AMLR 2003 Weekly
Report No. 7
23 February 2003
1.
Our current position is approximately 15 nautical miles south of King
George Island,
where we are conducting the second of two surveys of bio-oceanographic
conditions in the vicinity of the South Shetland Islands.
The West and Elephant Island
areas of the survey have been completed. An extensive band of pack ice and
icebergs pushing northward out of the Weddell Sea forced
a change in the conduct of the survey in the Joinville
area allowing only minor penetration into the area. During the first survey of
this year the heavy ice conditions reached appreciably into the Elephant
Island area hindering operations.
The fact that we completed the Elephant
Island on our second survey
illustrates a dramatic retreat on the part of the Weddell Sea
ice in the past four weeks. Weather has been mild to moderate allowing the time
lost to a patch of heavy weather earlier in the survey to be made up.
2. Krill,
salps and other zooplankton. Post larval krill
were present in 42 of 48 (88%) Elephant Island samples. Mean and
median abundance values (95 and 9 per 1000 m3) were ca. 33% those
observed here during Survey A. This was
associated with decreased numbers of small juveniles (<30 mm lengths) and a
modest increase in older mature krill >40 mm in length. Overall proportions of juvenile, immature and
mature stages was 21%, 52% and 27% compared to 42%, 39% and 19% the previous
month. Reproductively advanced female
stages (i.e., gravid and spent) were scarce, comprising <1% of total krill
abundance in the area. The gaunt appearance
of mature individuals gave the impression that spawning was delayed or
minimized due to poor feeding conditions.
Larval krill were present in 43% of samples and had a slightly higher
mean abundance than last month (6.9 vs. 4.6 pre 1000 m3). While most of these were early calyptopis stages (1 & 2), 12% were furcilia
stage 2 indicating a prolonged, possibly sporadic, spawning season.
Salpa thompsoni abundance
(mean 62 and median 10 per 1000 m3) was essentially the same as
during last month. Greatest
concentrations were offshore. The
aggregate stage comprised 92% of individuals.
Only 10% of these were <20 mm in length indicating negligible chain
production during the intervening period.
The median length (34 mm) was 8 mm larger than during Survey A and
conforms to a ca. 0.44
mm per day growth rate. These
observations suggest retention of a relatively stable salp
population within the Elephant Island
area during the 2003 summer. This could
be explained by the sluggish circulation indicated by the drifters deployed in
outer shelf and offshore areas.
Increased abundance of solitaries was due to new production, as
indicated by the fact that 75% were <25 mm.
Production of this over wintering stage, together with negligible chain
production, signals an early end to the summer production cycle.
As
during Survey A, copepods numerically dominated the Elephant
Island zooplankton catches. As in the West area, copepod abundance had a seasonal
increase with the mean value 3× and median value 2× those in January; this
increase was primarily due to coastally derived Metridia gerlachei (62% of total). Post larvae of the euhpausiid
Thysanoessa macrura
followed copepods in overall abundance.
3. Krill
biomass and dispersion. Seven transects were completed over the Elephant
Island Area during the week ending 23
February 2003. Highest densities
of krill biomass were mapped along the westernmost transect and to the north
and north east of Elephant Island,
along the shelf break, and again to the east and northeast of Clarence
Island. Krill densities for each
transect ranged from 0.95-27.72 g/m-2. Mean krill density over the seven
transects was 12.1 g/m-2, which is significantly lower than the mean
density measured during Survey A of the 2003 cruise (22.1 g/m-2). Compared with Survey D results for 2002 and
2001, krill density is higher then 2002 (6 g/m-2) and on par with
2001 estimates (13 g/m-2). Myctophid mean
biomass densities for the West area are estimated to be 49.4 g/m-2
and 40.9 g/m-2 for the Elephant
Island area during leg III. These density estimates indicate an increase
in myctophid abundance as compared with the West and
Elephant Island areas during leg I (35.1 and 16.1 g/m-2
respectively).
4. Phytoplankton.
Chlorophyll concentrations in the Elephant
Island have increased since Leg I.
For 48 stations surveyed, 5 m chlorophyll was 0.63 +/- 0.42 mg m-3
and integrated (to 100 m) was 21.44+/- 13.69 mg chl m-2.
These values compared to 0.10 +/- 0.09 mg chl m-3
and 16.4 +/- 9.1 mg chl m-2 for 5 m and
integrated values, respectively, measured for Leg I. Leg II values are lower than
the February average (1990-2002) of 1.13 +/- 1.23 mg chl
m-3 for 5 m and 70.91+/- 64.43 mg chl m-2
for integrated (to 100 m) chlorophyll concentrations (572 stations). Highest chlorophyll
concentrations were observed in shelf and shelf-break waters (bottom depth less
than 1000 m), with 5 meter samples having 0.63 +/- 0.51 mg chl
m-3 (16 stations), compared with 5 m samples of pelagic stations
having 0.25 +/- 0.31 mg chl m-3.
