Weekly Report No. 2
22 January 2006
Our current position is approximately 60 nautical miles northwest of Elephant
Island where we are conducting a
survey of bio-oceanographic conditions in the vicinity of the South
Shetland Islands. The survey is divided into four strata: the West
Area north of King George and Livingston
Islands; the Elephant Island Area encompassing
the northern portion of the South Shetland archipelago; the Joinville
Island Area in the western portion of Bransfield Strait;
and the South Area in the central portion of Bransfield Strait
south of King George and Livingston Islands.
Survey operations in the West Area and half of the Elephant Island Area were
completed this past week. Agreeable weather and sea conditions enhanced
progress during the week.
Krill, salps and other zooplankton. Post larval krill
(Euphausia superba) were
present in 19 of 25 West Area samples (76%) with relatively high mean and
median abundance values of, respectively, 10 and 8 per 1000 m3. Greatest concentrations were along the inner shelves
of the South Shetland Islands where they were obviously grazing
on ample phytoplankton resources.
Virtually all individuals were large reproductively mature stages, 76%
of which were males and 22% were gravid and spent females. The vast majority of mature females were in
advanced reproductive stages (96%) indicating a favorable early spawning
season. Most of the krill were between
49 and 57 mm in length with a 52 mm mode.
These individuals represent the successful 1999/2000, 2000/01 and
2001/02 year classes now four to six years of age.
were the most abundant taxonomic category (14500 per 1000 m3 mean,
1230 per 1000 m3 median) and numerically dominated the zooplankton. Oceanic species Calanoides acutus, Calanus propinquus and Rhincalanus gigas were particularly abundant with their distribution
pattern reflecting the presence of the Antarctic Circumpolar Current Southern
Front (SACCF) along the outer island shelf area. Other zooplankton taxa
associated with the SACCF (e.g., larvae of the euphausiid
Thysanoessa macrura, chaetognaths and the pteropod Limacina helicina)
were also relatively abundant. In contrast,
the salp Salpa thompsoni was comparatively rare, with mean and median
concentrations of 72 and 7 per 1000 m3. Its distribution was in Drake
Passage north of the SACCF where postlarval
krill were scarce.
biomass and dispersion. We have completed preliminary estimation of the
biomass of krill along the west shelf of the South Shetland Islands
using the EK-60. Mean krill density along each line was very low ranging from
1.3 to 6.8 g/m2, and averaging just 2 g/m2. These numbers
are an order of magnitude lower than last year. Consequently, biomass along the
west shelf was also low, just ~100K tons. Likewise krill biomass in 2006 is an
order of magnitude lower than last year. We will estimate krill biomass using
the SDWBA approximation in the final report.
(C.D. Hewes, reporting). High concentrations of chl-a were found along the entire
northern coastal margin of the South Shetland Islands.
Shelf and shelf-break waters averaged 2.8 ± 1.2 mg m-3 chl-a at 5 m, with 102 ± 47 and 59 ± 17 mg m-2
for chl-a integrated to 100 meter and the euphotic zone (1% incident irradiance), respectively (15
stations). Highest values were at stations A23-07 and A14-08 where chl-a concentrations were measured over 4 mg chl-a m-3 in surface
waters. Pelagic waters (>2000 meter bottom depth; 10 stations) had lower
surface concentrations, averaging 0.5 ± 0.4 mg m-3, with integrated
values averaging 30 ± 15 and 23 ± 13 mg m-2 (100 meter and euphotic zone, respectively). Although these are relatively
low biomass values, they are considerably higher than the ~15 year average of
~0.2 mg m-3 found of the area. Pelagic Drake Passage
waters are associated with low surface chlorophyll concentrations, but a chlorophyll
maximum that typically occurs at ~75 meter depth. This season, only 3 of 10
pelagic Drake Passage stations (A1505, A1808, and A1909)
as well as two stations above the Shackleton Fracture
Zone (A0902 and A1101) had a deep chlorophyll maximum. However, the deep chlorophyll
maximums this year are occurring at 30-75 meters, and therefore are much
shallower than has been observed during past years. This corresponds with a
much shallower winter water remnant than found for average in Drake
Passage waters. This year,
in collaboration with AMLR, NSF- and NASA-sponsored phytoplankton studies are
also being conducted. Members and projects include:
1) Murat Řzturk, NTNU-TBS, Norway,
has collected 60 samples (Stations A09-02, A15-05, and A 20-10) for
measurements of particulate and dissolved iron as well as determination of Fe
speciation and organic ligands.
