Report of Activities on the RVIB N.B. Palmer Cruise 0202

3 May 2002

 

The N.B. Palmer began work on 3 May out in the deep ocean beyond the continental shelf. A half-moon with its light filtered by higher thinner clouds in the late night and pre-dawn held sway until the sun rose, shining through a broken cloud layer. Winds during the late night were around 12 kts out of the southwest and the barometer was rising well above the 1000 mlb mark (something that seems to happen very infrequently) as a large high pressure region moved in over the survey area. By mid-morning, the barometer had reached a high of 1007 mlb. Winds throughout the day remained in the 10 to 25 kt range, but the air temperature dropped from -1.8 in the morning down to -7.0C in the late evening, making work on the deck some what less comfortable.

 

Work was completed at broad-scale survey stations 68, 69, 70, and 71 including 4 CTDs and one XCTD, a 1-m ring net tow at station 70, and an APOP cast and a MOCNESS tow at station 71. The XCTD was cast at station 69, while the ship remained underway. The transits between stations 68 to 70 were short ones (5 to 8 nm), because they were part of the high resolution physical survey described in the 2 May report. BIOMAPER-II was in for transits between all of the stations including the long 36 nm run between stations 70 and 71, which took over 7 hours. Seabird and marine mammal surveys took place during the daylight period and one sonobuoy was deployed during the transit to station 71.

 

CTD Group report (John Klinck, Tim Boyer, Chris Mackay, Julian Ashford, Andres Sepulveda, Kristin Cobb)

The CTD group did four casts at three stations and an XCTD at a fourth station. These casts were in the Antarctic Circumpolar Current on the 220 line (survey line 8) and over the outer shelf on the 180 line (survey line 9). The oceanic stations were typical with temperature maximum between 200 and 300 m (upper Circumpolar Deep Water - CDW) and an salinity maximum around 800 m (lower CDW). The surface mixed layers were relatively thin with moderate chlorophyll (compared to shelf levels).

 

Station 68 (cast 71, 2499 m). A uniform mixed layer occurred to 40 m (-1.1C, 33.8 psu, 0.3 ug/l chlorophyll). Clear temperature reversals were present in the pycnocline to 300 m. The temperature maximum (1.8C) was at 250 m and there was a weak salinity maximum at 800 m.

 

Station 69 (deep XCTD, 2774 m). The deep XCTD ran to 1700 m. There was a uniform mixed layer to 50 m (-1.2C, 33.7 psu). A deeper isothermal layer at the surface extended to 80 m. The pycnocline ranged from 70 to 200 m. Temperature maximum (1.9C) was at 220 m.

 

Station 70 (cast 72, 73, 2941 m). One cast was made to 100 m with no bottles to acquire FRRF data. A uniform mixed layer occurred to 60 m (-1.1C, 33.8, 0.4 ug/l chlorophyll) and there was a thin residual Winter Water layer at 70 m. Small-scale temperature and salinity variability was present in the pycnocline. The temperature maximum (1.9C) was at 250 m and a weak salinity maximum was at 800 m.

 

Station 71 (cast 74, 417 m). Three well mixed surface layers were present. The upper extended to 25 m (-1.6C, 33.5 psu, 0.15 ug/l chlorophyll). The next to 50 m was warmer, saltier, and higher in chlorophyll. The third extended to 80 m and was again slightly warmer, saltier, and higher in chlorophyll. Small-scale temperature and salinity variability was present in the pycnocline, to 300 m. The deep temperature was uniform (1.6C)

 

Phytoplankton Primary Production (Wendy Kozlowski, Kristy Aller)

During the third week of sampling (27 April 3 May), the primary production group has completed six more simulated in situ (SIS) productivity experiments at stations 48, 51, 55, 57, 58, and 63. Preliminary results through station 55 show a decreasing level of production, both onshore and off. The lowest production for the cruise seen to date was at station 51, which corresponds with low mixed layer fluorescence seen with the CTD. At five of the six SIS stations, the FRRF was deployed, with a total of thirteen casts for the week. A second, 100 m cast for the FRRF was done at deep station 70 for more off shelf data.

 

Chlorophyll samples continue to be taken at all stations where the CTD was deployed, and samples for particulate carbon have been preserved at ten more stations. Similar to production levels, chlorophyll levels are decreasing as we head south on the grid. Average weekly 100 m integrated chlorophyll a levels have fallen from 77.4 ug/m2, to 63.5 ug/m2, to 42.6 ug/m2, respectively.

 

New ice was seen again this week in the vicinity of station 54, and an older, multi-year floe was sampled in the vicinity of station 55. A personnel basket was lowered to the floe, and snow and ice thickness was sampled and a core taken. A second large floe (diameter estimated to be 20 m) was sampled in the same manner in the vicinity of station 57. At both locations, there were significant amounts of fresh snow on the surface of the ice, and thick (10-35 cm) layers of granular ice at the snow/ice interface. At Ice Station 4, three 1-m extensions were used on the 1- m core barrel and still the bottom of the flow was not reached. As the ship headed out of the older ice and into newly forming pancakes, a sample was collected over the side with a bucket between stations 59 and 60. The table below summarizes some of the measurements taken at the two core sites.

