The Southern Ocean is an extremely dynamic environment. The annual expansion and contraction of the pack ice is the largest seasonal process in the World Ocean. Approximately 16 million km2 of the ocean is covered with seasonal ice during the winter and ice free during the summer. Seasonal migration of the ice cover has a profound impact on the pelagic community in the upper portion of the oceanic water column, where the interactions between ice cover and apex predators, such as seabirds and mammals, are most intense.
In addition to seasonal pack ice, Antarctic pelagic communities are affected by free-drifting icebergs that break off from glaciers and ice shelves. These chunks of ice may range in size from a few meters across to immense objects more than 100 km across and hundreds of meters thick. The largest iceberg ever measured was over 310 km long x 100 km wide, larger than the nation of Belgium! Icebergs are dynamic structures that drift, spin, lose pieces through ablation, and eventually melt as they move through the Southern Ocean.
Effects of Pack Ice on Epipelagic Communities in the NW Weddell Sea during a Single Season
My research in the Antarctic began in 1992 with a pilot project designed to examine differences in the composition and vertical structure of epipelagic (near-surface) communities in areas of the Weddell Sea with and without ice cover. This initial study was carried out over a three week period during a single field season in September 1992. We found that individual targets detected with upward-looking acoustic instruments were larger and more numerous in open water than beneath seasonal pack ice. The largest acoustic targets beneath the ice were detected only at night, and pelagic trawls in the ice at night captured species of large, vertically-migrating mesopelagic fishes that were not collected in the same area during the day. Large numbers of scavenging lysianassoid amphipods were collected with baited traps deployed through holes in the pack ice. Deeper traps beneath the ice and similar traps in open water contained no amphipods, suggesting the presence of scavengers associated with the underside of the ice. The results of this study were published in the journal Marine Biology in 1995 (pdf).
Effects of Pack Ice on Epipelagic Communities in the NW Weddell Sea over an Annual Cycle
The results of that initial study led to a second research project in the northwestern Weddell Sea to examine the effects on the epipelagic macrozooplankton/micronekton community of pack ice in a single area that was covered by ice during the winter and ice free during the summer. This study took place on three cruises from September 1995 through December 1996 and focused on the uppermost 100 m of the water column. Sampling was conducted with a 10 m2 MOCNESS trawl. During the first cruise in September-October 1995, the study area was covered with heavy pack ice, and the community was dominated during the day by krill (Thysanoessa macrura and Euphausia superba) and chaetognaths and at night by siphonophores (Diphyes antarctica) and the same two krill species. During the second cruise in April-May 1996, the ice cover was lighter, and the community was dominated by large numbers of salps (Salpa thompsoni) and Antarctic krill (Euphausia superba). Total biomass of macrozooplankton/micronekton collected during the second cruise exceeded that from the first cruise by 30-300x. During the final cruise in November-December 2006, there was no pack ice at the study site, and the community was dominated overwhelmingly by Euphausia superba, though large numbers of Salpa thompsoni and Diphyes antarctica also were collected. The results of this study were published in the journal Marine Biology in 2004 (pdf).
Measurements and surveys on seasonal cruises provide valuable information. Ideally, however, it would be possible to collect information year-around, beyond the time limitations imposed by individual cruises. For this purpose, my doctoral supervisor and long-time collaborator, Dr. Ken Smith of the Monterey Bay Aquarium Research Institute, has worked to develop instrumentation that can be deployed in situ to make measurements with high temporal resolution over long periods of time. One of our intentions while working in the Weddell Sea was to monitor epipelagic communities with upward-looking acoustic instruments that could measure biomass and vertical distribution of animals during the periods between cruises. However, one of the challenges of working in the northwestern Weddell Sea is the prevalence of icebergs that break off from ice shelves and continental glaciers then pass through the so-called "Iceberg Alley" on their way into the Antarctic Circumpolar Current. These icebergs can damage moored instruments, and large icebergs can extend hundreds of meters below the ocean surface. In fact, two acoustic instruments were lost during the second Weddell Sea study. After this experience we began to consider working in an area that facilitated long-term study of the plankton community as well as protection from icebergs.
