R.S. KAUFMANN* and K.L. SMITH, JR., Marine Biology Research Division, Scripps Institution of Oceanography, La Jolla, California 92093-0202
*Present address: Marine and Environmental Studies Program, University of San Diego, San Diego, California 92110
The southern ocean is one of the most productive areas of the marine environment, supporting an abundant and diverse pelagic community (e.g., Lancraft, Torres, and Hopkins 1991; Hopkins et al. 1993; Voronina, Kosobokova, and Pakhomov 1994). One of the most important features structuring communities in this part of the ocean is the antarctic ice sheet, which covers up to 20 million square kilometers during the austral winter but contracts during the summer to less than 4 million square kilometers (Zwally et al. 1983; Laws 1985). The magnitude of this seasonal variation, arguably the most dramatic seasonal process in the ocean, may have substantial effects on the biota inhabiting the underlying water column (Eicken 1992; Loeb et al. 1997; Nicol and Allison 1997). In particular, the presence/absence of pack ice may substantially affect the trophic coupling between surface predators (e.g., seabirds, marine mammals) and their pelagic prey (Ainley et al. 1986). Until recently, little was known about the community living beneath seasonal sea ice in the southern ocean, due in part to the logistic difficulty of sampling in areas covered by pack ice (cf. Kaufmann et al. 1995). As a result of these logistical constraints, previous studies have been confined to brief periods of time that had broad spatial coverage but poor temporal coverage and low temporal resolution.
As part of a study to sample the pelagic community beneath seasonal pack ice with high temporal resolution over a complete annual cycle, we conducted a series of trawls during September and October 1995 in an ice-covered area of the northwestern Weddell Sea near 63°S 46°W. The primary goal of this effort was to sample the epipelagic community with high temporal resolution during a period when our study area was covered by seasonal pack ice, for later comparison with samples taken during seasons characterized by different degrees of ice coverage. Samples were collected using a multiple opening-closing trawl with six nets [MOCNESS, with a 10-square-meter mouth opening (Wiebe et al., 1985), 4-millimeter circular mesh in the main body, and 505-micrometer (m) mesh cod ends]. Sampling periods lasted 1 hour and covered a 50-meter (m) deep portion of the epipelagic zone, defined here as the region of the ocean between the surface and 100 m depth. Volumes of water sampled during the 1-hour sampling periods ranged from 24,274 to 46,832 cubic meters.
The most abundant species collected with the MOCNESS trawl were the euphausiids Euphausia superba and Thysanoessa macrura and the salp Salpa thompsoni . Also abundant was the siphonophore Diphyes antarctica , and lesser numbers of hyperiid amphipods ( Cyllopus lucasii , Hyperiella dilatata , and Primno macropa ), polychaetes ( Vanadis antarctica and Tomopteris carpenteri ), pteropods ( Clio sp.), and chaetognaths were collected as well. A noteworthy component of our collections was a number of individuals of the pelagic medusa Periphylla periphylla . Although this species typically occurs at greater depths (e.g., Lancraft et al. 1991), we collected five medusae in the epipelagic zone. The largest of these had a fresh wet weight in excess of 4 kilograms, and all but one of the others were larger than 2 kilograms, by far the most massive organisms collected in our nets.
A temporal pattern in biomass was observed at this site; reduced values were recorded in the upper 50 m of the water column between 0800 and 1800 hours and maximum values between 2100 and 0100 hours (figure 1). During the period covered by this study, sunrise occurred between 0430 and 0530 hours (ship time) and sunset between 1700 and 1800 hours. The temporal pattern observed between the surface and 50 m depth was less evident between 50 and 100 m depth; however, a slight elevation in biomass was observed 3-4 hours after sunset (figure 1). It should be noted that trawl-based biomass from 0 to 50 m was consistently lower than between 50 and 100 m (figure 1), possibly as a result of surface perturbations due to the passage of the ship ahead of the trawl.
A portion of this temporal pattern may be explained by diel variation in the depth distribution of the abundant euphausiid Euphausia superba . Although virtually absent from surface waters during the day, substantial numbers of E. superba were collected in the epipelagic zone between 1800 and 0200 hours (figure 2). A similar pattern was observed for Salpa thompsoni , although estimated salp biomass was substantially lower than estimated krill biomass. Thysanoessa macrura exhibited an opposite pattern: elevated biomass in surface waters during the day and a reduced presence at night. Total biomass fluctuations were strongly influenced by T. macrura , as exemplified by the correspondence between total biomass (figure 1) and T. macrura biomass (figure 2) peaks at 1000 and 1400 hours between the surface and 50 m depth.
Our data indicate that biomass distribution within the epipelagic zone in the presence of seasonal pack ice was influenced primarily by krill species, specifically Euphausia superba and Thysanoessa macrura . The results thus far agree well with data collected to the northwest of our sampling site, near Elephant Island, that show a predominance of krill, compared to salps, during the austral spring and summer of 1995-1996 (Loeb et al. 1997). Our sampling program included trawl collections in the same area during April and May 1996 and November and December 1996. Results from these operations indicate substantial differences in community composition among seasons: trawls from April and May 1996 contained up to 100 times more biomass than the samples from September and October 1995. Collections from November and December 1996 were intermediate in size between the other two sampling periods. We will continue to analyze the data from these three cruises and compare the results among periods characterized by different degrees of ice cover, to generate a greater understanding of the role of seasonal ice cover in structuring antarctic epipelagic communities.
We are grateful to C. Scott, T. Lehmann, R. Sliester, K. Chen, D. Stokes, E. Dufresne, J. Drazen, N. Ash, M. Binder, and J. McCloskey, as well as the captain and crew of the R/V Nathaniel B. Palmer for their invaluable assistance at sea. Laboratory support was provided by M. Blakeley-Smith, J. Long, R. Marcos, A. Parker, and C. Witkowski. This research was supported by National Science Foundation grant OPP 93-15029 to K.L. Smith.
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