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
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