Report of Activities on the RVIB N.B.
Palmer Cruise 02-04
On 4 September, work in the
northern sector of the Southern Ocean GLOBEC survey grid was focused on
stations 10 and 16. BIOMAPER-II was deployed at the end of the work at station
11 at
An under-ice SCUBA dive took
place in a large lead in the mid-afternoon and a series of CTD casts were done
while the divers were in the water. A
balky outboard engine made it difficult for the divers in the Zodiac to make it
back the last hundred meters; paddles were needed to get the boat to the Palmer's starboard side. In the early
evening, ice collections were made on a large floe and the ROV surveyed its
underside for krill furcilia and other plankton.
Completing the work was a 1-m MOCNESS tow to about 500 m and a Tucker trawl to
collect live animals for experimental work. Just before
September 4 was another day that
provided excellent working conditions and good viewing. In the morning, low
billowy clouds covered most of the sky, but the sun was trying to burn through
so it was bright and the visibility was good. Later in the morning, the
mountains of
CTD Group report (Baris Salihoglu, Eileen Hofmann,
Bob Beardsley, Chris MacKay, Francisco (
In the morning of 4 September, the
last CTD cast on survey transect 2 was completed at station 10. In the afternoon, we started working on transect
3 and completed the first station (survey station 16) on this transect. At this station we first did five consecutive
casts to 350 m with the last cast down to the bottom done according to standard
CTD procedure. Eighteen Niskin bottles were closed to obtain water samples for
nutrients and biological properties. The repeated casts were made to observe
the variability in microstructure in the water column (to be described in a
future CMiPS report).
At station 10, surface waters
were below freezing (-1.837ºC) and the well-mixed Winter Water layer extended
to 75 m. Below this, temperature increased to a maximum of 1.49ºC at 378 m,
after which it again decreased to 1.28ºC at the bottom (446 m). The salinity at
the temperature maximum was 34.70 and that at the bottom it was 34.72.
At station 16, the surface waters
were above freezing at -1.79ºC. The well-mixed Winter Water extended to 80 m.
As observed at station 10, temperature increased to a maximum of 1.43ºC at 250
m with a corresponding salinity of 34.67. Below this depth, temperature
decreased to 1.08ºC at the bottom. Bottom salinity was 34.71.
The vertical temperature section
for transect 1 which extended from off-shelf towards the northern side of
The other water masses that were
observed off-shore of the shelf break were UCDW (>1.6ºC) and Lower
Circumpolar Deep Water (~1.5ºC) around 200 to 500 m and 800 to 1000
respectively. At the offshore end of the transect, the
water temperatures were observed to increase to 2ºC between 200 and 250 m.
The vertical temperature section
for transect 2, which is located 40 km to the south of transect 1, showed
similar intrusions of modified UCDW and of UCDW. In this case UCDW was not as shallow as was
observed on transect 1 but it was below 300 m (whereas at transect 1 it
extended up to 200 m at the shelf edge). This structure also
pushed the isohalines upward towards off-shelf on transect 1. The isohalines were tilted downwards on
transect 2. This may be an indication that the UCDW is intruding onto the shelf
from the northern side of the study region.
Nutrients (Yulia
Serebrennikova and Steve Bell)
The nutrient hydrography
for GLOBEC survey grid lines 1 (stations 1-4) and 2 (stations 10-13) was
essentially the same as for the transects completed in
the southern and central parts of the grid. Overall, it was very similar to
that found during GLOBEC II (last year's winter cruise - NBP01-04).
The concentrations of nitrate and
phosphate were 29 and 2.0 micromolar, respectively,
in the upper mixed layer (50-100 m) and 33 and 2.3 micromolar,
respectively, in the deep water. Silicic acid concentration increased monotonically with
depth from 80 micromolar right below the mixed layer
to 140 micromolar at 3000 m.
