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

11-12 September 2002

 

The convoy with the L.M. Gould ended on 11 September when we reached the location chosen for the time-series station north of the northern tip of Renaud Island (-65 11′S; -65 35′W) and about 100 nm north of the northern most survey line on the SO GLOBEC station grid. The Gould remained in the area long enough to make some ice collections before heading off toward Palmer Station. The station was located very close to a small iceberg surrounded by pack ice. Water depths ranged from 530 to 735 m. At this station, a series of five sets of CTD casts were done within a 24-hour period. Simultaneous with the CTD casts was the acquisition of acoustic data with BIOMAPER-II, the ADCP, and the Simrad EK 500 echosounders. Both pairs of the BIOMAPER-II 120 and 200 kHz transducers were facing downward and the towed body was held at 10 m depth. In periods between the CTD casts, there were two ROV deployments and two 1-m MOCNESS tows, one each during the day and during the night. An under-ice dive was done in early afternoon. While the divers were away from the ship, BIOMAPER-II was deployed for calibration using standard target tungston carbide balls. In the late evening after the second MOCNESS tow, the pair of 1-m ring nets was deployed in the upper 10 m off the stern to collect furcilia for experimental work. The time series began at 0500 on 11 September and ended at 0400 on 12 September.

 

Steaming to the north resumed with the next stop being Palmer Station, located on Anvers Island. The Gould had reached the station earlier in the morning and was tied up at the dock when the Palmer arrived in Arthur Harbor about 1100 on 12 September. The arrival took place with very good viewing conditions. A mix of sun and clouds provided ideal lighting with the white of the snow and nearby glacier contrasting with the dark blue of the Butler-style station buildings. The Palmer Station manager, Joe Petit, came to the Palmer around noon to give a briefing on the do's and don't's of the station. In the early afternoon, while a number of the investigators on board the Palmer went to the station to tour the buildings and hike the glacier, BIOMAPER-II was deployed off the stern for a final acoustic calibration session. Another group took a second Zodiac and went for a SCUBA dive in the vicinity of the wreck of an Argentinian ship. In late afternoon, the HTI towed body was put in the water for additional calibration work.

 

The hike up the glacier along a flagged route was especially nice given the long period spent at sea. The low sun angle and variable cloudiness gave rise to a complex lighting that made the views from the top especially rewarding. The route led past a Weddell seal that had just given birth to a pup in an isolated place at the head of a small ice covered inlet. During the evening, personnel from both the Gould and the Palmer attended a barbecue at the very kind invitation of the station manager. It was a terrific spread and a great evening to share the cruise experiences with the station personnel and between personnel from the different vessels.

 

September 11 was a gray day with low clouds and no sunshine. The winds were light (<10 kts out of the west-northwest) and the air was relatively warm (-1 to +0.4C). There was a mist in the air throughout the day, but when the ice collectors were out on the ice, they stripped down to shirt sleeves because of the relative warmth. The visibility was low in the fog and mist. The barometric pressure rose from 1010 mb in the early morning to 1017 mb in the evening and then plateaued. The high pressure continued into 12 September with a peak of 1018 mb reached about 0900. Temperature on the 12th continued to be remarkably mild reaching a high of +4C while the N.B. Palmer was anchored in Arthur Harbor. Winds, mostly out of the east-northeast, reached 15 knots around 1000 and then dropped to less than 5 knots for the afternoon. As noted above, visibility was excellent. All in all, the weather contributed to the excellent working conditions both at the time-series station and at Palmer Station.

 

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

September 11 was a busy day for the hydrography group. We began work at the time-series station off Renaud Island in the morning with two CTD/Rosette casts. This was followed by two more CTD/Rosette casts about mid-day, four casts in the afternoon, three casts in the late evening, and two casts the next morning for a total of 13 casts. For each block of CTD/Rosette casts, the first casts were to either 400 m or 350 m for microstructure sampling with CMiPS. The last cast in each block was to within a few meters of the bottom, which varied between 540 m and 730 m, and water samples were collected with the Niskin bottles on the upcast. The only exceptions were the casts in the first block when no water samples were taken and the final casts in the fourth and fifth blocks which went to only 500 m and used lowering speeds in the upper 100 m that were appropriate for FRRF sampling. During all casts, BIOMAPER-II was parked off the stern to collect acoustic data during the casts. Some previously observed scattering layers appeared to be due to physical causes perhaps associated with fine/microstructure, thus the desire to co-sample.

