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

26 August 2002

The Palmer arrived at station 40 about 0900 on 26 August. This station, situated in the middle of the entrance to Marguerite Bay on survey line 6, was over the Marguerite Trough. The Trough is a deep channel or canyon that cuts across the continental shelf from north to south and runs into Marguerite Bay. The inner portion of the Bay can be defined by a line that extends from Adelaide Island in the north to Alexander Island in the south. What is interesting about the trough is that its maximum depth (around 1500 m) is just to the north of Alexander Island in the vicinity of the inner Bays entrance and it has much shallower depths on the outer continental shelf. Lower Circumpolar Deep Water from offshore makes its way into the deep inshore reaches of the Marguerite Trough in spite of the shallower trough depths at the continental shelf edge and understanding how it does this is a part of the GLOBEC study.

Coming into station 40, the lead system was increasingly blocked by pack ice. Just prior to reaching the station position in a small open water area, a particularly resistant set of ridges was encountered, forcing a great deal of backing and ramming. During one of the backings, BIOMAPER-II’s tow cable somehow became caught on a portion of the stern door closing off the fantail under the A-frame. The A-frame was moved inboard and a crowbar was used to help get the cable free. It only took a minute or two to free the cable and apparently there was no damage. But there was more ice ahead so the towed body was retrieved.

At station 40, a pair of CTD casts was made in the late morning and then a SCUBA under-ice dive took place between 1130 and 1330. Although other work was scheduled for this area, it was postponed until later so that seabird and marine mammal surveying could be done in the afternoon light while steaming towards station 28, located on survey line 5 off the southwest corner of Adelaide Island. BIOMAPER-II was deployed for the transit toward station 28.

Progress in leaving station 40 and heading north for station 28, however, came to a standstill after a couple of hours. We got into floes and ridges that were very tightly compressed by northwesterly winds that had come up early in the morning and persisted during the day. During one of the initial encounters with a tough ice ridge, BIOMAPER-II’s tow cable again suffered some damage in the process of breaking through and the towed body was brought on deck after repairs to the cable were made. The late afternoon and evening were spent attempting to break through one ridge system after another with very little success no matter what direction we tried to go in.

The wind shift to the northwest took place in the late evening of 25/26 August and by midnight winds of around 10 kts were from 315º. In the afternoon of the 26th, the speed increased to around 15 kts with gusts over 20 kts, enough to significantly change the dynamics of the pack ice and making it much harder to move through. The barometer rose from 998 mb in the wee hours to 1002.5 in the late morning and then declined back down to 999 mb by 2000 hrs. Skies were cloudy throughout the day and snow began falling in the late morning about the time the divers in the Torres group set off for their dive in the Zodiac. The cloud ceiling dropped down in the afternoon and the visibility was on the edge of OK for the survey work. The day ended with the winds increasing out of the northwest to 22 to 28 kts. The snow was being driven horizontally across the frozen pack ice.

The 26th of August was also a day in which discussions began between the science groups on the Gould and the Palmer as to when the move to the third process site would take place along with the beginning of the survey of the northern sector of the Southern Ocean GLOBEC grid. Between the time the Gould established its second process site near station 43 on 20 August and the morning of 26 August, the ship had drifted a net distance of 58.5 nm miles in 138 hrs. Their average speed over the period was 0.4 kts. This was very similar to the net drift speed of the floe that was the location for process site 1. The discussion centered on how soon the Palmer could finish up work in the central sector and join the Gould. A tentative date for a rendezvous and the start of a third convoy was 30 August. There was, however, a possibility that the Gould might be able to move within their area of interest unassisted and this might change the timing of these events. This was going to be tested on 29 August when their work at the site would be completed.

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

In the morning of 26 August we arrived at station 40 which is in the inner portion of survey transect 6. The nominal bottom depth at station 40 is about 863 m, which places the station in the southern portion of Marguerite Trough just before it extends into Marguerite Bay. This sampling location is likely the closest to Marguerite Bay that will be reached on this cruise. As such, it provides an important data point for the hydrographic distributions.

