Report of Activities on the RVIB N.B. Palmer Cruise 02-04

10 September 2002

 

The end of the Southern Ocean GLOBEC survey of the Western Antarctic Continental Shelf in the vicinity of Marguerite Bay came in the early morning hours of 10 September. The work at station 5 was completed with a pair of CTD casts made between 0200 and 0330 and a 1-m MOCNESS tow, which ended at 0600. The end of work on the grid set the stage for another rendezvous with the L.M. Gould, which had been working at their third process site which began at station 4, but had moved to the north about 40 nm.

 

The two ships began steaming towards the rendezvous site around dawn and met at 1000. There was enough open water in leads to allow the exchange of personnel to take place by Zodiac. The chief scientist on the Gould, Dan Costa, along with MPC, Karl Newyear, and Captain Roberts came over for a meeting to discuss plans for the remainder of the cruise. The party of three that came over to the Palmer from the Gould just before the start of the survey of the northern sector of the grid returned to the Gould. About 1300, the two ships began the convoy to the northeast.

 

The Palmer headed for a site off the northern tip of Renaud Island (-65 11′S; -65 35′W). This site was chosen for a 24-hour time-series study to look at the pattern of spatial change in the turbulence structure of the water column in relationship to the vertical distribution of the species living in the water column. The area was near where penguins that had been tagged with satellite transmitters were located. The criteria used in selecting the study site were that pack ice was present, that an iceberg was located nearby, and that water depths were between 400 and 300 meters. The site was approximately 80 nm from the rendezvous point and it took more than 15 hours to make the transit. Along the route, the seabird observers on the Palmer (Chris Ribic and Erik Chapman) spotted a very large group of Adlie penguins. Estimated numbers were around 130. There was a protracted attempt to maneuver the L.M. Gould into a position to put people on the ice in order to get diet information from some of the penguins. The penguins proved to be too mobile and the ice too fragile for the work to be done and after about 45 minutes, the convoy moved on.

 

In the evening of 10 September, King Neptune and his court paid the N.B. Palmer a visit to officiate at a ceremony bringing the polliwogs (those crossing the Antarctic circle for the first time) on board into the ranks of the shellbacks. The polliwogs entertained the King with skits, poetry, and multimedia presentations. It was a good time for those who chose to participate in it.

 

This was another day in which the sun failed to make an appearance. Low clouds prevailed along with a dense fog and mist that turned to a light snow in the afternoon. The air temperature remained relatively mild rising from around -2C to above the freezing mark (+0.6C) for the second time since arriving in the GLOBEC study site. Winds on 10 September were moderate to low. In the morning they decreased from around 25 kts to less than 5 kts by noon and then picked up to between 10 and 15 kts in the afternoon and evening. This corresponded to a gradual shift in wind direction from northeast to northwest and modest change in barometric pressure, which varied between 1006 and 1009 mb.

 

CTD Group report (Eileen Hofmann, Bob Beardsley, Baris Salihoglu, Chris MacKay, Francisco (Chico) Viddi, Sue Beardsley)

In the early hours of 10 September we completed two CTD/Rosette casts at station 5, which is in the inner portion of survey transect 1. This station marked the end of the CTD/Rosette work on the survey grid. The first cast at station 5 was to 350 m for microstructure sampling with CMiPS. The second cast extended to within a few meters of the bottom, which was at 675 m.

 

The surface water temperature at station 5 was -1.84C, which is the coldest observed in the northern part of the survey grid. Surface salinity was 33.99. The cold well-mixed Winter Water layer extended to almost 100 m, which is 20 m to 40 m deeper than observed at other stations in this portion of the survey grid.

 

Below the Winter Water layer, temperature increased to a maximum of 1.49C at 315 m, after which it decreased to 1.29C at the bottom. Salinity reached a maximum of 34.72 at 560 m and remained at this value to the bottom. These thermohaline characteristics indicate the presence of Upper and Lower Circumpolar Deep Water.

