AMLR 2004 Weekly Report No. 10
14 March 2004
1. The
ship is currently moored alongside the pier in Punta Arenas, Chile. The passage across
the Drake was uneventful. The three days in port will conclude our 2004 season.
2. Krill
biomass and dispersion. Surveys were
completed on March 6th.
3. Zooplankton
and krill. Overall krill abundance and maturity stage composition during
January-February was quite similar to that during January; both surveys were
characterized by widespread distributions dominated by equal proportions of
immature and mature stages. Juveniles represented
less than 5% of the catch each survey indicating poor recruitment from the
previous season (2002/2003) compared to the highly successful 2000/2001 and
2001/2002 year classes. Greatest
differences in krill populations between the two surveys were due to seasonal onshore
migration of large mature individuals into the Elephant Island and West areas resulting in a polymodal
length-frequency distribution around 33, 43 and 50 mm modes and substantial
increases in proportions of advanced reproductive stages. During Survey D, 83% of mature females in the
Elephant Island area were in advanced stages (gravid and spent) compared to
12% during Survey A; fully mature males increased to 43% from 11% of the krill
catch between the two surveys. Larval
krill were also widely distributed during both surveys but their numbers
increased by ca. 20× over the two month period.
Proportions of more advanced furcilia stages
(particularly F2 and 3) relative to early calyptopis stages
(notably C1) increased to 56% vs. 5% during this time. These results, taken in conjunction with the
active spawning observed during Survey D, indicate that the 2003/2004 krill
spawning season was unusually prolonged.
The
absolute abundance of Salpa thompsoni
decreased between the two surveys; this decrease was most obvious in the West
and Elephant Island areas where largest concentrations were located. These areas were characterized by unusually
large proportions of the solitary (over wintering) stage across the survey
period. Small solitaries contributed ca.
10% of the salp catches here during Survey A compared
to ca. 50% during Survey D. Substantial
increases in the over wintering population in conjunction with decreased
abundance of the summer-dominant aggregate stage suggest a relatively early
termination of the summer production cycle.
The high latitude salp, Ihlea racovitzai, persisted throughout the survey
period, its distribution over the northern shelves of the South Shetland and Elephant Islands and the deep basins of Bransfield Strait indicative of Polar Slope water influence from the
east. Abundance of this salp decreased in all but the Elephant Island area between the two surveys suggesting a more northerly
vs. westerly advection with the advancing season.
The most
dramatic change over the survey period was evidenced in total zooplankton
abundance and species richness.
Increased abundance was largely due to copepods, chaetognaths,
larval Thysanoessa macrura and krill,
and radiolaria.
Order of magnitude increases occurred in the abundance of dominant
copepod species, Metridia gerlachei, Calanoides acutus, Calanus propinquus, Paraeuchaeta Antarctica, as well as chaetognaths
and T. macrura
larvae; abundance of oceanic copepod species, Rhincalanus gigas, increased by two orders of
magnitude. A total of 104 zooplankton taxa were identified during Survey D compared to 89 the previous
month. Associated with this change, the
relative importance of Salpa thompsoni
decreased from 17% to 2% of total mean zooplankton abundance.
Within
the long-term Elephant Island data set post larval krill abundance during the
2003/2004-field season conformed to the long term (1992-2004) average. Low krill concentrations relative to those of
the 1995/1996 and 2002/2003 field seasons result from poor recruitment success
from the previous year class. Although Salpa thompsoni abundance
values during the past two field seasons have been low relative to those of
1993-1994 and the entire 1997-2002 period they remain at least 3× greater than
the lows observed during 1995-1996. Large
copepod concentrations of February-March 2004 were similar to the highs
observed during late summer 1996 and the 2000-2002 period. These results (along with abundance
fluctuations of other dominant zooplankton taxa)
reflect multi-year trends within the Antarctic
Peninsula ecosystem.