 

Sample

Number

Lat

Lon

Snow Depth (cm)

Total Core Ice Length (cm)

Measured Ice Thickness (cm)

Samples Collected

Ice3

-68.885

-68.967

110

233

253

0-83 cm

83-233 cm

Ice4

-68.998

-69.429

~165

293

387

0-126 cm

126-207 cm

207-293 cm

slush/brine

 

We thank the RPSC and ECO crews for the assistance in selection and sampling of ice floes.

 

 

Marine Mammal report (Debra Glasgow)

The marine mammal survey started on 3 May at 1028 on transit to station 71. Visibility was excellent for a change though very cold at - 4.2C, causing the windows inside the ice tower to continually ice up and have to be scraped clean throughout the day. The Beaufort sea state was 3-4 all day. At 1059, three fur seals were seen at 67 35.09S; 75 11.79W, porpoising alongside the starboard bow of the ship, accompanying us briefly. Another fur seal was recorded at 1406 resting on the surface, flippers extended in the wind. These were the only marine mammal sightings for the day. Continuous survey was kept until poor light at 1632. To date on this voyage, there have been 23 cetacean sightings with a total of 54 animals.

 

Sea Birds (Erik Chapman and Matthew Becker)

May 3 was an unusually calm day with welcome breaks of sunshine as the ship transited from station 71 to 72 off the continental shelf. Observations were made during 6 straight hours during the transit before the sun went down. It was the fifth survey off the shelf and the fourth where low bird abundance has been documented. Relatively high bird abundance was observed during just the first survey off the shelf on the northern most grid line.

 

Today's survey indicated some of the lowest bird densities of the cruise. Most of the birds we observed were following the ship throughout the survey and no feeding behavior was seen. BIOMAPER-II also indicated low scattering overall, particularly at the surface. A surface tow at station 71 found diatoms and a relatively large amount of copepods, but no amphipods or larval krill in the top layer of water. Considering the limited potential food for birds at the surface, it is not surprising that few birds were in this area.

 

It is interesting that a relatively large number of Antarctic Petrels have been seen during this cruise. Antarctic Petrels breed in colonies of tens of thousands of birds far from the Antarctic Peninsula, mainly along the Antarctic Coastline east of the Weddell Sea. At sea, Antarctic Petrels are sometimes seen in huge flocks of up to 1,000,000 birds. Very little is known about the breeding biology of these birds because they nest in remote rock outcrops that are difficult to study. Their at-sea distribution and winter movement patterns are even less well understood. The nearest colony to Marguerite Bay has 10,000 breeding pairs and is located on the eastern shoreline of the Ross Sea. It is possible that surveys are detecting a movement of these birds to wintering grounds on the Peninsula or further north. During SO GLOBEC I last year, which was two weeks later in the winter than this cruise, very few Antarctic Petrels were observed in the study area, and these sightings were limited to the northern most lines of the grid. The surveys during SO GLOBEC I may have detected the tail end of the movement of birds through this part of the Peninsula.

 

A summary of the species and number of individuals of birds and seals within the 300 m transect during 6 hours of daytime surveys between consecutive stations 71 and 72 is the following:

 

Species (common name)

Species (scientific name)

Number observed

Cape Petrel (Pintado Petrel)

Daption capense

2

Southern Fulmar

Fulmarus glacialoides

1

Antarctic Petrel

Thalassoica antarctica

21

Blue Petrel

Halobaena caeulea

3

Southern Giant Petrel

Macronectes gfiganteus

4

Snow Petrel

Pagodroma nivea

7

 

 

Material Properties of Zooplankton Report (Dezang Chu, Peter Wiebe)

On May 3, measurements of sound speed and density contrasts were made at station 71 on E. crystallorophias. This was the second experiment with this species and we wanted to see whether the depth-dependent pattern of the sound speed contrast would be similar to that observed on April 29. The measurements were again a combination of shipboard and APOP cast measurement conducted as on April 29. The experimental animals were collected by Kendra Daly on the Gould on April 23 and April 30. Their mean length was 32 mm and the standard deviation was 3 mm, similar to those used in the previous measurements on April 29. The cast was made at station 74 (68 05.995S; 74 47.369W). This time the data quality of acoustic measurements was very good, since all of the received signals (time series) throughout the measurements had high signal-to-noise ratios and basically no dispersion was observed, indicating that bubbles were absent.

