Report 18 -- GLOBEC Background

## GLOBEC Southern Ocean Plans

### Overall GLOBEC Science Goals

The overall goal of the Global Ocean Ecosystems Dynamics (GLOBEC) program is "to understand how physical processes influence marine ecosystem dynamics in order to predict the response of the ecosystem and the stability of its food web to climate change." The linkages between the planktonic stages of marine life and physical forces are likely vulnerable to changes associated with variations in climate. For example, long-term shifts in the amount of seasonal sea ice formed around the Antarctic continent will affect physical mixing which in turn will influence primary productivity. These changes in food abundance will affect the entire food web.

The U.S. GLOBEC program has identified the following specific objectives (U.S. GLOBEC Report No. 12):

1. To determine physical influences and biophysical interactions in planktonic communities

2. To understand the dynamics of zooplankton (i.e., holoplankton, meroplankton, and icthyplankton) and their interactions with both lower and higher trophic levels

3. To identify probable changes in living marine resources resulting from climate change.

To accomplish these objectives, U.S. GLOBEC has identified several marine ecosystems types for detailed process studies. The studies will focus on describing the basic characteristics of each system, including variability of populations and their environment, quantify biological and physical rates, characterize the sensitivity of the ecosystem to climate forcing, and determine the critical global change processes. For each system, GLOBEC will design a monitoring system that will link modeling and observations. At present, U.S. GLOBEC has conducted studies at Georges Bank, and is just beginning studies of the Southern Ocean. Upcoming studies include the California Current as well as critical regions of the open ocean.

### Science Issues

Discussions at the initial workshop held in June 1991 highlighted the importance of the annual formation and retreat of pack ice in influencing the structure and function of the Antarctic marine food web. Observations that include the austral winter and extend over several ice cycles were given high priority as components of a Southern Ocean GLOBEC program. In particular, the need for seasonal observations of the abundance and distribution of Antarctic krill (Euphausia superba) and the salp (Salpa thompsoni), which were identified at the workshop as key target species, was noted. Other suggested target species included a commercially harvested fish species (e.g., Champsocephalus gunnari), a non-harvested holopelagic fish species (e.g., Pleurogramma antarctica) and a non-harvested nearshore fish species (e.g., Notothenia neglecta). The workshop discussions also focused attention on the importance of top predators in the Antarctic marine ecosystem. Hence, several penguin species (e.g., Adelie and Chinstrap penguins), the crab eater seal, and the Antarctic fur seal were recognized to be very important and recommendations were made to include these as key species. Benthic communities were also considered to be an important component of an Antarctic GLOBEC program and recommendations were made to include as target species bivalves, echinoderms and crustaceans that have both pelagic and benthic larval stages.

The Southern Ocean workshop convened in 1993 as part of International GLOBEC had as its objective defining the key science question for a Southern Ocean GLOBEC program. These questions were designed to provide a basis from which to develop elements of research programs and to select core study sites. Southern Ocean GLOBEC is focused on understanding population dynamics of certain key species and how these are affected by physical (environmental) variability. Hence, the first part of the workshop focused on narrowing the species of interest. The key zooplankton species were identified as: Euphausia superba, Calanoides acutus and Metridia gerlachei. Salpa thompsoni was recognized also as an important species since it tends to dominate in years and/or locations when Euphausia superba is scarce. The primary target species for the top predator component were defined in terms of the degree of association with ice cover or the ice edge, the degree of dependence on krill, the availability of data from existing and historical studies and the feasibility of study. These species are: Crabeater seal, Adelie penguin, snow petrel, Antarctic petrel, fish, and squid.

The key questions for the zooplankton component of a Southern Ocean GLOBEC program are focused on factors influencing ecological cycles:

- zooplankton overwintering strategies

- seasonal and geographical variations in the distribution of Southern Ocean key zooplankton species, especially in relation to the physics of the environment

- factors affecting successful reproduction

- factors relating to larval survival and recruitment to the adult population

- the distribution of Southern Ocean zooplankton in relation to the distribution of food biomass and reproduction.

Similarly, the key science questions for the top predator component of a Southern Ocean GLOBEC program are focused on:

- effects of variability in the physical and biological environments on predator population dynamics

- the role of ice in affecting foraging performance, reproductive success and survival of top predators

- krill variability and its allocation between several top predator species

- the effect of predator foraging activities on altering the distribution and abundance of krill

- the nature of the functional relationships between krill availability and performance and survival of its predators.

The key science questions for zooplankton and top predators require that a Southern Ocean GLOBEC program study the population dynamics of a variety of organisms that range from small copepods to large seals. Modeling is recognized as an important component of all GLOBEC programs. Many of the important scientific questions identified for Southern Ocean GLOBEC require modeling approaches or input from model simulations. It was suggested that many of these questions be examined within the context of a conceptual model that would be developed for the Antarctic prior to the development of a field program. The conceptual model would also provide a framework for the field program. There are also unique features of the Antarctic ecosystem (sea ice, krill swarming) that need to be included in models. The primary recommendations are that:

- a conceptual model of the Antarctic physical environment and marine food web be developed

- GLOBEC International should sponsor a workshop on observation, theory and modeling of krill swarms

- existing circulation, ice and biological models of the Southern Ocean be evaluated for use as a point of departure for future modeling efforts

- models of krill allocation (as a key species) among various predator species be developed.

### Site Selection and Implementation

During the initial Southern Ocean GLOBEC workshop in 1991, considerable attention was given to the selection of research sites for a Southern Ocean GLOBEC program. Characteristics that were considered to be important for potential sites included the existence of identifiable populations of the key target species, reliable sea ice cover, and accessibility by ships and from shore-based laboratories. One region that meets these criteria is the area that extends west of the Antarctic Peninsula to the eastern portion of the Bellingshausen Sea. Other areas that were also discussed as potential sites were the southeastern Weddell Sea, the northern part of the Atlantic sector of the Southern Ocean, the Ross Sea and the Indian Ocean.

During the implementation meeting in June 1994, it was decided that Southern Ocean GLOBEC would consist of two major field studies, each a minimum of six months duration. The summer study will focus on foraging and recruitment; the winter study will focus on overwintering strategies. These field studies will take place in three areas: (1) the Antarctic Peninsula region, (2) eastern Weddell Sea, and (3) the Indian Ocean sector (Figure 1). The sampling strategy in each region will consist of two elements: a synoptic, mesoscale Time-Series Survey in an area of about 40,000 km2, and Process Studies aimed at understanding phenomena and mechanisms of crucial importance within the survey area. These two elements would alternate at 2-week intervals, providing a continuous research effort over a minimum period of 6 months. A series of standardized measurements will be employed, allowing for comparison of results both within and among the three study sites. A variety of new technologies will be employed for sampling and analysis.

Modeling studies are to be initiated, prior to the advent of field programs, in three key areas: the development of (1) a conceptual model of the ecosystem, (2) circulation models, and (3) biological models. Regarding circulation, mixed-layer and sea-ice models are required, with emphasis on site specific models. Biological models of trophic transfer and krill swarming are particularly needed. Data assimilation methods should be developed.

It is anticipated that modeling studies will begin in 1995, well in advance of field studies, which are anticipated to begin no later than 1997. In fact, the U.S. National Science Foundation-Office of Polar Programs has recently issued an announcement of opportunity for modeling studies in support of Southern Ocean GLOBEC field initiatives. Detailed logistic and scientific plans will be made in a series of three Regional Planning Meetings, one for each study site, held in 1995.