The deep water on the shelf exhibits elevated silicic
acid concentrations relatively to those at the same depth off shore. And again
110 micromolar concentration of silicic
acid was found both on the shelf bottom and at 900-m depth off the shelf break.
The mixed layer silica concentrations were 55 micromolar at station 1 and 75 micromolar
elsewhere. A silica minimum (60 micromolar) was
observed at station 2 at 75-100 m.
Nitrite mixed layer
concentrations were 0.1 micromolar
for most locations and 0.14 for station 1. A subsurface maximum, 0.16 micromolar, was found at station 2
below the mixed layer and coincided with silica minimum. Ammonium
concentrations were 0.2 micromolar
at station 1 and 0.4-0.5 micromolar elsewhere.
Seabirds (Chris Ribic and Erik Chapman)
Seabird and Crabeater seal
surveys were conducted for almost six hours on 4 September as the ship moved
south from station 10 to station 16. The
survey took place about 35 miles off
Very few birds were observed
today compared to the numbers recorded off the continental shelf where there
was more open water. The Snow Petrel was
the most abundant species, followed by the Adélie Penguin. Snow Petrels were observed flying low over
cracks between ice floes, apparently looking for fish or zooplankton. In general, we have not seen many birds over
the continental shelf, midway between shore and the shelf-break. In previous studies in the Antarctic, birds
have been shown to concentrate near the ice-edge. This appears to be the case during this
cruise as well. We continue to see few
Crabeater Seals in the northern sector of the grid and no seals were recorded
today in the survey transect.
A summary of the birds and marine
mammals observed on 4 September (YD 247) during 5 hours, 42 minutes of survey
time as the ship traveled between stations 10 and 16 is the following:
|
Species (common name) |
Species (scientific name) |
Number observed |
|
Snow Petrel |
Pagodroma nivea |
13 |
|
Antarctic Petrel |
Thalassoica |
2 |
|
Southern Giant Petrel |
Macronectes giganteus |
1 |
|
Adelie Penguin |
Pygoscelis adelii |
8 |
Krill distribution,
physiology, and predation (Kendra Daly, Kerri Scolardi,
Emily Yam and Jason Zimmerman)
We have continued to use tandem deployments of the 1-m Reeve and 1-m ring nets at the last rendezvous site with the Gould and at stations 4, 13, and 11 to collect live animals for shipboard experiments. The depths of the nets were generally about 5 and 10 m below the surface.
The zooplankton composition was
similar at most of the sites. Euphausia superba
furcilia and juveniles and E. crystallorophias furcilia,
juveniles and some adults occurred in all nets tows. Ctenophores, adult males
and females of the copepod, Calanus propinquus, fish larvae, chains of salps,
and amphipods also were common. At station 16, we completed another SCUBA dive,
where we collected some E. superba furciliae, 4 ctenophores, and several samples of sea ice
biota from the ice-water interface where furcilia
feed.
Ingestion and egestion
rates and assimilation efficiencies were measured for the furcilia
collected in the Reeve and Ring nets. We also routinely measure and stage E. superba from net samples, as well as
freezing individuals for dry weight and carbon and nitrogen content.
ROV report (Scott Gallager, Phil Alatalo, Alec
Scott)
ROV 20 was a late afternoon
deployment at 1730 on 4 September at Station 16 (-66º 44.746′, -70º
10.327′). Ice conditions were 9/10 ice cover with 10 to 50 m diameter
floes and compressed ridge lines. The ice team was working on a large floe next
to the ship as the ROV was deployed and began a transect
out to a ridge line about 100 m to starboard. As the ROV traversed the
undersurface of the floe, we observed a smooth undersurface and no furcilia larvae. Just as the edge of the floe and junction
with the ridge line was approached, swarms of furcilia
appeared in view. The larvae were small, possibly stage 4 or 5 and formed dense
aggregations in direct contact with the ice. The ice itself was different in
that the light from the ROV was absorbed and not reflected back to the cameras
as observed under most conditions. Internal reflectance is common in ice in the
process of melting and forming brine channels.
