0. Preface

The following document is a report of a Workshop held in Bermuda in April, 1994, to consider the US JGOFS contribution to future JGOFS research in the North Atlantic. Two particular themes emerged from this meeting and subsequent considerations by the US JGOFS Steering Committee:

1. a Control Volume Experiment (CVE) focussing on the carbon budget near the Bermuda Atlantic Time Series (BATS) site was discussed in some detail.

2. a larger-scale, regional study aimed at constraining meridional carbon fluxes, to be carried out in concert with the WOCE-JGOFS CO2 Survey, was also considered.

This report mainly concerns the former study. The regional survey was to have been the focus of a subsequernt, major US JGOFS Workshop. However, due to the expectation of insufficient resource availability in later years, and the growing realization that resources need to be devoted to Synthesis activities, US JGOFS has recently curtailed planning of that study. As the following report shows, a CVE could be performed in coordination with BATS operations, and would enhance the scientific value of BATS and of JGOFS' understanding of local, 1- and 3-dimensional carbon budgets. This report is being placed on the US JGOFS Web Pages to elicit further discussion of scientific issues and priorities for JGOFS as it begins to consider its accomplishments and future needs. The final report will be published in the US JGOFS Planning Report Series following asuitable period of public discussion.

Hugh Ducklow

28 November, 1995

1 Findings and Recommendations

1. Local, vertical (1-dimensional) carbon budgets cannot be constructed with better than factor of 2-3 accuracy at the best-studied sites (e.g., Bermuda). The uncertainty is caused by some unspecified combination of conceptual and observational inadequacy, but there are few data for the design of future studies addressing this shortcoming.

2. Current estimates of carbon uptake in the North Atlantic are based on coarsely gridded (5-degree boxes, seasonally-resolved) data. Observations in much of the basin are still sparse.

3. Control volume experiments (CVE) provide a powerful approach to better understanding and quantification of carbon flux processes, but require heavy investment in moorings, shiptime and PI support.

4. A powerful array of modeling techniques now exist to enable design and implementation of CVE's optimized for addressing questions about carbon flux.

5. CVE's probably require prior time series and survey information in order to optimize site selection and timing, and allow proper interpretation of the results.

6. At the present time, Bermuda has the richest archive of observational and modeling data, but flux signals there are low relative to advection.

7. Higher latitude sites (e.g., the Irminger Sea south and east of Greenland) have stronger signals and possibly less advective interference, but almost no data for proper design of future studies.

8. A properly designed and implemented CVE would contribute to narrowing the uncertainties in carbon budgets at time series sites, and appears to be a logical extension of time series operations. The CVE model is ideal for answering focused questions about processes. But CVE's alone cannot serve as strawman models for the entire basin.

9. US JGOFS should contribute toward time series and survey operations now being planned by European JGOFS investigators. In addition, a limited and focused selection of specific process studies (e.g., zooplankton grazing and egestion, DOC cycling, export processes) should be undertaken to sharpen questions and improve models.

2 Introduction

The report contains the record of a U.S. JGOFS planning workshop for future process-oriented research in the North Atlantic Ocean, held at the Bermuda Biological Station for Research, 5-8 April, 1994. The U.S. JGOFS planning has proceeded in parallel with, and in response to international planning activity carried out by the JGOFS North Atlantic Regional Planning Group. Some of the background is summarized here, followed by an Executive Summary of the Bermuda meeting.

One of the first major activities of JGOFS was the North Atlantic Bloom Experiment (NABE), carried out along longitude 20°West in 1989-1991. U.S. JGOFS participated in 1989 only, executing a pilot field study with 3 process cruises. Germany, The Netherlands and the United Kingdom (BOFS) continued the program with cruises in 1990-91. Initial results of the 1989 studies have been reported in a special volume of Deep-Sea Research (Ducklow and Harris, 1993). The comprehensive results of the recently completed U.K. BOFS Program are summarized in a Final Report (NERC, 1994). A special Discussion Meeting sponsored by the Royal Society of London in September 1994, titled "The Role of the North Atlantic Ocean in the Global Carbon Cycle" provided a chance for another look back to NABE and ahead toward emerging issues about the North Atlantic. Proceedings of the Royal Society meeting were published in Philosophical Transactions of the Royal Society (Eglinton et al., 1995). These published reports contain much of the scientific background on which any subsequent research in the North Atlantic will be based.

