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

17 August 2002

 

The transit between stations 72 and 65, which was started about 1800 on 16 August, was completed about 0800 on 17 August, in the vicinity of the L.M. Gould. The Palmer averaged 2.8 kts to travel those 39 nm, an indication of the difficulty in moving through the pack ice in the area. Although the Gould had been left at first time-series process station near station 76, the ship and pack ice they were studying drifted approximately 50 nm to the northeast to station 65 in the seven day period. This was fortunate for us, because it meant that we did not have to steam back south to get them for the next stage in the science program.

 

Upon arriving at station 65, we immediately began an ice collection transect on a large floe and an ROV under-ice survey. This was followed by a pair of CTD casts to 400 m. Although we reached station 65 having often to back and ram through 9/10 and 10/10 pack ice, the keel water line subsequently opened up into a lead that extended for more than 4 miles from the station. This made it very easy to conduct the 1-m and 10-m MOCNESS tows to obtain quantitative samples for analyses of zooplankton and nekton and a Tucker trawl to collect live animals for experimental work. The final activity before rendezvousing with the Gould was a BIOMAPER-II towyo along the towing path.

 

August 17 was a mild day with reasonably good working conditions. The temperature began to rise in the late evening of the 16th, and by 0700 it was -3.6C. During the day, there was a slow decline to -4.2C at 1430 and -7.6C by 2200. The barometric pressure remained around the 1000 mb mark most of the day. The winds were west-northwest in the morning at about 15 kts, turned to the southwest by early afternoon, and dropped in speed to less than 10 kts. Flurries occurred in the morning and, in the afternoon, there were periods of light wet snow or sleet with the concomitant reduction in visibility.

 

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

On 17 August, we completed two CTD casts at station 65, which is on the outer end of transect 5. The first cast was for microstructure sampling and went to only 300 m. The second cast went to within a few meters of the bottom (404 m).

 

The vertical distribution of temperature and salinity showed a well mixed layer that extended from the surface to about 100 m. Below this, temperature and salinity increased to values of 1.53C and 34.718 at the bottom. The temperature profile showed 1.8C water at about 280 m which indicates that the station was at the edge of the southern boundary of the Antarctic Circumpolar Current. The vertical property distributions, especially salinity, showed considerable fine structure that is indicative of mixing and interleaving of different water types. The data from the microstructure profiler will be very useful in further defining and understanding these features.

 

The vertical dissolved oxygen profile at station 65 showed the expected decrease with depth as Circumpolar Deep Water was encountered. An interesting feature in the dissolved oxygen distribution is that values in the upper 10 m are lower by about 0.1 ml/L to 0.2 ml/L than those just below and throughout the well mixed layer, i.e., 6.23 ml/L versus 6.31 ml/L. This surface decrease has been observed in other CTD casts.

 

Initially the lower oxygen at the surface was thought to result from the oxygen sensor not being fully equilibrated at the start of the cast. However, the oxygen sensor is soaked at the surface until it does equilibrate before a cast is started. Also, the lower dissolved oxygen values show up in the titrated oxygen samples taken from the surface Niskin bottles. We are now trying to understand how this feature develops and how it persists.

 

Sea Ice Studies (Frank Stewart and Jenny Boc)

During week two (August 10-17) of cruise NBP 02-04, members of the ice collection group sampled the sea ice biota at four stations in the southern sector of the SO GLOBEC survey grid: stations 77 (-68 54.75′S; -75 07.07′W; 12 Aug), 82 (-68 46.70′S; -76 46.84′W; 14 Aug), 72 (-68 35.24′S; -74 05.72′W; 15 Aug), and 65 (-68 05.49′S; -74 44.72′W; 17 Aug). Sea ice at stations covered 9 to 10/10th of sea surface area and was dominated primarily by consolidated first-year (FY) floes ranging in thickness from 42 cm (station 82) to 141cm (station 77). A zone of second year ice or old (3-4 months) FY ice was sampled at station 72. Satellite imagery suggests that this zone of older ice, evidenced by heavy ridging, deep snow cover (~30 cm), and increased ice thickness (~2 m), may have been advected into the area from the east or south.

 

Consolidated ice cores and interstitial water (brine) were collected at all stations and processed aboard ship for algal and bacterial biomass and for bacterial community composition. Mean chlorophyll a concentration (a proxy for algal biomass) in ice core and brine samples (n = 60 from 27 complete cores and 9 brine samples, including samples from Crystal Sound ice stations visited during week one) was 1.21 mg l-1 (range: 0.004 mg l-1 in the interior of FY ice at station 72 to 6.87 mg l-1 in FY ice at station 77). Chlorophyll concentrations in second year ice at station 72 (range: 0.004-1.18 mg l-1) were not markedly higher than concentrations in younger FY ice at other stations. Clear vertical trends in the distribution of algal biomass were not apparent in the ice cores analyzed thus far. Bacterial data are not available; however, preliminary observations of stained bacteria suggested an increase in large rod and filamentous cells relative to last year.

 

In conjunction with field sampling, members of the ice collection group, with the generous help of F. Viddi, R. Beardsley, and S. Beardsley, continued shipboard sea ice observations (according to ASPECT protocol) on an hourly basis during transit. Ice conditions in the southern sector varied from those in Crystal Sound and along the north-south transit route traversed during week one. Specifically, mean ice floe thickness increased from ~40-60 cm north of ~68S latitude (week one) to ~70-90 cm in the southern grid area (week two). Likewise, snow depths on floes and percent floe area occupied by ridges also increased; snow depths approaching one meter and ridge coverages of 80 to 90% of floe surface area were recorded while en route to station 77. Similarly, ship-based observations of algae in overturned floes indicated a general southerly increase in sea ice microbial abundance; the percentage of overturned floes (thickness > 0.2 m) with algal coloration at either the surface, interior, or bottom of the ice increased from 25% during week one to 37% during week two. Observations also suggested that algal biomass was most densely concentrated at the bottom of floes; however, analysis of sectioned core samples did not support this trend. Latitudinal increases in ice thickness, snow depth, ridge coverage, and algal abundance are evidence of a general southerly increase in sea ice age.