 

One objective of the science plan for September 11 was for the CTD group to sample close to an iceberg while the other groups characterized the biological distributions in the vicinity of icebergs. The first block of CTD/Rosette casts was made roughly 1 nm from an iceberg at about 0643 with the ship at -65 11.04′S; -65 33.86′W. Two other icebergs were within 1.5 nm and a total of 11 within 2.5 nm. To keep on the science plan schedule, just two casts were made to 400 m and no water samples were collected. These casts showed considerable structure in the upper 200 m, but the ice conditions there were no good for an ROV under-ice survey and ice sampling, the next two sets of measurements scheduled. Therefore, the decision was made to make a 1-m MOCNESS tow back along the ship's course towards the SW where thicker ice had been observed. Near the end of this tow, the ship again passed a small iceberg. When the tow was completed, the ship steamed back to this iceberg and set up for the ROV and ice sampling. The L.M. Gould was just completing ice sampling at a site near this iceberg, and reported that the conditions were good for ice sampling, since the ice had some flooding with interesting biology.

 

After the Palmer finished its morning ROV and ice sampling, the ship moved closer to this iceberg to start the second block of CTD/Rosette casts. The remaining blocks of casts were made at different sites close to this iceberg. Visual and radar measurements of this iceberg show it to be roughly oval in plan view, approximately 140 m long by 105 m wide, with an average height above sea level of 10 m. Assuming a constant plan form in the vertical, this iceberg should have extended down to roughly 90 m. For each block of casts, the Palmer was docked in the ice close to the iceberg, with radar ranges to the leading side of the iceberg varying from 0.159 nm (290 m) to 0.234 nm (430 m). These positions were located on different sides of the iceberg. While this iceberg was small, the waters around it were teaming with life. Dense scattering layers were observed, large samples of krill were collected, and seals and Emperor penguins were observed off the stern.

 

The depth of the Winter Water layer varied over the casts from a minimum of about 30 m to a maximum of almost 70 m. The shallowest Winter Water layer observed during the cruise, 30 m, was seen during the second set of CTD/Rosette casts The vertical temperature and salinity distributions from the second set of casts showed a gradual increase with depth rather than a homogeneous distribution that changes abruptly at the bottom of the Winter Water layer, as is typically observed. The different structure of the Winter Water layer at this site is suggestive of mixing of the upper water column with the deeper waters. Also, considerable small-scale variability in temperature and salinity, consistent with mixing, was observed in the Winter Water layer.

 

Below the Winter Water layer, temperature increased to a maximum of 1.43 between 550 and 630 m and remained at this temperature to the bottom. The salinity of the deeper waters was 34.69. These properties are characteristic of Upper Circumpolar Deep Water that has been modified by mixing with the overlying Winter Water and the Antarctic Surface Water that is present in the summer and fall.

 

Microplankton report (Phil Alatalo, Gustavo Thompson, and Scott Gallager)

With the exception of under-ice and slush-ice sampling at dive sites and ice stations, the microplankton observed during the period 2 to 11 September has been typical of the winter assemblage seen thus far. The following stations have most recently been sampled in the northern sector of the survey grid: stations 13, 12, 11, 10, 16, 15, 14, 23, 22, 21, 20, 19, 18, 17, 9, 8, 5, and special 24-hr. Renaud Island samples, RI2 and RI5. At nearly all stations, surface samples contained a low to moderate number of particles with little motion. On occasion, pycnocline or bottom samples showed a slight increase in numbers, but motility of the plankters was low. Average particle concentrations were less than 10/ml. Mean speeds were low and particle diameter ranged 14-31 μm. Mesodinium or a small Meso-like ciliate continued to be found in off-shore/slope surface waters (stations 12, 11, and 14). They were also present at station 9, which was located close to Adelaide Island. Under-ice samples taken at station 16, 14, 18, RI2, and RI5 typically showed very high particle loads, but usually very limited motility. Some variability was observed at Renaud Island, where the morning CTD sample contained a moderate number of particles with a few ciliates and flagellates present, while the evening sample was much reduced in particle density and motion.

 

MOCNESS Report (Phil Alatalo, Peter Wiebe, Ryan Dorland, Dicky Allison, Scott Gallager, Gareth Lawson)

MOCNESS tow #18 occurred at dawn on 11 September at a special station west of Renaud Island, 100 nm north of SO GLOBEC survey line 1. With the ice pack becoming thinner, successful tows have been more routine and tow 18 fished to 500 m without incident. The biomass pattern closely matched the previous tow except for very distinct layers of euphausiids. Copepods and chaetognaths were significant below 350 m. Thysanoessa macrura was dominant between 350 and 50 m, but biomass was very low at these depths. Very high biomass of Euphausia superba occurred between 50 and 25 m.

 

The oblique net 0 contained mostly E. superba, copepods, and chaetognaths, with several siphonophores present. Individual specimens of krill were frozen for A. Bucklin and a bulk sample of krill was frozen for D. Costa. While copepods and chaetognaths were most abundant between 500 and 350 m, a diverse assemblage of ostracods, siphonophores, tomopterid worms, and radiolarians were observed. Net catches were sparse between 350 and 50 m. T. macrura were present between 200 and 75 m, with furcilia stages dominant between 100 and 75 m. Limacina sp. pteropods were particularly abundant between 150 and 75 m. Net 7 caught very high numbers of E. superba and a few amphipods. The surface net (25 and 0 m) contained very little zooplankton.

