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

7 September 2002

 

After a wonderful string of very sunny days, the fact that 7 September was a gray day was not surprising. There was a heavy fog around the ship all day and there was never a hint of sunshine. One reason for the fog was the occurrence of a huge expanse of open water that was encountered as the Palmer steamed into the region just to the southwest of the southern end of Adelaide Island. This made it extremely easy to towyo BIOMAPER-II between stations 20 and 19, although the transits between stations 21 and 20 became progressively easier because of the increasing size of leads as the Palmer headed towards Adelaide Island. Along the transit routes, seabird and marine mammals observations were made.

 

The work at station 21, which was started in the evening of 6/7 September, continued into the earlier hours of the 7th with the completion by 0630 of an ROV under-ice survey, a pair of CTD casts, and a 1-m MOCNESS tow. The steam to station 20 took about 3.5 hours. BIOMAPER-II was left in the water at this station to conduct time-series acoustical measurements while a pair of CTD casts was done. This was done to compare the spatial variability of acoustic features associated with the water column with those observed with the CMiPS turbulence sensors on the CTD.

 

The Palmer arrived at station 19 around 1800 after a 6 hour run. This station was located in the vicinity of Johnston Passage, a channel running north to south off the southwest corner of Adelaide Island. This area was charted by the British in 1963 and was named after the master of one of the vessels involved in the survey. After the usual shallow and deep CTD casts, a pair of 1-m ring nets was deployed in the upper 10 m off the stern. A 10-m MOCNESS tow to 500 m was done to the north in the Passage. This tow ended at midnight. A 1-m MOCNESS tow was done soon after on the reverse course, also to 500 m, and then BIOMAPER-II was deployed. Its transit took the towed body over the region that had just been sampled with the nets. This overlap will allow the animal types and their distributions in the water column to be compared with the acoustical properties.

 

In addition to the pack ice being easier to move through and a good portion of the day being spent in open water, the near absence of wind and temperatures above -10C for much of the day also contributed to good working conditions. The temperature rose from -14C in the early morning to a peak of -4C in mid-afternoon and then declined to-9C in the evening. Wind speeds were generally <10 kts until night fall, then rose to 15 kts in the early evening. They stepped up to around 27 kts near midnight. Winds shifted from southwest to north-northwest after sunrise and then backed around to the southwest by evening. Barometric pressure began a slow decline from 1015 mb to 1012 mb during the course of the day.

 

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

Throughout 7 September we worked our way along the inner part of survey transect 4. In the early morning, two CTD casts were made at station 21 for CMiPS sampling and to sample the water column to the bottom. The second cast also included FRRF sampling, which requires slower lowering speeds in the upper 100 m.

 

The next station on survey transect 4 was number 20. However, this station was moved a short distance from the original position so that it would be possible to remain in a large lead that the ship had been following since leaving station 21. The displacement from the original site located the station, renamed station 20A, in the center of Marguerite Trough, which is a deep bathymetric feature that runs from the shelf break across the shelf and into the southwestern part of Marguerite Bay. Four CTD casts were made at this location; three for microstructure sampling and one to sample the full water column. The first two CMiPS casts were from the surface to 350 m. The third CMiPS cast was from 50 m to 150 m. This station provided the fourth time series designed to capture microstructure variability.

 

The inner-most station on survey transect 4, station 19, was occupied in early evening. Bottom depth at this station is about 315 m, which is shallow enough to allow FRRF sampling. Therefore, two casts were made. The first was for microstructure sampling with CMiPS and the second was for sampling the full water column. The lowering speeds in the upper 100 m were slowed to accommodate FRRF sampling.

 

The most striking surface features observed during the work at stations 21, 20A, and 19 were the lack of sea ice and the large areas of open water. Thus these stations were located in the coastal polynya that is known to form along the western coast of Adelaide Island. The surface temperatures at the three stations sampled today were above freezing or just at freezing, being -1.77C, -1.81C, and -1.76C, respectively. The well-mixed Winter Water layer extended to almost 100 m at station 20A in Marguerite Trough. At the two other stations this layer was eroded and extended to only about 60 m to 70 m.

