What a difference a day makes! Yesterday was a very good day for observational oceanography in Marguerite Bay. The partly cloudy skies and considerable sunlight made for good visibility. This combined with low winds and seas, made for an optimal work environment. Today we have very different conditions. Although the air temperature has not changed much (now -0.7°C), gale force winds (35 to 45 kts with gusts over 50 kts) out of the North northeast (030) started up about 0100 and continue unabated. Our current position is -67 39.950°S; -71 40.527°W (18:45 local - 8 May) and we are headed for Station # 38.
We completed work at Stations 31, 32, 33, and 34 and along the trackline between the stations including deployment of one satellite tracked drifter and five Sonabuoys, five CTD casts, one MOCNESS tow, and bird and marine mammal survey observations. Steaming between stations was slowed significantly because Marguerite Bay has many shoal areas that are poorly charted. The Seabeam bathymetry data being collected on this cruise will help remedy this, but for now the Officers on the Bridge are exercising appropriate caution as we move through the uncharted areas.
Eileen Hofmann reports (for both May 7 and through mid-day on May 8) that the CTD group has completed casts at 35 stations and the quality of the data acquired on these casts remains high. Comparisons of the values obtained from the conductivity and oxygen sensors with those obtained from discrete water samples shows little difference. At this point no corrections are indicated for the CTD data. Howard Rutherford and Rebecca Conroy are continuing to take nutrient samples at each CTD station.
Weather conditions at survey station 36 were marginal for a CTD cast due to the movement of a strong storm front into the study region. This station was completed, but with the breaking of one bottle on the Rosette as it was being brought back on board the ship. By station 37, the weather had deteriorated further and it was decided not to deploy the CTD. Instead we did an XCTD to acquire data on the vertical temperature and salinity distribution. No water samples were obtained at this station, except for a surface sample from a bucket. The gale-force weather conditions made the XCTD drop exciting.
Preliminary analysis of the distribution of the horizontal temperature maximum below 200 meters shows clearly a large intrusion of Upper Circumpolar Deep Water moving onto the west Antarctic Peninsula continental shelf over the northern portion of the study grid. The axis of the intrusion is along a deep depression that extends from the outer shelf edge into Marguerite Bay. The existence of this bathymetric feature has been confirmed by Sea Beam data. Along the inner portion of the survey grid the southwesterly flowing coastal current is present. This current appears to turn into Marguerite Bay at the southern tip of Adelaide Island.
The ADCP data that are provided by Susan Howard support the circulation patterns that are indicated in the hydrographic data. On shelf velocities associated with the intrusion are on the order of 8-10 cm/sec. Velocities associated with the coastal current are about 5-8 cm/sec. The horizontal temperature distribution at 20 m shows cooling of the surface waters as the survey progresses southward. This indicates the break down of the seasonal pycnocline and the transition from summer to winter conditions. The largest cooling of surface waters is at the outer part of the study grid. Inner shelf waters and those within the inner portion of Marguerite Bay remain warmer.
The surface salinity distribution shows more saline water offshore overlying the region of the Upper Circumpolar Deep Water intrusion. This may be an indication of upwelling/mixing of the deeper water with the Antarctic surface water. Surface salinity associated with the southwesterly flowing coastal current and that within Marguerite Bay are low, relative to that offshore.
