Report of Activities
on the RVIB N.B. Palmer Cruise 02-02
7 May 2002
The count down has started as the end of the third Southern
Ocean GLOBEC broad-scale survey is in sight.
On 7 May, the Palmer worked
from near the outer end of survey line 11 to the inner most station, leaving
only two relatively short survey lines to go. The weather continued to treat us
nicely in the sense that it was another day of relatively moderate winds,
except for a period in the early evening when they picked up and there were
gusts to 30 kts. This was about the time the ROV was
to be deployed. For the most part,
however, wind speeds were 18 to 21 kts or lower. The
barometer readings fell during the day from 1000.5 mlb
around 0130 to 987 mlb in the late evening and the
clear skies of yesterday gave way to a heavy dark overcast. There was snow
during the morning and poor visibility.
The snow ended before
On May 7, work was completed at broad-scale survey stations
82, 83, and 84. Four CTD casts were made
(two at station 84). The ring nets,
which are towed from the starboard side of the Palmer, have become very difficult
in the pack ice, but a 1-m ring net tow for surface zooplankton and a 1-m Reeve
net tow for live animals were completed at station 82. A MOCNESS tow was completed at station 83 and
an ice collection was made at station 84.
An APOP cast was also done at station 84 using animals caught with the
Reeve net. An ROV under ice survey,
scheduled for station 84, was scrubbed because the ice was too thin and the
wind too strong (gusts up to 30 kts) to hold the ship
in place without significant use of the ship's thrusters. BIOMAPER-II was in
for transits between the all of the stations. Seabird and marine mammal surveys
took place during the daylight period and 2 sonobuoys
were deployed during the transit to stations 83 and 84.
CTD Group report (John Klinck, Tim Boyer, Chris Mackay,
Julian Ashford, Andres Sepulveda, Kristin Cobb)
On 7 May, the CTD group did four casts at three stations in the middle and inner shelf along line 100 (survey line 11). The mixed layers were between 50 and 100 m thick with several isothermal layers. The deep temperatures were cold (1.0 to 1.5ºC). There was little variation in conditions over the southern third of the grid except for minor changes in the structure of the surface layer.
Station 82 (cast 85, 455 m). There
were two constant temperature layers (0-20 m and 30-60 m) near the
surface. A deep Winter Water (WW) layer was
centered at 130 m. The pycnocline extended from 70 to 250 m with sharp changes in
temperature and salinity with thicknesses up to 15 m. A 30-m thick temperature
maximum layer (1.5ºC) occurred at 350 m.
Station 83 (cast 86, 337 m). The surface mixed layer
extended to 80 m with two uniform temperature regions separated 35 m. Clear temperature reversals (10-15 m thick)
were seen to 200 m. The bottom
temperature was 1.1ºC with no temperature maximum.
Station 84 (casts 87, 88, 633 m).
The first cast was to 100 m with FRRF.
The surface mixed layer extended to 40 m (-1.8ºC, 33.4 psu, 0.05 ug/l chlorophyll). Temperature and salinity increased to 100 m,
which was the top of the pycnocline. Some minor temperature layers occurred to
about 250 m and the deep temperature was nearly uniform (1.0ºC).
Marine Mammal report (Debra Glasgow)
The marine mammal survey began at 0944 on 7 May during the
transit to Station 83 in poor light due to overcast conditions, snow and fog. Visibility
was variable, but improved as the day wore on.
The Palmer was in 8-9/10 open
pack ice all day consisting mainly of newly formed young grey and white ice,
and rafted pancake with many pools often covered in nilas.
Some areas included a few first year floes. As we towed the MOCNESS at Station
83, we began seeing more and more seals on floes, continuing until just before
survey ended in poor light at 1630 on the transit to Station 84. A total of 9
fur seals, 8 crabeater seals, and 5 leopard seals were recorded, but many more
unidentified seals were seen on distant ice floes too far away to identify. At
1438, 69 01.52ºS; 75 03.14ºW, an ice floe passed less than 5 meters to
starboard with fresh bright orange seal feces looking like it contained krill.
No cetaceans were seen today.
Sea Birds (Erik Chapman and Matthew Becker)
Seabird surveys were conducted on 7 May during the ship's
transit between stations 82 and 84 approaching
Snow Petrels continued to dominate the seabird species
assemblage today. Many of the birds were
following the ship and Snow Petrels numbers appeared to decrease throughout the
day as we moved further into the pack closer to land. A number of Snow Petrels
were observed feeding throughout the day, some of which fed along the open
water leads where wind drove small wavelets, which presumably carried plankton,
over the edge of the sea ice. It appears that Snow Petrels were in highest abundance
offshore, toward the ice edge, in nilas, pancake, and
grease ice that was probably only days old.
Perhaps copepods and other grazing zooplankton move to the surface in
large numbers under new ice becoming available as food to Snow Petrels. Snow Petrels may key in on the development of
new ice that is a particularly high quality foraging habitat due to this
process. However, this is merely
speculation and an investigation of zooplankton communities associated with
different ice types will have to wait until results from surface tows and
MOCNESS tows can be analyzed. Antarctic
Petrels were also scattered throughout the survey and were also following the
ship.
