AMLR 2004 Weekly Report No. 6

15 February 2004

1. Our current position is alongside the pier in Punta Arenas, Chile, having arrived yesterday afternoon. While in port the ship will take on fresh provisions and exchange a portion of the scientific party. Leg II of the 2004 AMLR cruise will begin on Tuesday, 17 February. This past week, the Cape Shirreff survey was completed, M. Goebel was embarked from the Cape Shirreff field camp, D. McWethy and S. Corsolini were embarked from the Copacabana field camp, P. Winogrodzki was embarked from Arctowski Base, and the ship took its departure from Admiralty Bay bound for Punta Arenas at approximately 0000 on 12 Feb.

2. During Leg I, personnel, mail and fresh provisions were delivered to two field camps, a survey of the bio-oceanographic conditions in the vicinity of the South Shetland Islands was conducted, 10 drifter buoys were deployed in the vicinity of the Shackleton Fracture Zone, the shipboard acoustic system was calibrated, three instrumented moored buoys were deployed off Cape Shirreff, 15 female fur seals tagged as pups at Seal Island were re-captured, and a high-resolution survey in the vicinity of Cape Shirreff was conducted. Planned operations for Leg II include a second survey of bio-oceanographic conditions, a second calibration of the acoustic system, recovery of the moored buoys, and closure of the field camps.

3. Cape Shirreff survey. As noted in last week's report an instrumented Zodiac, the R/V Ernest, was deployed with J. Warren and A. Jenkins to conduct a survey of the inshore end of a submarine canyon east of Cape Shirreff. The R/V Ernest was moored each night off the field camp while the ship conducted sampling further offshore. This is the third in a series of similar surveys, the first two conducted in the 2000 and 2002 field seasons. Despite losing 3 of the scheduled 7 days to unfavorable weather conditions, the 2004 survey was the most thorough coverage of the eastern canyon to date. The primary reason for this was the implementation of a transom mount for the acoustic echosounder, which allowed for survey effort to be conducted under a variety of sea states and weather conditions. Six cross-canyon transects were undertaken with an average survey speed of 5 knots. Weather conditions were typically quite poor with ubiquitous rain, wind speeds typically 15-25 knots, and sea states ranging from 2 to 5 m. All equipment and data acquisition systems performed without error or breakdown, a first for the inshore survey program. The calibration of the acoustic system was difficult due to the new position of the echosounder and the fact that additional personnel were not on board the Ernest to assist in lowering and locating the calibration sphere in the acoustic beam pattern. During this same time period, the R/V Yuzhmorgeologiya conducted two transects up and down the eastern canyon and one along-shore transect near the shelf break. Additionally, stations were conducted in the middle of the eastern canyon to examine temporal variability in the acoustic scattering record, hydrographic properties, and zooplankton population.

The presence of numerous icebergs around the Cape Shirreff area severely altered and limited the planned coverage of the shipboard survey. Despite this, 19 IKMT net tows were conducted and analyzed and 20 CTD casts were performed. Initial results from the R/V Ernest survey are similar to previous years. There were large aggregations of acoustic scatterers, believed to be krill, at the edges of the canyons and in the shallow waters adjacent to the canyon. The presence of these patches was often indicated by the observation of penguins, fur seals, and a fin whale during the survey. Zooplankton tows from the R/V Yuzhmorgeologiya found krill in the canyon during both night and daytime stations. Other animals frequently present in the area included: large concentrations of copepods, amphipods, salps, euphausiids and other crustaceans. Hydrographic station data provided evidence of warm, sub-surface water in the eastern canyon, consistent with results from the 2002 inshore survey. Initial inspection of Acoustic Doppler Current Profiler data from one of the instrumented buoys indicates that the currents in the eastern canyon have a tidal signal, as well as the expected diel migration of acoustic scatterers (likely krill). Two of the buoys remain deployed to continue to collect additional information and will be recovered during Leg II.

4. Instrumented buoys. The 70 kHz buoy continues to successfully log data to Cape Shirreff, and the battery was recharged during good sunlight conditions on Tuesday and Wednesday. The 38/200 kHz buoy has reestablished radio contact with Cape Shirreff, possibly due to a shift in the position of a 300 × 300 m iceberg blocking the line of sight between the buoy and the shore camp. However the buoy's timing sequence has been corrupted and the Cape Shirreff personal have been trying to reset the cycle via the shore radio. Before the ship left Cape Shirreff a decision was taken to leave the buoys deployed to obtain a longer time series of data. When the ship returns to Cape Shirreff on 20 February the ice collision risk and buoy battery voltage and performance will be reassessed and a decision will be taken to retrieve the buoys, or leave them deployed until the end of Leg II.

