Southern Ocean Work Begins

contributed by Polly Penhale and Eileen Hofmann

The long-range goal for the U.S. GLOBEC program is to understand the interactions between physical processes and marine animal populations with an emphasis on predicting the effects of global change on population abundance and variability in marine ecosystems. Long-range goals for the U.S. JGOFS program are to evaluate and understand on a global scale the processes controlling the fluxes of carbon and associated biogenic elements in the ocean and to develop a capability to predict the response of oceanic biogeochemical processes to climate change. The Southern Ocean provides an opportunity to combine the goals of these two programs to address issues of climate change effects on biogeochemical cycling and marine food web processes and how these interact to control and regulate biological production. Modeling provides one approach for addressing many of the issues related to the long-term goals of both programs. Consequently, the decision was made to issue a joint request GLOBEC-JGOFS request for proposals for modeling work in the Southern Ocean.

The Southern Ocean activities planned as part of the U.S. Global Ocean Ecosystems Dynamics (U.S. GLOBEC) and the U.S. Joint Global Ocean Flux Study (U.S. JGOFS) programs are proposed to begin in the late 1990s. As part of starting these activities, the United States National Science Foundation's (NSF) Office of Polar Programs and Division of Ocean Sciences announced in early 1995 a call for proposals for modeling studies related to the developing science programs in the Southern Ocean. The purpose of the announcement was to encourage modeling studies that will advance the understanding of the biogeochemistry and the interactions between marine populations and physical processes in Southern Ocean ecosystems. In particular, modeling studies were encouraged that would advance the planning and design of multidisciplinary field programs. The goal was to develop the capability to predict the response of oceanic biogeochemical processes and marine animal populations to, as well as their influence upon, climatic change.

Following the recommendations of the national and international workshops and those from the Scientific Steering Committees for U.S. GLOBEC and U.S. JGOFS, proposals for modeling studies were solicited in advance of field programs in the Southern Ocean. It was hoped that modeling studies would provide guidance for the design and implementation of the field programs, both by addressing issues of sampling strategy, and by highlighting key processes and measurements necessary to understand the coupling among physical and biogeochemical processes. Modeling studies were solicited in the areas of (but were not limited to):

In addition, studies that addressed issues that could advance the state of knowledge of modeling as well as provide understanding of the Southern Ocean system were encouraged. Such studies might include ecological models for data assimilation and management, and modeling techniques for matching scales between models.

The Southern Ocean modeling request for proposals resulted in submission of twenty-two proposals, which were split between GLOBEC and JGOFS studies. Of these proposals, three GLOBEC-related proposals were funded from fiscal year 1995 funds. It is anticipated that additional proposals from this competition will be recommended for funding from fiscal year 1996 funds. Below are the abstracts for the funded GLOBEC proposals.

Aggregation Dynamics of Antarctic Krill, Euphausia superba Dana (Mark E. Huntley, Scripps Institution)

Patchiness of zooplankton and micronekton is a feature of central importance in marine ecosystems. In the Southern Ocean, aggregations of krill (Euphausia superba) are of particular interest. The distribution and dynamics of such aggregations are critical to determining the transformation of organic matter (e.g., carbon flux) and the fate of populations in the sea. These phenomena are especially important in the mesoscale and sub-mesoscale domains, where patchiness is most strongly expressed. If the means to predict patch dynamics is lacking, then so is the means to adequately predict carbon flux and population dynamics at these scales. Traditional models of zooplankton patch dynamics generally treat animals as Lagrangian particles whose aggregations are determined solely by processes of advection and diffusion. This approach ignores behavior induced by biotic and abiotic forces and manifested as purposeful motion--motion that clearly is not governed by advection and diffusion. Attempts to acknowledge behavior in models of plankton motility have been successful at the level of the individual animal, but even the most powerful computers cannot run individual-based models to predict aggregation dynamics of n individuals. This proposal takes a new approach to modeling aggregation dynamics, based on "bio-continuum" theory, and provides for model verification against benchmark field data. Rather than relying on traditional advection-diffusion equations, which ignore behavior, the bio-continuum theory recognizes behavioral forces in the context of statistical mechanics. Model output provides information on animal behaviors, manifest as swimming velocities, that are absent from other models of patch dynamics. All key model variables are measurable using common sampling techniques, such as acoustic Doppler and multiple net systems. The proposed research consists of studying both the internal and external forces that act on aggregations of Euphausia superba. First, the internal forces of autocoherence (that act between animals to maintain patch integrity) will be measured in krill aggregations observed in the Gerlache Strait region in 1992. Our database consists of more than 20 such aggregations observed by ADCP and MOCNESS. Second, the effect of external physical forcing on krill aggregations will be studied by embedding krill swarms of typical scales in numerically modeled flow regimes that are typical of the Gerlache Strait region, by combining the Princeton circulation model with our aggregation model. This research provides a novel, dynamic theory of animal aggregations in the sea. A study of the fundamental theory, coupled with model realizations that can be compared to observed aggregations of Euphausia superba, may lead to more realistic predictions of krill patch dynamics in the Southern Ocean. Such predictions are critical to more accurate measurements of carbon flux and the population dynamics of krill.