The ROV continued down under the
ridge and began a transect to the south along the axis
of the ridge. Throughout this transect, no furcilia
were observed. It was as if they had disappeared. The ROV turned again 90º to
starboard on a bearing taking it back across the open floe surface. As the
junction between the ridge line and flow was approached, again there were
masses of furcilia forming just below the ice
surface. The ROV continued back to the stern of the ship and began a fourth
transect at a bearing heading for where the ice team was working and had established
a transect line of bore holes. The first bore hole was encountered with help
from the bridge calling out positions of the ROV strobe light seen through the
1 m thick ice. A pole was inserted from above as the ROV documented depth
penetration and characteristics of the ice surrounding the bore hole. The
bridge again called out a bearing to the next bore hole on the
transect and the ROV began the search. This was repeated five times
until the last bore hole was located about 90 m from the ship at the floe-ridge
junction. Chris Fristen had an auger extending
through the last hole, which was drilled through multiple layers of rafted ice. Krill furcilia were
abundant in the area around the hole as the auger was repeatedly removed and
replaced in an attempt to suck some furcilia through
the hole. Ctenophores and a few larval fish were also present where furcilia were abundant.
These observations support the idea of a combination of biophysical processes which control the under ice distribution of larvae. It is likely that as the water flow across the flat surface is disturbed by the ridge line, small eddies and counter currents develop extending 1 meter or more from the ice. The swimming behavior of the larvae interacts with the physical flows to produce aggregations at certain positions. Furcilia larvae are rheotactic, meaning that they orient into and swim against a water current just as fish do in a stream. As the furcilia swim against the local water flow generated by rapid changes in ice topology, their retention time in that particular part of the flow increases causing a localized increase in larval concentration. This increase in local concentration is what we refer to as an aggregation. The larvae become concentrated in the same area of the flow simply because they have similar swimming behaviors. Whether there are other biological cues involved in initiating or maintaining such aggregations are unknown. Aggregations of larvae are in contrast to the large masses of adult krill referred to as swarms. Biophysical processes similar to those described for the larvae may initiate a swarm, but intensification and persistence of the swarm is most likely augmented by both chemical and mechanical signals generated as a result of swimming and for reproduction.
MOCNESS Report (Phil Alatalo, Peter Wiebe, Dicky
Allison, Ryan Dorland, Scott Gallager, Gareth Lawson)
The MOCNESS tow at station 11
(-66º 13.5′S; -70º 23.6′W) on 3 September produced an interesting
set of samples. The tow position was mid-shelf in the northern sector of the
sample grid in continuous pack ice. Here
small copepods and small chaetognaths were abundant
at depth, decreasing in abundance nearing the surface. Euphausiids, salps, and large chaetognaths
commanded the bulk of the tow at depths closer to the surface. A surprising number of fish were caught at
all depths. Blown fuses and low batteries caused some technical difficulties at
the start of this deployment.
Unfortunately, the strobe, which is used to blind krill and
fast-swimming organisms, was not operational on this night tow.
Biomass was high in the oblique
tow to 425 m (net 0). Copepods were by far
the most numerous, with chaetognaths, euphausiids, and a medium sized fish, Electrona, making up the rest of
the sample. The deepest sample (425-350 m)
was surprisingly low in biomass, with no euphausiids present.
A few more organisms were caught between 350 m and 250 m, consisting of another
Electrona
fish, many small copepods, and radiolarians.
Again, slightly more biomass was present at the next interval (250-150 m),
with salps, euphausiids,
and chaetognaths together with copepods comprising
the main taxa. This mixture of taxa
was maintained until about 75 m, where copepods decreased in abundance dramatically. Salps, euphausiids, and chaetognaths
continued to be the main taxa present. Pteropods,
including gymnosome larvae of Clione
were notable between 150 and 25 m. Ostracods continued to be found in nearly all samples above
150 m. An unusual-looking icefish was caught between 50-25
m.
