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

6 May 2002

May 6 was a day of transition for the continental shelf waters off of Alexander Island. The cold temperatures of the past several days combined with sea surface temperatures right around the freezing point (-1.79ºC) set the stage for a rapid set up of sea ice almost all the way to the edge of the continental shelf. Sea ice has been a common element of the work at the stations closest to shore, but on the transits along survey lines 8 and 9, there was mostly open water once away from the inner most stations. But on the run out to the edge of the shelf on survey line 10, newly formed sea ice was with us nearly all the way to outermost station (80). This transition was no doubt aided by the low winds of the past week as the area has been dominated by high pressure.

The 6th of May was also notable for the remarkably clear skies that stayed the day. Although early morning found the Palmer some 60 nm from land, the mountains of Alexander Island could seen in the distance silhouetted in the predawn light. Later in the day with the Palmer further offshore, they were still cloud free and cloaked in white. Visibility was excellent. Winds were somewhat fresher varying from 18 to 25 kts predominantly out of the southwest. The barometer again rose above the 1000 mlb mark reaching a high of 1003.2 mlb about 2100. Air temperatures varied between -9.0 and -5.0ºC. There were clear skies overhead during the evening enabling the myriads of stars to be seen, a decidedly uncommon event this cruise.

Work was completed at broad-scale survey stations 78, 79, 80 and 81 including 4 CTDs, an ice collection at station 78 and a 1-m ring net tow at station 80. BIOMAPER-II was in for transits between the all of the stations. Seabird and marine mammal surveys took place during the daylight period and two sonobuoys were deployed during the transit between stations 79 and 80.

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

The CTD group did four casts today at the outer shelf and over the slope. The mixed layer at all casts was variable reflecting the changing ice cover and varying, although, weak winds. The two deep stations were typical of other casts in the ACC. The two shelf stations were typical of stations in the southern part of the grid with no indication of recent influence of oceanic intrusions.

Station 78 (cast 81, 488 m). There was a uniform mixed layer to 50 m (-1.75ºC, 33.5 psu, 0.07 ug/l chlorophyll) and a relic Winter Water (WW) layer centered at 100 m. Energetic temperature reversals occurred from 100 to 150 m. The bottom temperature was 1.3ºC with no temperature maximum.

Station 79 (cast 82, 427 m). A uniform mixed layer occurred to 30 m (-1.8ºC, 33.45 psu, 0.1 ug/l chlorophyll) with two deeper uniform temperature layers below (75 to 125 m and 140 to 160 m). The temperature maximum (1.4ºC) was at 350 m.

Station 80 (cast 83, 2009 m). The surface layer had increasing temperature and salinity to 50 m and a relic WW layer was centered at 80 m. The temperature maximum (1.8ºC) was at 250 m and a cold layer, with lower salinity was at 550 m. Temperature declined with depth to the bottom.

Station 81 (cast 84, 1176 m). A uniform mixed layer went to 70 m (-1.8ºC, 33.8 psu, 0.2 ug/l chlorophyll). A second uniform temperature layer extended to 100 m. Some small-scale variability was present in the pycnocline. The temperature maximum (1.8ºC) was at 250 m and temperature declined from there with depth to the bottom.

Marine Mammal report (Debra Glasgow)

The sun rose this morning at 0957 on 6 May to a colorful pancake ice coated ocean with an air temperature of -7.8ºC. The mountains of Alexander Island dwindled behind us clothed in pale pink as we steamed towards Station 79. The survey was started a little early in poor light at 0908, but the light was sufficient by 0930. The first of the day's two seals was seen slumbering at the surface amidst the 8/10 pancake ice at 0955. At 1104, Capt. Joe Borkowski saw our first whales of the day while the Palmer was stopped at Station 79. From then on we were sighting whales fairly regularly until the sun set at 1600. There were 9 sightings throughout the day -18 whales in total. 15 were humpback whales, one was a large unidentified baleen whale (either sei, fin, or blue) and one was a small unidentified whale (possibly minke, but more likely a beaked whale). Most of the whales sighted today were repeatedly blowing 2-3 times then diving for several minutes and resurfacing in the same general area, suggestive of feeding at some depth. No socializing was witnessed at any time today. Excellent visibility and a fairly clear sky aided the survey today, ending with yet another spectacular sunset at 1558 on Station 80.

Sea Birds (Erik Chapman and Matthew Becker)

Observations were made today, May 6, as the ship transited off the shelf break south of Marguerite Bay. New pancake ice, probably formed over the previous 2 to 3 days of calm, cold weather, covered between 8 and 10/10ths of the ocean surface. The ship traveled through the ice edge into streaks of new grease ice that covered 2/10ths of the surface before entering open water just off the shelf break. BIOMAPER-II reportedly found little scattering in this area, and a surface tow conducted at station 80 off the shelf indicated diatoms, copepods, and some larval krill at the surface.