Chris Ribic and Eric Chapman surveyed birds for 5 hr 37 min. During our second day in Marguerite Bay, they again saw no Blue or Antarctic Petrels, a small number of Southern Giant, Snow and Cape Petrels and a relatively large number of Kelp Gulls. However, unlike their first day in the Bay, they recorded a large number of Southern Fulmars. They conducted one night survey in the morning and two in the evening yesterday. The following table summarizes today's observations:
|(Day) 5:37||(Night) 1:30|
|Antarctic Petrel (Thalassoica antarctica)||0||0|
|Cape Petrel (Daption capense)||11||2|
|Southern Fulmar (Fulmarus glacialoides)||107||0|
|Blue Petrel (Halobaena caerulea)||0||0|
|Southern Giant Petrel (Macronectes giganteus)||1||0|
|Snow Petrel (Pagrodoma nivea)||13||0|
|Kelp Gull (Larus dominicanus)||81||0|
Whale observations by Ari Friedlander began this morning at 0845 in transit to station #33. Weather conditions were very good with calm seas and wind. Just before station, at 1046 local time, 3 minke whales were seen approaching the vessel. The whales remained 300 meters from the ship and circled half way around before moving on. Observations were then halted until 1200 local time for the transit to station #34. Similar conditions prevailed. At 1422 local time, a single minke whale was seen, and it too approached the ship. The whale swam to the bow of the ship and remained there for 1 minute before
diving beneath and surfacing on the starboard beam. It then swam slowly away from the ship surfacing several times with its head completely out of the water, presumably to get a look at the ship. Photos and video were taken of this whale. Observations ended for the day when we reached station #34 at 1525 local time. As well as the whales, 6 fur seals were seen throughout the day. Weather conditions like this are always welcome and appreciated.
BIOMAPER-II/MOCNESS report (P. Wiebe, C. Ashjian, S. Gallager and
During May 7, BIOMAPER-II remained under repair in the "Garage van" on the stern of the main deck of the Palmer. It took most of the day and several phone calls to check out the repairs to 16 channel relay board done the day before. By mid-evening, the checks were finished and the echosounder was reassembled for testing in the van. Nine of the twelve channels in the echosounder passed the noise tests; hand rubbing of the transducers with the transmitters turned off produced output on the echosounder. But this test demonstrated that the three channels that had failed first several days ago had not been fixed. So an alternate configuration was put into place to enable 8 of the 10 transducers to be active, and in this case, all 8 channels passed the noise test. The upward looking 200 kHz transducer was then placed in a tub of water and the transmitter enabled to get it to ping in the water. It worked. After a final tie-wrapping of the cables, the echosounder was re-installed in the underwater case, and the air in the underwater unit was purged with nitrogen to eliminate any moisture. Another test of the system was done before moving BIOMAPER-II out on deck, and while the noise this time was the same as previously, a reverse fault was showing up on the power and ground fault monitor. In spite of that, the towed body was moved out of the van and onto the deck where it was hosed down with sea water to see if any other faults showed up. None did. After some debate, a decision was made to deploy BIOMAPER-II and see what it did while being towed. About 0100 on May 8th, the fish was back in the water and when turned on, another noise test was done. All eight of the transducers showed a noise response. Then the moment of truth - the transmitters were turned on. The lower frequency transducers worked and produced respectable echograms. But no signal appeared on either 420 kHz transducer, just a faint trace of noise above the noise threshold. It appears that there must be a problem with the high frequency transmit board that is independent of the relay board, and there is more trouble-shooting ahead. But for now data will be collected in the current configuration with the three operational pairs of transducers (43 kHz, 120 kHz, and 200 kHz).
A MOCNESS tow (#9) was taken at Station 34 in Marguerite Bay on 7 May. This tow was made in some highly variable topography. To do the tow, advantage was taken of the fact the Seabeam system had produced a very good bathymetry chart of the ocean bottom as we came onto the Station. So to make sure that there would not be any surprises about what subsurface peaks might be encountered along the tow path, the tow was conducted on the reverse course. This was possible because of the low winds and seas. Normally, tows must be into the wind and seas. This was a highly successful tow which resulted in the very large catch of adult krill (Euphausia superba). This was the first location where we saw high abundances of adult krill. The adults were found primarily in the 50-75 m depth interval, with much lower abundances in both the 25-50 m and the 75-100 m depth intervals. Larval krill were found in both the 25-50 m and the 0-25 m depth intervals, overlapping with adults in the 25-50 m interval. Hence, the two life stages appear to have different depth distributions.