 

The second MOCNESS tow at the Renaud Island Station and the last one for the cruise took place in the evening of Sept. 11th. Results of tow 19 were similar to the morning tow except that a thick layer of krill at the surface resulted in extremely high surface biomass. While a full complement of other taxa was present at depth, in general biomass below the surface depths was low. Euphausiids dominated biomass and abundance at nearly every depth. Copepods were never abundant. While fishing net 4 (150-100 m) ice caught the towing wire, causing the instrument to drop to 202 m. Otherwise, the MOCNESS and OPC systems were successful.

 

Much biomass was obtained from net 0. Euphausiids, primarily E. superba, were dominant. One large ctenophore was given to K. Scolardi and several krill were frozen from this net for A. Bucklin's genetic analysis of Antarctic zooplankton. Euphausiids were also frozen for D. Costa and D. Chu for biochemical study and acoustical study back in the USA. Between 325 and 200 m, a few krill, radiolarians, and chaetognaths made up the low biomass collected for this depth. Low biomass was characteristic of all nets between 200 and 75 m. Thysanoessa macrura and pteropods comprised most of the biomass. A large salp was noted between 100 and 75 m. Above 75 m, E. superba replaced T. macrura and biomass increased slightly. Sampling between 50 and 25 m produced many krill with pteropods. Results from the top net yielded 1.25 gallons of nearly pure E. superba!

 

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

On September 11, we deployed the BIOMAPER-II six times, as part of the 24-hour time-series study done off Renaud Island. One of these casts was used to calibrate the BIOMAPER-II acoustic transducers. Calibration is done by suspending a standard sphere under the transducer and measuring its target strength. By knowing the density and diameter of the ball and assuming it is perfectly spherical, its target strength can be predicted from first-principle physics. Comparing measurements of the ball's target strength to this prediction then allows us to determine how well the transducer is performing. Ideally, the ball should be positioned exactly in the center of the acoustic beam, at least 5 m from the transducer. Getting the ball into such a position, however, can be very tricky. In this case, we were able to appropriately position the ball and thoroughly calibrate both of our 120 kHz transducers. These transducers measured weaker than expected target strengths for the calibration ball. Unfortunately, we were not as lucky in positioning the ball under the 200 kHz transducers, and were only able to collect low quality data at this frequency.

 

The other deployments made at the station were done in conjunction with repeated CTD casts. This allowed us to compare our acoustic measurements of the water column to CTD microstructure profiles, while simultaneously making observations of organisms as they passed under our transducers. Since the BIOMAPER-II was kept stationary at a depth of 10 m for the duration of each deployment, we did not attempt to capture any images with the VPR. The deployments were one to two hours long, and were performed at 0520, 1210, 1710, 2245, and 0230 (i.e., in the morning of September 12). Each time, noise from the bow thrusters being used to keep the ship in position for the CTD prevented us from making observations deeper than 150 or 200 m. Nonetheless, there was a strong indication of enhanced scattering in a thin band associated with the pycnocline.

 

During the first deployment, very little biological scattering was observed. By the second run, a backscattering layer had developed centered at 50 to 60 m. In the third deployment, a weak layer of backscatter was present very close to the towbody, but it wasn't until the fourth run that we saw any very strong signal. On this deployment, a very dense layer developed quite suddenly, extending from the depth of the BIOMAPER-II downwards by 5 to 23 m. For much of the time that this layer was evident acoustically, people working the CTD in the Baltic Room were able to actually see the krill swarm at the surface. During the fifth run, a somewhat weaker layer was evident between 14 and 30 m. On all deployments, the layers that we observed were highly variable in density and position in the water column.

 

While we were moored in Arthur Harbor off of Palmer Station on September 12, we took advantage of our stationary position and the calm sea conditions to complete the calibration of the 200 kHz transducers. On this second attempt, we were able to position the calibration ball appropriately under the transducers and make a number of measurements of the ball's target strength. These measurements were very close to what was expected, suggesting that the 200 kHz transducers are performing properly. Further analysis of all of the calibration data will be invaluable in interpreting the acoustic observations we have made over the course of the cruise.

 

Current Position and Conditions

The trek back to Punta Arenas, Chile has begun in earnest. We are currently steaming down the Gerlache Strait while conducting some of the final work of the cruise - a CTD transect in a region where waters from the north of Weddell Sea origin meet and mix with waters from the south of Antarctic circumpolar current origin. Our current position at 2043 on 13 September is -64 28.096′S; -62 14.093′W. It is cloudy and it is snowing. The air temperature again is -2.2C and the barometric pressure is 1012.4 mb. Winds are 10-12 kts out of the east-northeast (066). The strait has numerous small icebergs, growlers, and bergy bits, but little to no pack ice.

 

Cheers, Peter