U.S. GLOBEC Georges Bank Program Phase III Prologue

The primary objective of the U.S. GLOBEC Program is to understand the physical and biological processes controlling the abundance of marine animals in time and space. Studies under the GLOBEC banner focus on early life stages and recruitment as keys to explaining variations in fish and zooplankton stock abundances and distributions. GLOBEC seeks to demonstrate the links among climatic variation, physical processes active in the sea on shorter time scales, and recruitment rates (U.S. GLOBEC Report No. 6, 1992).

The U.S. GLOBEC program selected the Georges Bank area of the Northwest Atlantic as the first site for intensive study. The reasons were: (1) Georges Bank is in a boundary region with respect to climate, ocean currents and faunal patterns. Therefore, ecosystems in and around Georges Bank can be expected to be highly sensitive to climatic variability. (2) Secondary production on Georges Bank supports a large, commercially valuable fishery. (3) Sea temperatures (and perhaps advective transport) in the region are predicted to change more with climatic variation than are other areas of the North Atlantic. (4) The circulation on Georges Bank is characterized by a distinctive, regionally closed flow field. The spatial and temporal scales of the circulation patterns enable distinct, trackable populations to develop and persist on site for periods long enough to permit meaningful and logistically feasible study by time-series sampling. The NW Atlantic/Georges Bank Study is not restricted to the continental margin and shelf, but encompasses also the processes and phenomena of the larger oceanic boundary region that affect the Bank.

Within the overall objective outlined above, the NW Atlantic/Georges Bank Study has four general goals:

A) To determine the processes that control the Georges Bank circulation and transport of biological, chemical, and geological materials in a strongly tidal- and buoyancy-driven system and to determine the physical and biological processes that control the population dynamics of the target species that are key elements in the planktonic assemblages of the Georges Bank region. The target species are the pelagic larval stages of codfish (Gadus morhua) and haddock (Melanogrammus aeglefinus), and the copepods Calanus finmarchicus, Pseudocalanus moultoni, and Pseudocalanus newmani.

B) To embody this understanding in diagnostic and prognostic models capable of elucidating ecosystem dynamics and responses on a range of time scales, including inter-annual fluctuations.

C) To understand the effects of climate variability and climate change on the distribution, abundance, and production of the target species in the Northwest Atlantic/Georges Bank region.

D) To apply the understanding of biophysical processes which affect distribution, abundance, and production of the target species to the identification of critical variables that support ecosystem-based forecasts and indicators, as a prelude to the implementation of a long-term ecosystem monitoring strategy.

Phases I and II of the Georges Bank program have followed a four-pronged approach consisting of modeling, retrospective/synthesis analyses, broad-scale studies, and process-oriented studies. This approach will be continued in Phase III. The field programs have been structured to have alternate years of intensive process-oriented studies in 1995, 1997, and 1999. Broad-scale studies (via ship, drifter, mooring, and satellite observations) were/will be conducted in each of the intensive process-study years and in the intervening years (1996 and 1998). The continuity of the broad-scale observations over 5 years (1995-1999) will provide the longer-term context for the process-oriented investigations and permit analysis of inter-annual variability in physical forcing and biological responses.

In Phase I, important interactions were established between the physical characteristics of water column stratification on Georges Bank and the timing of biological events which result in the observed distributional patterns of planktonic prey (microplankton and copepods), planktonic predators, and larval fish. For example, adult cod spawn on the Northeast Peak at a time when the water column is well-mixed and microplankton prey are most abundant. Since newly hatched larval cod rely on soft-bodied microplankton for nutrition during absorption of the yolk-sac, the coincidence of spawning with abundant and palatable microplankton is critical to larval survival and, possibly, recruitment. After this early period of larval development (about 10 days), larvae feed on the abundant copepod nauplii, particularly Calanus finmarchicus, which have been advected onto the Bank from the Gulf of Maine. Grazing by numerous predators on the microplankton community reduces their numbers by a factor of 10 or more between the months of January and April. As larval patches are advected around the Bank towards the Southern Flank, insolation and warming of the surface waters induces the water column to stratify with respect to temperature. The now isolated mixed layer becomes a region of rapid microplankton growth and secondary production, particularly in the region of the pycnocline where micro-scale mixing is at a minimum and microplankton form dense patches. Larval cod tend to range vertically between the pycnocline and the surface, feeding extensively at a subsurface maximum of about 15 to 20 m where light intensity is optimal. The burst in production due to stratification of the water column is short-lived because of the lack of new nutrients from off-bank. This leads to a climax Calanus community entering diapause just at the time when cod larvae are metamorphosing into the juvenile stage and taking up a benthic existence.

