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

6 August 2002

The sixth of August found us working for a second day in Crystal Sound. In the first light of the day, we could see that the L.M. Gould had made it to the work site using the back route to the sound and the investigators were now able to work in the area. Our effort during the night and early morning was spent doing a CTD section across the mouth of Matha Strait. At the end of the section, a joint ice collection and ROV under-ice survey was done. The Palmer then steamed to a site just south of the Barcroft Islands (-66º 30′S; -67º 01′W). Along the route, we passed within a quarter of a mile of the Gould and could see a group of investigators working on a crabeater seal that they had managed to sneak up on and anesthetize. They were making physiological and biochemical measurements and attaching a small tag equipped with sensors and a satellite telemetry system to the back of the seals head. At the Barcroft site, a calibration of an HTI acoustic system was undertaken during the latter portion of the morning. A search for Adélie penguins that were hauled-out on the ice after feeding was started around 1300 and within a short time 10 individuals were spotted on large floe. The Palmer was able to move slowly up to the edge of the flow without the penguins leaving the area and a party of investigators were deployed onto the ice with the personnel carrier operated from the ship's crane on the bow. Three of the penguins were captured and diet-sampled before being released. Late in the afternoon, the Tucker trawl was used to make collections of live zooplankton, especially krill, for use in feeding, growth, and physiological experiments over the next few days. Two back-to-back tows were done near the entrance to Crystal Sound with the second tow catching substantial numbers of adult krill.

The early evening was spent steaming to a rendezvous point where an exchange of equipment and supplies took place between the Palmer and the Gould. The pack ice limited the drift of the vessels and made it possible to position the ships with the bow of the Palmer within a few meters of the stern of the Gould. The exchange was made using the bow crane on the Palmer to move cargo nets with the gear between the Palmer's bow and the Gould's stern deck. The transfers took about an hour, after which the Palmer steamed south to a location north of Laird Island (-66º 41.8′S; -67º 08′W) where a second CTD transect was begun to define the hydrographic characteristics of water flowing into Crystal Sound through Matha Strait.

In the morning before sunrise (0730), the skies were cloudy and the air temperature was -5.2ºC. The barometric pressure (1017.4 mb) was down a bit from the last day or two and the wind was out of the south (177) at about 5 kts. The sea surface temperature was -1.795ºC and salinity was 33.845 psu. The morning turned beautiful with the sun breaking through the overcast skies so that there was a hazy cloudiness filtering the sunlight. In the mid-afternoon, there was a low thin foggy mist to the atmosphere and the sun, shining through, was a yellow ball with blurred edges. What little wind there had been in the morning died and it was calm throughout the afternoon and evening. During the transfer with the Gould, a very fine misty snow fell lightly and the air temperature was around -7ºC.

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

In the late evening of 5 August, we began a CTD section along a transect that extended across Matha Strait, the opening into Crystal Sound between Adelaide Island to the south and Watkins Island to the north. The purpose of the transect is to look at water inflow into Crystal Sound, especially the input of Circumpolar Deep Water (CDW). Of particular interest is flow in a deep trough that extends from the continental shelf into Crystal Sound. Also, during the GLOBEC III Gould process cruise in April, Meng Zhou did an ADCP survey along a similar transect. The section done on this cruise can be compared to the earlier one.

The transect consisted of 3 stations at 5 nm intervals. The depth at the center station was about 570 m, which puts it in the area of the deep trough. CTD casts were made to the bottom at all stations. The transect was completed in the morning of 6 August.

Temperature increased with depth at all stations, with the warmest bottom temperature (about 1.2ºC) occurring at the central station. The vertical oxygen profile showed an oxygen minimum at about 200 to 300 m. Salinity increased to 34.71 at depth. These properties are characteristic of modified CDW, which forms from mixing of CDW with the overlying Antarctic Surface Water. Thus, it appears that CDW extends across the west Antarctic Peninsula continental shelf and into Crystal Sound.

At all stations, the Winter Water layer was 80 m to 100 m thick. Surface temperatures were slightly above freezing at about 1.75ºC. It may be that upwelling/mixing of the deeper warmer water contributes to the surface waters remaining above freezing.

During the first CTD cast, problems with the data acquisition software were encountered. Large spikes appeared in the data at random intervals. On the second cast the spiking in the data became so bad that it was not possible to observe the profiles produced by the CTD sensors on the computer screen. After checking numerous potential problems, it was determined that the computer used for data acquisition was bad. This computer was swapped out for another and this seemed to fix the problem.

However, the fix was short-lived. On the next cast, the spiking returned. Examination of the processed data files showed that these had been corrupted by the large spikes in the data, but the data collected on tape were good. This provided further evidence that the problem was with the acquisition computer and/or software. After considerable effort on the part of the ship's ETs and ITs, it was determined that the ship's computer system had become infected by a virus and that this was causing the software/computer problems. This has now been fixed and we have recovered the CTD cast files from the back-up tapes.

We thank Fred Stuart, Todd Johnson, and Paul Huckins for their help and effort in fixing the problems with the CTD computer system.

