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

25 August 2002

The move from the outer continental shelf toward the outer reaches of Marguerite Bay and the deep water region of the Marguerite Trough (with depths of more than 1000 m) continued on 25 August. Using the lead system that we were fortunate to locate just east of station 62 allowed easier passage to station 48 and onto to station 49. After a series of net tows were done in the lead during the evening of 24 August, BIOMAPER-II was deployed just after midnight and completed the run to station 48 about 0400. A single CTD cast was done at this station and then the towed body was re-deployed for the transit to station 49. Again we took advantage of a series of large leads that occasionally were closed off by an isthmus of pack ice. Most of the time, the Palmer had no difficulty moving through these ice barriers, but one proved to be particularly tough, requiring considerable effort to get through. During that encounter, the BIOMAPER-II towing cable suffered some damage as described below.

Station 49 was reached about 1330 and a pair of CTD casts was followed by ice collection and an ROV under-ice survey. During this latter pair of activities, three Emperor penguins appeared in the open water around the stern and then swam up to the edge of the floe where the ice collection was being done. They popped up onto the floe right next to one of the collectors much to her surprise, stayed for a few minutes, and then disappeared back into the water. Minutes later, two again popped up onto the floe, and proceeded to waddle-walk and slide on their bellies first over to within a few feet of the ice collectors, who were mesmerized, and then over near the stern of the ship on the starboard side. It was a tremendous show not only for the ice collecting party, but also for those on the fantail taking in the scene.

Work at station 49 stopped for a short period of time as the science party and crew took time to celebrate the fact that 25 August was hump day and the second half of the cruise was beginning. A barbeque was set up out on the helicopter hanger deck and the grill was ablaze for several hours while the chicken, beef, pork, and sausage were prepared. The helo hanger was decorated with strings of lights and Ryan Dorland fabricated a piñata for the occasion. It was a festive couple of hours around dinner time.

The work at station 49 re-commenced with a series of net tows in the lead in which the station was located. A 1-m ring net (also called the Reeve Net) with a large cod-end designed to bring up plankton alive and undamaged was towed vertically. Another conventional 1-m ring net was towed to make a quantitative collection of plankton in the upper 50 m. Two back-to-back Tucker trawls were done to complete the station work around 2130. BIOMAPER-II was deployed as the Palmer set off for station 40 on survey line 6.

During the morning of 25 August, it was cloudy, and cold. The day began with light winds, which picked up in the late morning to 18 to 20 kts out of the west, and then subsided to 5 to 15 from various directions the rest of the day. During the late night, the temperature dipped to -16ºC, rose to about -12ºC by mid-morning and then declined back down to -16ºC by the time of the hump-day celebration (1700). The skies became partly cloudy around noon and then the sun came out in the mid-afternoon. The barometer stayed between 993 and 995 mb until about 1400 and began a steady climb to 999 mb by midnight.

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

On 25 August CTD casts were completed at survey stations 48 and 49. These stations are on survey transect 7, with station 48 being in the central part of the transect and station 49 being at the edge of Marguerite Trough near the entrance to Marguerite Bay. Bottom depths at the two stations were 303 m and 463 m, respectively. At each station, two casts were made with the CTD. The first was to obtain data with the CMiPS sensor and extended to 200 m. The second cast went to the bottom and the lowering speed in the upper 100 m was slowed to accommodate sampling with the FRRF.

At station 48, a maximum temperature of 1.51ºC was encountered at the bottom (292 m) and the corresponding salinity was 34.70. The warm water at this location, especially above 300 m, suggests the recent intrusion of Upper Circumpolar Deep Water. At station 49, the maximum temperature of 1.43ºC was at the bottom (430 m) and was associated with a salinity of 34.71. The oxygen minimum at this location was at 300 m. These properties indicate the presence of modified Upper Circumpolar Deep Water. The observations from these two stations indicate that the onshore boundary of intruded Upper Circumpolar Deep Water is between stations 48 and 49.