A summary of the species and number of individuals of birds and seals within the 300 m transect during 3 hours, 4 minutes of daytime surveys between consecutive stations 82 and 84 is the following:
|
Species (common
name) |
Species (scientific
name) |
Number observed |
|
Snow Petrel |
Pagodroma nivea |
59 |
|
Antarctic Petrel |
Thalassoica Antarctica |
7 |
Material Properties of Zooplankton Report (Dezang Chu, Peter Wiebe)
In the early morning on May 7, a Reeve Net tow cast was conducted at station 82 (68 45.980ºS; 75 61.241ºW). The water depth was around 450 m. There was a light scattering layer between 300 to 350 meters observed on the Simrad EK500 38 kHz echogram. Since the layer depth exceeded the depth range for other two channels of higher frequencies (250 m maximum depth for both 120 and 200 kHz), we could not get independent confirmation on this light scattering layer. The Reeve Net was deployed to about 400 meters of wire out and towed at about 300 m depth for a few minutes. The animals caught from the net tow were mostly copepods (Calanus). There were a number of amphipods and a few krill juveniles, as well as krill furcilia.
In the late evening, an APOP cast was made at station 84 (69 14.010ºS; 74 11.628ºW) with the copepods collected from the Reeve net tow made in the morning. The mean length of the animals was 3.2 mm, with a standard deviation of 0.3 mm. The measured density contrast of the animals was 0.996, indicating that they were lighter than the seawater. This result is consistent with both the previous measurement (see daily report on May 2) and the observation that they were mostly floated on the surface in the containers that held them. The mean sound speed contrast from the cast was 1.023, with a standard deviation of 0.002. The mean value was greater than unity, indicating that sound traveled faster in the animals than in the surrounding seawater. This result is different from the result obtained on May 2 (sound speed contrast was 0.949) when the same species was used in the measurements. The inconsistency in sound speed contrast suggests that acoustic estimates of biomass and/or abundance of this species (Calanus) could be very challenging and may potentially result in much larger biased estimates than we previously believed.
There was no statistically significant depth dependence
observed from the acoustic data. The difference between the down and up casts
was very small, about 0.001. However, a slight trend in sound speed contrast
for down cast below 125 m could be identified. The sound speed contrast decreased
slightly, but monotonically with depth, about 0.0014 over 80 meters (from 125
to 205 m). Based on the measurements on this species so far, the results are
still not very conclusive and more measurements are definitely needed.
Zooplankton (MOCNESS/BIOMAPER-II) report (Carin Ashjian, Peter Wiebe)
MOCNESS tow #20 was conducted on 7 May at mid-shelf at station # 83, which is located on the 11th broad-scale survey line. The region was ice covered with very soft pancakes and loose unconsolidated ice so that towing was quite easy and the ship was able to open a path through the ice with no effort. The bottom depth was 360-380 m and the tow was conducted to 329 m. Relatively little was collected in the net that fished the entire water column or in the bottom most depth interval (250-329) and copepods were the dominant taxa in these nets. Krill furcilia were collected from 75-200 m, and were most abundant in the 150-200 m interval. Copepods were collected at 200-250 m and from 50-0 m. Of the copepods, Metridia was obvious because of the brilliant blue bioluminescence it emitted while being collected in the sieve; this copepod was very abundant in the 50-75 m interval. This was the most Metridia that Phil Alatalo has ever seen in a plankton net. Chaetognaths were collected from 50-100 m and then again at 150-200 m. Very low abundances of copepods were seen in the 25-50 m interval and this was the only taxa obvious there. The upper 25 m caught quite a few small chunks of ice that were suspended in the water column, as well as plankton, so that the cod end resembled a snow cone when it was brought into the laboratory for processing. The abundance of plankton collected in this net was very low and the taxonomic composition consisted of a few juvenile krill, krill furcilia, and copepods.
The survey of krill and other zooplankton and nekton with BIOMAPER-II continued unabated on 7 May between stations 81, 82, 83, and 84. There were no major problems with the sensor systems on towed body that required sidetracking it for repair. When the operating optimally, as it has the past few days, around one gigabyte of processed acoustics data, more than a gigabyte of Video data, and a smaller amount of environmental sensor data are logged. The daily accumulation of raw acoustic and video data files, which are stored on DAT and SVHS tapes, is much larger.
Along the trackline between the
shelf break station 81 and outer shelf station 82, there was a consistent deep
layer of volume backscattering starting at 250 m and extending to the bottom.
Occasional discrete patches of high intensity targets were present about 60 meters
depth. Towing in pancake ice presented
no problems. Between stations 82 and 83, there was very typical scattering for
a mid-shelf region. There was a bit of
scattering at the surface with isolated very small, but intense patches similar
to those seen further offshore, a mid-water layer about 140 m down, and a
bottom layer that extended up about 100 m above the bottom. This deep layer had
an intensified zone about 20 meters above the bottom. Surface ice conditions
were still moderate making for easy towyoing. Thicker ice and slower going occurred during
the transit from station 83 to 84. During this passage, some more intense near
surface patches occurred and in one case the VPR recorded images of krill on
the VPR. A zone of intense krill-like scattering as had been seen on the previous
three survey lines did not appear by the time the inner most station (84) had
been reached. This station, however, is located further offshore and may have
been seaward of the zone where krill-like layers and patches have now come to
be expected.
Cheers, Peter