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

28 August 2002

For the past eight days, we have been working in the waters of the continental shelf to the west of Marguerite Bay, which constitutes the central sector of the Southern Ocean GLOBEC survey grid. During the first five or six days, the pack ice covering the region had been a mix of rather large leads and small- to medium-sized floes surrounded by one to two meter tall ridges that were often extremely tough to get through. We were able to take advantage of a system of leads that were bridged by relatively small pack ice blockages after finishing work offshore and moving inshore to station 40. The change in winds and ice conditions during the past two or three days resulted in the leads being broken up into smaller units with much larger areas of intractable jumbles of upturned floes in the ridges separating them. During 28 August, we spent a second day backing and ramming our way toward station 26 located near the seaward end of survey line 5, since we had given up trying to get to station 28. The net distance traveled during the day was 35.5 miles in the 23 hours that were steamed for an average speed of 1.5 knots.

A pair of CTD casts was done about 0600 in the vicinity of station 41 primarily for the purpose of making microstructure measurements. Other tasks that would have been done (e.g., Tucker trawls for live animal collection and towyoing BIOMAPER-II) had ice conditions been better were dropped for lack of sufficiently large leads in which to do the work and the slow progress getting to station 26. Enough time had to be available for work at station 26 before heading north to assist the Gould in reaching their third process station site. During the afternoon, the Gould left its second process site and was attempting to move in nearby leads in search of Adélie penguins. During the afternoon radio communications, it was opined that it was unlikely they would be able to get to their third site on their own.

During the morning of 28 August there were high clouds that cleared for the most part by noon leaving only a thin cirrus cloud layer. The sun was very bright with added reflection of sunlight off the pack ice. The visibility was such that the mountains of Adelaide and Alexander Islands were visible some 60 to 70 nm off to the east. There was a low cloud layer around the foot of Adelaide Island and a darker band of clouds on the horizon to the north. In the late afternoon, the low cloud layer reached us and the visibility was markedly reduced. The air temperature in the early morning was around -5ºC, but fell rapidly during the early afternoon to about -10.5ºC. The barometric pressure increased throughout the day from 995 to 1012 mb. Winds were out of the west at 15 to 25 kts in the morning and then shifted to southwest and decreased to 5-10 kts by evening. In spite of the slow progress through the pack ice, it was a very nice day for steaming between stations.

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

Most of 28 August was spent trying to reach station 26, which is along the outer part of survey transect 5. The heavy and extensive sea ice made it difficult to maintain headway and as a result progress was slow. However, in the early part of the day we were able to complete two CTD casts at survey station 41 in the mid-portion of survey transect 6. This station is shallow (380 m) and is located along the southwestern edge of Marguerite Trough.

The two casts at this station were done in rapid succession in order to obtain microstructure profiles with CMiPS. The first CTD cast at station 41 was to 350 m and the second was to 361 m. Thus, both casts covered the full extent of the water column. On the second cast, the CTD was lowered at a slower speed in the top 100 m to allow for sampling with the FRRF.

Surface water at station 41 was -1.792ºC, which is above freezing, and the surface salinity was 33.84. The well-mixed Winter Water layer extended to about 60 m, which is shallower than encountered at other locations sampled in this region of the survey grid. The above freezing surface waters may be the result of upward mixing of the warmer water at depth, as was observed in other parts of the survey grid that included shallowing topography. This was observed in the XBT-derived temperature section done during the first transit at the start of this cruise (see August 9 CTD report). This section showed warmer surface waters and breaching of the Winter Water layer as the bathymetry shallowed on either side of Marguerite Trough.

Below the well mixed surface waters, temperature and salinity increased to values of 1.37ºC and 34.70, respectively. These values are associated with modified Circumpolar Deep Water that covers the west Antarctic Peninsula below 200 m in regions not affected by bottom intrusions. This water mass likely represents the typical shelf water in this region.

During the time occupied by the attempts to reach station 26, we were able to finish processing the discrete oxygen samples taken from the CTD casts done to date. Comparison of the titrated and CTD-derived dissolved oxygen concentrations show a clear offset, with the CTD-derived values always being the lower of the two. The magnitude of the offset increases with decreasing depth and it is a maximum at the surface. We are now developing an algorithm based on temperature, which is a proxy for depth, that can be used to correct the CTD-derived oxygen values. The standard approach of deriving a linear relationship between titrated and CTD-derived dissolved oxygen values will not work for correcting these data.