2.1 International Planning

Implicit in the identification of NABE as a pilot study was the expectation that JGOFS would revisit the North Atlantic in later years. Active planning for further JGOFS research on the carbon cycle in the North Atlantic began with the formation of the North Atlantic Regional Planning Group (NAPG), chaired by Richard Lampitt (IOS-Wormley) in May, 1992. U.S. JGOFS representatives included Hugh Ducklow (VIMS), Catherine Goyet (WHOI) and John Marra (LDEO). A larger workshop was held at the Institut fur Ostseeforschung, Warnemuende, Germany in May, 1993 (JGOFS Report 12). The third meeting of the NAPG took place at the Station Biologique, Roscoff, France in May, 1994. At that meeting Mike Fasham (Rennell Centre-Southampton) succeeded Lampitt as Chair of the NAPG. At the Roscoff workshop, the elements of a science plan were sketched out. At the Roscoff meetings, European JGOFS delegates began to discuss preparation of proposals to be submitted to the Commission of the European Communities Marine Science and Technology (CEC-MAST) Program for support of fieldwork beginning in early 1996. Implementation of the MAST proposal was the focus of a followup NAPG meeting held at the University of Bergen, Norway, in May 1995. Subsequent to the Bergen meeting, the MAST proposal was declined. Further plans for coordinated research by JGOFS in the North Atlantic are uncertain.

2.2 U.S. JGOFS Planning

U.S. planning for a possible future process study in the North Atlantic began with the formation of an informal planning group, and a short meeting to discuss the scientific issues was held in Oxford, MD in March, 1993 (U.S. JGOFS News 4(3), pg. 6). The purpose of the Oxford meeting was to define the principal aim of future U.S. JGOFS work in the North Atlantic. This objective is to gain a more precise estimate of the size of the atmospheric carbon sink in the North Atlantic. At the meeting in Warnemuende, this objective was adopted as the overall goal of coordinated JGOFS research in the region. The goal is stated as follows:

"To reduce the uncertainties in the estimates of the size of the

carbon sink in the North Atlantic. This is to be achieved

by improving our understanding of biogeochemical and physical

processes regulating the net uptake of atmospheric CO2."

Several elements of future process studies were discussed in Warnemuende. Control Volume Experiments (CVE) were identified as one possible tool for constructing a carbon budget, and therefore, determining the relative importance of various processes in a specified volume of upper ocean. Several of the European nations expressed interest in this research direction and so the scientific and logistical details of implementing a CVE were set as the main topic of discussion at the Roscoff meeting. In Roscoff it was decided not to pursue a CVE internationally, but to concentrate on a regional study in the Imrmiger Gyre (see above). However, the scientific rationale and implementation needs for performing a CVE were the focus of the second U.S. North Atlantic Planning meeting, held in Bermuda a few weeks prior to the Roscoff meeting.

A group of scientists representing physical, chemical and biological oceanographic viewpoints as well as representatives from IGAC and GLOBEC met at the Bermuda Biological Station, 5-8 April, 1994. The group took as its starting point the goal agreed to in Warnemuende and considered the relevance of a control volume experiment to the goals of US JGOFS in the North Atlantic Ocean and discussed some of the scientific and logistic aspects of such a study.

3.0 Executive Summary

3.1 Summary of Findings

The North Atlantic is unique among the major ocean basins as a strong CO2 sink, characterized by marked seasonal drawdowns of DIC and pCO2, which are driven by a spatially- and temporally-varying balance of physical and biogeochemical processes. In spite of intensive, but temporally- or spatially-limited studies of these large signals, carbon budgets have not been closed or successfully balanced, even to a first order. We are still unable to reconcile different geochemical and biological estimates of key processes of carbon fixation, transformation and export in the upper ocean. The problem is mainly due to poor conceptual understanding of the fundamental biogeochemical processes and their linkages to the physical climate system, which lead to inadequate sampling strategies. Uncertainties in the measurements, which in some cases are close to or even larger than the expected signals, also contribute to uncertainties in carbon budgets.

A new generation of coupled observational and modeling studies is likely to be the key to significantly improving our understanding of the processes driving the carbon cycle, refine flux estimates, and balance local- to basin-scale carbon budgets. Such studies are ideally broadly collaborative among various national JGOFS and other global change programs (GLOBEC, IGAC, WOCE, EOS, CLIVAR, etc.), and last more than one field year. They ideally include a mix of the elements of the original JGOFS components of process studies, time series and survey operations, and the use of state of the art technologies including remote sensing and autonomous chemical and optical sensors.

