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

20 August 2002

 

August 20 was day three of the search for a L.M. Gould process station site. The circuitous route taken to find a suitable location to serve as a base finally came to an end when we reached the vicinity of station 43. Open leads and a mixture of different pack ice types were present. The convoy arrived at the location before dawn and like the day before we had to wait until daylight to survey the area and locate an appropriate site in which to base the Gould. In the intervening time, the Palmer completed the work scheduled for station 43 and then about mid-day, escorted the Gould to a place of their choosing. The work included ice collection for sea ice biota studies and an ROV under-ice survey, which became an extended run because the vehicle's tether became caught in a crevice, trapping the ROV for a while. A CTD cast to the bottom (~400 m) was also done. During the morning, the Gould also completed a CTD cast and a SCUBA dive to determine if krill furcilia were present under the ice. Moderate numbers were seen both in the ROV cameras and by the divers, who collected some of them for experimental work.

 

With a site selected, the Gould steamed into a large floe and once situated lowered their gangplank onto the floe. Before committing to the site, some testing of the pack ice was done to make sure the floe was suitable for the multi-day studies. About 1500, the Palmer was given the OK to leave and we immediately set sail for station 44 located on the edge of the Continental shelf on survey line 6. Upon leaving station 43, BIOMAPER-II was deployed and towyoed all the way to station 44 with the trackline often following a series of leads. Upon reaching the station at about 2030, a Tucker trawl to collect live animals was done. The day ended with the start at 2200 of a 10-m MOCNESS tow to 1000 m. This tow was completed in the wee hours of 21 August.

 

The weather on 20 August was not particularly pleasant. The air temperature (-15C) during most of the day was warmer than the previous day, but it was still cold! The barometric pressure held for the day around 989 mb, up substantially from yesterday's minimum of 966 mb. Winds had dropped from gale force to 16-20 kts out of the southwest in the morning and decreased additionally to < 10 kts out of the south by late evening. Interestingly, sea surface temperature was not quite at the freezing mark (-1.786C), which may be a reason there were so many open leads along the route to station 44. Skies were cloudy all day and there was a light snow in the afternoon, which reduced the visibility and made the search for the process site difficult. In the late evening, the skies cleared and the moon (nearly full) provided a bright illumination that reflected off the pack ice. Over the open water of the leads, there was a diffuse fog caused by evaporation of the seawater. It was a very nice night to be working at station 44.

 

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

The first potential site selected for the second process station was in the vicinity of survey station 43. We were able to do one CTD cast at this station on 20 August while discussions were ongoing about the suitability of this as a process site. The CTD cast was to 393 m and included the FRRF and CMiPS sensors. The vertical distribution showed the maximum temperature and salinity at 360 m of 1.37C and 34.70, respectively, which are characteristic of modified Circumpolar Deep Water. The surface layer was well mixed to about 80 m and was above the freezing point (-1.79C to -1.78C).

 

At this station, oxygen samples were collected from all Niskin bottles (total of 22) and duplicate samples were taken at some depths. These samples will be used to further develop a calibration curve for the CTD-mounted oxygen sensor. The results of this calibration exercise will be described in future reports.

 

We have now completed enough stations on the survey grid to begin construction of maps of the horizontal distribution of properties such as the maximum temperature below 200 m and the distribution of the depth of the oxygen minimum. Both properties are tracers for Circumpolar Deep Water. A preliminary map of the temperature maximum, constructed from the stations occupied during the survey of the southern-most part of the grid, shows that the southern boundary of the Antarctic Circumpolar Current is located along the edge of the outer continental shelf. The survey of the central part of the grid, which we are now beginning, will allow us to determine the extent to which the southern boundary of the Antarctic Circumpolar Current is impacting this portion of the west Antarctic Peninsula continental shelf.

 

Sea Birds (Chris Ribic and Erik Chapman)

Surveys were conducted for almost 3 hours on 20 August as the ship moved between station 43 and 44. This transit brought the ship further offshore, near the shelf-break adjacent to the mouth of Marguerite Bay. Snow limited visibility, which fluctuated between 300 m and 800 m from the ship. The ship traveled through recently frozen leads with some small patches and leads of open water. Five Snow Petrels were the only birds observed in 9/10ths ice coverage with vast floes as the primary ice type. Crabeater Seals were also seen in relatively small numbers today, with just 4 individuals observed.

 

A summary of the birds and marine mammals observed on 20 August (YD 232) during 2 hours, 58 minutes of survey time as the ship traveled between stations 43 and 44 is the following:

 

Species (common name)

Species (scientific name)

Number observed

Snow Petrel

Pagedroma nivea

5

Crabeater Seal

Lobodon carcinophagus

4

 

 

