Report of Activities on the RVIB N.B.
Palmer Cruise 02-04
It was another in a string of
great working days in the Southern Ocean GLOBEC northern sector of the survey
grid because of the low winds and sunny skies. Work at offshore station 23 that
was started in the late evening of 5 September was completed by 0800 on 6
September. This included an ROV under-ice survey, a pair of CTD casts in 1800 m
of water and a 1-m MOCNESS tow to 800 m. The steam towards shore to station 22 was
done towyoing BIOMAPER-II. Station 22 began at
For the most of the day, ice
conditions were easy to steam through with the only difficult time being the
approach to station 21. This station, located in the middle of the continental
shelf, had large thick floes and deep ridges which required some backing and
ramming in order for the ship to proceed. BIOMAPER-II, which was towyoed from station 22, had to be retrieved from the water
a couple of miles short of station 21 in order for the ship to maneuver
effectively to make it through the last set of ridges. The work at station 21
began around 2200 with a the pair of 1-m ring nets deployed in the upper 10 m
off the stern to collect live krill larvae. A large floe was chosen for the sea
ice measurements and ice collection, and the team of ice collectors were
deployed on the floe about
As implied above, it was another
spectacularly clear day. For most of the
day there were no clouds at all and the mountains of
CTD Group report (Eileen
Hofmann, Bob Beardsley, Baris Salihoglu,
Chris MacKay, Francisco (
In the early morning of 6
September we arrived at station 23, which is at the outer-most end of survey
transect 4 in more than 3000 m of water.
Discussions during the cruise and analysis of CMiPS
data acquired to date resulted in giving a high scientific priority to obtaining
microstructure observations at a site along the shelf edge to the bottom. The outer part of survey transect was the
last opportunity during this cruise to obtain such a profile. However, a depth
greater than 3000 m at station 23 prevented using CMiPS
over the full cast. Thus, the decision
was made to move the CTD cast for station 23 further in towards the continental
shelf edge in order to obtain a CMiPS cast in deep
water to the bottom along the shelf edge. The CMiPS/CTD cast
was made in about 1800 m of water at the shelf edge. At this station, two casts were made: one to
350 m and one to within a few meters of the bottom. Both casts used CMiPS
lowering speeds in the upper 200 m and as a result there are two microstructure
profiles from this site for the upper water column as well as one cast for the
full depth. No FRRF sampling was done at
this station.
The second station completed on 6
September was station 22, which is 40 km inshore of station 23 on survey
transect 4. Two casts were made at this
site similar to what was done at station 23.
No FRRF sampling was done at this station.
After the microstructure time
series done on 5 September, CMiPS was discovered to
have a slight oil leak. Chris MacKay
drained the head and replaced the rubber finger and refilled the head with
mineral oil (instead of olive oil) before the station 23. The fix worked because CMiPS
preformed well at stations 22 and 23, recording high resolution temperature,
conductivity, and pressure data.
The temperature and salinity
properties observed at station 23 indicate that this station was along the
southern boundary of the Antarctic Circumpolar Current (ACC). The southern boundary of the ACC is indicated
by the 1.80ºC at about 200 m. At station
23, this isotherm was at 225 m. The
temperature maximum at this site was 1.89ºC at about 250 m. The water mass structure below the permanent pycnocline was as expected, with Upper and Lower
Circumpolar Deep Water present. The Winter
Water layer was well mixed to 60 m and had a temperature of -1.81ºC.
Station 22 was shallower than
station 23 at only 470 m. However, the water
mass structure at this site was similar to that observed at station 23. Both
Upper and Lower Circumpolar Deep Water were present and the maximum temperature
of 1.63ºC was at 275 m. The vertical temperature
and salinity profiles at station 22 showed considerable small scale structure
that is suggestive of mixing and interleaving. This in turn suggests that there is mixing and
exchange between oceanic and shelf waters at this site. The Winter Water layer extended to only about
43-45 m. This is the shallowest observed
to date during the survey.
The presence of the ACC at
station 23 and the warm temperature below 200 m at station 22 provides further
evidence that an intrusion of Circumpolar Deep Water is occurring in the
northern portion of the survey grid.
