NWA - Chapter 6

FIELD PROGRAM LOGISTICS

The GLOBEC Georges Bank field study is anticipated to begin in FY 1992 and to continue, in some aspects, for three years. In general terms, it is recommended that there be three principal and complementary sampling modes:

Bank-scale Synoptic Survey Cruises;
Process-oriented Cruises; and
Biological/Physical Moorings.

Physical oceanographic studies and biological investigations of all taxa would be carried out in each of these sampling modes. Where possible, attempts should be made to coordinate these cruises with ongoing programs in the region (e.g., OPEN). Cruises should be scheduled in the first and third years of the field program, with moorings operating continuously for the duration. This will permit results from the first year to help formulate altered strategies for the subsequent field program.

6.1 Bank-scale synoptic survey cruises

6.1.1 Survey domain and resolution

Survey cruises would cover a grid of approximately 20 km interline spacing, extending over the entire Bank to the 100 m isobath. In addition, selected transects should be run into the deep basins of the Gulf of Maine and the slope waters to the south in order to study exchange processes and to enable comparative studies of regionally disparate populations.

6.1.2 Timing

In Years 1 and 3, a total of 6 survey cruises, each of 2-3 weeks duration, should be conducted monthly from January through June. This amount of time is required to canvas the region at the appropriate resolution. Cruises of similar duration, but at lower frequency, may be sufficient to characterize the system for the remaining 6 months of the field year.

6.1.3 Critical measurements

Working groups agreed that survey sampling procedures of mutual interest would require the following general types of collections:

Vertical distributions, abundance and physiological condition of target species, with an emphasis on differences above and below the seasonal thermocline;
Vertical distribution and abundance of food (the definition of which depends upon the species of interest);
Vertical distribution and abundance of predators;
Physical, chemical and biological properties, focusing on vertical profiles of salinity, temperature, inorganic nutrients, beam transmission, downwelling irradiance and current velocities.

Key instrumentation may involve multiple net samplers (e.g., MOCNESS, BIONESS), optical plankton counters, acoustics, silhouette photography and image analysis, and sonar for fish schools. Physical, chemical and biological properties could be sampled by a consortium of instruments including CTD, radiometer, beam transmissometer, ADCP, and chemical analysis of water samples collected by rosette. Innovative methods in biochemistry/biotechnology which are sufficiently developed would also be incorporated.

6.2 Process-Oriented Cruises

6.2.1 Domain and resolution

Locations of the process-oriented cruises would be determined on the basis of preliminary results garnered from initial survey cruises. Sites which have already been identified as being of key interest are major spawning sites.

6.2.2 Timing

Similar to the survey cruises, process-oriented cruises would be conducted primarily in Years 1 and 3, with an intermediate year permitted for data analysis and interpretation. The exact timing of the cruises will depend to a great extent on the determination of which periods are deemed to be critical to understanding population dynamics of the target species being studied.

6.2.3 Critical measurements

Measurements should focus primarily on processes at different time and space scales. Certain types of studies are envisioned as essential, and can be described with respect to general location:

Fish larvae:
1. A key spawning site, at which a cohort of larvae can be identified, followed using drogues, and sampled at frequent intervals;
2. A location where feeding and growth can be measured with respect to development of seasonal stratification;
Zooplankton:
1. Near the northwestern boundary of the Bank, on the border of the Gulf of Maine, where intruding waters in late winter are thought to transport Calanus as it breaks out of its overwintering state;
2. Measurements of physiological rates conducted in conjunction with drogue and stratification studies on fish larvae;
Benthos:
1. Major spawning sites, for as many as six different species, from which drogue studies can be used to follow aggregations of larvae until settlement and losses by diffusion and other physical processes to the settled patch could be estimated.

Measurements would concentrate particularly on shipboard and in situ estimates of physiological rates such as egg production, growth, and feeding, as well as key population dynamics parameters such as mortality. The theme of these cruises would be to attempt to follow localized aggregations and to monitor key changes in their demography, physiology, and distribution as it is affected by physical processes and behavior through time. Of great interest is the relationship of these population processes to the development of seasonal stratification. A variety of background physical, chemical and biological measurements made during the process-oriented cruises would provide continuity with data generated from the survey cruises.

6.3 Moorings

6.3.1 Domain and resolution

Approximately five mooring sites are desirable: four on Georges Bank and one in the Gulf of Maine. The four Georges Bank sites include one on the western edge adjacent to the Great South Channel, one on the northeast corner, one on the southeast corner, and one in the center of the Bank (Fig. 6-1).

6.3.2 Timing

Moorings could be deployed in fall/winter of the first field season and maintained for three years. It is anticipated that moorings might be relocated or brought on line at new locations as the field program progresses.

6.3.3 Critical measurements

It is recommended that each of the five mooring sites include a central, heavily instrumented, mooring and an array of four peripheral current meter moorings. Central moorings might include Acoustic Doppler Current Profilers (ADCPs) and a variety of hydrographic, bio-optical and bioacoustical sensors (bio-sensors), whereas peripheral moorings would include only current meters. Mooring arrays are necessary to properly quantify tidal components of the circulation.

Selected process studies should be conducted in the vicinity of mooring sites; similarly, survey cruise transects should be brought fairly close to these sites as well.