 

The transition between the Winter Water layer and the Circumpolar Deep Water at depth was very sharp at station 20A, occurring over about 20 m. The sub-pycnocline waters at station 20A was composed of Upper and Lower Circumpolar Deep Water, with a temperature maximum of 1.55 at 263 m. Salinity reached a maximum of 34.72 at 560 m and remained at this value until the bottom. Considerable fine structure was observed in the temperature and salinity vertical profiles between 100 m and 250 m, which suggests that this is an area of active mixing. This was the motivation for the three CMiPS casts at this site.

 

The sub-pycnocline waters at station 21 were composed of Upper Circumpolar Deep Water that had been modified by mixing with the overlying Antarctic Surface Water and Winter Water. The maximum temperature observed was 1.35C at the bottom with a corresponding salinity of 34.70. At the equivalent depth, the waters below 150 m at station 19 were somewhat warmer. The maximum temperature at station 19 was 1.39C at 384 m.

 

The above freezing surface temperatures at stations 21 and 19 likely resulted from considerable mixing with the deeper warm waters. The surface temperature at station 20A was just at freezing and the surface salinity at stations 21 and 19 was higher that would be expected for the observed temperature. The higher salinity may result from cycles of freezing, with corresponding brine rejection, and thawing as the coastal polynya opens and closes. The higher surface salinity may be a factor that allows the polynya to remain ice free once it does open, i.e., more cooling is needed to reach the freezing point. The role of sensible heat and latent heat processes in the formation of this coastal polynya is a subject for further research.

 

The presence of Upper and Lower Circumpolar Deep Water in Marguerite Trough clearly indicated that this bathymetric feature provide a conduit for bringing oceanic water onto this portion of the west Antarctic Peninsula continental shelf.

 

Seabirds (Chris Ribic and Erik Chapman)

The seabird and Crabeater seal survey was conducted for 8 hours and 8 minutes as the ship moved between stations 21, 20 and finished the day approximately 9 miles from the Southwest shore of Adelaide Island at station 19. The ship followed a large lead approximately 1 km wide through most of the day and ended up in a massive open water area southwest of Adelaide Island approaching station 19. A dense fog limited visibility throughout the survey to between 300 and 600 m from the ship. Our survey transect extends 300 m from the ship and we were able to collect data within this strip throughout most of the day. Outside of the lead, the ice was in vast floe, and within the lead and in the large open-water area, new gray ice covered between 2 and 8/10ths of the ocean's surface.

 

Few birds were observed in the lead before reaching the open-water near station 19. A small number of Snow Petrels were observed in the lead, but no other birds were observed there. However, once the ship entered the large open-water area, we began to see a relatively large number of Snow Petrels along with a few Adlie Penguins and two Kelp Gulls and Antarctic Petrels. Crabeater Seals were also abundant in the lead where they were mainly in the water, presumably foraging for krill. Therefore the open water area, along the shore and inside the pack ice appears to have a strong biological signal from top-predators, mainly seals which can forage at relatively great depths. Indeed, BIOMAPER II indicated krill swarms at depths greater than 100 m in this area, well within the diving range of Crabeater Seals.

 

A summary of the birds and marine mammals observed on 7 September (YD 250) during 2 hours 46 minutes of survey time as the ship moved between stations 21 and 20 and 5 hour 22 minutes as the ship traveled between 20 and 19 is the following:

Species (common name)

Species (scientific name)

Number observed

Snow Petrel

Pagodroma nivea

52

Kelp Gull

Laru dominicanus

2

Adlie Penguin

Pygoscelis adelii

10

Antarctic Petrel

Thalassoica antarctica

2

Crabeater Seal

Lobodon carcinophagus

85

 

 

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

In the last few days we have collected samples during two dives and from two Reeve/ring net deployments. The dive at station 14 yielded several Euphausia superba furcilia, 2 ctenophores, and 3 samples of sea ice biota from the under-ice surface and one from brash ice. Few krill larvae were seen in the vicinity. The divers at station 17 observed only a few juvenile krill and some furcilia swimming under a second year ice floe. However, a small aggregation was seen feeding under some brash ice between the larger floes. In addition, we collected four ctenophores and three samples of sea ice biota under the brash where the furcilia were feeding, as well as samples of surface slush for our program and for Scott Gallager's group. The Reeve/ring nets at station 14 sampled the edge of a large krill aggregation and collected about 40 L of juvenile krill. Acoustic echograms indicated that the aggregation extended from near surface to about 50 m in depth and it was about 900 m in horizontal extent. At station 21, no krill were collected, but the copepod, Calanus propinquus, continued to be very abundant. Pteropods, a few salps, and amphipods also were collected. At station19, we collected a few furcilia and juvenile krill, some Thysanoessa, pteropods, amphipods, and the usual large number of C. propinquus. The detrital aggregations again consisted of radiolarians and mats of chain diatoms, with veliger and trochophore larvae, small pteropods and many small copepods stuck together.