The
2003/2004-field season was unusual in certain respects. Copepod and larval krill abundance typically
demonstrate a 2-5-fold increase over the January-March summer season. Their respective 10× and 20× increases this
year are rivaled only by increases observed during the 1994 and 1995
summers. The abruptness and magnitude of
these changes, in conjunction with increased abundance of other zooplankton taxa and overall diversity, suggest a hydrographic rather
than biological cause.
While
relatively large proportions of furcilia stage krill
larvae were observed during the early summer surveys of 1998, 2002 and 2004 the
large contributions of advanced furcilia during
February-March 2004 are unequalled in the long-term data set. This advanced development
along with high larval krill abundance suggest potentially strong
recruitment success of the 2003/2004-year class. In light of the successful 2000/2001 and
2001/2002 year classes, this potential is unique within the long-term data set,
which previously indicated successful krill recruitment every ca. 4-5 years.
4. Oceanography
and meteorology. A well-defined Antarctic Polar Front was crossed between
Latitude 58° 54.4639´S and 58°14.218´S on the 13th with sea surface
temperature and salinity increasing from 3.62°C to 6.8°C and 33.57 ppt to 33.86 ppt respectively. Wind blew mainly west to north-west during
the week at an average speed of approx 15 knots, but switched to easterly for
short periods on Tuesday and Thursday with a corresponding decrease to 5 knots.
Air temperature rose steadily from Thursday reaching a maximum of 11º C towards
the end of the transit.
5. Phytoplankton. The conclusion of the 2004 survey season was highlighted
by the highest Surface Chlorophyll values as predicted in the preceding
report. Station D08-10 in the South Area
had a measured surface Chlorophyll value of 6.40 mg m-3. The North East corner of the Bransfield Strait thus had the highest integrated Chlorophyll of the survey
area with values at 30 mg m-2 to 45 mg m-2. A total of 98 CTD stations were sampled in
this 2 Leg of the 2004 survey, Chlorophyll was sampled at all depths within the
top 200m of the water column. In addition
to the standard AMLR Chlorophyll, IOP profile data was collected at 35
stations, Radiometer profile data (PRR 600 Free Fall Radiometer) was collected
at 14 stations, and absorption spectra, HPLC Pigments, MAA and POC data was
collected at 53 stations. Several underway
stations were also sampled on the transect back to Punta Arenas for HPLC and POC while flow thru Fluorometer
and AC9 data was also logged continuously.
6. Seabird
and marine mammals observations. A visual seabird
survey using a standardized 300-metre strip transect
was conducted during daylight hours while the ship was transiting to Punta Arenas. In addition, all marine mammal sightings were recorded on
an opportunistic basis. A total of 190 NM of track line was surveyed during 15
hours of observation effort, from south of the Polar Front (60°27´S) to about
37 NM NNE of Rio Grande, Isla Grande de Tierra del Fuego (53°35´S). The high average sea state (Beaufort 5)
particularly on 12 March, was not conducive to marine mammal detection; hence
the low number of sightings (8 representing 6 species). Very few seabirds were
seen during the Drake crossing, only 65 individuals of 6 species, 78%
consisting of unidentified diving-petrels and Soft-Plumaged Petrels (40 and 11 individuals
respectively). Surprising was the complete lack of Wandering Albatrosses,
White-chinned, Cape and Blue Petrels and Black-bellied Storm-Petrels. On 13 March, 467 seabirds representing 16 species were seen, 73% were
Black-browed Albatrosses and Sooty Shearwaters, concentrated at the south end
of Estrecho de le Maire.
Very few birds were seen north of Cabo San Diego. An
exciting find was a Shy (White-capped) Albatross 13 NM northeast of Cabo San Diego, the first sighting of this species on an
AMLR survey. This albatross, which nests on New Zealand sub-Antarctic islands,
is commonly seen off South Africa during the non-breeding season, but is very
rare, if not unknown, off the east coast of South America. Also unusual, but
not unprecedented, was an adult Pomarine Jaeger on 13
March.
Submitted by Adam Jenkins