Two CTD casts were completed at station 40. The first was to obtain a microstructure profile with the CMiPS sensor and extended to 200 m. The second was a full CTD cast to within a few meters of the bottom.

As observed at other sampling locations, the surface waters at station 40 were near freezing at -1.81ºC and had a salinity of 33.92. The surface salinity was somewhat higher than that observed at stations 48 and 49, which are south and west of station 40. Thus, this station may be just at the edge of the coastal current that flows out of Marguerite Bay. The temperature and salinity characteristics extended to about 100 m.

The maximum temperature of 1.41ºC was observed at 400 m with a corresponding salinity of 34.70. This thermohaline structure is associated with modified Upper Circumpolar Deep Water. This water mass forms from mixing of Upper Circumpolar Deep Water with the overlying Antarctic Surface Water and Winter Water layer.

Below the temperature maximum, temperature decreased monotonically to 1.31ºC at 830 m. Salinity increased slightly to 34.72. These thermohaline properties are characteristic of Lower Circumpolar Deep Water. The source of this water mass is the deeper waters (500 m to 700 m) off the continental shelf. This water mass flows onto the west Antarctic Peninsula continental shelf via Marguerite Trough, which provides a deep conduit between the outer continental shelf and the inner shelf and Marguerite Bay proper. Thus this system represents one in which the environmental structure of the inner shelf waters may be controlled by forcing that allows deep oceanic water to move onto the continental shelf. How this water mass gets onto the continental shelf and its role in structuring the thermohaline and potentially biological distributions on the west Antarctic Peninsula continental shelf and Marguerite Bay remain to be determined.

Phytoplankton Ecology/Primary Production (Kari Sines and Frank Stewart)

During the second week of sampling (11 August 17 August), the primary production group completed six more simulated in situ (SIS) experiments, at stations 76, 80, 82, 74, 72, and 65. The fast repetition rate fluorometer (FRRF) was deployed at another nine stations. A separate cast to 100 m allowed collection of data at deeper water stations 72 and 74 as well as those under 500m.

Chlorophyll samples continued to be taken at all stations, and samples for particulate carbon were preserved at seven more stations. Chlorophyll measurements show mixed layer levels ranging between 0.013 and 0.034 μg/l.

During the third week of sampling (18 August 24 August), the primary production group completed three more simulated in situ (SIS) experiments at stations 42, 44, and 64. Due to temperatures of -15ºC and below only the experiment at station 64 ran to completion. Preliminary results through station 64 show production levels <0.2 mgC/m³/d. The highest production for the cruise seen to date was at station 44. At two of the three SIS stations the FRRF was deployed, with a total of four casts for the week.

Chlorophyll samples were taken at all stations where the CTD was deployed, and samples for particulate carbon were preserved at five more stations. Integrated 100 m chlorophyll a levels for the week range from 2.7 μg/m², to 4.89 μg/m².

The Phytoplankton Group would like to thank the RPSC and ECO crews for the assistance with the deck incubators during this past, very trying, week.

Seabirds (Chris Ribic and Erik Chapman)

On August 26, the seabird survey was conducted for over 4 hours as the ship approached station 40 in the morning and then left for station 28 in the afternoon. Station 40 is near the mouth of Marguerite Bay and will be the final station on this line before the ship moves north to complete the study grid lines west of Adelaide Island. The going was slow as the ship pounded against thick ice packed into the mouth of Marguerite Bay by steady winds out of the northwest. Snowfall limited visibility, but remained around 600 m and we were able to observe within the 300 m transect off the port side of the ship.

Two more Adélie Penguins were observed during the survey and have been recorded during each of the last three days. A small number of Snow Petrels were observed over leads in the survey as well. Crabeater Seals were recorded in relatively high numbers in the afternoon as the ship moved toward station 28 from station 40.

