Return to the Table of Contents for this chapter.
Salps (Salpa thompsoni) and other macrozooplankton taxa are important components of the antarctic pelagic ecosystem and food web. Salps are of particular interest because during the spring and summer months of some years, they may undergo explosive population growth and numerically dominate the zooplankton in the upper water column. During those times, salps may be a major competitor of krill for phytoplankton food resources (Siegel and Loeb 1995).
Macrozooplankton were collected along with krill using a 1.8-meter (6-foot) Isaacs-Kidd Midwater Trawl (IKMT). Sampling specifics are presented in Loeb and Siegel (Antarctic Journal, in this issue). All samples were analyzed onboard using fresh material. All salps were removed from the samples. For entire samples of fewer than 100 individuals, the two forms (aggregate/sexual and solitary/asexual) were enumerated, and internal body length (Foxton 1966) was measured to the nearest millimeter (mm). Representative subsamples of 50-150 individuals were analyzed for larger catches. After salps, krill, and adult fish were removed, the remaining zooplankton samples were analyzed. All larger organisms (e.g., amphipods, other euphausiids) were sorted, identified to the lowest taxon possible, and enumerated. The smaller constituents (e.g., copepods, euphausiid larvae) in representative aliquots were then enumerated using dissecting microscopes. Abundance estimates are expressed as numbers per 1,000 cubic meters (m3) of water filtered. Data are presented for the entire large-area survey during each leg of the cruise.
Seventy taxonomic categories were identified in Survey A samples (23 January to 4 February). Copepods were present in all 91 samples and constituted the most frequent and abundant category (794.4 per 1,000 m3; table). Larval stages of the euphausiid Thysanoessa macrura were second in abundance (308.5 per 1,000 m3) and occurred in 90 percent of the samples. Among the larger zooplankton constituents, T. macrura postlarvae were most frequent (99 percent of tows) and ranked fourth in abundance. Postlarval T. macrura were most abundant in Bransfield Strait and waters adjacent to King George and Elephant Islands, whereas the larvae were most abundant in the Drake Passage ( figure 1). These distributions are almost diametrical, as indicated by a significant negative correlation (Spearman's R = -0.40, p<<0.001) between the larval and postlarval abundance values at each station. Salps occurred in 65 percent of the samples and were the sixth most abundant taxon (20.4 per 1,000 m3). They were most frequent and abundant in the Drake Passage ( figure 2). Lengths ranged from 4 to 135 mm; the majority (74 percent) were 22 to 50 mm ( figure 3). Virtually all (98 percent) were the aggregate form. Although chaetognaths were relatively frequent (68 percent of samples), they were not numerous (12.5 per 1,000 m3) and ranked seventh in mean abundance. Early calyptopis stages of krill larvae were present in 22 percent of the samples, indicating successful spawning about 3-4 weeks earlier (Ross, Quetin, and Kirsch 1988). The pteropods Limacina helicina and Clione limacina were also relatively frequent and abundant.
Sixty-seven zooplankton taxa were identified in Survey D samples (24 February to 8 March). Copepods again numerically dominated; they were present in 90 samples and their mean abundance value (1,387 per 1,000 m3) was nearly twice that during January (table). The frequency of occurrence as well as mean and relative abundance values of postlarval and larval stages of T. macrura were similar to those observed during Survey A. Their distributions were also similar to those during January and negatively correlated (Spearman's R = -0.44; p<<0.001). Chaetognaths increased in frequency of occurrence (37 percent) and mean abundance (fivefold) over the previous month. Although krill larvae were encountered with greater frequency and abundance than during Survey A, they were not among the dominant taxa, ranking seventh in mean abundance. Calyptopis stages were most common (57 percent of stations, 85 percent of total larvae); early furcilia stages occurred at 18 percent of the stations. The pteropods L. helicina and C. limacina were notably less frequent and abundant than during Survey A.
Although mean salp abundance (28.2 per 1,000 m3) was higher than during Survey A (table), the median catch of 0.9 per 1,000 m3 was only 20 percent that in January indicating an overall abundance decrease across the area. Salps were again most frequent and abundant in the Drake Passage. Solitary forms made up a considerably larger proportion than during Survey A (21 percent). The overall size distribution was much more complex than during January (figure 3). Although a broad size range was again represented, small and large individuals were present in substantially greater proportions. A strong mode was centered around 7 to 12 mm lengths (25 percent of total); these were mostly aggregate forms (82 percent). Individuals larger than 54 mm made up 15 percent of the total; most of these (70 percent) were solitary forms. The 37 to 50 mm central size mode was due primarily to aggregate forms (88 percent). Cluster analysis indicated that the small-sized individuals were primarily distributed in northwestern portion of the survey area, whereas predominantly larger sizes were distributed to the south and east of these.
The overall median salp abundance decrease and composition changes between Surveys A and D were associated with the seasonal reproductive cycle and ontogenetic vertical migrations of S. thompsoni (Foxton 1966; Casareto and Nemoto 1986). Mean salp abundance values for the January through March survey periods in 1996 were similar to those of 1995; these values were two to three orders of magnitude lower than during the 1993 and 1994 "salp years" (table). Relatively low salp abundance in 1996 and 1995 followed winters with extensive sea-ice development; the 1993 and 1994 "salp years" followed winters with little or no sea-ice development, a condition apparently favoring explosive springtime salp population growth. The overall salp length frequency distribution during January 1996 was similar to that during January 1994, but differed significantly from that in 1995 (Kolmogorov-Smirnov test p<0.01; figure 3). Size composition differences probably result from differences in source areas (e.g., Bransfield Strait vs. Drake Passage) and length of the growing season.
The mean abundance values of postlarval T. macrura during 1996 were similar to those during 1994 and 1995 (table). Abundance of T. macrura larvae in January 1996 was an order of magnitude greater than in 1995, whereas that during February and March was 50 percent greater suggesting an earlier seasonal spawning period and increased larval production during 1996. The distinctly different larval and postlarval distribution patterns were previously reported for the February and March 1990 survey (Nordhausen 1991). Low numbers of krill larvae during February and March 1996 relative to 1995 suggest a later peak spawning period.
This work was supported by National Oceanic and Atmospheric Administration Contract Number 50ABNF600014.
References
Casareto, B.E., and T. Nemoto. 1986. Salps of the Southern Ocean (Australian Sector) during the 1983-84 summer, with special reference to the species Salpa thompsoni, Foxton 1961. Memoirs National Institute of Polar Research, Special Issue, 40, 221-239.
Foxton, P. 1966. The distribution and life-history of Salpa thompsoni Foxton with observations on a related species, Salpa gerlachei Foxton. Discovery Report, 34, 1-116.
Loeb, V., and V. Siegel. 1996. AMLR program: Krill demography in the Elephant Island area, January to March 1996. Antarctic Journal of the U.S., 31(2).
Nordhausen, W. 1991. Abundance and distribution of the Antarctic euphausiid Thysanoessa macrura in the eastern Bransfield Strait and around Elephant Island. AMLR 1990/91 Field Season Report (Southwest Fisheries Science Center Administrative Report LJ-91-18). La Jolla: Southwest Fisheries Science Center.
Ross, R.M., L.B. Quetin, and E. Kirsch. 1988. Effect of temperature on developmental times and survival of early larval stages of Euphausia superba Dana. Journal of Experimental Marine Biology and Ecology, 121, 55-71.
Seigel, V., and V. Loeb. 1995. Recruitment of antarctic krill Euphausia superba and possible causes for its variability. Marine Ecology Progress Series, 123, 45-56.