Ecosystem Studies at Deception Island
The location that was selected for investigation was Port Foster, the flooded caldera of Deception Island, an active volcano and one of the South Shetland Islands, situated west of the Antarctic Peninsula. Port Foster has a maximum depth of ~170 m, and the deepest basin in the center of the caldera is approximately 5 km long x 2-3 km wide. This study involved a series of five cruises between March 1999 and November 2000. It is common for oceanographers to name their cruises, and we chose to call this series ERUPT in reference to working inside an active volcano that erupted most recently in 1967, 1969 and 1970. The ERUPT study included a number of investigators who studied weather, currents, oceanographic conditions, water chemistry, carbon cycling, plankton communities, fishes, sediment composition, benthic communities, seabirds and marine mammals in and around Port Foster. My involvement in this project centered around macrozooplankton/micronekton communities, including a study of Euphausia superba, and the structure and dynamics of epibenthic megafaunal communities on the sea floor. A special volume of the journal Deep-Sea Research II (vol. 50, issues 10-11) was dedicated to the results from this study. An overview of the study, including a description of the site, objectives and major findings, can be found here.
From March 1999 through February 2000, the macrozooplankton/micronekton community in Port Foster was dominated by two krill species, Euphausia crystallorophias and E. superba, which collectively made up more than 94% of total pelagic biomass on a dry weight basis. In May and November 2000, community composition was strikingly different, with cydippid ctenophores comprising roughly one-third of total pelagic biomass on a dry weight basis. The decline in E. crystallorophias representation within the community was especially dramatic, while E. superba proportions increased so that total krill biomass remained relatively stable. The biomass mode of the community shifted deeper during 2000, primarily due to the deeper distribution and absence of diel vertical migration in ctenophores, compared to krill. The complete results of this portion of the study can be found here.
The physiological condition of E. superba from Port Foster was studied using several biochemical indicators. RNA:DNA ratios, indicative of protein synthesis rates, were highest in E. superba during seasons of abundant phytoplankton. Activities of the metabolic enzymes lactate dehydrogenase (LDH) and citrate synthase (CS) were significantly higher in males compared to females of similar size, indicating that male krill may have elevated swimming performance compared to females throughout the year. Highest activities of LDH were observed during the summer for both sexes, and highest CS activities were measured during the summer (females) or autumn (males), with lowest activities for both enzymes in both sexes during the winter. These results suggest that E. superba in Port Foster reduce their activity during the winter, when food supplies in the form of phytoplankton are scarce, and elevate their activity during the summer when food supplies are more abundant. Complete results of the krill physiological study can be found here.
Epibenthic megafaunal communities in Port Foster were studied with a towed camera sled and bottom trawls. The ophiuroid Ophionotus victoriae, the regular echinoid Sterechinus neumayeri and one or more sponge species were the most abundant taxa during the sampling period. The abundance of O. victoriae was highest during June 2000 and was correlated positively with sedimentation rates. By contrast, the abundance of S. neumayeri was relatively constant throughout the year, with a slight peak in June 2000. Peaks in the abundance of these two species occurred during times when the populations included large numbers of small individuals, suggesting that abundance maxima may have been the result of recruitment events. Complete results from this study can be found here.
Studies of Free-Drifting Icebergs in the NW Weddell Sea
One consequence of climate change has been the breakup of Antarctic ice shelves and the resulting proliferation of large icebergs in the Southern Ocean. Little is known about the impacts of free-drifting icebergs on pelagic ecosystems. Existing information suggests both enrichment and depletion of biological and chemical activity in the vicinity of large icebergs, but few studies have directly addressed the effects of icebergs on pelagic systems. In December 2005, I was part of a research team that studied two free-drifting icebergs in the Weddell Sea. The smaller iceberg, W-86, was ~2 x 0.5 km in area with an aerial height of 41 m and a submerged depth of at least 300 m. The larger iceberg, A-52, was 21 km long with a maximum width of 5 km, an aerial height of 25-32 m, and a submerged depth of at least 230 m.
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Last modified 11 May 2011 by Ron Kaufmann