A summary of the birds and marine mammals observed on 26 August (YD 238) during 4 hours, 23 minutes of survey time as the ship traveled near station 40 is the following:

Species (common name)	Species (scientific name)	Number observed
Snow Petrel	Pagodroma nivea	5
Adélie Penguin	Pygoscelis adelii	2
Crabeater Seal	Lobodon carcinophagus	25

Marine Mammal report (Chico Viddi)

Marine mammal surveying during 25 and 26 August resulted in only 3.8 hours of effective effort out of 15.8 hours of observation. Most of the effort was done in Sunday the 25th, a day that was characterized by some fog patches and an overcast to partly cloudy sky. There were floes of thick first year ice, and shuga and gray young ice varying from 8 to 10/10ths coverage. Variable sized leads of open water or ones freshly ice covered were present. In general, the day presented good viewing and weather conditions, but the survey was only performed during the morning; the afternoon was spent at station 49. Incidental observations were made at that time. Only two Crabeater seals were counted during the morning (0906) and one or two seals were seen while at station during CTD cast. Almost no effective effort was made on Monday the 26th, since the morning was spent at station 40 and once on the way to station 28, the day was characterized by a dense snow/fog which lasted all afternoon. Nevertheless, almost eight hours of observation were made. Twenty-three crabeater seals were observed and three minke whales were seen while at station and during the CTD cast (at 1020, -68º 02.00′S; -70º 23.76′W). As we have seen with some seals, one of the whales was observed to approach the vessel to within 20 m of the stern, as if it were attracted to the CTD.

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

In the central sector of the survey grid, water sampling occurred first along the shelf break and at the offshore stations before covering the expansive shelf west of Marguerite Bay in the period 20 to 26 August. Little has changed in going from the southern sector to the central sectors in terms of composition and motility of the microplankton, except that the highest concentrations of particles tended to occur at the top and bottom boundaries of the water column. Shelf stations 43, 48, 49, and 40 all displayed a moderate concentration of particles at the surface (9-14/ml), with much reduced numbers at depth. Only a few flagellates and ciliates were observed there. Shelf-break stations 44, 46, and 62 showed very few particles, except for higher concentrations at the bottom of station 46 (11/ml). A sub-surface ice sample taken by the Daly group at station 62 offered a moderate number of non-motile particles. Diatoms were observed in the surface sample from the CTD here. Off-shelf Station 45 seemed to show the greatest number of microplankton with ciliates and flagellates present from the surface to 290 m. Stations 64 and 63 offered some activity at shallower depths, but only low to moderate numbers of particles (6/ml) below 200 m in Upper and Lower Circumpolar Deep Water.

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

MOCNESS-01 Tow 8 occurred on the outer shelf shallow water (370 m) between stations 62 and 48 on the evening of August 24th. While towing conditions were favorable in a nice lead, technological problems marred the results. The OPC lost communication several times, temperature and salinity sensors were initially frozen, and the flow-meter failed to function for 6 nets. One mystery from the last tow was solved however: all nets had remnants of the previous tow, explaining the very low catch in Net 3 from Tow 7 (washing the nets after tow 7, could only be done with buckets of water because water in the hose normally used was frozen). Still, a diverse catch was acquired, with small amphipods abundant in shallow water and the presence of furcilia and pteropods distinguishing this tow.

The deepest layer sampled, 275-250 m, yielded large Paraeuchaeta, gelatinous pieces, and some Euphausia tricantha. The next two depth intervals (275-150 m) were very different with small copepods most abundant and Thysanoessa, chaetognaths, ostracods, pteropods, worms, and radiolarians present. Above 150 m, Euphausia superba and Metridia copepods were dominant. Between 100 and 75 m, large Paraeuchaeta, radiolarians, and ostracods predominated. The upper 75 meters was characterized by small amphipods and more large copepods including Paraeuchaeta, Metridia, and Calanus propinqus. Salps, krill furcilia, and large chaetognaths distinguished the top 25 meters. One fish larva was found between 75-50 m and several different types of pteropods were found throughout the water column. Biomass overall was relatively low and the volume filtered will have to be estimated using distance traveled for most nets.

