The Antarctic marine food web is unique among ocean ecosystems in that 1) it is characterized by dependence largely on a single key species, Antarctic krill (Euphausia superba), and 2) many species of the food web are dependent on sea ice during some or all of their life history. For these reasons the Southern Ocean marine ecosystem may be especially vulnerable to perturbations caused by changes in environmental conditions (e.g., climate), pollution stress, or exploitation of natural resources. Consequently, documentation of natural population fluctuations and understanding of the mechanisms underlying this variability is critical if prediction of the effects of natural or anthropogenic changes on the Antarctic marine ecosystem is a goal.
The Antarctic marine food web is more complex than the simple linear food chain (phytoplankton-krill-higher consumers) that has often been described for this system (Marchant and Murphy, 1994). However, linkages in the Antarctic food web can be short and may be dominated by few species. The short trophic connections arise because the basic prey types (e.g., Antarctic krill) available to predators are limited and because among the basic prey types, predators tend to concentrate on core groups of species, such as the abundant euphausiids and fish near the base of the food chain. It has been suggested that because of the apparent close coupling between trophic levels, long-term studies focusing on these predator-prey relationships and their environment will not only be critical to understanding variability in the Southern Ocean ecosystem in general, but may ultimately form the basis for monitoring the effects of man-induced perturbations on the system (see Sherman (1994) for a discussion).
Long-term fluctuations in krill abundance are well documented and years of low krill biomass have been attributed to krill redistribution by physical process (Priddle et al., 1988). However, the mechanisms controlling the abundance and recruitment of Antarctic krill are not well known. Similarly, long-term fluctuations in the abundance of top predators have been documented and have been attributed to habitat modifications brought about by changes in environmental conditions (e.g., Fraser et al., 1992), as well as biological interactions. As with krill, the processes underlying the observed changes in top predators are not well understood.
The strong coupling between the Antarctic marine food web and the physical environment, especially the dependence on sea ice, makes the Southern Ocean an ideal environment to test many of the GLOBEC core hypotheses on the role of physical variability on marine animal population dynamics. Many of the scientific concerns and objectives of this program are relevant also to those of the International Whaling Commission.
In the 1980s Southern Ocean research shifted to attempting to understand the processes that controlled primary production. Programs (e.g., JGOFS) have been undertaken to determine the role of circulation, mixed layer depth, stratification, micronutrient (especially iron) limitations, and grazing by protozoa and metazooplankton on limiting primary production in the Southern Ocean. Also, the ecology of sea ice and the impact of seasonal ice advance and retreat on water column biology have received attention. The importance of sea ice as a winter refuge for many pelagic organisms, including krill, as a component of the survival of certain top predators (e.g., Adelie and Chinstrap penguins) and as a productive region during periods of ice melt has become apparent. Also, in addition to krill, copepods and salps are now recognized as important metazoan grazers in the Southern Ocean. The results from recent multidisciplinary Antarctic programs indicate that the pelagic ecosystem is far more complex than the diatom-krill-whale paradigm. Moreover, considerable regional and interannual variation has been observed in the Antarctic marine food web which appears to result from environmental effects (Fraser et al., 1992; Murphy et al., in press).
Studies on krill distribution and population dynamics to date have not resolved the factors enabling maintenance of the enormous krill stock. Various hypotheses have been put forward to explain this unique feature of the Southern Ocean. The hypotheses are not mutually exclusive and none of them alone is sufficient to account for the available observations of krill occurrence. It appears that krill are capable of exploiting a wide variety of food resources in habitats ranging from open water to sea-ice and benthos. The regions where high krill concentrations have been frequently observed share common features, for instance their proximity to frontal zones separating major water masses. However, the reasons why krill congregate there and the underlying mechanisms of swarm formation and dispersal remain obscure.
Food supply is a major factor regulating the abundance and productivity of top predators in the Southern Ocean. For many of these species krill is the primary food source, and despite ecological segregation of many of these species competition for this resource potentially exists. It has been suggested that changes in abundance and population characteristics of some top predator species have come about as a result of food (krill) made available by the reduction in whale numbers (Laws, 1985). For example, Figure 1 shows estimated krill consumption by Antarctic predators before and following an approximately 90% reduction in baleen whale biomass. However, the evidence in support of this hypothesis is inconclusive (Kock and Shimadzu, 1994).
While whaling did no doubt produce changes in the Southern Ocean marine food web, the role of environmental conditions in either mitigating or exacerbating these changes cannot be dismissed (Kock and Shimadzu, 1994). Long term changes have been documented in sea ice cover, atmospheric systems, and current systems. These potentially affect all parts of the Antarctic marine food web through regulating food sources, changing patterns of dispersal, or changing habitat characteristics, for example. To simply attribute changes in prey availability to increases or decreases in predator stocks (e.g., the whale reduction-krill surplus hypothesis) ignores evolutionary processes that have produced strong linkages between the components of the Antarctic marine food web and their environment.
The inability of current hypotheses, such as whale reduction-krill surplus, to adequately explain observed changes in Southern Ocean top predator stocks suggests that the processes responsible for these changes have not been represented in current thinking about this system. The complex nature of the Antarctic system argues for a holistic and integrated approach for studying its response to changes. It is through a research program that includes studies of the environment as well as the organism that the cause and effect underlying changes in the Southern Ocean marine ecosystem will be understood.
At a Southern Ocean GLOBEC workshop held in June 1993, criteria were set forward for selection of top predator target species. The criteria were:
Several of the whale species that are found in the Southern Ocean, such as the minke whale, fit these criteria and were discussed at the workshop as possible target species for U.S. GLOBEC studies. However, at that time, the decision was made to exclude whales as target top predator species because it was believed that the IWC was developing a program for monitoring and studying whales in the Southern Ocean.
Nevertheless, the importance of whales in the Southern Ocean food web has been recognized in Southern Ocean GLOBEC planning. As a result, it was recommended that Southern Ocean GLOBEC develop and maintain ties with the IWC (GLOBEC, 1993). The goal put forward by the IWC of understanding the processes that regulate whale populations in the Southern Ocean makes interfacing with Southern Ocean GLOBEC desirable, as many of the scientific issues are of mutual interest to both programs.
To begin discussions between GLOBEC and IWC it is recommended that a joint GLOBEC-IWC working group be established. This group would be tasked with:
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