Nutrients (Yulia Serebrennikova and Steve Bell)

Twelve CTD stations have been done in the central portion of the grid. Stations 62-64 of survey line 8 were all off the shelf break, but CTD casts completed on lines 6 and 7 give an idea of the hydrographic properties distribution from offshore to the entrance to Marguerite Bay.

Nitrate and phosphate profiles for these stations are about the same as those for the stations in the southern sector of the grid. The highest concentrations of both nutrients are found between 200 and 400 m and are generally associated with temperature maxima. They are 34 and 2.35 micromolar for nitrate and phosphate respectively. The concentrations of both nutrients decrease below these depths until they reach minima of 32.5 and 2.20 micromolar respectively, between 800 and 1200 m. Then they increase slightly in the deeper water. The phosphate upper mixed layer concentrations are 2-2.05 micromolar. Nitrate exhibits 28-31 micromolar concentrations in the same water layer.

Nitrite concentrations are within 0.16-0.20 micromolar range for most stations except for the on-shelf stations on survey line 6. There the concentrations of nitrite do not exceed 0.08 micromolar. Ammonium hasn't shown any surprises so far. Ammonium concentrations are lower than those found during GLOBEC II in this area; they range from 0.2 to 0.7 micromolar.

The silicic acid distribution on lines 6, 7, and 8 exhibits the same features as found on lines 10-12. The intrusion of Circumpolar Deep Water onto the shelf can be traced with 110 micromolar concentration of silicic acid found both in the shelf bottom water (400-500 m depth) and in the deep water (1000-1200 m) off the shelf break. The upper mixed layer concentrations of silicic acid, approximately 65 micromolar offshore and 80 micromolar on the shelf, are higher than those found in the central portion of the grid last year on GLOBEC II, while the deep water concentrations are essentially the same.

Marine Mammal report (Chico Viddi)

Marine mammal surveying now totals 200.9 hours of observation, of which 91.7 hours were categorized as “effective effort”. There were three effective hours of surveying on 27 August and 8.5 hours on the 28th. Weather conditions during these two days were variable. On the 27th, viewing conditions were mainly affected by dense fog patches and snow, while on the 28th, the viewing conditions for surveying were optimal. Nevertheless, ice floes of first year ice of about 90 to 120 cm thick, 10/10ths coverage, affected the vessel track considerably. Few cracks and small leads of open water were seen today. Only a few observations of marine mammals were made during these two days. Twenty crabeater seas were counted, 14 on 27 August (6 were seen in the water) and 6 on the 28th. Even though viewing conditions on Tuesday were bad, a Minke whale was sighted at 1536 (-67º 58.30′S; -70º 43.96′W, 9º to port and 0.3 nm from the ship). It was seen only twice; first right ahead of the Palmer's bow and the second, far behind its stern.

Krill distribution, physiology, and predation (Kendra Daly, Kerri Scolardi, Emily Yam and Jason Zimmerman)

We collected live zooplankton using a Reeve net at station 49 and at a site between stations 40 and 28. The Reeve Net has a large non-filtering bucket at the base of the net to protect the collected live animals. The first net collection caught one small ctenophore. The second one caught a ctenophore, several juvenile Euphausia superba, several Thysanoessa, many copepods, and some larvaceans, amphipods, pteropods, and ostracods. There were also small aggregates of radiolarians. We also participated in a dive at station 40, where we collected samples under the ice surface to assess food availability for krill. Unfortunately, no krill or ctenophores were observed at this site. In addition, we have completed another experiment to estimate the efficiency with which krill assimilate carbon and nitrogen from their ingested food. The samples from these experiments will be analyzed back in our laboratory after we return from the field.

Current Position and Conditions

We finally made it to station 26 and spent the day working on the site. We are now underway for the fourth rendezvous with the L.M. Gould, which is about 80 nm away. Our current position at 0014 on 30 August is -67º 08.029′S; -72º 17.761′W. The air temperature is -3.7ºC and the barometric pressure is 1004.9 and falling. Winds are 25 to 30 kts out of the northeast (045). It is cloudy and we are steaming though 10/10 pack ice.

Cheers, Peter