Large amounts of water volume
filtered during 1-m MOCNESS tow 11 contributed to a good OPC data set. OPC data showed relatively high counts
densities between 500 and 1200 individuals/m3 below the approximate mixed layer
depth of 100 m, with highest densities between 300 and 350 m. Net 4, which sampled from 150-100 m was unique, dominated by small count-sizes below 0.8 mm
with very few large individuals present in comparison to other nets. The surface nets had fewer counts, but
contained predominantly larger individuals between 1.0 and 2.25 mm. Biovolume values
ranged between 20 and 100 mm3/m3 with high deviation
below 300 m. Acoustics showed moderate
scattering above 100 m from the 153 kHz ADCP and high scattering between 300
and 350 m on the 38 kHz Simrad Echosounder. Currents from the ADCP were weak during the
tow ranging between 5-10 cm/s to the east-southeast.
BIOMAPER-II group report (Gareth Lawson, Peter Wiebe, Scott Gallager, Phil Alatalo, Dicky Allison, Alec Scott)
Our first towyo
of September 4 was between stations 11 and 10 along broad-scale survey transect
2. Later in the day, we had the
BIOMAPER-II in the water for a second towyo as we
steamed from station 10 on transect 2 to station 16 on transect 3. A shallow
scattering layer was evident for most of both tows, generally centered at 25 m
and approximately 50 m in vertical extent. VPR observations indicated that the
plankton community between 25 and 100 m depth again consisted of a diverse
species assemblage that included krill, copepods, radiolarians, and diatoms. We
also observed a few pteropods.
As was the case yesterday, the shallow layer was often partitioned into vertical bands, alternating between high and low levels of backscatter at intervals of about 200 m. In order to test whether these bands relate to density-driven circulation cells (as we speculated in yesterday's report), at one point in the second towyo, we kept the towbody at a fixed depth to look for any fluctuations in density. If circulation cells were present, we might have expected to see distinct variations in density as we passed in and out of the cells. Unfortunately, no such fluctuations were evident, although they may have been beyond the sensitivity of our environmental sensors. During this short experiment, the VPR observed numerous large copepods. Further analysis of the VPR data will indicate whether the abundance of particular taxa showed alternations between highs and lows as we passed through these possible cells.
Over the course of the two tows,
we observed a number of very dense (up to -55 dB) krill-like patches within the
mixed layer, often just below the surface. These patches varied from 25 to 50 m
in height and 75 to 170 m in length. The BIOMAPER-II never passed directly
through any of the patches to make VPR observations of their species
composition. At one point, however, the towbody
passed five meters under one patch and did capture an image of a krill.
We also observed some enhancement
in scattering levels below 300 m. A 1-m2 MOCNESS tow conducted in the vicinity
of station 11 sampled many small copepods, one myctophid
fish (Electrona),
and five large krill (Thyssanoessa)
in the 250 to 350 m depth range. These latter two groups of organisms are
relatively large, and likely are responsible for the observed elevated
backscatter. On the one occasion that the bottom was within range (350 m
depth), a dense bottom layer was present.
Current Position and
Conditions
The N.B. Palmer is currently a station 23, the deep offshore station at
the seaward end of SO GLOBEC survey line 4.
Work started at the station about 2130 and will continue for about 8
hours. Our position at 2334 on 5
September is -66º 35.876′S; -72º 06.545′W. The air temperature is -9.8ºC and the barometric
pressure is 1008.5 mb and holding steady. Winds are light at around 10-12 kts out of the east (107). Skies are cloudy and a light
snow has been falling off and on during the evening, but visibility if
good. The pack ice is loosely aggregated
with mostly small floes and newly formed pancakes. An ocean swell is propagating through the
area.
Cheers, Peter