AMLR 2003 Weekly Report No. 3

26 January 2003


1. Our current position is approximately 20 miles northeast of Elephant Island. The South and West Areas of the first of two surveys of bio-oceanographic conditions in the vicinity of the South Shetland Islands have been completed. An extensive band of icebergs north of Elephant Island has hampered the conduct of the survey in this area and forced several changes to the survey design.


2. Krill, salps and other zooplankton. Post-larval stages of Antarctic krill (Euphausia superba) were present in all 33 IKMT samples thus far collected in the Elephant Island survey area.  The largest catch (ca. 24,500 of 30,500 total individuals) was made offshore northwest of Elephant Island.  Mean and median abundance values (330 and 26 per 1000 m3, respectively) reflect the broad krill distribution across this area.  Lengths ranged from 13-53 mm with distinct modes around 24-27 mm and 34-37 mm; these conform to juvenile and predominantly immature stages representing 1 and 2 year old krill.  Juveniles contributed 48% and immatures 32% of the total indicating strong recruitment success of the last two year classes (2001/2002 and 2000/2001).  Of note are the scarcity of individuals >40 mm, virtual absence of mature individuals >50 mm and paucity of advanced female reproductive stages. Relatively small numbers of early calyptopis stage krill larvae occurred in l2 samples (36%).  These observations suggest a delay in onshore migration and seasonal spawning activity by older year classes.

Copepods and post-larvae of the euphausiid, Thysanoessa macrura, were also present in all samples.  Copepods comprised the most abundant taxon over all (535 and 217 per 1000 m3) while T. macrura mean and median values (295 and 125 per 1000 m3) ranked 3 and 2 relative to krill.  Like krill, larval T. macrura were scarce, again suggesting delayed seasonal reproductive activity.  The same observations were reported from sampling within the South and West Areas.

Frequency of occurrence and abundance of the salp, Salpa thompsoni (85% of samples; mean 75 and median 13 per 1000 m3), was comparatively low.  The predominantly aggregate stages (98% of individuals) had a median length of 25 mm and resulted from a productivity peak during mid-November.  Relatively low abundance and small sizes suggest, as for other taxa, a delayed seasonal production period.


3. Krill biomass and dispersion. Highest densities of krill were mapped in the vicinity of the shelf break north of Cape Shirreff, Livingston Island, along the shelf break between King George and Elephant Islands, and offshore at the northern margin of the survey grid near the Shackleton Fracture Zone. Preliminary estimates of mean biomass density in the West Area was higher than that in the South Area and both show increases over 2001 and 2002 for the same survey areas and time periods.


4. Phytoplankton. Chlorophyll concentrations in the West and Elephant Island Areas are among the lowest in years. In the West Area (24 stations), 5 m concentrations averaged 0.18 +/- 0.09 mg chl m-3, and in the Elephant Island Area (33 stations), they averaged 0.10 +/- 0.09 mg chl m-3. These values compare with 11-year averages for 5 m concentrations of 0.74 mg chl m-3 for Elephant Island Area and 0.77 mg chl m-3 for the South Area. Lowest January values for the Elephant Island area were found in 1992, 1994 and 1998, and averaged 0.49 mg chl m-3. Integrated (to 100 m) chlorophyll were 16.4 +/- 9.1 and 12.25 +/- 6.8 mg m-2 for the Elephant Island and West Areas, respectively. This year thus appears to have the lowest phytoplankton crop as measured around Elephant Island by AMLR surveys since 1990. Recent SeaWiFS satellite imagery of chlorophyll distribution also indicates dwindling phytoplankton crops since December, with development of a large Blue Water Zone in the Drake Passage. The lowest 5 m chlorophyll values (mg chl m-3) measured in Drake Passage waters were 0.02 in the West Area and 0.05 in the Elephant Island Area. The elevated chlorophyll concentrations located in the eddy north of the Shackelton Fracture Zone, as observed from mid-December SeaWiFS images, have decreased considerably through mid-January; however, the area still maintains some of the highest phytoplankton biomass along the bordering regions of the AMLR survey grid. Twelve drifter buoys were released during the past week, with preliminary data indicating a strong southwesterly flow along the continental self north of the Shetland Islands.


5. Oceanography and meteorology. CTD casts in the West and Elephant Island Areas show the normal presence of ACC water extending north-eastward along the shelf edge and deflecting northward north of Elephant Island. Some Weddell water (Type V) was noted in the north-eastward corner of the survey grid, but the extent southward is unclear since ice prevented further stations to the south and east. A Weddell water intrusion surrounds the South Shetland Islands, with a narrow band of Bransfield water (Type IV) separating the ACC from the Weddell water on the northern side of the islands. The southern area is dominated by Bransfield water. Transition water (Type II) is seen only on the fringes of the survey area, mainly in the north (east of the Shackleton Fracture Zone) and in the south-west near the islands and no Type III has been observed. Delineation between Type I, IV & V is distinct with little or no Type II water in between.

Weather this week has been good with air temperatures mostly 0.5°C - 2.5°C and a brief low of 0°C on Sunday morning.  Warmer conditions from Thursday to Sunday reached 3°C. A gradual increase in air pressure until Thursday was followed by a sharp drop until Saturday afternoon when it rose steeply. We experienced almost daily sunny periods, with some overcast, and some rain and fog in the evenings later in the week as we probed the ice field. Winds swung steadily from the east thru south, west and north during the course of the week, rotating through the full extent of the compass until they reached the east again on Sunday. Stronger winds were experienced this week, reaching over 30 knts on Monday and Tuesday (SE), before settling to 5-15 knts midweek and gradually increasing again toward Saturday and dropping on Sunday.


6. Predator diet studies. To date, 25 Antarctic fur seal scats and 1 enema have been processed. The appearances of otoliths and squid beaks are more common the recent samples (8 containing otoliths and 3 containing squid beaks). However the majority of scats contain krill only.  Additionally, carapace length and width have increased, representing krill from 35-60mm in length. 37 fur seal milk samples have been processed. 70 fresh krill have been sexed, measured for total length, and measured for carapace length and width.


7. Bird and marine mammal observations. Chinstrap penguins were conspicuous in the pelagic waters north of the South Shetlands, particularly in the vicinity of dense krill swarms north of Livingston Island.  In contrast there were few observations of Adélie and gentoo penguins in this week's survey area.  Royal and wandering albatrosses were observed in the West Area. Antarctic prions and blue petrels, absent in the Bransfield Strait, were observed north of the insular shelf.  Whale sightings included Humpback (fewer than were in the Bransfield Strait), minke, long-finned pilot, and southern bottlenose whales.