Report of Activities on the RVIB N.B.
Palmer Cruise 02-04
August 20 was day three of the
search for a L.M. Gould process
station site. The circuitous route taken to find a suitable location to serve
as a base finally came to an end when we reached the vicinity of station 43.
Open leads and a mixture of different pack ice types were present. The convoy
arrived at the location before dawn and like the day before we had to wait
until daylight to survey the area and locate an appropriate site in which to
base the Gould. In the intervening time, the Palmer completed the work scheduled for
station 43 and then about mid-day, escorted the Gould to a place of their choosing. The work included ice
collection for sea ice biota studies and an ROV under-ice survey, which became
an extended run because the vehicle's tether became caught in a crevice, trapping
the ROV for a while. A CTD cast to the bottom
(~400 m) was also done. During the morning, the Gould also completed a CTD cast and a SCUBA dive to determine if
krill furcilia were present under the ice. Moderate numbers were seen both in the ROV cameras
and by the divers, who collected some of them for experimental work.
With a site selected, the Gould steamed into a large floe and once
situated lowered their gangplank onto the floe. Before committing to the site,
some testing of the pack ice was done to make sure the floe was suitable for
the multi-day studies. About 1500, the Palmer was given the OK to leave and we
immediately set sail for station 44 located on the edge of the Continental
shelf on survey line 6. Upon leaving station
43, BIOMAPER-II was deployed and towyoed all the way
to station 44 with the trackline often following a
series of leads. Upon reaching the station at about 2030, a Tucker trawl to
collect live animals was done. The day ended with the start at 2200 of a 10-m
MOCNESS tow to 1000 m. This tow was completed in the wee hours of 21
August.
The weather on 20 August was not
particularly pleasant. The air temperature
(-15ºC) during most of the day was warmer than the previous day, but it was
still cold! The barometric pressure
held for the day around 989 mb, up substantially from
yesterday's minimum of 966 mb. Winds had dropped from
gale force to 16-20 kts out of the southwest in the
morning and decreased additionally to < 10 kts out
of the south by late evening.
Interestingly, sea surface temperature was not quite at the freezing
mark (-1.786ºC), which may be a reason there were so many open leads along the
route to station 44. Skies were cloudy
all day and there was a light snow in the afternoon, which reduced the
visibility and made the search for the process site difficult. In the late evening, the skies cleared and
the moon (nearly full) provided a bright illumination that reflected off the
pack ice. Over the open water of the
leads, there was a diffuse fog caused by evaporation of the seawater. It was a very nice night to be working at
station 44.
CTD Group report (Eileen Hofmann,
Bob Beardsley, Baris Salihoglu,
Chris MacKay, Francisco (
The first potential site selected
for the second process station was in the vicinity of survey station 43. We were able to do one CTD cast at this
station on 20 August while discussions were ongoing about the suitability of
this as a process site. The CTD cast was
to 393 m and included the FRRF and CMiPS
sensors. The vertical distribution
showed the maximum temperature and salinity at 360 m of 1.37ºC and 34.70, respectively,
which are characteristic of modified Circumpolar Deep Water. The surface layer was well mixed to about 80
m and was above the freezing point (-1.79ºC to -1.78ºC).
At this station, oxygen samples
were collected from all Niskin bottles (total of 22)
and duplicate samples were taken at some depths. These samples will be used to further develop
a calibration curve for the CTD-mounted oxygen sensor. The results of this calibration exercise will
be described in future reports.
We have now completed enough
stations on the survey grid to begin construction of maps of the horizontal
distribution of properties such as the maximum temperature below 200 m and the
distribution of the depth of the oxygen minimum. Both properties are tracers for Circumpolar
Deep Water. A preliminary map of the
temperature maximum, constructed from the stations occupied during the survey
of the southern-most part of the grid, shows that the southern boundary of the
Antarctic Circumpolar Current is located along the edge of the outer
continental shelf. The survey of the
central part of the grid, which we are now beginning, will allow us to
determine the extent to which the southern boundary of the Antarctic
Circumpolar Current is impacting this portion of the west
Sea Birds (Chris Ribic and Erik Chapman)
Surveys were conducted for almost
3 hours on 20 August as the ship moved between station 43 and 44. This transit brought the ship further offshore,
near the shelf-break adjacent to the mouth of
A summary of the birds and marine
mammals observed on 20 August (YD 232) during 2 hours, 58 minutes of survey
time as the ship traveled between stations 43 and 44 is the following:
|
Species (common name) |
Species (scientific name) |
Number observed |
|
Snow Petrel |
Pagedroma nivea |
5 |
|
Crabeater Seal |
Lobodon carcinophagus |
4 |
Krill distribution,
physiology, and predation (Kendra Daly, Kerri Scolardi,
Emily Yam and Jason Zimmerman)
During the past week we completed
4 Tucker Trawls, 1 dive, and several physiology experiments. The Tucker Trawl at station 74 (a shelf-break
station in the southern part of the study area) collected numerous juvenile and
adult krill where a large layer 50-100 m in depth was detected by acoustic
systems. The Tucker Trawl at Station 72 (mid-shelf)
collected gravid Thysanoessa macrura (a euphausiid) bearing spermatophores,
indicating that this species may spawn in the near future. We also collected Euphausia triacantha, another euphausiid species normally found below 200 m off the
shelf. The occurrence of this species
was correlated with the presence of Circumpolar Deep Water from the Antarctic
Circumpolar Current, which had come onto the shelf in this area (Personal
Communication, E. Hofmann). Other organisms collected included a few E. superba furcilia, larvaceans,
pteropods, chaetognaths,
small medusae, several small amphipods, and numerous copepods,
such as Calanoides acutus, Calanus propinquus, and Paraeuchaeta spp. The Tucker
Trawl at station 65 (outer shelf) collected numerous ctenophores, some copepods
and amphipods and a few juvenile E.