 

The mean value of the sound speed contrast from the shipboard measurement was 1.026, similar to that we obtained on April 29 for a different group of individuals of the same species (1.027). The mean value from the cast was 1.030 (1.025 on April 29), with a standard deviation of 0.01. This time, the difference in the mean value between down and up casts was 0.013, a significant difference for weakly scattering objects. Compared to the measurements made on April 29, similar depth-dependent patterns were observed again, both for the down cast and the up cast! The sound speed contrast from the up cast had a negative gradient from the surface to about 100 m and a positive gradient below 100 m. A minimum was around 100 m in contrast to 85 m observed in the previous measurements. This time, however, the gradient was about twice as large as the previous one (0.02% per meter vs. 0.01% per meter). This type of depth dependence has not been observed on E. superba thus far on this cruise, suggesting that it is likely that such a depth-dependent pattern is indeed animal dependent. It might be due to the different response rate to the pressure change between two different krill species. It might also involve a possible combination of biological and biophysical reaction, as well as biochemical reaction to a sudden change in environmental conditions.

 

The density measurement on the same animals was conducted right after the APOP cast was completed. The density contrast was 1.0003 (the fourth digit was not reliable, though) as compared with 1.009 obtained on April 29 for the same species, indicating they were nearly neutrally buoyant.

 

ROV Under-Ice Investigations (Andrew Girard, Amy Kukulya, Gaelin Rosenwaks, Philip Alatalo)

The Sea-Rover ROV was used again the night following its first deployment, to characterize the under-ice surface and to survey for organisms. This seasonal habitat provides an important cover for our target organism, krill furcilia. These young krill tend to congregate beneath the sea ice feeding on blooms of algae and microzooplankton located on and in the ice substrate. Clearly the ice provides protection from the prying "eyes" of scientists using ring nets, MOCNESS nets, and BIOMAPER II; with the exception of divers, some type of ROV or AUV is the only means to view the ice-water interface. Using the Sea-Rover, we can "sneak up on" krill and other organisms in order to document their abundance and behavior.

 

Four people are necessary to carry Sea-Rover to the aft crane, which lifts the ROV over the starboard side into a patch of water that the ship has made for us by careful maneuvering. Underwater, the ROV is capable of "flying" underneath the ice, guided by the navigational camera and its directional instruments, the CTD, ADCP, and SONAR.

 

ROV-02 deployed on April 30th at 1840 at Station 58 in Marguerite Bay. In its logged dive of more than an hour, Sea-Rover spent most of its exploratory time under a snow-covered iceberg 25 m in diameter located starboard of the ship. Underwater, this berg extended 10 m deep. This and adjacent bergs were surrounded by brash ice. A series of short horizontal survey lines were conducted at selected depths along the steep sides of the iceberg. The ice surface appeared as large rugged pieces, full of knobs and protrusions. The general texture suggested erosion. Sometimes these pieces formed crevices; as ice crevices were located, the ROV maneuvered closer to allow visual inspection.

 

Several krill furcilia were seen on this survey, singly or in small groups. Towards the end of the survey, an amphipod was sighted twice swimming rapidly in the brash ice. Two ctenophores were observed on the ROV's navigational camera. These gelatinous zooplankton tend to drift or swim slowly, trolling for smaller zooplankton with their tentacles. After approximately an hour, the ROV was retrieved at 18:49 (local).

 

Zooplankton (MOCNESS/BIOMAPER-II) report (Carin Ashjian, Peter Wiebe)

MOCNESS Tow #17 was conducted on 3 May at the offshore end of Transect #9, which is located at the shelf break. The tow was conducted in the early night, ending just prior to midnight, and sampled down to 450 m. Krill were observed sporadically throughout the water column, being found from 150-450 m, 50-100 m, and near the surface (0-25 m). Chaetognaths were observed at depth (350-450 m). Myctophids were collected from 200-350 m, four of which were sampled for otoliths and sex by Julian Ashford. Copepods were abundant from 200-150 m and then again in the upper 100 m. Amphipods were seen mainly in the surface net (0-25 m). A few fish larvae were collected from 50-75 m.

 

BIOMAPER-II was towyoed through the nighttime of 2/3 May between stations 67, 68, and 69 and in the early morning to station 70, which was out in the Antarctic Circumpolar Current in 3000 meters of water. Much of the time, there wasnt much backscattering in the water column in the offshore waters on the lower 4 frequencies with light scattering in the pycnocline just below the surface layer. Much deeper at mid-water depths (250 to 440 m), there was a very diffuse low level layer of backscattering. Superimposed on that were intense individual scatterers or patches of them that sometimes occurred fair numbers and at other times absent altogether. There were also some individual scatterers in the pycnocline, starting about 100 m and going down to 160 meters, along with thin layers of much less intense scattering. There was not much scattering in the surface waters just below the ships turbulent zone, although occasionally patches of higher backscattering would occur there as well. The 1MHz echograms showed higher backscattering at the base of the surface mixed layer and top of the pycnocline.

 

 

Cheers, Peter