Snow Petrels were observed in high densities throughout the survey. A small number of Antarctic Petrels were also seen, but in much lower numbers. The Snow Petrels appeared to be drawn to the ship from nearby leads, and between 20 and 40 birds were recorded as followers throughout the day. As the ship left the pancake ice and entered grease ice, the followers remained, and they were continually joined by additional birds. Clearly, this is a particularly attractive habitat for Snow Petrels. Surveys in similar ice concentrations elsewhere on the grid have not indicated high Snow Petrel numbers. However, previous surveys in ice have been in areas with year-old ice floes, and the differences in petrel abundance may reflect differing biology between the under-ice zooplankton community associated with new ice and that associated with year-old ice. It will be interesting to look at results from surface ring net and MOCNESS tows to see if these sampling efforts indicate a difference in surface zooplankton communities associated with these two ice types.

A night survey was conducted in the late evening as we headed toward station 81. We had again entered light pancake ice with about 7/10 coverage, however Snow Petrel numbers had dropped significantly, with only a handful of birds around the ship. While some birds are perhaps attracted to the NPB's spotlights as the ship scans for icebergs, we witnessed repeated feeding behaviors in the dark water off the portside. Feeding consisted of the birds hovering just above the surface (facing into the wind) and making repeated dips into the water with their bill, without actually landing on the surface. Presumably, this unknown prey item existed in high numbers locally, as the petrels were observed making as many as ten feeding attempts in one sequence (due to the winds, subsequent water conditions, and methodical feeding behavior it seems unlikely that these attempts constituted repeated aggressive pursuits of a single prey item). How the birds are able to discern their prey in near total darkness is unknown. In addition, why such feeding behavior was not observed during the day, and why the numbers of snow petrels dropped off so dramatically at night is unknown, but probably concerns changes in prey distributions and abundance. It is possible that the birds observed feeding at night had encountered a very localized nocturnal migration of zooplankton in the water column. Some of these zooplankton species are capable of bioluminescence, which might be an easily discernable trait on the water's surface to the well-trained avian eye.

A summary of the species and number of individuals of birds and seals within the 300 m transect during 4 hours, 47 minutes of daytime surveys between consecutive stations 78 and 80 is the following:

Species (common name)	Species (scientific name)	Number observed
Snow Petrel	Pagodroma nivea	179
Antarctic Petrel	Thalassoica Antarctica	12
Southern Giant Petrel	Macronectes giganteus	1

Water Sampling for Microzooplankton (Phil Alatalo)

Survey Line 8 began in George VI Sound in very thick ice. Surface samples at Stations 56 and 57 were characterized by dense numbers of particles, Mesodinium, small diatoms, and large ciliates. Particle abundance decreased as the survey progressed out of Marguerite Bay, with Mesodinium still present, but general motility of microzooplankton very low. The bottom samples of nearly all CTD samples typically consisted of very fine particles and low motility. The bottom waters of nearly all CTD stations along this transect consist of abundant, tiny particles, very few diatoms, and no or low motile microplankton. Presumably the particle load is due to scouring bottom currents. Few swimming microplankton may reflect the apparent lack of food in these cold, dark regions.

Along the shelf, at stations 60 through 63, tintinnids and ciliates became more common as did the diatom, Corethron. Here, the water surface was free of ice. Mesodinium continued to be restricted to surface waters. Ciliate protozoans were present at all three depths sampled at Station 62.

In deeper waters, diatoms (Chaetoceras, Corethron, Nitzschia, Rhizosolenia spp) were common in surface waters, along with the ciliate, Mesodinium. This agrees with the increased chlorophyll values (0.3-0.4 ug chl/l) reported by the CTD group. Particle abundance and motility declined sharply in the pyncnocline at Station 66, but further offshore, assemblages similar to the surface were present as deep as 200 m at Stations 68 and 70.

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

No MOCNESS tow was taken during 6 May

BIOMAPER-II was towyoed along the survey tracklines on 6 May beginning at mid-shelf station 78 and running out to shelf-break station 80 at the end of survey line 10 and then over to station 81, another shelf-break station on the seaward end of survey line 11. During the transit along the mid-shelf region, there were regions of strong scattering from 30 to 60 m depth and varying from 60 to 100 m in vertical extent, then grading into lower backscatter at greater depths. There were occasional very intense patches of backscatter in this depth zone. Further out on the shelf between stations 78 and 79, there was low scattering throughout the mixed layer, a horizontal layer in the upper pycnocline, a zone of very low scattering at mid-water depths, and a zone of higher backscattering starting at the bottom, which was around 500 m, and extending up about 200 m. Intense small targets periodically appeared in the mixed layer. Towards the end of this transit, thin layers developed in a double pycnocline structure that was present.

On the run to the end of the line (station 80), there was light backscattering throughout the water column with isolated discrete targets at various depths. About this time, there were observations of whales making deep dives near an iceberg. Steaming along the shelf break from station 80 to 81, the deep scattering layer became somewhat stronger for a time, but scattering was low near the surface and in the pycnocline around 65 m.

Cheers, Peter