The reality of winter in the Antarctic is with us again today (9 August). Blowing snow and winds in the mid-twenty knot region kept the visibility low most of the day. The temperature, however, was much warmer than over the past few days. Late last night, we finished up work in Marguerite Bay and we steamed out to the middle of the continental shelf to continue work on transect 6 that was started four days ago after we moved with the Gould to station #42. We are currently (1911) just finishing up work at broad-scale station #43 (-67° 48.542S; -71° 04.522 W). Winds are out of the west (270) at 30 to 35 kts. The air temperature is -2.5°C and the barometer is 977.6 mlb.
On 8 August, the work centered on the northern reaches of Marguerite Bay and in the shallow water area to the south of Adelaide Island where a coastal current was observed on the April/May broad-scale cruise. The deep Laubeuf Fjord was a hot spot for krill on the previous cruise and we were hoping to find the high concentrations of krill that we sampled then. Adult krill were present in the MOCNESS net tow collections, but they were not nearly as abundant as previously. This was a disappointment to the experimentalists on board who are hoping to collect sufficient numbers of adult krill to enable some experiments to be done. The sea ice conditions in this area were much more favorable to the scheduled work and none of the towing operations had to be cancelled because of the ice thickness. Work was completed at stations #35, 34, 33, 31, and 30. Station #32 was dropped from the schedule because its position was in the middle of a shoal area. On the previous cruise, it had been moved to the south, but that move had not been taken into account in preparing the positions for this cruise. Moving it again to the south did not fit well with the transits to the other stations in the area, so it was dropped. The work on 8 August included 5 CTD casts two phytoplankton tows, one 1-m2 MOCNESS tow, one 10-m2 MOCNESS trawl, one Tucker Trawl and one ROV under-ice deployment. BIOMAPER-II did shallow towyo's between stations #35 to 33, and part way to station #31. Collision with a chunk of ice passing under the ship again brought an end to the towing and more repair work. Bird and mammal observations were made between stations during daylight transit periods.
John Klinck reports that on 8 August, the CTD group did three stations in a line across the entrance to Laubeuf Fjord (discussion will include station #33 taken late on 7 August). Two of the stations were in a deep depression in which water conditions were uniform; bottom temperature is about 1.0°C, so there is no indication of recently intruding warm oceanic water.
Station #33 (147 m) at the southern tip of Adelaide Island has a uniform mixed layer to 50 m followed by a gradual transition to the top of the pycnocline which is on the bottom. Steppy structure is clear between 50 m and the bottom.
Station #34 (577 m) has almost uniform deep (below 250 m) water properties. The deep oxygen is about 3.8 ml-l and deep temperature is 1.2°C. The surface layer extends to 70 m, but there is a salinity step at about 40 m. There is a temperature reversal (about 20 m thick) in the center of the pycnocline indicating lateral intrusion of water. The source and direction of intrusion is unclear at present.
Station #35 (761 m) has similar deep characteristics as station #34, in particular with the same constant deep conditions. The surface layer extends to 100 m, but there is a salinity gradient in the lower half. The surface layer temperature increases with depth and there is clear indication of layering in the lower half of the mixed layer. Additionally, there is clear steppy structure for oxygen and temperature in the pycnocline.
Station #36 (355 m) has a mixed layer to about 70 m. The deep oxygen maximum is at 250 m, but there is no temperature maximum.
Ana Sirovic reports that on 8 August, she deployed 1 difar buoy, as we were starting to do MOC-10 on station #34. She was able to monitor it for 6 h 13 min before it scuttled (stopped transmitting and sank). Apart from the ship noise and ice breaking, nothing could be heard.
Chris Ribic and Erik Chapman report that on 8 August (JD-220), they
surveyed for 1 hour and 30 minutes between station #35 and 34 and 34 and
33. Approaching station #34, we traveled through mostly new gray ice that
probably formed in leads in the cold weather of the past few days. There
was still some open water and we saw a good number of Adélies during
the 30 minutes that we surveyed in this area. We also saw Snow Petrels
cruising over the new gray ice, checking out the small holes of open water.
When we began to retrace our cruise track towing the MOC-10, we saw Snow
Petrels and Adélie penguins along the edge of the ice and over the
open water along the track we had created. Snow Petrels were feeding in
the open water and three were seen diving fully under the water, probably
30 cm beneath the surface. We also surveyed for a short time leaving station
#34 in heavy snow. Visibility was poor and we saw just a single Snow Petrel
during this time. A summary of the days results is the following:
They surveyed for an hour last night traveling to station #35 and saw
three Snow Petrels during that time.