 

The presence of these water masses so far inshore on the continental shelf brings up the issue of how they got there. The observed thermohaline characteristics suggest little mixing, exchange, or dilution of the oceanic water with the shelf waters. This in turn implies recent arrival of the oceanic water on the inner shelf or little exchange of the oceanic water with the overlying Winter Water and inner shelf water. The pathways by which the oceanic water is brought to the inner part of the west Antarctic Peninsula continental shelf and the exchange of this with shelf waters are research issues of interest to SO GLOBEC.

MOCNESS Report (Phil Alatalo, Peter Wiebe, Ryan Dorland, Dicky Allison, Scott Gallager, Gareth Lawson)

MOCNESS Tow #16 was a morning deployment at Station 9 on 9 September, close to shore, northwest of Adelaide Island. Although the water depth was relatively shallow (332 m), the separation of krill and copepods seen at Station 19 was present here also. Thecosome (shelled) pteropods were present in increased numbers at all depths, while their Gymnosome (unshelled) predators were recorded at nearly every depth interval as well. Overall, biomass collection was low for all nets. The strobe light batteries were too low to operate the strobe; however, the OPC experienced no problems while in the water.

 

Copepods were the most abundant taxa in the integrated net, with a large siphonophore and krill contributing to the biomass. Net 1 (300-250 m) hosted a fair amount of copepod biomass, mostly Calanus propinquus and gravid Paraeuchaeta sp. Chaetognaths were numerous here and present throughout the water column. Net 2 (250-200 m) contained mostly copepods, amphipods, and pteropods. All nets above 200 m produced low biomass catches and were composed mostly of krill (Thysanoessa macrura and Euphausia superba) and chaetognaths. Krill furcilia and pteropods were abundant between 75 and 25 m. The surface net contained mostly pteropods, a few large copepods, and some juvenile krill. A few fish larvae were found in the upper 50 m and an 8-cm ctenophore, caught between 150-100 m, was given to K. Scolardi since these creatures preserve very poorly and are usually unrecognizable back in the laboratory.

 

MOCNESS Tow #17 at Station 5 sampled the shelf waters along the first broad-scale survey line. This tow was conducted near dawn on 10 September in water 670 m deep. Copepods continued to remain dominant in deep water while krill provided most of the biomass above 200 m and below 50 m. Large calanoid copepods joined krill in the upper 50 m as dominant taxa. While not contributing much in biomass, thecosome pteropods were found in increasing numbers at all depths. The scattering layer detected by the ADCP between 0-100 m was likely a result of krill, copepods, and pteropods. The SIMRAD scattering layer between 200-350 m was attributed to high densities of small copepods found in net 2. Net 0 captured nothing due to the cod-end bucket getting into the mouth of the net just after the launch of the system thus fouling the net. The OPC functioned well.

 

The deepest depths sampled contained high abundances of copepods, chaetognaths, siphonophores, and ostracods. Copepods were much smaller in size between 350-200 m. Here also were found specimens of Euphausia crystallorphias. Biomass decreased considerably above 200 m. Thysanoessa macura provided the bulk of the biomass between 200 and 50 m. All stages were present. E. superba juveniles were found in the top 25 m, along with large Calanus propinquus copepods. Pteropods were found everywhere, especially between 150-100 m and above 50 m. Salps, radiolarians, ostrocods, amphipods, and gymnosome pteropods were noted at several depths.

 

Current Position and Conditions

The work at the time series location (Renaud Island) is nearing completion and by early morning we will be steaming north for a short stopover at Palmer Station. Our current position at 0049 on 12 September is -65 07.700′S; -65 30.372′W. It is cloudy and, as was the case yesterday, there is a light snow falling. The air temperature again is -0.6C and the barometric pressure is 1016.9 mb. Winds are 5-6 kts out of the north-northwest (023).

 

Cheers, Peter