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The Physical Oceanography of Georges Bank and Its Impact on Biology

Robert Beardsley (WHOI), Ken Brink (WHOI), Dick Limeburner (WHOI), Jim Churchill (WHOI), Jim Ledwell (WHOI), Changsheng Chen (UMassD), James J. Bisagni (UMassD), Charles Flagg (BNL), Peter Smith (BIO), Ron Schlitz (NEFSC), Jim Lerczak (WHOI)

This research project has three primary objectives that all serve the broader GLOBEC NWA synthesis effort. First, we seek to more fairly understand the physical dynamics and interactions of several specific processes (e.g., those associated with the seasonal evolution of stratification on the Bank, the crucial flow field over the Northeast Peak, and cross-frontal exchange within the tidal mixing and northern flank fronts) that are thought to play critical roles in zooplankton and fish recruitment. Second, we propose to combine these observationally based process synthesis studies into model-based studies to provide our best descriptions of the Bank's physical environment and its variability on time scales from minutes to monthly to seasonal for the GLOBEC field years. These model studies will use the finite-volume coastal circulation model (FVCOM) developed by C. Chen for coupled physical/biological studies. The model solutions, generated by hindcast and data assimilation approaches, will be used to define and quantify key physical mechanisms and physical/biological interactions on the Bank. Third, we want to provide other Phase IV investigators with as complete a description and understanding of the basic physical processes affecting their observations as possible.

The work has two long-term goals: (a) to refine and quantify the new physical paradigm and the physical/biological interactions that impact the target species, and (b) to develop with Franks, Chen et al. the FVCOM coupled physical/biological model system to understand the coupled physical/biological system on the Bank, including why one year might differ from another biologically. These goals are clearly related, since the proposed data synthesis work will guide model evaluation and refinement, and the model simulations (both process and seasonal prognostic) will provide process understanding and realistic property and flow fields that are essential for quantitative biological modeling.

Zooplankton Population Dynamics on Georges Bank: Model and Data Synthesis

Peter Franks (SIO), James Pringle (UNH), Changsheng Chen (UMassD), Ted Durbin (URI), Wendy Gentleman (UW)

This work will gain a mechanistic understanding of the influences of climate variation on the population dynamics and production of target zooplankton species on Georges Bank (Calanus finmarchicus, Pseudocalanus moultoni, P. newmani, and Oithona similis) through its effects on advective transport, temperature, food availability, and predator fields. Using data analysis and models as tools, results acquired during the first three phases of GLOBEC will be incorporated into a new synthesis of the physical and biological processes regulating zooplankton abundance on the Bank. Physical models will be forced with measured daily, interannually variable data, and coupled to biological models synthesizing the detailed observations collected during the GLOBEC program.

Specific issues to be investigated include: wind control of the advective supply of the target zooplankton species to Georges Bank during January-April; interannual and/or event-level variations in the advective flux of Calanus finmarchicus to Gulf of Maine basin diapausing populations during June-April; interannual and/or event-level variations in advective losses of copepods from Georges Bank and bank subregions; the influence of stratification on the planktonic ecosystem, and how this affects the population dynamics of the target zooplankton species through food and predation. As a link to Phase IV synthesis studies on target ichthyoplankton, our investigation will provide mechanistic insight into the factors determining production of copepod prey for larval cod and haddock on the Bank.

Patterns of Energy Flow and Utilization on Georges Bank

Dian Gifford (URI), James J. Bisagni (UMassD), J.S. Collie (URI), E.G. Durbin (URI), Michael Fogarty (NEFSC), Jason Link (NMFS), Larry Madin (WHOI), David Mountain (NMFS), Debbie Palka (NMFS), Michael E. Sieracki (BLOS), John Steele (WHOI), B.K. Sullivan (URI)

The overall objective of the research is to provide a broad ecosystem context for interpretation of the population dynamics of the Georges Bank GLOBEC target species. The proposed research will synthesize key aspects of production and energy flow, based on US-GLOBEC studies in the Northwest Atlantic, and augment the US-GLOBEC data with information from other sources on production processes at the lower and upper levels of the food web. The primary objectives are to examine several alternate model outcomes of GLOBEC and GLOBEC-related studies that will help to address a number of outstanding issues and to reexamine patterns of energy flow on Georges Bank. The proposed research will enhance and expand the findings of previous investigations, with explicit consideration of factors not addressed in earlier models of this system including:

  1. the microbial food web,
  2. consideration of new and recycled primary production,
  3. spatial heterogeneity of primary and secondary production on Georges Bank,
  4. changes in biomass and production at higher trophic levels, and
  5. the effects of environmental forcing on production processes.

