combined with measurements of biomass and distribution to compute the rate of total fecal production. The approach addresses those factors that are most likely to produce the greatest variability in zooplankton- produced biogenic flux:

Seasonal contribution of nano- and microzooplankton to antarctic food web structure in the southern ocean (U.S. JGOFS). David A. Caron, Woods Hole Oceanographic Institution and Darcy Lonsdale, State University of New York at Stony Brook. A major role for nanoplankton (2-20 Tm) and microplankton (20-200 Tm) protozoa in pelagic food webs of the world ocean has been firmly established in recent years. Neritic and coastal ocean ecosystems have been extensively studied in this regard, and considerable information is also accumulating on the microbial processes of tropical and temperate oceanic ecosystems. In contrast, the roles of nano- and microzooplankton in the flow of energy and elements through polar communities is less clear because of the difficulties associated with sampling and working in these environments and because of the prevailing dogma that microbial processes may be overshadowed by "classical" phytoplankton-zooplankton-fish food webs in these environments. Clearly, there is now a great deal of information that is contradictory to this dogma, and numerous studies in recent years have documented an abundant and active protozoan fauna in polar ecosystems. Nevertheless, there are still substantial gaps in our understanding of the overall importance of the microbial loop in polar environments.

We will examine questions concerning the ecological role of nano- and microplanktonic protozoa in the water column of the Ross Sea:

The carbon dioxide system in the southern oceans. Frank J. Millero, University of Miami. Our experiment will be conducted aboard the research ships Nathaniel B. Palmer and Roger Revelle. The flux of gaseous carbon dioxide from the atmosphere to the ocean is controlled by the air-sea difference in the partial pressure of carbon dioxide. In the ocean, the dissolved carbon dioxide is in chemical equilibrium with carbonate and bicarbonate ions. The carbonate ion concentration controls the rate of dissolution and precipitation of calcium carbonate and the rate at which inorganic carbon is delivered to the sediment. What is not well known is how these inorganic relationships are affected by biological processes in a region where production and transformation processes are quite large. As part of our research, we will make direct measurements of the carbonate system on seven cruises between 1996 and 1998. This will involve measuring total inorganic carbon dioxide (TCO2), the partial pressure of gaseous carbon dioxide (pCO2), and total alkalinity (TA) in the surface waters, as well as the pCO2 in atmosphere, continuously along the cruise tracks. We will also make complementary depth profiles of the oceanic parameters at some stations in cooperation with the carbon-related measurements planned by other groups participating in the experiment. The resulting carbon-based and nutrient measurements should provide the necessary field data that will be needed to examine the flux of carbon dioxide across the air-sea interface and the changes resulting from primary productivity and the oxidation of plant material. (S-253)