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

7 May 2002

 

The count down has started as the end of the third Southern Ocean GLOBEC broad-scale survey is in sight. On 7 May, the Palmer worked from near the outer end of survey line 11 to the inner most station, leaving only two relatively short survey lines to go. The weather continued to treat us nicely in the sense that it was another day of relatively moderate winds, except for a period in the early evening when they picked up and there were gusts to 30 kts. This was about the time the ROV was to be deployed. For the most part, however, wind speeds were 18 to 21 kts or lower. The barometer readings fell during the day from 1000.5 mlb around 0130 to 987 mlb in the late evening and the clear skies of yesterday gave way to a heavy dark overcast. There was snow during the morning and poor visibility. The snow ended before noon, but a heavy overcast remained. Air temperatures varied between -5.7C to -2.5C. Sea surface temperature was -1.794C and salinity was 33.120 psu on the inner shelf during the approach to station 84.

 

On May 7, work was completed at broad-scale survey stations 82, 83, and 84. Four CTD casts were made (two at station 84). The ring nets, which are towed from the starboard side of the Palmer, have become very difficult in the pack ice, but a 1-m ring net tow for surface zooplankton and a 1-m Reeve net tow for live animals were completed at station 82. A MOCNESS tow was completed at station 83 and an ice collection was made at station 84. An APOP cast was also done at station 84 using animals caught with the Reeve net. An ROV under ice survey, scheduled for station 84, was scrubbed because the ice was too thin and the wind too strong (gusts up to 30 kts) to hold the ship in place without significant use of the ship's thrusters. BIOMAPER-II was in for transits between the all of the stations. Seabird and marine mammal surveys took place during the daylight period and 2 sonobuoys were deployed during the transit to stations 83 and 84.

 

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

On 7 May, the CTD group did four casts at three stations in the middle and inner shelf along line 100 (survey line 11). The mixed layers were between 50 and 100 m thick with several isothermal layers. The deep temperatures were cold (1.0 to 1.5C). There was little variation in conditions over the southern third of the grid except for minor changes in the structure of the surface layer.

 

Station 82 (cast 85, 455 m). There were two constant temperature layers (0-20 m and 30-60 m) near the surface. A deep Winter Water (WW) layer was centered at 130 m. The pycnocline extended from 70 to 250 m with sharp changes in temperature and salinity with thicknesses up to 15 m. A 30-m thick temperature maximum layer (1.5C) occurred at 350 m.

 

Station 83 (cast 86, 337 m). The surface mixed layer extended to 80 m with two uniform temperature regions separated 35 m. Clear temperature reversals (10-15 m thick) were seen to 200 m. The bottom temperature was 1.1C with no temperature maximum.

 

Station 84 (casts 87, 88, 633 m). The first cast was to 100 m with FRRF. The surface mixed layer extended to 40 m (-1.8C, 33.4 psu, 0.05 ug/l chlorophyll). Temperature and salinity increased to 100 m, which was the top of the pycnocline. Some minor temperature layers occurred to about 250 m and the deep temperature was nearly uniform (1.0C).

 

Marine Mammal report (Debra Glasgow)

The marine mammal survey began at 0944 on 7 May during the transit to Station 83 in poor light due to overcast conditions, snow and fog. Visibility was variable, but improved as the day wore on. The Palmer was in 8-9/10 open pack ice all day consisting mainly of newly formed young grey and white ice, and rafted pancake with many pools often covered in nilas. Some areas included a few first year floes. As we towed the MOCNESS at Station 83, we began seeing more and more seals on floes, continuing until just before survey ended in poor light at 1630 on the transit to Station 84. A total of 9 fur seals, 8 crabeater seals, and 5 leopard seals were recorded, but many more unidentified seals were seen on distant ice floes too far away to identify. At 1438, 69 01.52S; 75 03.14W, an ice floe passed less than 5 meters to starboard with fresh bright orange seal feces looking like it contained krill. No cetaceans were seen today.

 

Sea Birds (Erik Chapman and Matthew Becker)

Seabird surveys were conducted on 7 May during the ship's transit between stations 82 and 84 approaching Alexander Island, south of Marguerite Bay. Despite some morning snow and fog, visibility was excellent during the 3 hour survey. Ice conditions were between 7 and 9/10ths concentration, shifting from pancake to new gray ice, and finally to 3 to 5 m diameter floes at station 84, closest to shore. A surface tow at station 82 revealed diatoms, larval krill, and numerous copepods, including a large number of bioluminescent Metridia. Night survey observations of feeding Snow Petrels reported in yesterday's daily report, may have been a response to the presence these bioluminescent copepods migrating to the surface at night.

 

Snow Petrels continued to dominate the seabird species assemblage today. Many of the birds were following the ship and Snow Petrels numbers appeared to decrease throughout the day as we moved further into the pack closer to land. A number of Snow Petrels were observed feeding throughout the day, some of which fed along the open water leads where wind drove small wavelets, which presumably carried plankton, over the edge of the sea ice. It appears that Snow Petrels were in highest abundance offshore, toward the ice edge, in nilas, pancake, and grease ice that was probably only days old. Perhaps copepods and other grazing zooplankton move to the surface in large numbers under new ice becoming available as food to Snow Petrels. Snow Petrels may key in on the development of new ice that is a particularly high quality foraging habitat due to this process. However, this is merely speculation and an investigation of zooplankton communities associated with different ice types will have to wait until results from surface tows and MOCNESS tows can be analyzed. Antarctic Petrels were also scattered throughout the survey and were also following the ship.