Incorporation of these elements into the modeling effort will permit a more detailed understanding of production processes on the Bank. The first four elements will help provide the broader ecosystem context, while the last provides the link to one of the US-GLOBEC program's principal themes, climate change. The latter will be addressed by comparing several different decadal-scale time periods that reflect differing environmental and fish community regimes:

  1. the cold 1960s characterized by abundant groundfish stocks fished by distant water fleets;
  2. the 1970s, characterized by "average" water temperatures, increased domestic fishing effort and depletion of groundfish stocks;
  3. the 1980s, characterized by "average" water temperatures, overfishing of groundfish stocks, and increases in elasmobranchs; and
  4. the "average" temperature, lower salinity 1990s, characterized by reduced fishing mortality, rebuilding of groundfish stocks, and increases in elasmobranchs and pelagic fish.

Because of large-scale changes in the fish community structure as a result of over-exploitation, a full understanding of the population dynamics of the target species cannot be attained without consideration of changes in other ecosystem components. Individual model networks will be formulated initially to represent each of the above periods. Subsequently, dynamic modeling will be developed to describe the transformations or shifts between these regimes.

Tidal Front Mixing and Exchange on Georges Bank: Controls on the Production of Phytoplankton, Zooplankton and Larval Fishes

Robert W. Houghton (LDEO), Dave Townsend (UME), Changsheng Chen (UMassD), R. Gregory Lough (NEFSC), Lew Incze (BLOS), Jeff Runge (UNH)

Georges Bank supports a rich fishery because: (1) large portions of the bank are shallow enough that light-limitation of phytoplankton is usually not important; (2) deep waters rich in inorganic nutrients are available for mixing onto the bank; and (3) the Bank's clockwise circulation can retain the planktonic stages of important fish species.

The tidally mixed front (TMF) is central to the productivity of Georges Bank through the processes of nutrient injection in the north and retention of larvae on the south flank. These two regions are connected by a circulation pathway along the front in which nutrients lead to phytoplankton and zooplankton growth, creating a donut-shaped region of high production surrounding the crest. We suggest that the productivity of this pathway is the result of northern edge nutrient injections and is susceptible to climatic influences on nutrient supply in that region.

The overall objective of this proposal is to understand the processes within the TMF that sustain the biological productivity of Georges Bank and the success of the target species, cod and haddock. This requires that we understand how mixing and circulation within the TMF supply new nutrients, support primary production, and retain larvae. GLOBEC dye tracer experiments have for the first time measured directly the near-bottom Lagrangian circulation and mixing in the TMF. Results show that vertical mixing in the front, and the on-bank flow through the base of the TMF, are dynamically connected. Our study examines the 3-dimensional dynamics of the TMF based on these measurements. Models will help us assess how the strength of the across- and along-isobath circulation sets time and space scales compatible with the development of cod and haddock larvae.

This project will consist of a mix of data analysis and modeling activities. First, dye dispersion data and simple shear dispersion models will be used to understand the link between cross-bank flow and vertical mixing. Second, a finite-volume coastal ocean model (FVCOM) will be used to calculate the temporal and spatial structure of nutrient flux into the TMF, contrasting northern and southern flank inputs. A coupled FVCOM-NPZ (nutrient-phytoplankton-zooplankton) model will be used to test the following hypotheses: (i) Nutrient injections in the north are advected around the crest of the bank and lead to a plume of elevated phytoplankton and zooplankton production. (ii) The plume enriches the area of larval entrainment on the south flank. If the above statements are true, then production in the plume, can be altered by the nutrient content of source waters in the Northeast Channel of the Gulf of Maine, and these changes will affect the feeding environment of larval cod and haddock. Finally, models incorporating the measured 3-D flow and turbulence fields will be used to examine spatial patterns of larval retention and define the kinds of environmental transitions that larvae experience during this process.

