AMLR 2005 Weekly Report No. 4
6 February 2005
1. Our current position is off Cape
Shirreff,
Livingston Island.
We are conducting bio-oceanographic operations in support of NSF and AMLR funded
research to describe and quantify the importance of local bathymetric canyons
to the local abundance of krill. The mechanisms responsible for the local
abundances will be used to compare feeding success of seals and seabirds in the
Cape Shirreff Vicinity. Operations will continue for
an additional two days.
2. Krill, salps
and other zooplankton. Zooplankton
assemblages in the Joinville
Island and South Areas differed
greatly from those of the West and Elephant Island Areas in that they were
numerically dominated by copepods and post larval Thysanoessa macrura. Here the mean and median abundance values
for Salpa thompsoni
(107-185 and 19-38 per 1000 m3) were ca. an order of magnitude lower
in the northern areas, apparently due to limited presence of Zone 1 water. Ihlea racovitzai, another salp
species that is a marker for Weddell Sea and polar slope
water, occurred mostly in the eastern portion of Bransfield Strait,
with greatest concentrations east and south east of Elephant
Island. Copepod concentrations in the two areas were
nearly five times greater than to the north due primarily to dense aggregations
of Metridia gerlachei and
Calanoides acutus. These were associated with frontal areas
within Bransfield
Strait. Post larval T. macrura were broadly distributed
across northern Bransfield
Strait while the larval stages were
concentrated to the south.
Elevated concentrations of post
larval krill across the Joinville Island Area (28 and
2 per 1000 m3 mean and median) were similar to those of the Elephant
Island Area. Within the South Area
aggregations were primarily encountered to the southeast of King
George Island
resulting in 50% lower mean and median values.
Krill length and maturity stage composition in the South was similar to
that in the Elephant Island Area: median
and modal lengths were 46-48 mm; juveniles made up ca. 3% of individuals; and
males outnumbered females by 50%.
Greatest differences occurred in proportions of immature stages which
constituted 20% in the South vs. 9% in the Elephant
Island Area. Krill within the Joinville
Island Area exhibited a bimodal length distribution centered around
27 mm and 45 mm lengths representing 1 and 3+ age classes. Juvenile and immature stages contributed ca.
22% and 27% of the total. This differed
greatly from the other areas which were strongly dominated by the larger,
mature age classes. Mean and median
larval krill abundance values in the Joinville Island
Area (31 and 14 per 1000 m3) were also higher than in the other
areas. Distribution patterns of these
larvae and small (<40 mm) post larval krill suggest eastward advection through Bransfield Strait, concentration at frontal features within
the Joinville Island area and subsequent northward
advection out of the survey area.
3. Acoustics. With the completion of the first broadscale survey encompassing the South
Shetland Islands, the acoustics research has focused on providing acoustic
data in support of an NSF and AMLR funded grant to Dave Demer
and Joe Warren. We have conducted more than 70 hours of acoustic sampling along
8 twenty nautical mile length transects in the vicinity of the Cape
Shirreff
field camp. While no analysis of these data has yet been conducted, large areas
of backscatter, within the frequencies used to target krill were detected. Over
the 7 days of the surveys, patterns seem to be consistent across the area,
suggesting that patterns of abundance have a residence time of at least a week.
4. Phytoplankton. South
Area chlorophyll concentrations have been completely processed at this time, 5
meter phytoplankton stocks averaged 1.67 ± 0.58 mg chl
m-3 (slightly higher than reported last week with only 9 stations processed).