Comparison of the on-shelf extent of Circumpolar Deep Water from this cruise with that observed in April-May 2002 (NBP02-02) shows that the warmer water has moved further onto the continental shelf. Whether or not the on-shelf intrusion of the warmer water has any correspondence to the large leads in the sea ice that we have been following for about the past 24 hours cannot be determined at this point. However, it is certainly a topic for investigation with more extensive data analysis combined with circulation and sea ice modeling studies.

The surface salinity at station 48 was 33.86, the surface temperature at -1.81ºC, and the well mixed Winter Water layer extended to 65 m to 70 m. Similarly at station 49, the surface waters were well mixed to 65 m, at the freezing point (-1.81ºC), and the surface salinity was 33.855. These surface salinities are the lowest encountered at the stations sampled to date during the survey. On previous SO GLOBEC cruises, low surface salinity in this part of the survey grid was associated with the coastal current that flows out of Marguerite Bay, around Alexander Island, and across the inner portion of the west Antarctic Peninsula continental shelf. Thus, the observations from stations 48 and 49 suggest that the coastal current may be present at this time.

Sea Birds (Chris Ribic and Erik Chapman)

On 25 August the seabird and Crabeater seal survey was conducted for over three hours between stations 48 and 49 in a driving snow that limited visibility to between 300 and 600 m. Ice conditions were 9 to 10/10ths concentration of new white ice and vast floes. The ship followed some leads, but also pushed its way through some thick first year ice.

Two Emperor Penguins were observed on the ice along the edge of the lead the ship traveled through. While the ship was stopped at station 49, three more Emperors were seen in the prop wash off the stern of the ship. These birds were apparently attracted to the ship, diving and calling at the surface for over an hour. Later, the three Emperors came out onto the ice, coming within a few feet of the ice team that was collecting ice cores at the time. Emperors seemed to be as curious about us as we were of them, and for an additional hour the birds remained on the ice near the stern of the ship. Earlier, seven Adélie Penguins were observed porpoising through a lead near station 49. The Adélies moved quickly away from the ship, and were not seen hauling out on the ice. We hope to attempt to diet sample Adélies if the opportunity presents in the remaining weeks of the cruise. Snow Petrels were observed in small numbers along leads and a single Crabeater Seal was seen in the survey.

A summary of the birds and marine mammals observed on 25 August (YD 237) during 3 hours, 19 minutes of survey time as the ship traveled between stations 48 and 49 is the following:

Species (common name)	Species (scientific name)	Number observed
Snow Petrel	Pagodroma nivea	6
Emperor Penguin	Aptenodytes forsteri	2
Adélie Penguin	Pygoscelis adelii	7
Crabeater Seal	Lobodon carcinophagus	1

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

In the last several days we have completed three more Tucker Trawls at stations 46, 64, and 48, and collected water from the undersurface of sea ice at station 62 to characterize the food available to larval Euphausia superba at the sea-ice interface. To collect these samples, we make use of the core holes made by the sea ice group. After they extract an ice core, we lower tubing to just beneath the ice and pump up water using a diaphragm hand pump. Samples will be analyzed for size fractioned chlorophyll a, particulate organic carbon and nitrogen (POC/N), and microplankton community structure (in collaboration with Scott Gallager’s group). The sample at station 62 was collected under level first year sea ice, 83 cm in depth. In addition, we completed a Copepod Predation Experiment and two Assimilation Efficiency experiments for juvenile and adult krill, and started an experiment to measure Growth and Molting rates of small juvenile krill. Kerri Scolardi is collecting information on the predatory activities of the ctenophore, Callianira antarctica. She has completed several digestion time experiments, as well as documenting the gut contents and spatial distribution of this zooplankter.

The Tucker Trawl at station 46 collected primarily the euphausiid, Thysanoessa macrura, and the copepod, Calanus propinquus. Many individuals of this species are mature adults. Other zooplankton included Metridia (copepod), one juvenile E. superba, several small ostracods, 1 small salp and 2 small ctenophores (C. antarctica). The trawl at station 64 had a very similar composition. In the vicinity of station 48 in a large lead, we collected numerous larval and small juvenile E. superba, many copepods, such as C. propinquus, Metridia, and Paraeuchaeta, very small ostracods, and chaetognaths.

