27 January
2002
1. Current position is approximately 30 miles north of d'Urville and Joinville Islands, near the tip of the Antarctic Peninsula, conducting a survey of bio-oceanographic conditions in the vicinity of the South Shetland Islands. This is the first time that the US AMLR sampling grid has extended south into this area, which unlike the survey areas to the north is primarily composed of water from the Weddell Sea. The Elephant Island Area was completed this past week. An unscheduled break in the survey was required in the lee of Elephant Island in order to replace a power supply to the acoustic system and re-calibrate. Despite this loss of 7 hours, moderate weather has allowed us to maintain the planned survey schedule.
2. Krill, salps and other zooplankton. Krill abundance values in the 44 Elephant Island area IKMT samples were relatively low (mean, 39.0 per 1000 m3; median, 7.5 per 1000 m3) and only slightly larger than those observed during the January 2001 survey (respectively, 20.7 and 6.0 per 1000 m3). Largest concentrations were located over and offshore of the northeast Elephant Island shelf. The overall length frequency distribution was bimodal around 24 mm and 41-42 mm lengths representing juvenile and mature stages. Juveniles and reproductively mature forms contributed similar proportions (45%) to the total catch while immature stages (2-year old krill) comprised only 10%. These results suggest substantially greater recruitment success of the 2000/2001 vs. 1999/2000 year class. Females outnumbered males and most were in advanced reproductive stages (i.e., developing ovaries, gravid and spent) indicating normal seasonal spawning activity. Larval krill occurred in low numbers across much of the area, with largest concentrations located in the northeast corner. While most of the larvae were E. calyptopis stages, occurrence of more developed furcilia 1 and 2 stages in samples indicate that some spawning took place quite early in the season (e.g., early December). Mean larval abundance (35.8 per 1000 m3) was similar to that in January 2001 and, given favorable overwintering conditions, could likewise lead to modest proportional recruitment success.
Salpa thompsoni mean abundance was relatively high (410 per 1000 m3) due to localized concentrations associated with semi-permanent eddies northwest and southeast of Elephant Island. Due to generally small salp catches over the rest of the area the median value (86 per 1000 m3) was the lowest observed since January 1997. Large copepod concentrations occurred across the Elephant Island area. Greatest values (to 96000 per 1000 m3) were associated with an intrusion of Drake Passage water from the northwest. Median copepod abundance (2175 per 1000 m3) was one to two orders of magnitude greater than observed during any previous January survey. While abundance values of postlarval Thysanoessa macrura were typical for January surveys those of their larvae were also one to two orders of magnitude greater than previous years. These results suggest enhanced secondary productivity within the area during 2002.
3. Krill biomass and dispersion. Highest estimated densities of krill were mapped midway between King George and Elephant Islands, northwest of Elephant Island and north of the east end of Elephant Island. Krill were concentrated along the shelf break delineated by the 500 m isobat. Low densities were mapped elsewhere. Estimated mean krill biomass density in the Elephant Island Area was 19 to 40 g m-2 as compared to 16 g m-2 in this area during January 2001.
4. Phytoplankton. The pattern for surface chlorophyll concentrations in the Elephant Island sector closely followed bottom topography of the area. Five m chl averaged 0.57 +\- 0.29 mg m-3, and integrated (100 m) averaged 45 +\- 19 mg chl m-2 for the entire Elephant Island section (42 stations). The shelf and break area around Elephant Island (14 stations) averaged 0.69 +\- 0.32 mg chl m-3 as compared to 0.49 +\- 0.25 mg chl m-3 in the oceanic region (22 stations). Chlorophyll concentrations this leg are considered average compared with the 11 year January mean (5 m being 0.81 +\- 0.80 mg chl m-3; 100 m integrated being 43 +\- 35 chl m-2). The most phytoplankton rich area occurred in the central Bransfield Strait below Elephant Island, with stations 03-06, 04-06 and 06-08 having highest 5 m chl concentrations of 1.3, 1.1 and 1.1 chl m-3, respectively. The lowest integrated chlorophyll concentrations of <20 mg m-2 were found in the eastern Bransfield Stait (Stations 02-07, 02-08, and 03-08) where the influence of Weddell Sea water was apparent. The optical oceanography component provided satellite (SeaWIFS) algorythms to determine surface chlorophyll distribution as well as in-situ optical properties of the upper water column at 25 stations. This is being accomplished by deploying a free-fall in-situ radiometer and an inherent optical properties (IOP) package containing fast repletion rate fluorometery and a backscattering sensor.
5. Oceanography and meteorology. Some ambiguity in the position of the typical frontal region is becoming apparent with two of three CTD sensors (temperature and oxygen) agreeing that the sampling grid has delineated the front in the western area, while the salinity sensor indicates that the salinity is approximately 0.5 PSU above typical and that the front may lie further offshore. In the area north of Elephant Island, a plume of transition water lies from the south-west to the north-east with an eddy extending from the middle into the north-west quadrant. Weddell water dominates the south-eastern corner of this area.
Predominantly 25 knot north-westerly winds with short periods of light south wind on Tuesday, strong (40 knots) north-easterly early Friday and light south to easterly winds on Saturday and Sunday. Occasional light snowfalls were experienced as we moved east and south on the grid, accompanied by a drop in sea surface (2+°C to 0.5°C) and air (average 3°C to 0.5°C, peak of 5°C on Thursday) temperatures. Partly cloudy conditions with brief sunny periods were recorded from Monday to Friday with the rest being overcast.
6. Predator diet studies. Lipids have been extracted from 28 fur seal milk samples and frozen. Net caught specimens now include juvenile and adult krill (E. superba) and a wider range of myctophid fish species, caught in the Elephant Island area, (E. carlsbergi, K. andersonii, E. subaspera, G. braueri, G. nicholsi and E. antarctica) which were retained for lipid extraction. Twenty-two fur seal scats and 2 regurgitations were processed with many containing krill carapaces and chitin and fish otoliths; however, no squid beaks were present. Myctophid fish otoliths from scats were primarily those of E.carlsbergi and to a lesser extent, E.antarctica with a few containing G. nicholsi. The trend continued where scats that contained large numbers of otoliths had very few carapaces and chitin, whereas the scats with large amount of krill carapaces and chitin contained fewer numbers of otoliths.