BIOMAPER-II/MOCNESS report (P. Wiebe, C. Ashjian, and S. Gallager):
BIOMAPER-II was in the water for towyos between stations #36, 35, 34, and 33. The extremely variable topography made for very tense (in a psychological sense) towyoing throughout a good portion of the day (8 August). Only in Laubeuf Fjord was the water deep enough to allow us drop the towed body down into scattering layers that we wanted the VPR to sample to provide data on the zooplankton present that were causing the backscattering. In the variable topography regions leading into the fjord from the northeast and the areas to the west of it, tremendous changes in water depth occurred over kilometer intervals or less. As a result, only very shallow towyos were done and even they, on a couple of occasions, came very close to a rapidly rising sea floor. But keeping the towed body shallow runs the risk of encountering broken up chunks of ice that have passed under the ship as we steam ahead. Right after the towed body was put into the water for the towyo from stations #33 to 31 in the early evening (station #32 was dropped, too shallow), it got slammed by a big ice chunk moving under the ship while we were avoiding a very shallow bit of topography about 15 to 20 m below the ship. The VPR cameras gave us the first indication that there had been a hit when they stopped sending images. BIOMAPER-II was brought on board and we found several parts had structural damage. The tail fin had been damaged- fiberglass on the leading edge of the tail was broken and the tail fin split open exposing the balsa wood core. The aluminum rod support fixture was also bent. The heavy metal spacers that sit under the VPR framework and cameras had the ½ inch bolts that fasten the spacer to the top of the main tow body frame sheared off (three of them), and the VPR support frame was bent again. Miraculously, the cameras were not damaged seriously; they needed some minor repair and then, realigning and calibration. The tail fin was patched up with Marine Tex and the framework straightened and re-installed. By early morning, the repairs had been completed and the towed body was returned to service. A later look at the wire tension data and the roll, pitch, and yaw data to see how wire tension and pitch varied during the period when the towed body was damaged, showed that there was over 6000 lbs of tension on the wire for some seconds and the towed body pitch swung from -15 to -20 to +15 degrees at the time the record log showed something had occurred. Normally we see no more than -10 to +5 degrees of pitch and 1800 to 2200 lbs tension when towyoing.
In the northern area of Marguerite Bay, there was strong backscattering on the down looking 120 and 200 echograms starting about 140 m below surface and extending down to the bottom. The upper 130 meters was pretty devoid of scatterers, except for some isolated targets in the upper 40 or 50 meters. The strong backscattering was also present in the deep hollows between peaks in the topography. The VPR showed mostly copepods in the deep scattering layer and only a few adult krill.
MOCNESS tow #9 was conducted on 8 August at station #35, which is located within Marguerite Bay in the vicinity of Laubeuf Fjord. The tow was conducted to 500 m. Conditions were excellent for towing, with very thin ice that easily wafted away from the hull of the ship.
The deep layers (100-500 m) were characterized by relatively higher biomass than observed from all previous MOCNESS tows. Many copepods were observed from 100-500 m. Large jellies (siphonophores, medusae) were seen in the 350-500 m range, with a few adult krill. Large mysids and gammarid amphipods, including a species white in color with orange eyes, were seen in the 200-350 m range. Euphausiids and copepods were prevalent from 150-200 m. Adult krill were observed from 75-150 m. The 75-100 m range was characterized by intermediate biomass, with low biomass seen in the upper 75 m. Approximately equivalent volumes were sampled in these depth intervals so this difference likely reflected true conditions in the water column. Krill furcilia, some with full guts, were observed in the upper 100 m. Ctenophores and amphipods were seen from 50-75 m. Small fish (larval) were seen in the 25-50 m range. The upper 25 m was dominated by copepods, with some furcilia and 2 adult krill present as well.
Jay Peterson reports that on the Optical Plankton Recorder (OPC) mounted on the MOCNESS provided an OPC profile in tandem with the MOCNESS tow at station #35. Plankton counts ranged from 2100 to 2300 m3 below 200 m and 500 to 1000 m3 in the upper water column. These values are 2-3 times higher than those encountered on this cruise at the stations outside of Marguerite Bay. Peak abundances of zooplankton occurred between 200 and 350 m, with a secondary peak between 350 and 500 m. The size spectra of the particles detected showed a concentration at the size range of large copepods and furcillia. Visual inspection of the net contents in the aforementioned depth intervals revealed high concentrations of copepods mixed with amphipods and the furcillia stage of krill.