 

Experimental measurements included ingestion and egestion rates for furcilia and small juvenile krill at different food concentrations and copepod predation by adults. We started another experiment to measure growth and molting rates of the juvenile krill collected by the Reeve/Ring nets. A recently completed experiment to measure these rates in furcilia indicated that there is essentially no change in body length when they molt, consistent with observed rates measured last winter during GLOBEC II (NBP01-04).

 

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

On September 7 we managed to keep the BIOMAPER II in the water for 11 continuous hours, allowing us to make some very interesting observations. We first steamed along transect 4 from station 21 to station 20. At the start of this run, a very thin (1 to 3 m high) and faint line of backscatter was associated with the pycnocline at 60 m. By the time we arrived at station 20, the layer had gradually moved deeper in the water column, to 90 m. In the vicinity of these depths we observed large and small copepods, pteropods, radiolarians, and diatoms with the VPR. Only two small patches of high backscatter were observed, at 80 and 150 m. Some occasional enhancement in scattering levels was evident below 250 m.

 

At station 20, a series of casts were made with the CTD-Microstructure Profiling System (CMiPS), during which time we kept the towbody in the water. This allowed us to make acoustic observations concurrent to the collection of microstructure profiles. The thin layer observed at 90 m as we arrived at station 20 continued to be visible for much of the time that the CMiPS casts were being performed. At this depth, the first CMiPS profile revealed a dramatic (0.5 degree) vertical change in temperature. Later profiles were variable, but similarly suggested a potential association between thin layers of backscatter with regions in the water column of particular kinds of microstructure. More detailed analysis of the two datasets will help us determine more exactly whether the layers were associated with regions of high thermal gradients, or with isothermal portions of the water column right next to these gradients. Sound scattering occurs at the boundary between any two media of different densities, where the strength of scattering varies with the degree of contrast between the two densities. Organisms whose density is reasonably different from that of the surrounding seawater thus will reflect sound and be detectable by our acoustic systems. However, sharp density gradients in the seawater itself, such as at the pycnocline or within microstructure layers, can also reflect appreciable amounts of sound. The thin layers we are observing acoustically therefore may be due either to planktonic organisms, or to the physical structure of the water column. Distinguishing between these two possibilities is one of the ongoing goals of our research group.

 

Later in the day we transited from station 20 to station 19. Near the start of this leg of the towyo, a diffuse shallow scattering layer was present at 25 m. Throughout the tow, we observed occasional enhancements in scattering and transient layers between 65 and 150 m. On one instance, the BIOMAPER II passed through one of these enhanced scattering regions, and krill and copepods were seen with the VPR. Between 25 and 75 m, we captured additional images of radiolarians, copepods, and two large ctenophores. Most excitingly, on this towyo we observed the largest patches to date in the cruise. On the basis of their morphology and density, as well as the catches of 1- and 10-m2 MOCNESS tows conducted at station 19, we believe that these patches were composed of adult krill. The first such patch appeared at a depth of 300 m, at a point when the bottom was at 500 m. Further along the tow, the bottom shoaled to 300 m and a number more patches appeared in close association with the bottom. One final patch was seen at 275 m as the bottom dropped off again. These patches ranged in size between 650 and 3360 m in length, and 69 and 125 m in height. Backscatter levels reached -51 dB, and were on average -56 dB. For an average sized krill, and assuming that the patches were composed entirely of krill, this mean backscatter level corresponds to a density of approximately 30 krill/m3. We estimate the volume of the largest patch observed as roughly 0.8 km3. This patch thus may have contained as many as 23 billion krill, more than the human population of the entire globe!

 

Current Position and Conditions

Work at station 17, one of the inshore stations next to Adelaide Island, has just been completed and we are about to get underway for station 9 some 21 nm to the northeast. Our position at 0031 on 9 September is -67 00.648′S; -69 31.034′W. The air temperature is -7.2C and the barometric pressure is 1012.0 mb. Winds are around 20 kts out of the northeast (033). Skies are cloudy. The pack ice is made of large broken up floes intermixed with brash and open leads.

 

Cheers, Peter