We have completed three
microstructure time series studies (perhaps “Drift Stations” is more
appropriate) where the CTD with CMiPS was lowered at
40 m/min typically down to350 m every approximately 20 min, 3 to 5 times before
the last cast was made to the bottom at that station. These repeat casts showed significant changes
in vertical temperature and salinity structure between casts over the 20 minute
intervals. With typical upper ocean
currents of less than 10 cm/sec relative to the ship, as determined from the
ADCP, these changes are occurring on horizontal scales on the order of 100
m. We are hoping that detailed analysis
of the CMiPS and CTD data from these repeat-cast
stations will help identify what processes are contributing to the large
observed variability in microstructure properties.
Seabirds (Chris Ribic and Erik Chapman)
Seabird and Crabeater seal
surveys were conducted for over 4 hours on 6 September as the ship moved
inshore between stations 23, 22, and 21. The transit was around 60 to 70 nautical
miles from
Once again, the presence of top
predators supported our understanding that the area near the shelf-break,
relatively close to the ice edge may be an important biological area within the
pack ice. Adélie Penguins were abundant
in the survey, but appeared to be less abundant than during yesterday's survey
along the shelf-break. In contrast,
Crabeater seals were more abundant than in yesterday's survey and were seen
hauled out on the edge of leads. Snow
Petrels again were abundant and Antarctic Petrels were not observed in the
survey. It is interesting to note that
Antarctic Petrels have dropped off significantly as we have moved south within
the northern sector of the grid. Perhaps
these birds are cueing into a physical feature associated with the northern
sector, such as an intrusion of the Antarctic Circumpolar Deep Water guided by the
deep canyon, or trough that is located there.
A summary of the birds and marine mammals observed on 6 September (YD 249) during 2 hours 32 minutes of survey time as the ship moved between stations 23 and 22 and 1 hour 46 minutes as the ship traveled between 22 and 21 is the following:
|
Species (common name) |
Species (scientific name) |
Number observed |
|
Snow Petrel |
Pagodroma nivea |
28 |
|
Kelp Gull |
Laru dominicanus |
2 |
|
Adélie Penguin |
Pygoscelis adelii |
32 |
|
Crabeater Seal |
Lobodon carcinophagus |
19 |
Krill Physiology and Fish
Ecology (Jose Torres, Tom Bailey, Joe Donnelly, Melanie Parker)
In our last report we talked a
little about our physiology experiments to determine how krill and other
Antarctic species make it through the winter.
In this one we will describe our most difficult type of sampling:
under-ice SCUBA diving. Your first
question might be why we do it, when we have nets to sample with and vehicles
to look under the ice. Kind of a long
story, but here goes. Let's start with
the “how” we do it.
Diving beneath the pack ice
requires a considerable amount of preparation and specialized equipment. First, to deal with the ice cold water, a dry
suit is required. A dry suit is
basically a tough rubber suit with a valve to inflate it and a valve to let the
air out. It is made with feet attached,
so that the only parts of your body that stick out are your head and your
hands. Tight rubber seals at the neck
and wrists prevent water from entering the suit. Because it is dry, you can wear heavy
underwear underneath the suit to protect you from the cold. A tight rubber hood and protective gloves
complete the Antarctic SCUBA diving outer garments. The suit attaches to your compressed air
bottle via a pressure hose to allow you to inflate it.
Besides the cold, another big
problem confronts the Antarctic diver: underneath the pack ice, the ocean is
anywhere from 500 to 3500 meters deep.
You are essentially diving in the middle of the ocean, only with a roof
over your head that prevents you from getting to the surface and safety. Tricky business. To dive safely, we use a tether system. Each diver
is tethered to a “down line” that is in turn attached to a Zodiac. The tethers are 33 meters long, which gives
us room to maneuver, but we are only 33 meters from our dive boat, and safety,
at any time.
Once underneath the ice, the
vistas are glorious. Visibility during
the winter season is about 60 m. Is it worth the effort to take a look? You bet.
Keep in mind that the area beneath the ice is an ecosystem 30% larger
than the continental
Our diving has revealed that
larvae are much less abundant this year than they were last year. We have made 7 dives this season and have seen
very few baby krill in the sea ice. Last
year, we saw clouds of baby krill on half of our dives! Part of our research is
to try and figure out why there are such large differences from year to year
and if there is a way to predict years of high and low abundance.
MOCNESS Report (Phil Alatalo, Peter Wiebe, Dicky
Allison, Ryan Dorland, Scott Gallager, Gareth Lawson)
MOCNESS Tow 13 was taken at dawn
on 6 September at station 23, which was located along the continental slope in
the northern sector of the survey grid.
