We are now at -66 40.925°S; -73 21.421°W
(13:08 local - 4 May) and on Station # 22 about to do a MOCNESS
tow to 1000 m. We are on transect leg #5 headed southeast towards Marguerite Bay. The air temperature is about
-0.1°C and there is fog with visibility out several hundred meters. With the air temperature hovering around the freezing
mark, the snow on the upper decks is now melting some (the main deck is heated, so ice and snow do not accumulate
there). The wind is back up into the 24 to 30 kt range out of the north northwest (345), but the seas are not particularly
large- yet. Working conditions are OK as we now have become accustomed to working under the rough sea conditions
that have thus far been the norm on this expedition.
Yesterday was a windy, cold, and snowy day. Low clouds and fog dominated
cutting the visibility to a few hundred meters
for much of the day. More evidence of the impending winter conditions was the sighting of an iceberg as we approached
station 19. The station was offset to avoid coming to close to the berg and associated fledgling ice chunks. None of us
actually saw the iceberg, it was only visible on radar. Work was completed at Stations 16, 17, 18 and 19.
Eileen Hofman and CTD Group reported the following:
The CTD group continued to make casts during the day and have now completed the first three transects of the survey
grid. Enough data have now been acquired to determine that there is an intrusion of Upper Circumpolar Deep Water
(UCDW) onto the west Antarctic Peninsula continental shelf. Water temperatures in excess of 1.7°C were encountered
at the outer ends of transects 2 and 3. The intrusion seems to be following the isobaths that are associated with the
deep depression that extends across the shelf. Completion of the next two transects (4 and 5) should give us a better
indication of the spatial extent of the UCDW intrusion.
The upper water column shows evidence of considerable mixing. The winter
water layer is significantly eroded and the
seasonal pycnocline that forms in austral summer at about 40-50 m is almost gone at some sample locations. This indicates
that the transition to winter conditions has started.
Chris Ribic and Erik Chapman reported that despite low visibility because of fog, rain, and snow, that they were able to
survey for 5 hours and 51 minutes today as we transited between stations 380.150 (#17), 380.180 (#18) and 380.220 (#19).
Seabird species richness and abundance were dramatically lower compared to previous surveys. Although the data will
be edited to include observations recorded when visibility was at least 300 m from the ship, a summary of their observations
are listed below.
|Antarctic Petrel (Thalassoica antarctica)||2|
|Cape Petrel (Daption capense)||2|
|Southern Fulmar (Fulmarus glacialoides)||5|
|Blue Petrel (Halobaena caerulea)||8|
In addition to daytime surveys, evening surveys using night-vision binoculars
were made for two 30 minute periods this
evening. During these surveys, 20 Snow Petrels (Pagrodoma nivea) were observed. Surveys were conducted from the
02 deck on the starboard side of the ship and visibility with the night-vision binoculars was estimated to be within 150 to
200 m from the ship. Evening surveys will continue as weather and work schedule permit.
Ice tower report by Ari Friedlander:
Unfortunately, this was a weather day for the whale observations, visibility was less than 500 meters all day, therefore,
no proper survey was conducted. An incidental watch was kept from 0900 until 1545 local time. No whales were seen.
One fur seal was sighted, however. Hopefully weather will improve tomorrow and survey will commence at first possible light.
In the absence of whale sightings, Catherine Berchok elected not to deploy any Sonobuoys during the daylight period.
BIOMAPER-II/MOCNESS report (P. Wiebe, C. Ashjian, S. Gallager
and C. Davis):
BIOMAPER-II completed towyo's along all of the transect lines between stations on 3 May and the MOCNESS tow
was successfully completed at Station 19.
High concentrations of volume backscattering in layers near the surface
and between 100 and 300 m in the inner shelf regions,
and distinctly lower concentrations of scatterers on the outer margin of the shelf. The MOCNESS, which was towed
between 400 m and the surface, continued to catch high concentrations of larval krill in the surface nets. Biomass of plankton
was considerably lower below 50 m. A few large krill were caught in the nets which fished between 400 m and 200 m, and
several fish were caught in these depths as well. The Video Plankton Recorder documented the presence of pteropods
(Limacina) in the upper 50 m as well as the numerous krill larvae.
One interesting acoustic observation occurred at Station 18 in the middle
of the shelf. At this station there was a surface
layer of scatterers that was most intense on the higher frequencies (200 and 420 kHz) and a deeper layer of which was
most intense on the 120 kHz and continuous in the echogram as we drifted slowly during the station work. The CTD had
just been deployed and as it passed through the deeper layer, which could be seen as a distinct trace on the echogram, the
layer opened up around the trace creating a "void" or "hole" in the echogram. Once the CTD was deeper than the layer, the
hole disappeared and the layer on the echogram again became continuous. The same pattern occurred when the CTD on its
return to the surface, passed through the layer. Where the trace of the CTD occurred on the echogram, a hole developed,
and then disappeared when the CTD went on up to the surface. A few minutes later, another smaller hole appeared in the echogram
which was not associated with our sampling activities. Instead, in the middle of this hole were two larger targets. Our interpretation
of these observations is that the two targets in the smaller hole, probably predators, were being avoided by the animals
comprising the layer (probably krill). Our CTD was probably being treated as a predator and also eliciting an avoidance
response. Using ADCP data to estimate the currents at the station and GPS data to estimate the ship's drift during
the CTD deployment, we should be able to estimate the dimensions of the holes and therefore the
avoidance distance. This kind of behavioral information is largely unknown, but essential for building models of the
population dynamics of the species and predator/prey interactions.