Modeling the Transport and Exchange of Krill Between the Antarctic Peninsula and South Georgia (Eileen E. Hofmann and John M. Klinck, Old Dominion University)

Increasing evidence indicates that krill populations surrounding South Georgia are supplied by krill exported from the Antarctic Peninsula region. However, little knowledge of the potential krill transport pathways exists. General circulation patterns for the Antarctic Peninsula and Scotia Sea regions are known. However, recent observations have shown considerable mesoscale structure to the flow on the continental shelf west of the Peninsula, in Bransfield Strait, around Elephant Island and in the Scotia Sea, which potentially influences krill transport and retention. Moreover, local hydrographic and current conditions have considerable influence on the development and growth of krill. Hence, understanding and elucidating krill transport pathways or possible retention regions requires knowledge of the mesoscale current and water mass distributions. The overall goal of the research is to investigate transport of krill between the Antarctic Peninsula region across the Scotia Sea to South Georgia. To accomplish this general objective the following specific research objectives will be pursued: (1) implement a circulation model for the Antarctic Peninsula-Scotia Sea region; (2) interface an energetically based model for the development of krill from larva to adult with the circulation model; and (3) use the circulation-krill model to investigate the retention and/or transport of krill in the Antarctic Peninsula to South Georgia. This modeling study is a joint effort between E. Hofmann and J. Klinck at Old Dominion University and Dr. Eugene Murphy at the British Antarctic Survey (BAS) in Cambridge, England. It will provide a framework for analyzing, synthesizing and integrating the large environmental and krill data sets collected by BAS around South Georgia with those from the Antarctic Peninsula region that have come from historical sources (e.g., BIOMASS) and the Palmer Long-Term Ecological Research (LTER) Program and those from the Bransfield Strait and Elephant Island regions from the U.S. Antarctic Marine Living Resources (AMLR) program. Moreover, the proposed modeling studies are relevant to the key science questions set forth by U.S. GLOBEC (GLOBEC, 1990) and International GLOBEC (GLOBEC, 1993) for the Southern Ocean. In particular, it addresses issues related to the role of circulation and biological processes in structuring Antarctic krill populations. Also, quantifying the krill transport (flux) between the Peninsula and Scotia Sea has been identified as a high priority issue by the Convention for Conservation of Antarctic Marine Living Resources (CCAMLR).

Physical-Biological Interactions Controlling Larval Krill Development and Early Survival: Implications for Population Recruitment and Demography of Euphausia superba Dana (Peter J. Franks, Scripps Institution)

This project will investigate how spatial and temporal variability in physical-biological features affects the development, condition and survival of Antarctic krill larvae (Euphausia superba). It is believed that adult spawning behavior and regional differences in primary productivity and temperature are significant forces controlling krill mortality, population demography and recruitment. Using a modified stage-structured larval population model, the effects of spawning behavior and variations in stage durations and mortalities on demography and recruitment will be examined. The model results will be compared with observed larval distributions to determine which processes best account for the observed population structures. Using a detailed metabolic model with stage structure and realistic external forcing, we will determine how much of the variability in stage durations and mortalities can be explained by the effects of food availability and temperature. Larval lipid metabolism will be incorporated into the model for elucidating the influences of physical and biological variability of larval krill condition. Models will integrate the effects of multiple parameters and will be intimately coupled to field observations and laboratory experiments. This study will provide a valuable contribution to the understanding of interactions between marine populations and physical processes in the Southern Ocean ecosystem. The results from this study will be applicable to the concurrent research investigating the physical-biological interactions affecting Euphausia superba in the Souther Ocean and Euphausia pacifica in the California current. The ultimate intent is to quantify the impact of physical-biological patchiness associated with physical features and phenomena on larval condition, demography, and recruitment in euphausiid populations. Understanding species' responses to physical perturbations will elucidate how environments have evolutionarily constrained life-history patterns to maximize survival in inherently patchy and variable systems. Through this understanding, this study will provide insights into the potential effects of climatic change on euphausiid populations and their ecosystems.

(Polly Penhale is manager of the Polar Biology & Medicine Program at the Office of Polar Programs at NSF. Eileen Hofmann is at Old Dominion University and is Chairperson of the U.S. GLOBEC Southern Ocean Working Group.)