Distinct layers of organisms were present here. Copepods and large jellyfish were found in
deep waters, krill dominated the middle depths, and salps
together with large copepods were found in shallow water. Limacinid
pteropods were found at all depths, but were most abundant
above 50 m. Radiolarians were prominent
components of each net. The VPR on BIOMAPER-II recorded many pictures of these protozooplankters.
At 68 m, the MOCNESS and the Optical Plankton Counter (OPC) lost
communication with the underwater unit. It was restored for the MOCNESS, but
then lost again permanently at 25 m causing the net 7 to filter the top 50 m.
The oblique tow to 800 m resulted in a very diverse collection of organisms with high biomass. Chaetognaths, krill, and a large (7cm diameter) reddish jellyfish provided most of the biomass. Abundance at depth (800-400 m) was attributed to copepods and chaetognaths, but a small fish, a large red decapod shrimp, and a pair of 8cm diameter clear jellyfish contributed to the high biomass. Above 400 m, biomass dropped off considerably. Many small copepods, chaetognaths, radiolarians, and ostracods appeared to form the scattering layer observed on the Simrad between 350-450 m. Small krill, mostly Thysanoessa sp. were the dominant taxa between 200-50 m, but were especially dense at the 75-50 minterval. The ADCP showed a scattering layer centered at this shallower depth. Between 50 m and the surface, Calanus propinquus was almost the exclusive component of this high biomass surface net. Two large salps were noted above 75 m.
The OPC count densities for both
tows 12 and 13 were low despite higher biomass present in the nets. Average count densities were between 400-600
individuals/m3 within the OPC size range for both tows. Tow 12 had low biovolume
in the upper 250 m with a pronounced count minimum at 75 m. Deep tow 13 had a more uniform count density
distribution with depth, with some increase between 100 and 200 m. Simrad and ADCP
showed moderate scattering at 75 m and between 300 and 450 m. OPC data for tow 13 was lost above 68 m due
to a communication failure.
BIOMAPER-II group report
(Gareth Lawson, Peter Wiebe, Scott Gallager, Phil Alatalo, Dicky Allison, Alec
Scott)
Our first towyo
of September 6 lasted from 0900 until 1200, while we steamed from the endpoint
of the previous night's 1-m2 MOCNESS tow (east of station 23) to
station 22. The second towyo of the day began at 1745
as we transited away from station 22. This tow ended due to ice conditions at
2100, just before we arrived at station 21. At the start of the first towyo, a shallow scattering layer was present from 25 to 75
m, again partitioned into distinct vertical bands. A half hour into the tow,
the layer dissipated almost entirely. Towards the end of the second towyo, we observed some enhancement of scattering in the
mixed layer, but no vertical bands were evident. VPR observations in the 30
to100 m depth range indicated the presence of diatoms and pteropods,
as well as many small and large copepods (including Calanus).
Over the course of both tows, a
number of discrete patches of very dense backscatter were present, falling into
two general categories. Some patches were very tall (30 to 40 m), located at
depths of 40 to 125 m during the morning tow, and contiguous with the surface
during the evening tow. On one instance during the first towyo,
the BIOMAPER II passed through one of these patches and observed krill with the
VPR. The second category included patches that were much smaller in vertical extent
(11 to 14 m) and positioned at greater depths, between 150 to 250 m. The
specific composition of these two kinds of patches, and how this composition
relates to their shape and location within the water column are intriguing
questions.
In neither tow was the bottom (475 to 600 m) ever within range of the echosounder for us to determine whether a bottom layer was present. We did frequently observe enhanced deep scattering: below 300 m in the first tow, and below 250 m in the second. As on previous tows, these regions of enhanced backscatter often had a speckled appearance, suggestive of the presence of large single targets. A 1-m2 MOCNESS tow performed at station 23 sampled krill (Euphausia tricantha), a large chaetognath, and many copepods in the 200 to 350 m depth range. Just below these depths, one large myctophid fish also was captured. The large targets we observed acoustically thus may well have been krill and fish.
Current Position and
Conditions
A very large expanse of open
water provided easy steaming from station 22 to station 19 during the afternoon
of 7 September. We are currently finishing up work at station 19 with a 1-m
MOCNESS tow. Our position at 0042 on 8
September is -67º 23.054′S; -69º 32.983′W. The air temperature is -7.3ºC and the
barometric pressure is 1010.4 mb and falling slowly. Winds, which were around 14 kts this evening, have kicked up and are now around 28 kts out of the south (179). Skies are cloudy. There is
little to no pack ice in the vicinity of station 19.
Cheers, Peter