 

ROV report (Scott Gallager, Phil Alatalo)

Station 65 on 17 August was characterized by 10/10 ice cover, moderately high ridges on the order of 1 to 2 m, and small, compacted floes about 10 to 50 m across. The wind was low and the ambient temperature was only -8 C as the ROV was deployed off the starboard quarter at 0809. The ice team was deployed immediately before launching the ROV. The ROV descended to 20 m and began its transects at a heading of 45 off the starboard side towards the bow of the ship. Ship's gyro was 260. Upon reaching the end of the 150 m tether, the ROV turned to the right 90 and began sweeping the undersurface of the ice while following within 3 m of the ice interface. Six transects of about 70 m each were made parallel to the ship's axis in undisturbed ice. The smooth but pock-marked surface of floes gave way to deeply protruding ridges extending 4 m in depth. Although this was not the most extensive ridge field encountered on this cruise, the 1 m thick jagged edges standing vertical were impressive. Some brown ice layered with clear ice was encountered. The first transect presented little in the way of plankton, only a few ctenophores. The second, third, and fourth transects found numerous larval fish hovering about 0.5 m below the ice undersurface and not up in the nooks and crannies provided by the rafted ice. Although these larvae did not appear to be aggregated, they did occur in groups of about 3 to 10 at a time. A relatively strong current of about 1 kt made it difficult to stay with the larvae for more than a few seconds, but considerable stereo video footage was obtained for extraction of swimming vectors. No larval krill were observed on this deployment. Towards the end of the deployment, 1st Mate Mike Watson on the bridge asked to have a look at the opening to the moon pool and the covers to the new Simrad acoustic system. As ship's engineers John Pierce and Dave Munroe watched the ROV camera monitors and provided directions in what can be a complicated environment, the ROV traveled along the hull on the starboard side of the ship. Numerous devices used for cathodic protection were located along with areas of the hull apparently not painted during the recent dry dock due to positions of under-hull supports. The moon pool was jammed with ice chunks about 0.5 to 1 m in diameter that protruded below the hull's surface by about 0.75 m. The horizontal cover to the Simrad was located just forward of amidships. The cover was bright yellow and covered with flat ice chunks about 20 cm thick by 0.5 m wide. Attempts were made to push away the ice to get a better look at the cover but the buoyancy of the ice made it surprisingly difficult to move the chunks away from the cover. It was almost as if the ice was cemented in place. As the ROV was repositioned to the forward side of the cover, it began to be affected by the current generated by the ships forward thruster which was being used to keep the ship up against the ice. A quick dive ensued to get any slack tether out of harms way. The deployment ended by motoring back to the stern followed by an uneventful retrieval by the deck crew using the starboard knuckle crane.

 

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

MOCNESS #5 was a successful mid-day tow conducted on 17 August near the shelf break at Station 65. An ample lead allowed us to sample within 25 m of the bottom in an attempt to examine the SIMRAD scattering layer at 350 m. This layer appeared to be mostly copepods with Thysanoessa sp. juveniles, chaetognaths, and amphipods. Composition between 375 m and 100 m was very similar: a diverse mixture of copepods, juvenile krill (Thysanoessa sp.), chaetognaths, ostracods, siphonophores, pteropods, worms, amphipods, and radiolarians. An abundant catch of Euphausia superba with a large Beroe ctenophore marked a layer or patch at 75-100 m. No Euphausia were captured shallower than this, but another large ctenophore was. Above 75 m, very little was caught, mostly copepods and chaetognaths. A couple of fish larvae at the surface were of notable interest, given the low volume filtered at this depth. All other nets filtered large volumes, but biomass sampled was very low with the exception of the copepod layer at 325-375 m and the krill layer at 100-75 m. The OPC operated correctly and no communication problems occurred with the MOCNESS system.

 

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

In the early evening of August 17, we were able to do a short towyo with the BIOMAPER II along the tow path sampled earlier that day by the Tucker trawl and the 1-m and 10-m MOCNESS. We use acoustic backscattering as an index of zooplankton and fish abundance, and in this case, scattering was low throughout most of the water column. Close to the bottom, however, there was a weak scattering layer extending from the bottom (405 m) to 365 m depth, followed by a second layer between 365 and 315 m depth consisting of denser individual targets. Towards the end of the tow, we observed a beautiful and dense patch of krill-like high backscattering between 50 and 100 m depth that was 190 m in length. Over the course of the towyo, the VPR captured images of small copepods and radiolarians, but no krill.

 

Current Position and Conditions

The L.M. Gould and the N.B. Palmer are still together as the search for a suitable site for the Gould's process station #2 continues. Yesterday's convoy to station 41 was thwarted by heavy pack ice just east of station 42 and another site will be sought during the next 24 hours. We are currently completing work at station 42. Our position is -67 28.165′S; -71 27.35′W. The air temperature is -19.7C and with a wind speed of 35 to 42 kts out of the southwest (247), the wind chill is -50.6C. The work on deck is truly brutal.

 

Cheers, Peter