5. Zooplankton and krill. Many of the Survey A sample results indicate essentially average conditions within the long term AMLR data set. Numerical dominance of the zooplankton-micronekton assemblage by copepods, Salpa thompsoni, postlarval Thysanoessa macrura and krill, and chaetognaths occurred at or near their long term mean abundance values.

Absolute and relative salp and krill carbon biomass values conformed to the long term average. In terms of overall species abundance relationships within the Elephant Island area, this surveys results most resembled those from January 1997. Salp length frequency distribution and maturity stage composition were typical for late summer. Krill length-frequency distribution and maturity stage composition reflect strong recruitment success from the 2001-2002 and 2000-2001 year classes and little input from the 2002-2003 spawning season. The consequently large immature or maturing component of the krill population somewhat masks the degree of this years spawning effort. However, modest numbers and broad distribution of larval krill, including early calyptopis through mid-furcilia stages, indicate a prolonged albeit low level reproductive effort.

Two features of the January 2004 survey results, which were also common with those from January 1998 survey and set these two years apart from the rest, are: (a) relatively high frequency of occurrence and abundance of the high latitude salp, Ihlea racovitzai and (b) onshore presence of a species-rich and abundant zooplankton assemblage dominated by copepods, chaetognaths, ostracods, and I. racovitzai. While post-larval and larval stages of krill, Thysanoessa macrura, Euphausia frigida and salps were broadly and fairly uniformly distributed across the large survey area, the other typically “oceanic” zooplankters were concentrated onshore largely in association with Bransfield Strait Type 4 water. Here they occurred in close proximity to I. racovitzai, which appeared to be associated with an intrusion of deep Polar Slope water. These distribution patterns most likely result from atmospheric forcing and hydrographic mixing processes over the previous winter and spring.

6. Oceanography and meteorology. 35 kt southwesterly winds were experienced on leaving Cape Shirreff, but fair weather and clear skies were experienced at Admiralty Bay. Calm seas, sun, and a southerly wind, averaging 15 knts, accompanied the ship across the Drake Passage. The Antarctic Convergence was encountered at 59º 05´S and by 58º 05´S the sea surface temperature had increased from 3.4°C to 6.8°C. Salinity increased from 33.68 ppt to 34.04 ppt.

7. Predator diet studies. Scats samples have continued to shift from krill based scats to fish and squid based scats. Scats through half of week 6 have been processed (55 to date). This year there is the presence of an otolith that has not been seen in previous years and will be identified upon completion of this field season. Samples for Leg II are aboard and are ready for processing.

8. Bird and marine mammal observations. Seabird highlights include Soft-plumaged Petrel, Common Diving Petrel, and Arctic Tern. Soft-plumaged petrels were common north of the convergence zone, and sightings were recorded near Tierra del Fuego. Five species of Albatross have been observed, and Black-browed Albatross were most common during the inshore survey. Cape Petrels exhibited behaviors indicative of feeding, especially during the 9 February acoustic transit over the submarine canyon east of Cape Shirreff. A multi-species (9 species) feeding flock (100's of individuals) was encountered near the shoreward end of the canyon on 10 February, and the activities were recorded with a video camcorder. Three species of albatross were observed initiating the feeding frenzy, and a Wandering Albatross with a distinctive green paint spot on its breast was observed attending the flock. Giant Petrels were the most abundant aerial seabird observed feeding, and there were many instances of intra-specific behavior observed. Fur seals and Chinstrap penguins were also observed within the main feeding circle, and on the peripheral edge of the flock. Storm petrels also focused their efforts at the peripheral zone of the flock. The Fur seals were observed within the core of the feeding event, and were frequently observed porpoising up from the middle of the Giant Petrel feeding aggregation. Such behavior may suggest that the Fur seals were forcing evading prey to the surface, and therefore becoming available to non-diving surface feeding aerial predators. The following marine mammals have been observed during this portion of our surveys (in order of abundance): Cetaceans: Humpback, Fin, Minke, Southern Bottlenose, and Grays Beaked Whales; Pinnipeds: Antarctic Fur Seal, South American Fur Seal and Sea Lion. During an eastbound transit on Y3, we encountered numerous small groups (3-6 individuals) of Fur Seals. On 9 February, 14 Fin whales in groups of 4-5 were encountered, and a few cow-calf pairs were noted.

Submitted by Roger Hewitt