 

A summary of the species and number of individuals of birds and seals within the 300 m transect during 3 hours, 4 minutes of daytime surveys between consecutive stations 82 and 84 is the following:

 

Species (common name)

Species (scientific name)

Number observed

Snow Petrel

Pagodroma nivea

59

Antarctic Petrel

Thalassoica Antarctica

7

 

 

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

In the early morning on May 7, a Reeve Net tow cast was conducted at station 82 (68 45.980S; 75 61.241W). The water depth was around 450 m. There was a light scattering layer between 300 to 350 meters observed on the Simrad EK500 38 kHz echogram. Since the layer depth exceeded the depth range for other two channels of higher frequencies (250 m maximum depth for both 120 and 200 kHz), we could not get independent confirmation on this light scattering layer. The Reeve Net was deployed to about 400 meters of wire out and towed at about 300 m depth for a few minutes. The animals caught from the net tow were mostly copepods (Calanus). There were a number of amphipods and a few krill juveniles, as well as krill furcilia.

 

In the late evening, an APOP cast was made at station 84 (69 14.010S; 74 11.628W) with the copepods collected from the Reeve net tow made in the morning. The mean length of the animals was 3.2 mm, with a standard deviation of 0.3 mm. The measured density contrast of the animals was 0.996, indicating that they were lighter than the seawater. This result is consistent with both the previous measurement (see daily report on May 2) and the observation that they were mostly floated on the surface in the containers that held them. The mean sound speed contrast from the cast was 1.023, with a standard deviation of 0.002. The mean value was greater than unity, indicating that sound traveled faster in the animals than in the surrounding seawater. This result is different from the result obtained on May 2 (sound speed contrast was 0.949) when the same species was used in the measurements. The inconsistency in sound speed contrast suggests that acoustic estimates of biomass and/or abundance of this species (Calanus) could be very challenging and may potentially result in much larger biased estimates than we previously believed.

 

There was no statistically significant depth dependence observed from the acoustic data. The difference between the down and up casts was very small, about 0.001. However, a slight trend in sound speed contrast for down cast below 125 m could be identified. The sound speed contrast decreased slightly, but monotonically with depth, about 0.0014 over 80 meters (from 125 to 205 m). Based on the measurements on this species so far, the results are still not very conclusive and more measurements are definitely needed.

 

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

MOCNESS tow #20 was conducted on 7 May at mid-shelf at station # 83, which is located on the 11th broad-scale survey line. The region was ice covered with very soft pancakes and loose unconsolidated ice so that towing was quite easy and the ship was able to open a path through the ice with no effort. The bottom depth was 360-380 m and the tow was conducted to 329 m. Relatively little was collected in the net that fished the entire water column or in the bottom most depth interval (250-329) and copepods were the dominant taxa in these nets. Krill furcilia were collected from 75-200 m, and were most abundant in the 150-200 m interval. Copepods were collected at 200-250 m and from 50-0 m. Of the copepods, Metridia was obvious because of the brilliant blue bioluminescence it emitted while being collected in the sieve; this copepod was very abundant in the 50-75 m interval. This was the most Metridia that Phil Alatalo has ever seen in a plankton net. Chaetognaths were collected from 50-100 m and then again at 150-200 m. Very low abundances of copepods were seen in the 25-50 m interval and this was the only taxa obvious there. The upper 25 m caught quite a few small chunks of ice that were suspended in the water column, as well as plankton, so that the cod end resembled a snow cone when it was brought into the laboratory for processing. The abundance of plankton collected in this net was very low and the taxonomic composition consisted of a few juvenile krill, krill furcilia, and copepods.

 

The survey of krill and other zooplankton and nekton with BIOMAPER-II continued unabated on 7 May between stations 81, 82, 83, and 84. There were no major problems with the sensor systems on towed body that required sidetracking it for repair. When the operating optimally, as it has the past few days, around one gigabyte of processed acoustics data, more than a gigabyte of Video data, and a smaller amount of environmental sensor data are logged. The daily accumulation of raw acoustic and video data files, which are stored on DAT and SVHS tapes, is much larger.

 

Along the trackline between the shelf break station 81 and outer shelf station 82, there was a consistent deep layer of volume backscattering starting at 250 m and extending to the bottom. Occasional discrete patches of high intensity targets were present about 60 meters depth. Towing in pancake ice presented no problems. Between stations 82 and 83, there was very typical scattering for a mid-shelf region. There was a bit of scattering at the surface with isolated very small, but intense patches similar to those seen further offshore, a mid-water layer about 140 m down, and a bottom layer that extended up about 100 m above the bottom. This deep layer had an intensified zone about 20 meters above the bottom. Surface ice conditions were still moderate making for easy towyoing. Thicker ice and slower going occurred during the transit from station 83 to 84. During this passage, some more intense near surface patches occurred and in one case the VPR recorded images of krill on the VPR. A zone of intense krill-like scattering as had been seen on the previous three survey lines did not appear by the time the inner most station (84) had been reached. This station, however, is located further offshore and may have been seaward of the zone where krill-like layers and patches have now come to be expected.

 

Cheers, Peter