AMLR 2003 Weekly Report No. 2

19 January 2003

 

1. Our current position is approximately 30 miles north of King George Island where we are conducting the first of two surveys of bio-oceanographic conditions in the vicinity of the South Shetland Islands. The survey is divided into four areas: the South Area in the central portion of Bransfield Strait south of King George and Livingston Islands, the West Area north of King George and Livingston Islands; the Elephant Island Area encompassing the northern portion of the South Shetland archipelago; and the Joinville Island Area in the western portion of Bransfield Strait. Survey operations in the South Area were completed this past week.

 

2. Krill, salps and other zooplankton. 32 plankton samples have been collected from the South and West Areas. Antarctic krill (Euphausia superba) were present in 28 (88%) of these with a total estimated catch of ca. 5000 individuals. Greatest concentrations were encountered in the South Area reflecting a high degree of patchiness relative to the West Area; median catches in the South and West were respectively 1 and 7 per 1000 m3. Lengths ranged from 13-54 mm, but the majority was small juvenile and immature stages: 52% of individuals were 29-33 mm and 10% 18-22 mm in length. Overall, juveniles comprised 46% and immature forms 47% of the total catch. Large mature krill were represented in oceanic water offshore of the South Shetland Islands and, based on their scarcity in predator diets, may have recently appeared there from areas further north. While these krill were not actively spawning, some females appeared to have recently shed eggs. Presence of early larval stages in both South and West Area samples indicates that spawning was initiated during December.

 

The salp, Salpa thompsoni, was present in 26 of 32 samples (81%). Like krill their distribution was much patchier in the South compared to the West Area with median catches, respectively, of 2 and 34 per 1000 m3. Virtually all salps were in the aggregate stage. Individual animals ranged in length from 4-52 mm. The largest length suggests initiation of production in early October; median and modal lengths (26 and 28 mm respectively) suggest peak production during mid-November.

 

Copepods and post-larvae of the euphausiid, Thysanoessa macrura, were present in all samples; with median catches of 140 and 113 per 1000 m3, respectively, these numerically dominated the zooplankton. Larval T. macrura were scarce, suggesting that seasonal reproduction of this species along with that of krill and salps, was delayed during winter-spring 2002. Delayed reproduction could be related to thepersistence of extensive sea ice coverage well into spring (i.e., early November) during this period.

 

3. Krill biomass and dispersion. Krill were detected acoustically throughout the West Area with highest biomass densities mapped in the center of the Bransfield Strait south of Admiralty Bay, King George Island, and south of Greenwich Island. A preliminary estimate of mean krill biomass density for the West Area was 32 g m-2. This compares to estimated mean biomass densities of 15 g m-2 for the West Area during 2002 Survey A and 13 g m-2 during 2001 Survey A.

 

4. Phytoplankton. Phytoplankton biomass in the South Area were generally low, with 5 meter chlorophyll averaging 0.4 +/- 0.2 mg m-3 and integrated values (to 100 meter) of 22 +/- 8 mg m-2. This compares to a historical average of 1.32 mg chl m-3 for 5 meter depth in the South Area during January derived from 1993-2002 AMLR data. Lowest values occurred in 1993, 1994, and 1998, averaging 0.75 mg chl m-3, while highest values occurred in 1995 and 2001, averaging 2.20 mg chl m-2. The highest biomass thus far measured was at Station A10-12 having a 5-meter chlorophyll of 1 mg m-3. Very low biomass was also measured on the most western section of the West Area, with 5 meter values averaging 0.09 +/- 0.03 mg chl m-3 and integrated chlorophyll (to 100m) of 13 +/- 6 mg m-2. Although these chlorophyll values are preliminary (i.e., awaiting post-cruise calibration of instruments), the low biomass is corroborated by recent SeaWiFS chlorophyll distributions of the area (provided by M. Kahru, SIO), and profiling in-situ transmissometer and fluorometer data.

 

In cooperation with WOCE, two drifter buoys were released north of King George Island to provide information of water currents along the Shetland Islands shelf and into the Elephant Island Area. 10 other drifters will be released within the next week. Near-real time tracking will be provided by M. Zhou (UMB). O. Holm-Hansen, B.G. Mitchell, and P. Niiler (SIO) also provide land-based assistance and coordination for this program.

 

5. Oceanography and meteorology. Oceanographic conditions in the Bransfield Strait showed cool Weddell Sea influence from the south-east, extending south of King George Island where type IV (Bransfield Strait) and transition types became prevalent. Some localized variability of salinity and sea temperatures were noted on underway logging and during CTD casts due to extensive ice, particularly near the coastlines.

 

Fine weather conditions and calm seas, accompanied by a gradual rise in atmospheric pressure were experienced throughout the week. There were extensive sunny periods and moderate air temperatures around 2-3C with two short periods of 6.5C. Low wind speeds of 5-13 knts prevailed, with a short spell reaching 25 knts. Winds were predominantly from the N to NW.

 

6. Predator diet studies. 11 fur seal scats from Cape Shirreff have been processed to date. Otoliths were almost completely absent from the scat samples compared to this time last year; only one scat sample contained otoliths with a mixture of Gymnoscopelus nicholsi and Electrona antarctica. Additionally, one scat sample contained a single squid beak.

 

Krill carapaces were present in all scat samples to date, and overall were smaller compared to last year's carapaces (most common size class was 35-45 mm). 51 fresh krill have been sexed and measured for total length, carapace length and carapace width in order to verify equations used to infer length from exoskeleton remains in scat samples. Lipids have been extracted from 24 milk samples.

 

7. Bird and marine mammal observations. Systematic bird and marine mammal surveys were conducted during daylight hours in the Bransfield Strait. 15 species of seabirds were encountered and a strikingly large aggregation of Antarctic Fulmars (1000+) was observed sitting on the water while transiting the southwest portion of the Strait. Cetacean observations included a single southern right whale, two long-finned pilot whales, several Antarctic minke whales, and approximately 50 humpback whales. Pinniped observations included two Weddell seals and numerous Antarctic fur seals.