VALERIE LOEB, KIMBERLY PUGLISE, and DAWN OUTRAM,Moss Landing Marine Laboratories, Moss Landing, California 95039
Macrozooplankton were collected along with krill in the Isaacs-Kidd Midwater Trawl (IKMT) samples during AMLR 1997. Various macrozooplankton, particularly salps ( Salpa thompsoni ) and copepods are, along with krill, important components of the antarctic food web.
Sampling specifics are presented in Loeb ( Antarctic Journal , in this issue). Freshly collected samples were analyzed onboard. All salps were removed from the samples. For samples with fewer than 100 individuals, the two stages (aggregate/sexual and solitary/asexual) were enumerated and internal body length (Foxton 1966) measured to the nearest millimeter. Representative subsamples of 50-120 salps were analyzed for larger catches. After removal of salps, krill, and adult fish, the remaining zooplankton were analyzed. All larger organisms (e.g., amphipods, other euphausiids) were sorted, identified to species if possible, and enumerated. The smaller constituents (e.g., copepods, krill larvae) in representative aliquots were then enumerated using dissecting microscopes. Abundance estimates are expressed as numbers per 1,000 cubic meters of water filtered.
A total of 72 zooplankton taxonomic categories were collected at 105 stations during Survey A (table). Eight taxa dominated the collections. Copepods were the most abundant category, followed by salps, postlarval stages of the euphausiids Thysanoessa macrura and Euphausia superba (krill), chae-tognaths, larval stages of T. macrura and krill (93 percent calyptopis stages), and postlarval Euphausia frigida (table). Six hyperiid amphipod species were relatively frequent and abundant. These were Primno macropa, Cyllopus magellanicus, Themisto gaudichaudii, Vibilia antarctica, Hyperiella dilatata, and C. lucasii. The pteropod Spongiobranchaea australis was also relatively frequent and abundant.
Fairly large concentrations of salps (e.g., greater than 100 per 1,000 cubic meters) were distributed across the survey area (figure 1). Aggregate (sexual) stages constituted 80 percent, and solitary (asexual) stages 20 percent, of the total. The solitary stages were represented primarily by newly released embryos 4-7 millimeters in length (49 percent) and old large individuals (all salps 78-172 millimeters, 34 percent). The vast majority of the aggregate stages (98 percent) were less than 60 millimeters in length (figure 2). The aggregate stages demonstrated three distinct size modes around 7-17 millimeters (31 percent), 25-35 millimeters (23 percent), and 45-50 millimeters (10 percent), which suggest successive peak budding periods by the solitary stages during spring and summer months.
Various zooplankton taxa demonstrated significant correlations of abundance across the large-area survey as indicated by Kendall's Tau values greater than 0.30 (P<0.0001). Abundances of salps, E. frigida , and E. triacantha were all positively correlated, reflecting increased nighttime abundance in the upper 170 meters due to diel vertical migrations. Abundance of salps and C. lucasii, C. magellanicus , and V. antarctica were also positively correlated probably due to commensal relationships of these amphipods with salps (Madin and Harbison 1977). A significant positive correlation also resulted from the parasitic relationship between the amphipod H. dilatata and pteropod S. australis. Apparently, the toxicity of S. australis is used by H. dilatata as a predation detractor (McClintock and Janssen 1990). Because of shared distributional patterns in primarily Drake Passage water, abundances of larval krill and larval T. macrura were significantly and positively correlated with H. dilatata and S. australis . As during 1996 (Loeb and Outram 1996), the distributions of larval and postlarval T. macrura were diametrically opposed as indicated by a significant negative correlation.
In total, 37 taxa were represented in the 16 Survey D samples (table). This total was about half the number collected in Survey A and was due to the limited number of samples. Aside from the absence of primarily uncommon and rare taxa, the Survey D station locations provided overall results that were representative of the large-area survey (Loeb unpublished manuscript). The same eight dominant taxa from Survey A again dominated but with slightly different abundance relations. Copepods, salps, and postlarval T. macrura remained the three most abundant taxa; E. frigida replaced krill as rank 4 in abundance; and chaetognaths and larval T. macrura were ranked 7 and 8. Among these taxa, significantly higher mean abundances of salps (Z test, P<0.001) and E. frigida (P<0.05) indicate substantial population growth during late summer. As during Survey A, only calyptopis stages of krill larvae were collected. The absence of more advanced furcilia stages, together with similar mean abundance (P>0.05) of calyptopis stages, suggests relatively poor spawning success and/or poor egg and larval survival during January and February. The lower larval T. macrura abundance relative to Survey A is due to their primary distribution in Drake Passage waters outside the scope of Survey D (Loeb and Outram 1996). Significantly increased mean abundance of the amphipods V. antarctica (P<0.05) and C. lucasii (P<0.01) relative to Survey A was directly associated with increased salp population size.
Salps were present at all 16 stations at concentrations ranging from 280 to 4,350 per 1,000 cubic meters (figure 1). Aggregate stages contributed 92 percent of the total individuals; proportionately fewer solitary stages were collected than during Survey A (8 percent versus 20 percent). Most (66 percent) of the aggregate stages were 8-16 millimeters in length and resulted from relatively recent chain release by solitary stages. As during Survey A, the larger aggregate stages demonstrated a polymodal distribution (figure 2). Within the 17-82-millimeter aggregate size range, abundance peaks occurred around 26-38 millimeters (26 percent), 45-55 millimeters (26 percent), and 69-74 millimeters (9 percent). These size modes and increased proportions of individuals larger than 60 millimeters relative to Survey A suggest a net length increase of approximately 20 millimeters over the intervening 45-day period; this corresponds to a summertime growth rate of approximately 14 millimeters per month, twice the estimated solitary stage growth rate during winter (6-8 millimeters per month; Foxton 1966). The solitary stages were primarily represented by newly released embryos 4-10 millimeters in length (29 percent) and old, large (74-145 millimeters) individuals (51 percent).
Within the 1992-1997 Elephant Island area data set, the mean and median salp abundance values during Survey D were second only to those observed during February and March 1993 and, as in 1993, resulted from massive population growth during summer. These two years contrast with the two others which demonstrated declines in median salp abundance with the advancing season. The overall salp length frequency distribution and stage composition during the 1997 surveys differed markedly from those of previous years (figure 3), and the presence of distinct size modes, which allowed growth rate estimation, was unique. These differences reflect large interannual variations in conditions influencing the initiation, duration, and continuity of both aggregate and solitary stage production across the Antarctic Peninsula region.
This work was supported by National Oceanic and Atmospheric Administration contract number 50ABNF600014.
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. 1997. AMLR program: Krill demography in the Elephant Island area January to March 1997. Antarctic Journal of the U.S. , 32(5).
Loeb, V., and D. Outram. 1996. AMLR program: Salps and other macrozooplankton sampled, January to March 1996. Antarctic Journal of the U.S. , 31(2), 149-152.
Madin, L.P., and G.R. Harbison. 1977. The associations of Amphipoda Hydperiidea with gelatinous zooplankton—I. Associations with Salpidae. Deep-Sea Research , 24(5), 449-463.
McClintock, J.B., and J. Janssen. 1990. Pteropod abduction as a chemical defense in a pelagic antarctic amphipod. Nature , 346, 462-464.