2) Henrieta Dulaiova and Paul
Henderson (Charette group, WHOI) will estimate
vertical particle export from thorium-234 measurements, and horizontal water
transport in surface water from measurements of four naturally occurring radium
isotopes. The first stations were occupied to optimize sampling and measurement
conditions. 42 radium samples have thus far been analyzed, and 77 thorium-234
samples have been filtered for analysis in the near future. Preliminary beta
counter results indicate a classic disequilibrium in the surface mixed layer for
waters north of the South Shetland Islands.
Nigel Delaney and Brian Seegers (Mitchell group, SIO)
have acquired primary productivity verses irradiance data (30 station/depths), particulate
organic carbon/nitrogen (CHN) and photosynthetic pigment (HPLC) samples (26
station/depths), and particulate and soluble absorbance spectra (27 and 18
station/depths, respectively). Additionally, in-situ radiometric, backscatter
light and fast repetition rate fluorometry profile
data from 6 stations have been obtained.
and meteorology. Predominantly overcast skies with periods of rain, saw air temperatures ranging between 1 and 6°C during
the week, with only short periods of sunshine. Gradual barometer swings between
983 and 1007 millibars produced a northwesterly wind
speed averaging between 10 and 15 knots. The average wind speed increased to 18
knots when the wind swung around to the southwest on Friday. Three of the five wind powered autonomous
instrument buoys off Cape Shirreff continued to
transmit data during the week, but two of the buoys have switched into power
saving standby mode, due to the low average wind speeds experienced. 46 CTD
casts were completed in the West and Elephant Island Areas and an initial field
water typing assessment was done according to the Water Zone Classification
table. This showed classical, well defined Water Zone 1 (ACW) zones at the deep
offshore stations, Water Zone 2 stations along the shelf break, and on the
shelf itself, and north of the islands. The western end of the survey area
produced stations of transition between Water Zone 1 and 2. One Water Zone 3
station (11-07) was found on the shelf on the eastern end of the West Area.
diet studies. Predator diet studies
have been initiated for Antarctic fur seals (Arctocephalus gazella). Lipids have been extracted
from 28 milk samples using the FOLCH method. A total of 15 scats have been
processed; 5 scats from week one, 6 scats from week two and 4 scats from week
three. So far, only one myctophid otolith
from Electrona antarctica
has been discovered from week two of the scat collections. The scats are
dominated by large krill (Euphausia superba) ranging from 35-55 mm in length. No squid
beaks have been seen to date.
and marine mammal observations. Data on the distribution, abundance and
behavior of seabirds and mammals was collected during underway ship operations
in the West area. In total, 18 transits between
stations were collected equaling approximately 360 nautical miles of sampling
effort (75% of the West area). Feeding
activity by seabirds was fairly common throughout the shelf break region, especially
between the 500 and 1500 m isobaths.
Aggregations of Antarctic Fulmars, and Cape
Petrels were especially abundant on
the westernmost transects of the survey.
Black-browed and Gray-headed Albatrosses were common, and on a few
occasions they were observed in loose aggregations surface-seizing for
prey. As in past AMLR surveys, Blue
Petrels and Antarctic Prions were abundant along the
South Shetland Trough, and were observed flying predominantly into the headwind. There were 5 sightings of Humpback Whales,
and one sighting of a Southern Bottlenose Whale. The most exciting sighting of the survey
period was a pod of Killer Whales on the morning of 17 January while positioned
at AMLR station 17-09. Approximately 8
adults with at least 2 calves were traveling in a southeast direction. Photographs and video footage was collected.
A. Jenkins sends.