Three
additional (and final) drifter buoys to measure ocean
currents were deployed this week, thus 8 total drifters were deployed during
Leg II. Signals are still being obtained from the 6 of 12 drifters deployed during
Leg I. Preliminary inspections indicate that drifters deployed west of the Shackleton Fracture Zone (SFZ) are deflected in both northerly
and southerly directions to suggest that the easterly flow of the Antarctic Circumpolar
Current is interrupted by the SFZ. CTD casts made in the Elephant Island Area
have been of sufficient coverage to resolve a region of possible down-welling
parallel to and approximately 50 km east of the SFZ, ranging from at least
60.0°S, 57.5°W to 60.75°S, 56.5°W. This down welling condition was best
observed by a “plume” of higher concentrations of oxygen and lower temperatures
relative to surrounding waters to 750 m. As 60ºS is our most northerly survey
boundary, it is not known how much further north this region of possible down
welling extended.
With
aid of Local Area Coverage from SeaWiFS satellite, a
small patch of phytoplankton bloom (near Station D11-07) surrounded by a large
area of water having low chlorophyll concentrations was observed north of the eastern
end of King George Island. As this bloom was near our current position, an
extra CTD station (Station X10-07) outside the determined survey pattern was
made. 5 m chlorophyll was 1.73 and 2.31 mg chl m-3
and integrated chlorophyll (to 100 m) was 32.2 and 55.9 mg chl-2 at
stations X10-07 and D11-07, respectively. The high chlorophyll concentrations
at these two stations corresponded to a possible upwelling condition at these
two stations as evidenced by cooler, less dense, and more oxygenated water
column profiles than for surrounding stations. Water column properties (Type
III water) of these two stations suggest that they had been influenced by Bransfield
Strait water that had pushed northward
over the sill that separates King George and Elephant
Islands.
5. Oceanography
and meteorology. Northwesterly winds, averaging twenty knots with maximum
speeds reaching 35knts, were experienced for most of the week. Tuesday and
Wednesday saw the winds swing to northeast, with average speeds of fifteen
knots. Atmospheric pressure showed a gradual increase until midnight on Thursday, reaching ~1010 mbar, then a steady decrease
to 980 mbar on Sunday. Air temperatures ranged between 1 and 5°C with most days
being overcast.
48
CTD stations were occupied in the Elephant
Island area with two stations
(D09-08 & D08-08) only done at the end of EI07 transect, due to bad weather
at the beginning of the week. An extra CTD station X10-07, identified as a high
chlorophyll concentration area by a SeaWiFS satellite
image, was added between the West and Elephant
Island areas. This station and the
adjacent station (D11-07) showed higher concentrations of chlorophyll and
possible upwelling in the CTD traces than the surrounding stations in the area.
As per the Water Zone classification tables, Type 1 (ACW) water was found in
the Western side of the area and just below the Shackleton
Fracture Zone. Mixed waters of Type II & III are found on the upper eastern
side of the fracture zone, with the ACW becoming diminished. This can be seen
in the CTD traces in the area which shows lots of mixing occurring and some evidence
of down-welling taking place at some stations. Type V (Weddell Sea) water being
found on the extreme lower eastern side of the area. The lower part of
the area is mainly influenced by Type IV (Bransfield
Field Strait)
water in the west and around Elephant
Island shelf area.
6. Predator
diet studies. This week 10 additional scats were sifted and processed. All samples but one contained krill and 8 of
the 10 contained fish otoliths. Week 5 scats continue to show a reversal of otolith dominance from the previous week, with G. nicholsi
becoming much more prevalent than E. antarctica. In addition, week 5 scats contained a large
number of small, unidentified otoliths, most likely
eroded G. brauri. Five and a half squid beaks were discovered
and stored in ethanol. Twenty-four fur
seal milk samples had lipid extracted, and an additional 25 fresh krill were
gendered and measured for total length, carapace length, and width. Opportunistically, extra myctophids
and krill are collected from the net tows for later homogenization and lipid extraction.
7. Bird
and marine mammal observations. This report includes information regarding
the Elephant Island
area survey. Systematic observations of vertebrate
abundance and behavior were conducted during daylight hours in transit between
stations. A total of 34 transits were
sampled and 24 species of seabirds were recorded. Highlights included Thin-billed prion, Soft-plumaged petrel, White-headed petrel, Common
diving petrel, and Arctic tern.
Chinstrap penguins were highly conspicuous in the pelagic waters north
of Elephant Island
and were the most abundant species. A
strikingly large aggregation of Black-browed (300+) and Grey-headed (40+)
albatrosses were observed sitting on the water on transect line E4. Such a dense aggregation of albatrosses may
represent an abundant and predictable prey resource. On transect line E6, we encountered hundreds
of Antarctic prions every hour, and the majority of which
were flying in a westward direction.
Great numbers of Wilson and Black-bellied
storm petrels were observed on transit south of Clarence
Island. On transit to station 5.5-08, thousands of
Antarctic fulmars were observed flying north; most likely towards Gibbs, Aspland, Eddie, and O'Brien Islands, where there may be
nesting colonies for this species. A
total of 22 humpback whales were counted during transit to 5.5-08 as well. Other marine mammal observations included:
fur seal, fin whale, humpback whale, minke whale, southern bottlenose whale,
and hourglass dolphin.
A.
Jenkins sends