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

At 0730 on August 26, we completed the towyo started at 2200 the night before between stations 49 and 40. On this occasion, the shallow scattering layer was centered at more or less a constant 50 m depth, and varied from 35 to 75 m in height. The layer was partitioned into regions of higher and lower scattering intensity: at times it was barely distinguishable from background noise, while at other times it showed intensifications into structures that looked very patch-like, approaching backscatter values as high as -50 dB. Obviously here we run into difficulties with the terms layer and patch. The former implies some sort of contiguity over some depth range and reasonably long spatial scales, while the latter refers more to structures that are fairly distinct from background scattering levels and limited in spatial extent. How one distinguishes between the two, particularly when they appear co-exist, is not a straightforward question.

The VPR observed copepods, radiolarians, ostracods, and larvaceans in this shallow scattering region. Most excitingly, we also repeatedly captured images of large adult krill at between 20 and 100 m depth, generally associated with particularly intense regions of backscatter within the overall layer. Many of these krill were observed not to be of the genus Thysanoessa and were most likely one of the five common species of Euphausia. Within and above the regions where krill were observed, many copepods also appeared to be present. At 0600, the ship was traveling through an open lead without significant noise due to ice impaction. The VPR observed a dense layer of krill between 65 and 20 m with another dense layer of copepods above 20 m. Below the krill layer was a third layer extending between 107 and 65 m and consisting of ctenophores, copepods, and notably, Beroe, a large tentacle-less lobate ctenophore. Of these layers, only the krill layer was also observed acoustically, presumably because at low densities, copepods and ctenophores are not expected to scatter much sound at our operating frequencies of 120 and 200 kHz.

Whenever the bottom was within range of the echosounder during the towyo, a fairly dense 50 to 175 m thick layer was present next to the bottom. No VPR observations were made within the layer, but between150 and 200 m, we did capture images of copepods, medusae, and radiolarians.

We began a second towyo in mid-afternoon, as we moved from station 40 towards 28. This towyo lasted only two and a half hours before Ricky the winch operator noticed that just like yesterday, more strands had frayed on the tow cable. The point on the cable where the strands had broken was again repaired, but we will now not be able to pay out more than 150 m of wire, limiting our maximum towyo depth to between 100 and 150 m. One of the VPR cameras, cam4, briefly lost signal suggesting that one of the fibers in the cable was being stretched or bent too sharply. We will need to keep an eye on this to see if a switch over to another fiber will be necessary.

Prior to this event, we observed a number of small (10 to 20 m in height, 60 to 80 m in length) and dense patches of backscatter located at 225 m depth. At one point a diffuse and very long patch (or perhaps layer) was present at 200 m. VPR observations near this depth indicated the presence of copepods. Below 350 m, scattering was substantially enhanced; although the bottom was out of range of our transducers, it is likely that this layer was bottom-associated. While the cable was being repaired, the BIOMAPER II was hanging at 150 m depth. At two points during this period, dense patches passed right under the transducers and the VPR observed numerous large krill.

Current Position and Conditions

Leaving station 40 proved quite difficult given the condition of the pack ice and the change in wind direction to northwest on 26 August. The ordeal in leaving the station has passed and we are now moving effectively toward survey line 5, although the goal of reaching station 28 is no longer being considered. We are now heading for the more offshore stations instead. Our current position at 0021 on 28 August is -67º 54.585′S; -71º 05.081′W. The air temperature is -5.0ºC and the barometric pressure is 994.6 mb. Winds are 12-16 kts out of the west (263). It is cloudy, but visibility is good.

Cheers, Peter