Weekly Report No. 9
11 March 2002
1. Current position: the R/V Yuzhmorgeologiya is hove-to in Admiralty Bay, King George Island of the South Shetland Islands. The second survey
(Survey D) of bio-oceanographic conditions in the vicinity of the South Shetland Islands was completed over the
weekend. Weather conditions improved
from the beginning of Leg II allowing for the successful completion of survey
operations, the past two days have been dedicated to the closing of two field
camps. Five personnel were recovered
from the camp at Cape Shirreff, Livingston Island, and three persons from
the camp in Admiralty Bay of King George Island. The shipboard scientific party currently
totals 25 persons. The ship is preparing to steam into the Weddell Sea to conduct two stations
to characterize the oceanographic profiles and zooplankton assemblages in the Weddell Sea gyre which feeds into
the Bransfield strait. After completing these
stations the ship will make way for Punta Arenas, Chile.
2. Krill, salps and
other zooplankton. Survey D Results: An estimated 57750 krill were collected in
94 Survey D samples. While overall mean
krill abundance (281 per 1000 m3) was larger than during Survey A
(65 per 1000 m3) the median was substantially lower (0.5 vs. 2 per
1000 m3) due to much patchier distributions. The largest catches, estimated to be 11000 to
22000 individuals (between 7300 and 9300 per 1000 m3), were located over the northern Shetland Island shelf (West area) and
in western Bransfield Strait (South area). Here mean abundance values were 694 and 548
per 1000 m3, respectively.
Lowest mean abundance (4.3 per 1000 m3) was in the Joinville Island area. With exception of one large catch over the
north Elephant Island shelf, krill abundance within the Elephant Island area
was relatively low and mean and median abundance (respectively, 10 and 0.4 per
1000 m3) was markedly reduced from January Survey A (39 and 8 per
Krill lengths ranged from 18-59 mm with a
modal length of 27 mm; 87% of individuals were < 35 mm. Accordingly, juveniles comprised 73% and
immature stages 25% of the total.
Immature forms were primarily small (31-35 mm) males and females
and, like the juveniles, resulted from a relatively successful 2000/2001 year
Overall krill demography was highly influenced
by large catches in the West and South areas where juveniles comprised 72% and
76% and immatures 27% and 23%, respectively. Size and maturity classes were more
heterogeneous in the Elephant Island area where the size
distribution was polymodal about 29 mm, 42 mm and 52
mm lengths and juveniles comprised 39%, immatures 17%
and mature stages 44% of the total. This
composition is more or less similar to that represented in the Elephant Island area during Survey A
where 20-30 mm juveniles contributed 46% and > 45 mm mature stages 20%
of the total. All length and maturity
classes were evenly represented in the Joinville Island area.
Larval krill were present in 29% of survey
samples, with a mean value of 61 per 1000 m3. The largest catches were primarily offshore
of the South Shetland Island shelf and greatest mean
abundance in the West area was 134 per 1000 m3. Calyptopis 1
through Furcilia 2 stages were collected and
reflected a prolonged spawning season initiated in early December. Calyptopis larvae
made up 86% of the total larvae; Calyptopis stage 3
dominated (50%). Mean, median and
maximum larval krill abundance values within the Elephant Island area (50, 0
and 730 per 1000 m3) were only slightly elevated over those of
Survey A (36, 0 and 360 per 1000 m3).
Salpa thompsoni was the fourth most
abundant taxon overall following copepods, larval Thysanoessa macrura, and chaetognaths. Chain
forming aggregates comprised 95% of the salp
catch. Of these, 50% were < 13 mm and
resulted from budding over the preceding month.
Greatest salp concentrations, and of large
mature solitaries and recently budded aggregates, occurred in Oceanic waters of
the West and Elephant Island areas. This salp was most
frequent (91% of samples) and had greatest median abundance in the Elephant Island area where it also was
fourth most abundant taxon. As with larval krill, mean, median and
maximum salp abundance values in the Elephant Island
area (541, 251 and 2903 per 1000 m3) were only slightly elevated
over the January values (410, 86 and 2817 per 1000 m3).
Copepods were by far the most frequent (100% of
samples) and abundant zooplankton overall and within all four survey
areas. Calanoides acutus, Calanus propinquus, Metridia gerlachei and Rhincalanus gigas were the major constituents. Greatest concentrations (>10000 up to
175000 per 1000 m3) generally occurred offshore of the South
Shetland and Elephant Island shelf areas. However, several large catches were also made
within Bransfield Strait (South and Joinville Island areas) probably due to
retention with frontal zones or eddies.
Mean and median copepod abundance values in the Elephant Island (17500 and 7600 per
1000 m3) exhibited a 3X seasonal increase over Survey A.
2002 Field Season Overview
With respect to the long term AMLR Elephant Island area data set, mean and
median copepod abundance values during both 2002 surveys were by far the
highest: they were an order of magnitude
greater than those during 2001 and 2X the highs observed during February-March
2000. Survey A and D 2002 chaetognath and larval T.
macrura abundance values also were the highest
recorded, ca. 50% greater than peak values of 2000. Greatly elevated abundance of predatory
amphipod species, Themisto gaudichaudii
and Primno macrura,
relative to previous years further indicates a substantial increase in
secondary productivity. Survey A and D
results (advanced female maturity stages, krill larval abundance and stage
composition) indicate a prolonged, and fairly successful krill spawning season
during 2001/2002. Decreased postlarval krill abundance between these surveys probably
resulted from seasonal distribution shifts associated with termination of the
spawning season rather than a major stock decline. Mean and median January abundance increases
over those of 2001 resulted from relatively strong recruitment success of the
2000/2001 year class which offset decreased abundance (i.e., mortality) of the
highly successful 1995/1996 year class that has dominated Elephant Island demographics over the
past 6 years. Survey A and D larval
krill abundance values, while not extraordinarily high as during 1995, were
similar to those of 2001 and, like 2001, demonstrated a modest seasonal
increase in abundance and development.
