A certain monotony has set in as we experience the around the clock survey work at and between the stations, which are spaced about 40 km apart, and the seemingly endless cloudiness and fog, often accompanied by freezing mist or snow flurries. The shortness of the day is also the subject of conversation, especially for the bird and mammal surveyors who need the ship to be steaming between stations during a period when there is daylight to make their observations. Still we are only a day or so away from finishing the survey and moving onto more site specific experiments. The weather today (18 May) has moderated significantly from yesterday and the seas are much reduced enabling all of the programmed activities to take place. We are now headed for Station # 77 the last one on transect line 11. Our current position at 1429 local time is -69 08.354°S; -74 28.041°W. Winds are out of the southeast (137) at about 20 kts and the air temperature is -1.5°C.
A good portion of yesterday (17 May) was spent with near gale conditions (28 to 33 kts). The waves were spilling their white capped tops and foam lines were streaked across the blue water portion of the swells. The anemometers were not giving very accurate readings because of ice build up on the propellor blades last night. In the early evening (the winds dropped into the low 20's and working conditions improved enough to permit the CTD to be deployed. BIOMAPER-II, which was deployed at Station 69 on the 16th of May, remained in the water throughout the day and was towyo'd between the stations. During the day, work was completed at stations 71 to 74 and included two CTDs, two XCTDs, four XBTs, two sonabuoy deployments, two ring net tows (one ended up in the ship's propellor and the net was destroyed), three bird observation sessions, and the BIOMAPER-II towyo's.
Eileen Hofman reports that during 17 and part of 18 May the CTD group completed survey stations 71 to 74 on transects 10 and 11. The outer-most stations on each transect (73 and 76) were done with XCTDs. The weather conditions encountered at these stations were such that the CTD could not be lowered. In between the survey stations, XBTs were dropped to increase the spatial resolution of the temperature field.
The hydrographic observations from survey stations 71-76 show that the southern boundary of the Antarctic Circumpolar Current (ACC) is located along the shelf edge. The 1.8°C isotherm was encountered at about the 1200-1500 m isobath. The southern ACC boundary is confined to a narrow band that is about 20 km wide. The 1.4°C isotherm was encountered at survey stations 72 and 75, which indicates that modified Upper Circumpolar Deep Water (UCDW) is overlying the outer shelf region.
The coastal current that extends outward from Marguerite Bay was found between survey stations 71 and 70 on transect 10. The pattern in the isotherms below 200 meters suggests that this current is flowing across the inner shelf in a west-southwesterly direction. A significant drop in surface salinity was associated with crossing the front. Temperatures inshore of the coastal current are mostly less than 0°C, which suggests that UCDW is not reaching this part of the shelf. Thus, the coastal current may provide a mechanism for separating the inner and outer shelf habitats.
Susan Howard has produced ACDP-derived current maps for most of the region covered in the survey so far. The current distributions provide confirmation of the hydrographic patterns. These also suggest that the west Antarctic Peninsula continental shelf can be separated into different physical regimes. It will be interesting to see if the biological distributions follow these patterns.
Chris Ribic and Erik Chapman report that on May 17th they surveyed for 3 hours and 23 minutes in transit between stations 72 and 73. This survey covered the outer portion of the continental shelf and ended just before the shelf break. They saw few birds
today with the Southern Fulmar being the most abundant. In sharp contrast
to their previous day's survey along the western side of Alexander Island,
they did not see any Snow Petrels during today's transect. Results from
today's survey are summarized below:
|Species||Number (JD 137) 3hr 23min|
|Antarctic Petrel (Thalassoica antarctica)||3|
|Cape Petrel (Daption capense)||4|
|Southern Fulmar (Fulmarus glacialoides)||13|
|Blue Petrel (Halobaena caerulea)||6|
|Southern Giant Petrel (Macronectes giganteus)||1|
|Snow Petrel (Pagrodoma nivea)||0|
Catherine Berchok's mammal sound report:
May 15th. Two difar sonabuoys were thrown in today. The first was around 5 miles before station 66. There were just a couple of sounds heard, maybe seals. The sonobuoy must have had a defect, because I lost the signal suddenly at only an hour after deployment.
The second was deployed around 17 miles from station 67. This buoy picked up very faint 400-300 Hertz moans - upsweeps, downsweeps and other variations. It sounded like humpbacks, but I wasn't certain, because I didn't hear any of the high or low components of their calls. I continued hearing these sounds until about 7 miles from station 67 when I lost reception. I am happy to report an almost 11 nm range on the newly configured dipole antennae. This definitely improves not only listening time, but quality of the recorded signal.
May 16th: This day started off with a large group of crabeater seals being spotted by Chris Ribik at station 68. As soon as we left this station, I deployed a difar in hopes of getting some nice recordings of the crabeaters. After the ship noise died down, I started hearing the same 400 Hertz moans like those of yesterday evening. So I started thinking that maybe crabeater seals sound like humpbacks. Then after a half hour three things happened. First, I started to wonder why I seemed to only hear one animal, when there was obviously a large group of seals following the boat. Second, I was able to get a directional fix on the bearings of the ship and the sound source from the buoy , and they were almost 90 degrees apart, ruling out our band of hitchhikers. And third - I started to hear the high (greater than 1 kHz) and low (less than 100 Hz) components of the humpback songs. The 400 Hertz sounds seem to be the strongest which is probably why I was only hearing these yesterday. About 4 hours later, Ari spotted no less than 5 humpbacks when we were around 7 miles from station 69. These animals seemed to be in the right place to be the source of the sound - but I need to work out some of the bearings from the difar.
In total, I deployed 4 difar sonobuoys along the path between 68 and 69 and recorded around 10 hours of very loud humpback vocalizations. There was at times when at least 3 different individuals were 'singing'. Further analysis is needed to determine whether they were singing full complete songs, or just snippets. It was tough to tell because there were several singing at once. The best part of the day was when we were working station 69, I was able to receive signals from a buoy 4 nm away and another 8 nm away. It was possible to see the same calls appearing on the two buoys , and to see the delay time change and the amplitudes increase as the singers moved back up the coast towards the 8nm buoy. It could just be a daily migration, but I am curious to see what I will record when we pass by that way on the way back.
May 17th: One sonabuoy was deployed approximately 17 nm from Station
73. Possibly on minke whale grunt was heard in the 2 hours of recording.
Another was deployed at station 73 with a possibly another another minke
grunt. Normally minkes will have several of these grunts spaced out in
a series, and it is easy to say who is making the sound. But in this case
with only one, it could have been some freak movement of a wave or the
buoy, etc. Listening time for second buoy was two hours and ~10 nm range.
BIOMAPER-II/MOCNESS report (P. Wiebe, C. Ashjian, S. Gallager and
The 17 May BIOMAPER-II towyo's between stations 71 to 74 were more or less routine; the acoustics, video, and environmental sensor data keep rolling in. The bottom topography, which had been so variable earlier on this transect line, flattened out and got deeper for the most part and towyo's to 250 m were now possible. No large patches of krill were observed, although the volume backscattering was moderately high throughout the water column on the 120, 200, and 420 echograms in the mid-shelf region. Little showed on the 43 kHz echograms, except in the beginning of the morning when the fish targets were present. But mid-way along the transect, they disappeared. Late in the day the data were collected in the outer shelf region. A consistent pattern is developing of low volume backscattering off the shelf and increasing backscattering as we come further onto the shelf. Most, but not all, of the krill patches are close to shore in the coastal current. The VPR is beginning to be used in realtime to show where the different species groups are located. Linking them to the hydrography is also beginning to be possible.
There were no MOCNESS tows scheduled during 17 May.