Integration and Synthesis of Georges Bank Broad-Scale Survey Results Peter Wiebe (WHOI), Carin Ashjian (WHOI), Larry Madin (WHOI), Dennis McGillicuddy (WHOI), Dave Mountain (NMFS), J.R. Green (NMFS), Peter Berrien (NMFS), S.M. Bollens (SFSU), Dave Townsend (UMaine), Ted Durbin (URI), Bob Campbell (URI), Barbara Sullivan (URI), Ann Bucklin (UNH), Jeff Runge (UNH)

The GLOBEC NW Atlantic/Georges Bank study identified the pelagic early life stages of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) and the copepod zooplankton, Calanus finmarchicus and Pseudocalanus spp. as target organisms (GLOBEC, 1992) for an extensive and intensive effort to understand the underlying physical and biological processes that control the population dynamics of key populations of marine animals in space and time. Over a six year period, broad-scale surveys of the Georges Bank and adjacent waters were conducted to collect samples for cohort and survivorship analysis of the target fish and zooplankton populations. These surveys included the collection of data on hydrography, acoustics, phytoplankton chlorophyll, competitors, and predators, as well as the target species, in order to provide a description of the biological and physical environment in which the target species resided. More than 30 surveys of the Bank were conducted between January and June/July over the period June 1994 to June 1999.

Phase IV of the US GLOBEC Georges Bank program will synthesize the results from the program's earlier phases to provide an integrated understanding of the population dynamics of key, target species and evaluate how a varying climate may influence these populations. Our intent in this proposal is to capitalize on the very comprehensive broad-scale survey data sets that now exist to address two overarching questions:

  1. What controls inter-annual variability in the abundance of the target species on Georges Bank (e.g., bottom up or top down biological processes, or physical advective processes)?
  2. How are these processes likely to be influenced by climate variability?

Under this proposal, a team of principal investigators will bring together the broad-scale data sets for integrative studies. Most of the analyses to date have been done on an individual or project basis and an integrative approach is needed now. Two general methods of analysis will be used to identify and investigate these patterns and relationships: statistical analysis and inverse modeling using the adjoint method of data assimilation.

The broad-scale data sets represent a unique opportunity to explore the spatial and temporal patterns and relationships between the various measured biological and physical fields as they relate to the population dynamics of the target organisms. These results will provide a fundamental foundation for a complete interdisciplinary synthesis involving all components of the GLOBEC Georges Bank program.

Phase IV Support for the Scientific Investigators' Synthesis Symposia

Peter Wiebe (WHOI) and Robert Groman (WHOI)

The U.S. GLOBEC (GLOBal ocean ECosystems dynamics) research program on Georges Bank, which was initiated in 1994, conducted a three-phase broad-scale and process-oriented field study for a six year period ending in December 1999. During the same period, modeling and retrospective/synthesis analyses were also taking place. The field program has now been completed and many scientific papers describing the results of specific experiments and events have been published. However, a directed effort now is needed to enable investigators who participated in the program and other investigators to collectively bring about an integration and synthesis of the data sets in order to reach a new level of understanding about the physical and biological processes controlling the abundance of target species in the Georges Bank region and more generally of their predators and prey. Phase IV of the US GLOBEC Georges Bank program is thus focused on the synthesis of the results from the program's earlier phases. Each year a series of related workshops will be held to focus on a particular step in the synthesis. Each workshop will focus on a specific topic with a set of specific objectives. At the end of each year a symposium will be held to present the products of these integrated analyses. The last year of the synthesis will be dedicated to the production of a book that will present the overall results of the program and address the original programmatic goals articulated in the Implementation Plan (GLOBEC Report 6).