Given the strong proportional recruitment success of the 2000/2001 year
class, there is reason to anticipate at least modest recruitment success from
the 2001/2002 season.
The late season spurt of aggregate salp production during 2002 is reminiscent of the 1997
season which preceded a major salp year in 1998. This could be reason for concern were it not
for the facts that (a) this was associated with only a modest seasonal
abundance increase (there was an order of magnitude increase between the two
surveys in 1997) and (b) most of this late season production was located in
Oceanic waters and most likely not retained as overwintering
seed populations within the survey area.
Salp abundance within the Elephant Island area has remained
fairly stable over the past three years; no major changes are anticipated for
3. Krill biomass and dispersion. Highest krill
densities were mapped north of Elephant Island at the shelf break.
Krill was also observed in great abundance along the shelf northeast of Livingston Island and south of Snow Island. Overall krill biomass densities were
calculated to be 11 to 18 g m-2 during leg I and 5 g m-2
during leg II. Average krill biomass
densities for the West, Elephant Island, Joinville Island and South Areas were 3,
6, 4 and 6 g m-2 and have declined with respect to leg I (11-29,
19-40, 7 and 16 g m-2, respectively).
4. Phytoplankton. 5m chlorophyll averages for the Elephant
Island Sector were 0.97 +\- 0.58 mg m-3, and integrated
(100 meters) averages 67 +\- 44 mg m-2 for the entire Elephant Island section (37
stations). These surface values are
about 80% higher, while integrated values about 50% higher, than those found
during Leg I (January). For this sector, chlorophyll concentrations for
February are slightly higher than the 11 year Leg II averages (420 stations) of
0.89 +\- 0.98 mg chl m-3 for 5
meters and 50 +\- 45 mg chl m-2
for 100 meter integrated values. Phytoplankton biomass also increased over
January values for the Bransfield Strait (27 stations), with
surface chlorophyll values of 1.00 +\- 0.58 mg m-3 (a
45% increase) and integrated values of 58 +\- 34 mg chl
m-2 (about 18% higher). The Bransfield Strait phytoplankton biomass
for February is less than the 11-year average of 1.6 +\- 1.9 mg chl m-3 for 5 meters and 85 +\- 94
mg m-2 for integrated (100m) chlorophyll. For the Joinville Island area (6 stations), phytoplankton biomass
increased as well having surface values of 0.88 +\- 0.55 mg chl m-3 and integrated chlorophyll of 65
+\- 26 mg m-2. This is a new section so no comparison to
historical averages might be made. All equipment fairs well, with only
replacement of cables for the profiling fluorometer
and profiling transmissometers as needed.
5. Oceanography and meteorology. Wind varied between northeast and northwest
for the greater part of the week, changing to the southwest on Tuesday and then
gradually swinging back to the north over the following two days. Wind speeds
were variable reaching a maximum level of approx. 40 knots from the southwest
on Tuesday. Pressure cycled around 988 mb until
Wednesday and then decreased steadily to 972 mb by the
end of the week. Temperatures varied between approx. 4.5º and 1ºC, the minimum
being recorded in midweek. Conditions were generally overcast with drizzle
clearing partially on Sunday, the 3rd.
Light snowfalls were also experienced.
Water Zone II dominated the northern portion of
the Elephant Island area with Zone III
water passing through the northern shelf region between King George and Elephant Islands. Water Zone I
penetrated as a tongue, from the west (Stations D11-03 and D09-03),
into Zone II waters. Water Zone IV lay directly south, extending from just
below Elephant and Clarence Islands through the deeper portions of the Bransfield Strait to southwest of Deception Island. A thin
band of Water Zone III extended along the southern coastline of the South
Shetland Islands. Water Zone V extended
from north of Clarence Island (Station D02-05) along the east-most
transect to the Weddell Sea, and continued in a southwesterly direction hugging
the Antarctic Peninsula along its northern continental shelf (to station D15-15).
6. Predator diet studies. Both the milk lipid extraction and the scat
sorting portions of the diet studies were completed. Lipids were extracted from the remaining 25
prepared fur seal milk samples and were frozen.
An additional 9 whole milk samples were aliquoted
in duplicate, one set undergoing lipid extraction while the other was
frozen. The final eleven scats and three
vomits were sorted, and krill carapaces were measured and stored in ethanol. Scats from this week were mostly comprised of
krill chitin and contained far fewer otoliths,
predominantly G. nicholsi. However, one additional squid beak was found
and also stored in ethanol. Krill and myctophids caught during the last portion of the large area
survey were again saved. Approximately
five hundred krill and all of the myctophids will be
sent back to the lab for extra processing there.
7. Advanced Survey Technologies. The second multi-instrumented buoy was
retrieved following the completion of the large-area survey. The buoy, fitted with the dual-frequency
echosounder, was moored near the head of a canyon approximately 4.5 n.mi. offshore to the east of the field camp at Cape Shirreff. A cursory look at the multi-day time-series
indicates a strong diel migration pattern for krill
residing in the area. These data allow a
two-frequency krill identification technique to be implemented. The preliminary results are consistent with a
previous interpretation of backscatter data from the ADCP mounted on the first
buoy that was deployed in the area. Total target strengths (TTS) were measured
over a wide acoustical bandwidth (36-202 kHz) for multiple size classes
of Antarctic krill (Euphausia superba),
three species of myctophid (Electrona antarctica, Gymnoscopelus nicholsi, and G. braueri), and a squid.
The TTS data for krill are remarkably consistent with predictions based
on a distorted wave born approximation.
A. Jenkins sends