NWA - Chapter 9.3

## Zooplankton

Chairman: Charles H. Greene
Rapporteur: Stephen Bollens

### 9.3.1 Context

GLOBEC will attempt to understand how physical processes affect the dynamics of animal populations in the ocean. Such an understanding is essential to predicting the consequences of changing physical processes on animal populations in the context of global change.

### 9.3.2 Target species

Although there are numerous zooplankton species worthy of intensive study on Georges Bank, Calanus finmarchicus was chosen as the target species for zooplankton investigations during the GLOBEC field program. This species was chosen for the following reasons:

1. It dominates the zooplankton biomass on Georges Bank during the winter/spring;
2. Congeners often dominate the zooplankton biomass in other boreal and cold-water temperate ecosystems during the winter/spring;
3. It is an important prey item for the larval and pelagic juvenile stages of commercially important cod, haddock, and herring;
4. More physiological and ecological studies are available for the genus Calanus than for any other zooplanktonic taxon;
5. Reasonably good historical data sets are available for Calanus on Georges Bank (however, there is some confusion with regard to C. finmarchicus and C. glacialis in these historical data sets).

### 9.3.3 Field program objectives

The primary objectives of the field program are the following:

1. To examine the demographic processes of birth, growth, and mortality of Calanus in the context of physical transport processes on Georges Bank and
2. To examine the exchange processes between Georges Bank and adjacent deep waters (Gulf of Maine, Slope Waters) with regard to Calanus' population ecology on the Bank.

### 9.3.4 Field program approaches

The field program envisioned by the group will consist of three basic elements:

1. survey cruises,
2. process-oriented cruises, and
3. biological/physical moorings.

Where possible, attempts will be made to coordinate these efforts with the OPEN and other relevant field programs. It was suggested that cruises should be scheduled primarily in the first and third years of the program, with moorings operating throughout the duration of the program. The reasoning underlying this approach was that it would enable the research community to plan appropriate studies in year three based on analyses of data from cruises in year one and moorings in years one and two.

### 9.3.5 Survey cruises

Survey cruises will cover a grid extending over the whole Bank out to approximately the 100 m isobath. In addition, several cruise tracks out to the deep basins of the Gulf of Maine and the slope waters to the south will be conducted to study exchange processes and other factors regulating Calanus' winter/spring excursions onto Georges Bank.

Data sets to be collected on survey cruises might include the following:

- Vertical distribution, abundance, and physiological condition of Calanus
  1. stage distribution
  2. size (length, mass) distribution
  3. physiological and molecular genetic indicators (e.g., lipid content, gonad development, RNA/DNA ratio, gut fluorescence, digestive enzymes, allozymes, mitochondrial DNA)
- Vertical distribution and abundance of food
  1. phytoplankton
  2. microzooplankton
- Vertical distribution and abundance of predators
  1. invertebrate predators
  2. fish predators
- Physical properties - Vertical profiles of
  1. salinity
  2. temperature
  3. light transmittance
  4. downwelling
  5. light
  6. current velocities

Six survey cruises will be conducted monthly from January until June. It was suggested that each will require 2-3 weeks of ship time to complete. It was further suggested that NOAA might be able to provide the necessary ship time for this component of the field program.

### 9.3.6 Process-oriented cruises

Additional cruises will be carried out to conduct process-oriented studies associated with Calanus' population ecology. Cruise tracks will be determined on the basis of early survey cruise results. Processes to be studied on these cruises might include the following:

- Egg production rates
  shipboard incubations
  egg traps
- Development, growth rates:
  shipboard incubations;
  Lagrangian "drift" studies
- Mortality, loss rates:
  shipboard predator-prey experiments;
  Lagrangian "drift" studies
- Physical dispersion - Lagrangian drifters

Four or five process-oriented cruises will be conducted between January and June. It was suggested that each will require about 3 weeks of ship time to complete. It was further suggested that UNOLS should provide the necessary ship time for this component of the field program.

### 9.3.7 Biological/Physical Moorings

Five biological/physical mooring sites were tentatively chosen for the Georges Bank field program (Fig. 6-1). Four mooring sites will be located on Georges Bank and one in the Gulf of Maine. The four sites on the Bank include one on the western edge adjacent to the Great South Channel, one on the northeastern corner, one on the southeastern corner, and one in the center. The central mooring will include one acoustic Doppler current profiler (ADCP) and a variety of hydrographic, bio-optical, and bioacoustical sensors (bio-sensors); the other three Georges Bank moorings will include five ADCPs and the biosensors. The Gulf of Maine mooring will be situated in Georges Basin. It will include an ADCP and the bio-sensors. Moorings will be deployed in the autumn of 1992 and maintained for three years. It is anticipated that moorings might be relocated or new ones brought on line as the field program progresses.