## Northwest Atlantic Implementation Executive Summary

Rationale for the U.S. GLOBEC NW Atlantic/Georges Bank Study: The primary objective of the U.S. GLOBal ocean ECosystem dynamics program is to understand the underlying physical and biological processes that control the population dynamics of key populations of marine animals in space and time (Peterson and Powell, 1991). Most of the ecological work will be conducted on recruitment of zooplankton and fish. It is assumed that changes in the recruitment of fish and zooplankton populations are rooted in the early stages of life. Thus, we can understand the links between recruitment and climatic change only if the relationships between the physical and biological parameters affecting those early stages are described and understood.

The first intensive U.S. GLOBEC field study will be conducted in the Georges Bank region of the Northwest Atlantic starting in 1994 (GLOBEC, 1991c). Reasons for selecting this area are: Georges Bank is thought to be highly sensitive to climatic change because it is positioned in a faunal, climatic and oceanic boundary region; the primary and secondary production on Georges Bank support a large and commercially valuable fisheries; Georges Bank is a region predicted to be more heavily impacted by climate change than other areas in the North Atlantic Ocean; and Georges Bank is of sufficient size and has a physical circulation pattern which enables distinct, trackable populations to develop and persist for long periods that are amenable for time-series study.

The target taxa to be studied include pelagic stages of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus), and the copepods Calanus finmarchicus and Pseudocalanus spp., which are prey for early life stages of the fish larvae. The focus of the Georges Bank Study is to determine how biological and physical processes interact to control the population dynamics of these target species on the Bank. Information obtained can be used to assess the potential fate of these populations under various plausible global climate change scenarios.

This study is both a part of the international GLOBEC Program sponsored by SCOR and IOC, and the Cod and Climate Change Program of the International Council for the Exploration of the Sea. In addition, the study will complement many other national and international programs including the NOAA Atlantic Climate Change Program, the NOAA Coastal Ocean Program, the Canadian Ocean Production Enhancement Network (OPEN), and the Northern Cod Science Program (NCSP). Cooperation with OPEN, NCSP and Cod and Climate Change Program will provide the opportunity to compare results of similar studies from different bank and coastal systems of the North Atlantic Ocean.

Hypotheses: Specific hypotheses developed to address how physical processes affect the population dynamics of the target organisms on Georges Bank are:

- the horizontal and vertical structure of the circulation of water around Georges Bank leads to a circulation gyre; the residence time of water is long relative to biological time scales so that in situ growth rather than lateral exchange is the dominant process controlling the abundance of animals on the Bank;

- seasonal density stratification over the southern flank of the Bank causes prey aggregation in the pycnocline and thus increased survival of predator populations;

- temporal changes in mixing and stratification may control the abundance and species composition of phytoplankton which in turn may result in different rates of growth and production of herbivorous copepods in well-mixed, frontal, and stratified regions of the Bank;

- the occurrence of large, episodic exchanges of water and organisms on/off the Bank contributes to variability in population abundance;

- seasonal density stratification and the processes of turbulent mixing influence predator-prey encounter rates and thus the growth and survival of individual organisms;

- predation rather than starvation is the dominant source of mortality of fish larvae; predation rather than advective loss is the dominant source of mortality of copepods.

Implementation: The Georges Bank Study has four major components: broad-scale field studies, fine-scale process studies, modeling studies, and methodology/instrumentation development. The broad-scale studies will include ship board surveys, moored instrumention and analysis of satellite data. These studies will determine the distribution and abundance of the target organisms in relation to their physical environment over the December to August pelagic period of the gadid larvae. Experimental work on living animals will be carried out during these surveys as well, and will be directed at measuring vital rates of individuals relating to population dynamics, and at gaining new information on basic biology of the target species.

Fine-scale process studies will be nested within the broad-scale observations to investigate specific biological and physical processes. The approach anticipated to be used in most of these studies is to follow a drogue while investigating the physical-biological interactions. Processes associated with vertical mixing and stratification, and with cross-frontal exchange will receive the greatest attention. With respect to biological work, a vigorous field sampling program will be coupled with experimental work on living animals. Close cooperation and interaction will be required between the broad-scale and fine-scale components of the program in the execution of field sampling, with experimental work, and in analysis of the data.

Modeling studies will assist in the formulation and interpretion of the field studies, as well as in providing the context for integration of the results. U.S. GLOBEC already has some modeling studies underway and additional modeling studies may be required. Our goal of integrating the results of modeling with field sampling and experimental studies cannot be overemphasized.

As for methodology and technology, if we are to achieve the full objectives of the Georges Bank Study, we will require the application of recent developments in methodology and technology, both in the sampling systems used and in the analysis techniques applied to samples. Instrumented net systems, multi-frequency acoustical samplers, bio-optical and optical imaging systems, and moored instrumentation will measure the physics and biology on scales relevant to the controlling processes. Biochemical analysis of copepods to indicate growth rate and physiological state, otolith analysis to determine birth date and growth rate of fish larvae, and genetic analyses to determine possible sources of Georges Bank copepod populations are among the techniques that could be considered in this study.

With these different components the specific program objectives are:

1. Quantify abundance of target species in time and space on Georges Bank over the December/August period.

2. Measure vital rates of target species relating to population dynamics.

3. Quantify the physical processes that influence the vertical mixing and seasonal stratification of the water column; determine if the vertical distribution and vital rates of target species are correlated with mixing processes. Examine effects of mixing-driven changes in the phytoplankton food environment on copepod growth rates.

4. Quantify the rates of physical exchanges of water and biota across the boundary of the Bank; determine how these exchange processes are affected by vertical migration behavior and how they influence population abundance.

5. Determine how microscale turbulence interacts with micro-patchiness to affect predator-prey interactions and vital rates.

The field program is structured to have alternate years of intensive study (1994, 1996, 1998). The broad-scale studies will be conducted in each of the intensive years. The focus of the process studies, however, will change in each intensive year. In 1994, the priority will be on fine-scale studies of the vertical mixing and stratification processes; in 1996, the focus will be on processes controlling the inflow, retention and exchange of water and organisms from the bank; and in 1998, on frontal exchange processes. Depending upon funding constraints, some work in each process area may be undertaken in each year. A modest broad-scale program will be conducted in the intervening years (1995 and 1997) to maintain continuity of observations through the entire period of the program.

A number of data sets already exist which could help address some objectives of the Georges Bank Study. Analysis of these data sets is encouraged and could be supported as part of this program.

All data collected during the Georges Bank Study should be managed through a central site, combined with existing data sets for the region and made available to the program participants. Real-time telemetry of data from shipboard measurements and moored instruments, as well as preliminary model results will be made available to investigators in the field to aid in program response to events and unusual observations. Final data sets will be archived and made available to the scientific community. All participants must adhere to the data management distribution and exchange policies of U.S. GLOBEC and the U.S. Global Change Research Program.

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