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

During the past week we completed 4 Tucker Trawls, 1 dive, and several physiology experiments. The Tucker Trawl at station 74 (a shelf-break station in the southern part of the study area) collected numerous juvenile and adult krill where a large layer 50-100 m in depth was detected by acoustic systems. The Tucker Trawl at Station 72 (mid-shelf) collected gravid Thysanoessa macrura (a euphausiid) bearing spermatophores, indicating that this species may spawn in the near future. We also collected Euphausia triacantha, another euphausiid species normally found below 200 m off the shelf. The occurrence of this species was correlated with the presence of Circumpolar Deep Water from the Antarctic Circumpolar Current, which had come onto the shelf in this area (Personal Communication, E. Hofmann). Other organisms collected included a few E. superba furcilia, larvaceans, pteropods, chaetognaths, small medusae, several small amphipods, and numerous copepods, such as Calanoides acutus, Calanus propinquus, and Paraeuchaeta spp. The Tucker Trawl at station 65 (outer shelf) collected numerous ctenophores, some copepods and amphipods and a few juvenile E. superba in the vicinity of a very low density layer (about 90 dB) centered about 100 m in depth. The divers did not observe any krill or any other organisms under the sea ice in this region. We collected 3 samples from the under-surface of sea ice for chlorophyll and particulate organic carbon and nitrogen analyses, as well as for slide enumeration of autotrophs and heterotrophs. Another Tucker Trawl on 19 August in a lead near Station 42 (mid-shelf) collected numerous copepods, primarily C. propinquus and Metridia gerlachei, several individuals of Thysanoessa and pteropods, a salp, and a ctenophore. We also completed two more experiments assessing the growth and molting rates of juvenile and adult E. superba, 17-51 mm in length. Molting rates ranged between 28 and 34 days, which was similar to molting rates of krill collected further to the north in Crystal Sound.

 

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

Observations of microplankton from the first portion of our survey of the southern sector grid stations completed by 13 August, mimic for the most part samples taken in the Crystal Sound area. There were few particles and little motility. Stations 75 Gould, 76, 77, and 80 were nearly identical below 200 m. Fine particles, moderately abundant were present at the surface of mid-shelf station 76, whereas particle abundance increased only at 100 m further inshore at Station 77. Small diatom chains appeared in this station's pycnocline (100m). Between Stations 77 and 80, low numbers of small flagellates were motile in the surface sample. The CTD fluorometer continues to show low chlorophyll values throughout the water column.

 

Automatic particle tracking for this series of stations shows overall smaller mean diameter particles with lower mean speed and lower mean concentrations than samples run from the previous cruise NBP0202, in early winter. Also, net to gross displacement ratios for this cruise average ~0.9. This ratio calculated for plankton tracks measures the degree of circuitous motion present (non-motile particles travel in straight lines vs twists, turns, and jumps exhibited by swimming microzooplankton). For example a sinking diatom might displace 5 cm while sinking 5 cm (NGDR = 5/5=1) whereas a ciliate swimming a spiral might displace 12 cm, yet only end up 4 cm from its starting point (NGDR = 4/12= .33) A value near one confirms the presence of inert particles and low motility of plankters.

 

During the period ending on 20 August, little had changed in microplankton composition of sampled water depths as work was completed in the southern sector and we moved into the central grid sector. Stations 81, 82, 75, 74, 73, 72, and 65 were all similar with low particle concentrations the norm (5-10/ml). The exception was a sample at shelf-break station 65. There, an oceanic water intrusion at 200 m showed a higher abundance of particles (30/ml). Mean particle diameter for all stations was between 15-20 μm. A few flagellates and an occasional ciliate were typical for nearly all samples.

 

Station 42 (CTD cast 31) was sampled in the central sector of the Grid. All depths exhibited few particles with little motility, except for a very large strombiid ciliate at 75m.

 

A calibration of fluorescent 16-um beads was made to secure a relationship between the threshold used to track particles and the particle concentration. For most stations sampled on this cruise, lower thresholds were required to track the sparse concentrations of microplankton.

 

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

The intention of our group is to tow the BIOMAPER II continuously between the broad-scale survey stations, in order to provide a picture of the spatial distribution of krill and other organisms. To date, however, problems with ice and systems malfunctioning have meant that we have only achieved towyos of short duration. In the mid-afternoon of August 20, we were able for the first time to keep the BIOMAPER II in the water for the entire transit between stations 43 and 44. A thin and weak scattering layer was evident at depths that changed along-transect, varying between 80 and 120 m. Like yesterday, this layer was associated with the pycnocline and again composed primarily of copepods, according to images captured by the VPR. Towards the end of the tow, we observed a number of small and dense patches of backscattering between 20 and 55 m. The VPR made occasional observations of large krill at 20 m, suggesting that these patches may be composed of our elusive target species.

 

With the bottom at only 400 m, we were able to get the BIOMAPER II deep enough to make observations of a scattering layer in close association with the bottom. Early in the towyo, this bottom layer was fairly diffuse and extended approximately 100 m off the bottom. As we moved farther towards the continental shelf break, and also as it got dark, the top-most reaches of the layer coalesced into more sparsely distributed patches and groups of stronger targets. This resulted in two adjacent layers of very different structure, similar to what we observed farther south on August 17 at nearly the same time of day, depth, and position relative to the shelf break. Whether this change from one to two bottom layers represents a diurnal change in the aggregative behavior of organisms at these depths or an along-transect change in the near-bottom species composition is not clear at this point, but poses an intriguing question.

 

Current Position and Conditions

In spite of the very cold working conditions, the Palmer is making good progress in accomplishing the schedule of tasks at each station and we have not had any difficulty moving between stations during the past couple of days. We are currently working at grid station 45, the off shelf station on survey line 7. Our current position on 21 August - 2344 hrs is -67 11.022′S; -74 29.175′W). Air temperature is -25.0C and the barometric pressure is 986.1 mb. Winds are out of 235 (southwest) at 10 to 15 kts. Skies are clear.

 

Cheers, Peter