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

12 August 2002

 

The Southern Ocean GLOBEC grid on this cruise is composed of 85 station locations distributed along thirteen lines perpendicular to the coast. Line 1 lies at the northern end of Adelaide Island and line 13 lies offshore of Charcot Island some 500 kilometers southwest of line 1. The center of the grid includes Marguerite Bay. Because pack ice usually covers the entire grid area during the winter period, a ship like the RVIB N.B. Palmer with ice breaking capability is needed to get to the stations. In fact, some of the pack ice is so thick and formidable, that even the Palmer cannot get to some of the stations during the winter cruise. Our companion ship, the R/V L.M. Gould, has an ice strengthened hull, but lacks the kind of ice breaking capability needed to move through most of the pack ice in the area. The Gould can, however, move through most of the ice pack if following the Palmer. Thus, the strategy for this cruise has been to divide the grid into four sectors: southern, central offshore, Marguerite Bay, and northern. The plan is to convoy with the Gould to each of the sectors, find a central location that meets the Gould's scientific need for time series studies of ice structure and dynamics and marine predator work, and then to survey the sector with the Palmer. We are currently in the southern sector of the grid, which includes transect lines 9 to 13 and the Gould was positioned on transect line 11 a couple of days ago.

 

This southern sector of the grid has the oldest and thickest pack ice in the survey area and is proving very difficult to move around in. On 12 August, we steamed to station 76, the first grid station to be sampled on the grid after leaving the Gould late in the evening of 11 August. After work with the 10-m MOCNESS and the Tucker trawl was completed while en route, BIOMAPER-II was deployed about 0100. Only about 40 minutes later, it had to be retrieved when the ridging in the ice pack proved too tough for continuous forward movement and Palmer had to resort to backing and ramming frequently. Two back-to-back CTD casts were made at Station 76 and then the steaming began for Station 77 near the center of the continental shelf. Around 1000, we came into an area of open pools of water and fairly large and smooth floes that were about meter thick. The radar seemed to show more of the same ahead so we decided to deploy BIOMAPER-II. About the time we were ready to do the launch, a rubble field with substantial ridging was encountered that quickly brought the ship to a stop and also any idea of towyoing BIOMAPER-II. We never made it to the intended location of Station 77 because the ice pack was too tough to get through. Time constraints forced us to stop short and the station work was done in a large lead. Work completed included an under-ice dive, two CTD casts, a ROV under-ice survey, ice collection, and a 1-m MOCNESS tow. Because of the ice pack conditions, a decision was made to drop stations 78 and 79, stations located closer to shore. Late in the evening, we set off for Station 80 to the southwest instead.

 

The temperature moderated noticeably by mid-day on 12 August rising from around -20C about 0100 to -2.2C at 1300 as the barometric pressure dipped slightly from 995 to 987 mb. As the barometric pressure rose in the evening back up to 996 mb, the temperature dropped to -9.6C. Winds were a moderate 12 to 15 kts out of the WNW (303) in the early morning, shifted directions to west later in the morning, and picked up to 21 to 27 kts by early evening. It snowed over-night leaving about inch on the deck and in the morning there was a high overcast cloud layer and not much light from the sun. The contrast was very low making it difficult for the bridge to pick a course through the rubble and ridging in the ice pack to get to good floes. Periodically, a fog lowered visibility to only a few hundred meters.

 

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

Throughout 12 August, we continued to move inshore along survey transect 11. CTD casts were done at survey stations 76 and 77 during this time. These casts also included the FRRF and CMiPS. The CMiPS, which measures microstructure, seemed to work well at both stations.

 

The vertical profiles of temperature and salinity were similar at both stations. Bottom temperature and salinity were about 1.3C and 34.6, respectively, which indicates the presence of modified Circumpolar Deep Water. The flow description that is emerging is that bottom waters in this portion of the survey grid are 0.1C to 0.2C warmer than those observed in April-May on NBP02-02. Following completion of transect 11 and transect 12 over the next couple of days we should be able to determine the extent of the difference in bottom water properties between the fall and winter SO GLOBEC cruises.

 

Surface waters at stations 76 and 77 were at freezing and a well mixed layer extended to 100 m and 80 m at the two stations, respectively. The observed vertical structure is consistent with that seen at other sampling locations in this portion of the survey grid. This indicates that extensive cooling has occurred in this region.

 

The dissolved oxygen concentrations in the surface waters at the stations occupied to date range between 6.0 ml/L to 6.5 ml/L. Water of -1.8C and colder can hold dissolved oxygen of concentrations of about 8.5 ml/L. Thus, the values observed so far during NBP02-04 are 2.0-2.5 ml/L less than the saturation value. This may result from the combined effects of: 1) low primary production, 2) organic matter consumption in the upper water column, 3) input of low oxygen water from depth (modified Circumpolar Deep Water is low in oxygen), and 4) decreased exposure to the atmosphere because of sea ice cover. The regulation of dissolved oxygen concentration in west Antarctic Peninsula shelf waters in austral winter is a topic for further study.