3.2 Scientific Presentations

The first 1.5 days of the meeting was devoted to a series of presentations which are summarized here. Reports of each presentation are included in the remainder of this report. Presentations were divided between observational and modeling perspectives. The scientific rationale for a control volume study of carbon fluxes was presented by Tony Michaels and Nick Bates of BBSR. The term Control Volume Experiment has several different connotations to different people, including a very exact sense understood primarily by physicists. In the discussions at Bermuda, and in this report, it should be taken to mean any one of a class of linked observational and modeling studies seeking to estimate fluxes of mass through some three-dimensional volume of water, and transformations between chemical states within it. Observations of the annual carbon cycle at the BATS site and budgets constructed from sediment traps and other flux estimates suggest that our conceptual understanding of the carbon cycle, and/or our technical capability for observing that cycle, are still inadequate to close the budget. Well-designed CVE's, including real-time modeling studies were suggested as one path toward better closure of local carbon budgets.

Taro Takahashi and Catherine Goyet presented summaries of their observations of pCO2 and TCO2 variability in the North Atlantic and equatorial Pacific oceans. Alt6hough the variability cannot be predicted without a better understanding of the processes generating it, Takahashi and Goyet were proponents of the idea that the variability itself should be the objective in these studies.

Dennis Hansell contributed some ideas on the potential importance of the DOC pool as a short- or longer-term storage pool. Chris Garside emphasized the longer-term goal of JGOFS, a basin-scale understanding of carbon fluxes, and presented an approach to constructing algorithms with which nutrient distributions could be estimated from more finely resolved data like temperature and salinity. John Marra used observations made near 59°N, 20°W in the Marine Light-Mixed Layers program to demonstrate how well (or how poorly) incubation-based estimates of primary production compare with those inferred from bulk changes in water column properties. The latter kind of estimates might be better constrained in a CVE-type approach. Finally Jim Price discussed some of the physical oceanographic constraints on a CVE study.

Modelers Scott Doney, Dennis McGillicuddy, Don Olson and Allan Robinson discussed a variety of approaches to supporting CVE's with models. In particular, Robinson described the application of Observation System Simulation Experiments (OSSE's) which could be used to design field observation programs. As Robinson put it, "The ocean is a very complicated nonlinear system. It is hard to outguess it. Observed behavior must be used to construct the models." Doney and McGillicuddy described rather simple ecosystem models coupled to more sophisticated physical models. Olson addressed some ways in which the ecosystem models can be refined to treat complex plankton systems more realistically. A major problem with either approach is validation of the model behavior.

3.3 Interactions and Collaboration with Other Programs

The resource spectrum available for future work might be enhanced by collaboration with other programs. Alex Pszenny provided an overview of the International Global Atmospheric Chemistry (IGAC) Program. IGAC has a strong ocean component, with particular interest in better understanding of the sulfur cycle. The IGAC North Atlantic Regional Experiment will be carried out in 1997 in the region of the Azores with aircraft support. Tom Powell talked about GLOBEC, a program addressing the effects of global change on oceanic ecosystems at higher trophic levels (zooplankton and fish). Modeling is a strong common thread between JGOFS and GLOBEC. Joint efforts in the development of OSSE's need to be carried out at several sites, including the North Atlantic. In spite of many areas of common interest, the precise nature of interaction and collaboration among these 3 programs (and others) still need to be spelled out. Lack of space on ships, different time frames, and different priorities make close interaction difficult to achieve.

3.4 Discussions

The group did not, and has not reached a final conclusion regarding the priority that should be assigned to a control volume experiment in the North Atlantic. The reason for a lack of consensus is that the three program elements in JGOFS, time series, surveys and modeling, require rather different mixes of observational approaches, and proceed on different time and space scales. Time series observations provide local understanding of temporal variability. Wide areal surveys provide global (at least basin-wide) understanding of spatial variability. Process studies, including CVE's, are complementary of the other two modes of study. They combine some aspects of each and are typically performed on different temporal and spatial scales from the two other modes of study.

Elements of all three observational modes are required for improving our understanding of North Atlantic carbon fluxes. JGOFS time series observations have been limited to lower-latitude sites, and the temporal (seasonal) context in which to view processes and fluxes over much of the basin is still lacking. A seasonally- resolved survey with moderate spatial resolution could be carried out over some limited area. Together, these studies provide the spatial and temporal context in which a CVE aimed at better constrained carbon budgets could be most profitably conducted. Meeting participants articulated a varying priority of resources to assign to each of these study modes.