The 13 year average for surface phytoplankton stocks in the South Area is 1.27
mg chl m-3, therefore values for this
season are slightly higher than on “average”. With respect to the general distribution
of chlorophyll for the entire AMLR Survey for Leg 1, pelagic waters (bottom
depth >2000 meters) of highest surface phytoplankton concentrations were
found in the Joinville Island Area with one station
having 1.8 mg CHL m-3, intermediate concentrations were found for
the Elephant Island area averaging 0.45 ± 0.45 mg chl
m-3, and lowest in the West Area with 0.12 ± 0.18 mg chl m-3. With regard to shelf and shelf/break
areas (bottom depth < 2000 meters), the lowest phytoplankton stocks were
found in the West area with 0.68 ± 0.44 mg chl m-3,
intermediate concentrations of 1.13 ± 0.66 mg chl m-3
and 1.04 ± 0.45
mg chl m-3 in the Elephant Island and Joinville Areas, respectfully, and the South area having
the highest values of 1.67 ± 0.58 mg chl m-3.
Although not all samples have
been processed for the near-shore survey, general trends are found, with
chlorophyll concentrations highest off shore (~1.8 mg CHL m-3) and
slightly decreasing near-shore (~1.4 mg CHL m-3). Both extracted
chlorophyll and chlorophyll fluorescence profiles (nighttime measures) indicate
a slight chlorophyll maximum occurring 20-50 m for the
outermost stations. The outermost stations also roughly correspond to stations
A17-11 and A16-10, that measured ~0.8 mg CHL m-3
two weeks previous, and indicate an approximate doubling of phytoplankton
biomass during this time.
5. Oceanography and
Meteorology. Predominantly Westerly
to North Westerly winds, averaging 20 knots, were experienced during the week,
with winds peaking at 40 knots on Friday. The constant strong winds experienced during
the week brought with it increase swell size towards the latter part of the
week. Barometric pressure showed a steady decline from 995 to 965 millibars
through the week, rising steeply on Saturday to end the week on 990 millibars. Air temperature during the week remained around
1.5°C, rising to a maximum of 3.5°C on Saturday. 27 CTD stations were completed during the
week as part of the Near Shore Survey being conducted off Cape
Sherriff.
Four transect, of 5 CTD stations each, were completed on the eastern (Y8) and
western (Y2) ends of the survey area. The central transect (Y5) was cancelled
due to rough seas, with only 2 CTD stations being completed.
6. Predator diet studies. All scats samples have been processed from
weeks three through six (collection dates of 1/6-1/27/05). Fish otoliths from myctophid fish have
been present in the diet of Antarctic fur seals from week four through week
six. Otoliths from myctophid
Gymnoscopelus nicholsi are
the most abundant in ten scats out of 42 collected with Electrona antarctica present
in eight of the scats. However overall abundance of fish otoliths
is low compared to previous years during the same time. Squid beaks have been
observed in two scats. All 47 milk samples have been processed for lipid
extraction and will been taken back to the SWFSC for final fatty acid signature
determination. To date a total of 250
live krill have been measure for total length, carapace length and width and sexed.
Additionally, 24 various species of Antarctic fish have been caught in net tows
and will be measured for total length, weighed, dissected for stomach content
and fullness, sexed and have their otoliths removed.
These fish will be homogenized for lipid extraction in order to match the fatty
acid signatures to the milk samples collected and processed.
7. Bird and marine mammal
observations. This report includes information on observations collected
during the nearshore survey north of Livingston
Island. Approximately 600 nautical miles were
surveyed along transect lines, which represent two samples each for the west
and east hydrographic canyons. Cape
Petrels were the numerically
dominant feeding seabird recorded, and were primarily observed feeding by
plunge-diving for prey. Preliminary
mapping of seabird feeding aggregations suggest that they are forming along the
edges, and between the canyons. Fur
seals were primarily observed traveling in a northwest direction and
subsequently the greatest number (18) of animals recorded (per hour of transect
surveyed) was located at the northwest head of the west canyon. Humpback and Minke
whales were commonly encountered inshore and appeared to be feeding. Five species of Albatross were recorded
throughout the survey period; in decreasing order of abundance: Black-browed,
Gray-headed, Wandering, Royal, and Light-mantled Sooty Albatrosses. Blue Petrels, and
Antarctic Prions were patchy in distribution, and
were relatively restricted to the west survey lines. Other avifauna highlights include: one Common
Diving Petrel and four Arctic Terns.
Submitted by Christian Reiss.