 

ADCP/OPC/MOCNESS Studies (Ryan Dorland)

Several patches of high echo intensity (-60 db VTS) generally representing high abundances of krill and other zooplankton have been observed from ADCP data during the past several days. The average extent of these observed patches is 0.5 nautical miles along the cruise track and 50 m vertically, generally ranging within the upper 100 m.

 

On transit to station 75, approximately 6.0 nm of a nearly continuous patch were detected within the upper 150 m on 11 August starting at 0800 from -68 41.0′S; -76 19.0′W. This is the largest patch observed by acoustics on this cruise to date.

 

Results from Optical Plankton Counter (OPC) data processed from the first 1-m2 MOCNESS tow (MOC-01-001 on 7 August) showed the highest counts were recorded below 250 m, with around 1200 counts/m3 for net 1 (300-250 m). OPC counts/m3 for the corresponding surface nets ranged between 200 and 800 using volume filtered data from the MOCNESS. The ADCP data indicated two low-intensity layers (-80 db VTS) between 350 and 275 m and 100 and 75 m during the tow. The net system missed a high intensity patch (-60 db) situated between depths of 250 and 100 m by a horizontal distance of less than 500 m.

 

ADCP current data processed from Station 75 (-68 41.0′S; -76 04.0′W) showed a general NE flow of about 10-15 cm/s persistent with depth. The station area had relatively flat bottom topography, with water depths of around 400 m.

 

Sea Birds (Chris Ribic and Erik Chapman)

Surveys were conducted for nearly four hours on 12 August as the ship traveled between stations 76 and 77. The ship backed and rammed and slowly made its way through the 9 to 10/10ths coverage of 1 meter thick ice. Just two Snow Petrels were observed during the survey period and for the second day, no penguins were observed. It appears that even in areas with some open water, there are few birds deep within the pack ice and it will be interesting to see whether we begin seeing more birds as we approach the ice edge in the next few days.

 

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

Our group is investigating krill molting, growth, and feeding rates, as well as predation behavior. In the past week we completed 2 dive, 3 net, and 4 under-sea ice sample collections and several krill physiology experiments. The results of dive and net collections were briefly described in previous reports. This report gives a few more details on the results after preliminary analyses. In Crystal Sound at the northern end of the study area, our first Tucker Trawl sampled a near-surface krill aggregation, which extended from the surface to about 75 m in depth based on acoustic backscattering echograms. This aggregation was comprised of juveniles and immature females and males ranging from 17-50 mm in length. Physiology experiments indicate that these krill molt about every 30 days, but individuals were not increasing in length (i.e., growing). Feeding rates were very low on the sparse particulate matter from the water column. However, adult krill ingested about one copepod per day based on predation experiments. No larval krill were collected in net tows or observed by divers in Crystal Sound.

 

At the southern end of the study area (station 75), our Tucker Trawl collected krill from another aggregation, which extended from about 20 to 75 m in depth. This aggregation also was composed of juvenile (17-24 mm) and immature female and male (36-51 mm) krill. We are currently measuring growth and molting rates on this group. Feeding rates appear to remain very low. During a dive yesterday at station 77, low densities of larval krill were observed feeding along the under-surface of the sea ice. Collected individuals were Furcilia stages IV-VI and 7.5-10.5 mm in length. Several ctenophores, which are a potential predator of larval krill, also were observed under the ice. A Tucker Trawl this morning (13 August) at station 80, collected a number of small ctenophores and a few copepods, including Paraeuchaeta sp. and Calanus propinquus, in the upper 100 m of the water column. This low abundance was not surprising as there was no evidence of acoustically detected aggregations at this station.

 

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

The second 1-m2 MOCNESS tow for this cruise occurred on 12 August in the southern sector of the survey grid at Station 77. We were fortunate to squeeze a complete tow within a good lead amidst heavy ice. Thanks to Todd Johnson's efforts, the heater installed in the pressure housing kept the depth sensor functioning, though we temporarily lost the salinity sensor at the beginning of the tow and eventually lost communication with the underwater unit at the very end of the tow.

 

Biomass collected below 200 m was much greater than all other depths. Adult and furcilia of Krill, large copepods, and chaetognaths comprised the 350-200 m sample and were likely the targets exhibited in the layer at ~225 m displayed on the Simrad Echosounder. Furcilia (Thyasanoessa), copepods, and chaetognaths were the primary organisms present from 200 to the surface, with ostracods present at 150-100 m and small amphipods common from 100m to the surface. Bioluminescent Metridia sp. copepods were observed in net samples from 75 m to the surface. The presence of only one fish larva at 75-50 m was surprising given the novel ROV observation earlier of several fish larvae below the ice surface. Biomass from all nets above 200 m was very low.

 

Current Position and Conditions

A dramatic change in the weather has occurred as we steam toward station 82 located on the continental shelf break on survey line 12. The winds have picked up significantly and are now blowing at 25 to 35 kts with one gust that registered 50 kts a bit earlier. The temperature, which at 2030 was -2C, is now -10.6C and falling dramatically as a front moves through. The barometeric pressure is now rising and the skies have cleared. Our present position at 0023 on 14 August is -68 48.805′S; -76 37.612′W. We are still in 10/10 pack ice and often need to back and ram to move forward.

 

Cheers, Peter