ROV report (Scott Gallager, Phil Alatalo, Alec Scott)

The ROV under-ice survey on 24 August took place near Station 62 (67º 48.776′S; 74º 11.335′W) directly over the shelf break. The 10/10 ice cover was characterized by small (5 to 20 m diameter) second year floes composed of broken and re-frozen brash. Between floes, an extensive network of snow covered ridges extended to a height of about 1 meter. A few small open leads were present along with numerous small ice bergs with diameters on the order of 100 m. The ROV was not scheduled for this station, but we decided that the results of a short deployment would be useful to both the divers and trawlers in deciding what should be attempted at this station. The ship drove into an area of re-frozen floes and became stationary at a heading of 149º. The ROV was deployed at 0918 local time immediately after the ice team began their work on a small floe on the starboard side of the ship. Following a checkout of all systems, the release pin was pulled and the ROV headed towards the small ice flow on which the ice team was currently working. There were three seals frolicking in the pool behind the ship that took quite an interest in the ROV. Video footage was acquired on both the large area color pan and tilt camera and the 3D stereo cameras of the seals swimming by and checking out this odd looking co-habitant of their underwater world. The ROV dove to 30 m quickly and traveled under a series of ridge lines, then came to the under surface at 8 m and began a transect on a magnetic bearing of 220º. In all, four transects were conducted each approximately 40 m long and parallel to the ships axis. The seals did not follow the ROV under the ice ledge. The under ice surface was characterized by thick, flat plates rafted on top of each other and re-frozen into a solid mass. There were many complicated channels and domed chambers that the ROV explored as fully as possible. Once again however, not a single larval krill or other plankter was observed.

BIOMAPER II group report (Gareth Lawson, Peter Wiebe, Scott Gallager, Phil Alatalo, Dicky Allison, Alec Scott)

We conducted two towyos in the morning of August 25, the first of which continued yesterdays transit between stations 62 and 48, while the second was between stations 48 and 49. The two towyos were separated by only a two hour CTD station, and covered very similar waters. Near the start of the first tow, we passed over a series of very tall (24 to 30 m) and dense (up to -54 dB), but short in horizontal extent (37 to 76 m) patches of krill-like backscatter located at 25 m. Later in the tow a diffuse scattering layer formed at this depth, which by the second towyo had shifted to a distribution centered deeper in the water column (80 m) and had become more diffuse. Over the course of the transect, we observed a number of regions of intensified backscatter within this layer. In the upper 100 m, we made observations with the VPR of copepods, radiolarians, larvaceans, and diatoms. A fairly dense bottom layer also was present for the entire length of both towyos, extending from the bottom (300 to 400 m) up to between 25 and 95 m above-bottom. At times, a second layer of more sparsely distributed and larger individual targets was evident above the main bottom layer. Our deepest observations with the VPR were made at 275 m, at which depth we observed large copepods, but the BIOMAPER II was still well above the bottom layer at the time of these observations.

During the second towyo, the ship was doing a lot of backing and ramming to get through thick ice ridges. On one particularly bad occasion, Sam the winch operator called us on the radio to report that he saw some broken strands of wire in the tow cable. Upon checking, we found two strands had broken and begun to unravel. The wire was hauled in to a point where the break could be tied off with seizing wire and then taped such that the unraveled wire could be cut loose. What this means is that we will no longer be able to deploy cable beyond the point where the unraveling began. The maximum depth attainable by the BIOMAPER II thus will be about 200 m.

At 2200 we began another towyo that continued into the early morning of August 26, the results of which will be presented in tomorrow’s daily report.

Current Position and Conditions

Our current position at 2353 on 26 August is -67º 59.495′S; -70º 30.485′W. The air temperature is -4.6ºC and the barometric pressure is 996.7 mb. Winds are 22-28 kts out of the northwest (329). It is snowing moderately hard and we are being challenged by some extremely tough heavily ridged pack ice as we attempt to steam from station 40 to station 28.

Cheers, Peter