Sea Birds (Chris Ribic and Erik Chapman)

On the afternoon of 6 August, the ship headed toward the Barcroft Islands where we were hoping to use zodiacs to look for and catch Adélie Penguins returning from foraging trips. Although there appeared to leads, immediately surrounding some of the islands, the water was too shallow there for the ship to take us where we could launch the boats. As a back-up plan, we chose to move through the area looking for Adélies hauled out on floes large enough to walk on. Bird researchers on the L. M. Gould were also cruising the area looking for Adélies to diet sample and to fix with satellite tags. Both ships began surveying the area just west of the Barcroft Islands looking for birds. We had already seen several large groups of Adélies in the area and were hopeful that we would begin to see more hauling onto floes toward the end of the day as they finished foraging. It did not take long before we were heading toward a group of 10 Adélies hauled out on a large, 20 to 30 m diameter floe. The birds moved away from the ship, but remained on the floe as the Captain nudged the bow of the N.B. Palmer against the floe. The deck hands used the crane to lower our group onto the floe using a personnel basket. After setting up our diet sampling equipment, we managed to catch 3 of the 10 penguins before they moved across some very thin ice, to an adjacent floe. Two of the three diet samples contained fresh stomach contents. The third contained mostly digested contents and a small amount of krill that was still recognizable. Virtually all of the stomach contents of all three samples were krill. There were both juvenile and adult krill in the samples, ranging in size from 22 mm to 54 mm. In the two fresh samples, there were 2 or 3 amphipods and one of the samples contained a single larval fish.

These samples were taken about 1 mile from where 14 diet samples were taken in the Barcroft Island in May. These samples were quite different, containing a relatively large amount of amphipods and fish. At that time, there were a lot of icebergs in Crystal Sound, but there wasn't nearly as much sea ice, which ranged between 7 and 9/10ths coverage today. Although we didn't get many samples today, the differences between the two diet sampling periods suggest that there may be a seasonal, or ice-driven shift in the prey base available to the Adélies in this area. We are hoping to be able to diet sample more Adélies tomorrow afternoon before we leave Crystal Sound and head south to the survey grid.

ROV report (Scott Gallager, Phil Alatalo)

The objective of the ROV studies is to observe and quantify the distribution, abundance, behavior and size distribution of larval krill in association with the underside ice surface and sea surface hydrography. A Benthos SeaRover ROV was equipped with a variety of physical and biological sensors including a stereo camera system with a field of view of 1 m³, a synchronized strobe, a CTD, an Imagenix 881a 630 kHz-1 Mhz sector scanning sonar, an uplooking DVL Navigator 1200kHz ADCP, and the standard forward looking pan and tilt color camera.

The ROV is deployed off the stern with 200 m of tether paid out with a 50 lb. clump weight at a depth of 20 m. The ROV first descends to 20 m and travels at least 10 m away from the ship. The ROV then ascends to about 5 m depth or until the underside of the ice is observed in the pan and tilt camera. A trackline is established extending radially away from the ship out to a distance of approximately 100 m. As the ROV travels the trackline at a speed of about 2-10 cm/s, the stereo camera is used to image the under-ice surface and associated organisms such as larval krill and their Ctenophore predators. Precise positioning and sizing of targets within the 1 m³ is established through post-processing using a stereogrammetry algorithm. The forward speed of the ROV is established with data from the ADCP and used in conjunction with the image volume to calculate volume sampled per unit time. For example, at a forward speed of 10 cm/s, a new 1 m³ field will be imaged every 10 s. The ADCP also provides distance to the under-ice surface and backscatter intensity. The sector scanning sonar is used to evaluate distance from the ice and for locating krill swarms. The CTD provides backup data on ROV depth and documentation of hydrography. In addition to larval distribution, swimming behavior is also quantified. Stereogrammetry is used to measure swimming speeds and direction to obtain a vector for each individual every 1/30 s. To correct for background motion, the instantaneous vector for all particles in the field of view are ensemble- averaged and subtracted from each organism at 1/30 s intervals. Thus the swimming speed, direction and body posture, angle of attack, etc. is quantified as a function of body size and developmental stage.

Today's deployment (6 August) was the first ROV observations for this cruise. We deployed in Crystal Sound at 0528 hours local time at CTD Station 3 in conjunction with setting up an ice camp for obtaining cores. All systems operated correctly as the ROV traversed along three radial transects to the starboard side of the ship, each a distance of about 100 m. The ice was about 65 cm thick with a snow cover of another 50 cm. The underside was rough and jagged with protrusions extending 3 to 4 m downward interspersed with regions of relatively flat surface. Usually we find larval krill (furcilia stage 5 and 6) concentrated in the nooks and crannies along with swarms along the bottom edge of the deepest protrusions, but today not a single furcilia was observed. Only a single ctenophore about 1 cm in diameter drifted by during the deployment. These observations confirmed those of the dive team who also reported the striking lack of furcilia under the ice. The area of deployment was in the southern region of Matha Strait where a strong current typically runs into Crystal Sound. Although Crystal Sound typically has an abundance of furcilia this time of the year, the Strait may be a region of transport and not retention as found in the inner Sound. The next ROV deployment is scheduled for early on 7 August, just to the west of Station 3, so we will have additional information on this region of the Matha Strait.

Current position and conditions

Our work was completed in Crystal Sound during the evening of 7 August and we are now beginning to steam south to the southern portion of the SO GLOBEC grid in a convoy with the L.M Gould. Our current position at 0023 on 8 August is -66º 36.369′S; -67º 07.769′W. The air temperature is -6.4ºC and the sea temperature is -1.844ºC. Both anemometers appear to be frozen up, so the wind data are faulty. The barometer is at 1010.1 mb and falling slowly. The skies are cloudy.

Cheers, Peter