superba in the vicinity of a very low density layer (about 90 dB) centered
about 100 m in depth. The divers did not
observe any krill or any other organisms under the sea ice in this region. We collected 3 samples from the under-surface
of sea ice for chlorophyll and particulate organic carbon and nitrogen
analyses, as well as for slide enumeration of autotrophs
and heterotrophs.
Another Tucker Trawl on 19 August in a lead near Station 42 (mid-shelf)
collected numerous copepods, primarily C.
propinquus and Metridia gerlachei, several individuals of Thysanoessa and pteropods, a salp, and a
ctenophore. We also completed two more
experiments assessing the growth and molting rates of juvenile and adult E. superba, 17-51 mm in length. Molting rates ranged between 28 and 34 days,
which was similar to molting rates of krill collected further to the north in
Microplankton
report (Phil Alatalo, Gustavo Thompson, Dicky Allison, and Scott Gallager)
Observations of microplankton from the first portion of our survey of the
southern sector grid stations completed by 13 August, mimic for the most part
samples taken in the
Automatic particle tracking for
this series of stations shows overall smaller mean diameter particles with
lower mean speed and lower mean concentrations than samples run from the
previous cruise NBP0202, in early winter.
Also, net to gross displacement ratios for this cruise
average ~0.9. This ratio
calculated for plankton tracks measures the degree of circuitous motion present
(non-motile particles travel in straight lines vs
twists, turns, and jumps exhibited by swimming microzooplankton). For example a sinking diatom might displace 5
cm while sinking 5 cm (NGDR = 5/5=1) whereas a
ciliate swimming a spiral might displace 12 cm, yet only end up 4 cm
from its starting point (NGDR = 4/12= .33)
A value near one confirms the presence of inert particles and low
motility of plankters.
During the period ending on 20
August, little had changed in microplankton
composition of sampled water depths as work was completed in the southern
sector and we moved into the central grid sector. Stations 81, 82, 75, 74, 73, 72, and 65 were
all similar with low particle concentrations the norm (5-10/ml). The exception was a sample at shelf-break
station 65. There, an oceanic water
intrusion at 200 m showed a higher abundance of particles (30/ml). Mean particle diameter for all stations was
between 15-20 μm. A few flagellates and an occasional ciliate
were typical for nearly all samples.
Station 42 (CTD cast 31) was
sampled in the central sector of the Grid. All depths exhibited few particles with little
motility, except for a very large strombiid ciliate
at 75m.
A calibration of fluorescent
16-um beads was made to secure a relationship between
the threshold used to track particles and the particle concentration. For most stations sampled on this cruise, lower
thresholds were required to track the sparse concentrations of microplankton.
BIOMAPER II group report
(Gareth Lawson, Peter Wiebe, Scott Gallager, Phil Alatalo Dicky Allison, Alec
Scott)
The intention of our group is to
tow the BIOMAPER II continuously between the broad-scale survey stations, in
order to provide a picture of the spatial distribution of krill and other
organisms. To date, however, problems with ice and systems malfunctioning have
meant that we have only achieved towyos of short
duration. In the mid-afternoon of August 20, we were able for the first time to
keep the BIOMAPER II in the water for the entire transit between stations 43
and 44. A thin and weak scattering layer was evident at depths that changed
along-transect, varying between 80 and 120 m. Like yesterday, this layer was
associated with the pycnocline and again composed
primarily of copepods, according to images captured by the VPR. Towards the end
of the tow, we observed a number of small and dense patches of backscattering
between 20 and 55 m. The VPR made occasional observations of large krill at 20
m, suggesting that these patches may be composed of our elusive target species.
With the bottom at only 400 m, we
were able to get the BIOMAPER II deep enough to make observations of a
scattering layer in close association with the bottom. Early in the towyo, this bottom layer was fairly diffuse and extended
approximately 100 m off the bottom. As we moved farther towards the continental
shelf break, and also as it got dark, the top-most reaches of the layer
coalesced into more sparsely distributed patches and groups of stronger
targets. This resulted in two adjacent layers of very different structure,
similar to what we observed farther south on August 17 at nearly the same time
of day, depth, and position relative to the shelf break. Whether this change
from one to two bottom layers represents a diurnal change in the aggregative behavior
of organisms at these depths or an along-transect change in the near-bottom
species composition is not clear at this point, but poses an intriguing
question.
Current Position and
Conditions
In spite of the very cold working
conditions, the Palmer is making good
progress in accomplishing the schedule of tasks at each station and we have not
had any difficulty moving between stations during the past couple of days. We are currently working at grid station 45,
the off shelf station on survey line 7. Our current position on 21 August - 2344
hrs is -67º 11.022′S; -74º 29.175′W). Air temperature is -25.0ºC
and the barometric pressure is 986.1 mb. Winds are out of 235º (southwest) at 10 to 15
kts. Skies are clear.
Cheers, Peter