In Phase II (1997 field year which is just now ending), the primary focus of process studies was on the sources, retention, and losses of water and organisms from the Bank. Research again focused on the target species. Hypotheses concerning these species for Phase II involve retention and loss of water and animals from over and around Georges Bank. Tests of those hypotheses involved four main activities: (1) quantification of target species abundance patterns across Georges Bank and through their growth season; (2) measurement of the vital rates of target species; (3) quantification of physical exchanges of water and biota across the boundaries of the Bank; (4) determination of the interaction of vertical migration and vertical position maintenance behavior on retention and loss of planktonic animals on the Bank.

The primary focus for Phase III (1999 field year) will be on cross-frontal exchange processes. Although each phase has a different primary focus, information on all processes, e.g., stratification, retention and loss, and frontal processes, has been and will be collected during all intensive process-study years. The four major programmatic elements that form the basis of the final field phase of the U.S. GLOBEC Georges Bank Study are:

Broad-scale studies: The broad-scale studies include shipboard surveys, multi-disciplinary moorings, long-term drifter deployments, and analysis of satellite data. Ship-board studies are needed to determine the distribution and abundance of the target species in relation to their physical environment during the pelagic period of cod and haddock larvae (January to June). In addition, there is a continuing need for long-term mooring- and drifter-based observations and interpretation of regional satellite data. The fundamental importance of the broad-scale studies is to provide the basis for multiple inter-annual comparisons of population processes and their coupling to the physical structure and variability of the environment. The broad-scale studies will provide a regional context for the Phase III cross-frontal exchange study and provide further data to evaluate stratification and source, retention, and loss processes.

Process studies: To further identify and understand the physical and biological processes that control the population dynamics of the target species, the Phase III process studies will primarily focus on cross-frontal exchanges. The Georges Bank region has major frontal features around the periphery of the Bank (the shelf/Slope Water front along the southern flank of the Bank and a front between the Bank and the Gulf of Maine waters along the northern edge of the Bank) and a tidal mixing front located near the 60 m isobath which surrounds the well-mixed water over the shallow crest of the Bank. The exchange of physical and biological properties across these fronts can influence the supply of nutrients for primary production, the retention (loss) of the target species and their prey on (from) the Bank, and interactions between the target species, their prey, and their predators. Cross-frontal exchange is influenced by physical processes which determine the location, deformation, and movement of the front including tides, winds, seasonal heating/cooling, and offshore forcing, and by biological characteristics and behavior which may enhance or minimize exchange. Fronts often are regions of aggregation for marine plankton, both because of physical processes such as divergence or convergence and biological responses such as enhanced production or behavior (i.e., depth-keeping swimming). Such aggregations of plankton provide an enhanced food source for predators including larval cod and haddock. Fine-scale description of the physical and biological fields comprising fronts may reveal aggregations of phytoplankton and zooplankton associated with specific physical structures (e.g., density, temperature, and micro-structure) which provide for enhanced predator-prey contact rates and ultimately elevated production of the target species.

Modeling: The research conducted during Phases I and II will result in a significant archive of data concerning abundance and distribution of the target species, source regions, and vital rates. Modeling efforts initially focused on the circulation and progressed to include biological processes and the development of data assimilation capabilities (e.g., through inverse modeling). These archives and tools will provide significant opportunities for hypothesis testing concerning biophysical processes. Data-assimilative studies are expected to continue in hindcast mode throughout Phase III, and the emergent Phase III data relative to cross-frontal exchange will provide the opportunity to test data-assimilative models at finer spatial scales, with resultant improvement in the representation of these processes.

In addition, it is envisioned that a battery of quasi real-time process-oriented data assimilative models will be run on shipboard during the process cruises. Such models will allow continuously updated parameters including the 3D flow field, the vertical and horizontal fine and micro-scale distribution of plankton and cod larvae, in situ feeding and growth conditions for cod larvae and Calanus, and the prevailing prey field (nano and microplankton). Inverse modeling techniques will allow model verification leading to nowcasting and forecasting of the present and future conditions on the Bank.

Finally, the large number of studies of cod and Calanus being conducted across the North Atlantic (e.g., the TASC, CCC, and ICES programs) provide an opportunity for larger (basin) scale modeling of coupled biological/physical dynamics at the basin scale.

Synthesis/comparative analyses: Efforts to synthesize the results of U.S. GLOBEC NW Atlantic studies are encouraged to maximize the utility of GLOBEC research to Georges Bank resource managers and to other ecosystem-based research programs. In addition to examining data sets emerging from this study, studies are encouraged that will compare and integrate data from other regions (especially the Scotian Shelf and Northeast Atlantic) with data from this study. Such comparative studies could help separate those aspects of animal behavior and population dynamics which respond to basin-scale climate variability from those influenced primarily by local variability. Synthesis and comparative studies will be useful for investigating the physical and biological consequences of climatic variation and should be tied to resource management needs.

To Reviewers: please note that by agreement with the NSF program manager, this prologue does not count against the fifteen page project description limit.

Last modified: December 2, 1997