The good working conditions of the last two days have given way to less favorable conditions as we steamed out today to the edge of the continental shelf on the tenth transect of the broad-scale survey. During the early morning of the 17th of May, winds and seas began to pick up and for much of the day, the winds have been in near gale region (28 to 33 kts) with higher gusts. We have now finished transect line 10 and are now headed for Station # 74 to start transect line 11. Our current position at 1659 local time is -68 20.367°S; -75 49.223°W. Winds are out of the northwest (055) at 28 to 36 kts and the air temperature is -0.7°C.
May 15th was a relatively benign day compared to the preceding few days and a welcome respite from the marginal working conditions. Winds were steady at 10 to 20 kts throughout the day and low clouds continued to prevail, but there has been only limited precipitation in the form of snow flurries, usually at night. May 16th was the second day of reasonably good working conditions. Winds for the most part remained under 20 kts until the evening when they began to increase, and the swell was down from yesterday. We spent much of the day near the coast of Alexander Island working in the vicinity of Stations 68 and 69 and steaming in between them. As described below, we had an entourage of about 80 seals swimming along beside the ship after leaving Station 68 for several hours.
Work over the course of these two days included seven CTDs, three MOCNESS tows, seven sonabuoy and one satellite tracked drogue deployments, the bird and mammal surveys when possible, and BIOMAPER-II towyo's between each of the stations.
Eileen Hofmann reports that by the end of the 15th of May, the CTD group completed survey stations 64, 65, and 66 on transect nine. At survey station 65, a large swell arrived just as the CTD was being brought back into the Baltic room. The ensuing rock and roll resulted in kinks in the hydro wire on the CTD. As a result the CTD was re-terminated. Thanks go to Raytheon personnel Matt Burke, Jesse Doren and Jeff Otten for their efforts in getting this done.
The CTD is continuing to give high quality data. Comparisons between the values obtained with the oxygen sensor on the CTD and those obtained with titrations of samples taken from the Nisken bottles show a r2 value of 0.99. A similar correspondence is obtained between CTD-derived salinity values and those from measurements of discrete water samples made with a salinometer.
The temperature maximum below 200 m continues to show the strong influence of the Upper Circumpolar Deep Water on the west Antarctic Peninsula continental shelf. We are now moving inshore on transect nine and are now starting to see the influence of more coastal waters. This is manifest by lower temperatures below 200 and decreased surface salinities. Based on the observations from survey transect eight, we are anticipating being in the coastal current at the end of transect nine.
Susan Howard has produced current vectors plots for the surface (31-75 m) and at depth (200-300 m) from the transects in Marguerite Bay. The current distributions show flow into the Bay on the eastern side and flow out on the western side. The deeper current vectors give the suggestion of two circulation cells inside the Bay, perhaps in response to shallowing topography in the center of the Bay. Hopefully there will be time after the survey grid is completed to return to the Bay and investigate this further.
The mapping of the hydrographic structure of the waters overlying the west Antarctic Peninsula continental shelf continued on 16 May with a series of CTD and XBT casts. CTDs were done at survey stations 67, 68, 69, and 70. The temperature and salinity observations indicated that these stations were in the inshore coastal waters and in the frontal boundary between these waters and the continental shelf waters. Therefore, the resolution of the temperature observations was increased by using XBTs. This allowed us to get a good description of the inshore coastal front.
Susan Howard has provided ACDP-derived current distributions along the completed survey transects and inside of Marguerite Bay. These show the current structure that is indicated by the hydrographic distributions. The currents on the shelf are suggestive of a gyre circulation with a clockwise rotation.
Nutrient distributions for the first five survey transects have been plotted and examined. These data show considerable structure along and across the shelf. Regions of upwellling and downwelling are clearly evident in the nitrate and silicate distributions. The ratio of silicate to nitrate is being used to track the upwelling of Upper Circumpolar Deep Water on the continental shelf.
Chris Ribic and Erik Chapman report that on May 15th, they observed for 4 hours, 17 minutes while we transited between stations 65 and 66. Fog limited their visibility and they were unable to view the entire 300 m transect for most of the day. They saw low numbers birds and again saw the typical offshore species. Here are the summarized results for the day's survey effort:
|Species||Number (JD 135) 4 hr 17 min|
|Antarctic Petrel (Thalassoica antarctica)||5|
|Cape Petrel (Daption capense)||2|
|Southern Fulmar (Fulmarus glacialoides)||2|
|Blue Petrel (Halobaena caerulea)||2|
|Snow Petrel (Pagrodoma nivea)||1|
On May 16, they surveyed for 4 hours 17 minutes between stations 68 and 69. When the transit began, there were 80 Snow Petrels circling the ship. These birds continued along with us for the beginning portion of the transit, but had dropped away over the course of the transect. Overall, the Snow Petrel was the most abundant bird within the transect and there were a good number of Antarctic and Blue Petrels. Today's transect was located along the western shoreline of Alexander Island within the same cold coastal current that we sampled on the north end of Alexander Island on JD 132. Although we were sampling within the same current, there was more brash and new ice on the north end of Alexander Island than during today's transect. Nevertheless, Snow Petrels were the dominant species observed during both sampling periods (see below). They also saw more Antarctic and Blue Petrels and fewer Cape Petrels and Southern Fulmars within the coastal current west of Alexander Island than in the same current north of the island. Another difference was noted in the behavior of the Snow Petrels between the two sites. They saw many more Snow Petrels circling and following the ship today, while birds in the brash and new ice north of Alexander Island flew low over the ice seemingly unaffected by the presence of the ship. Perhaps the difference in foraging strategy reflects a difference in prey availability in the two areas that results from the presence of ice. Counts for the day are:
|Species||Number (JD 132) 4 hr 19 min||Number (JD 136) 4 hr 19 min|
|Antarctic Petrel (Thalassoica antarctica)||9||38|
|Cape Petrel (Daption capense)||18||6|
|Southern Fulmar (Fulmarus glacialoides)||48||18|
|Blue Petrel (Halobaena caerulea)||0||40|
|Southern Giant Petrel (Macronectes giganteus)||5||6|
|Snow Petrel (Pagrodoma nivea)||170||121|
Ari Friedlander reports that on 15 May only incidental whale observations were made as high winds and poor visibility created poor sighting conditions. On 6 May, the watch began as we departed station #68 at 1030. Sighting conditions were poor, with heavy fog limiting visibility to less than 0.5 nm. While at station #68, and for the first three hours of transit to station #69, approximately 80 crabeater seals swam with, around, under, and beside the ship. The seals remained extremely close to the ship throughout the transit, possibly because we were traveling with the swell, and thus it was easier for them to maintain the 5 knots that we were steaming. From 1315 to 1430 visibility lifted to 1 nm. We were steaming through an area with several medium sized ice bergs, and it appeared that there was more ice to port. During this time, three sightings were made of five humpback whales, and one sighting was made of two minke whales. The whales were all seen within 30 minutes of each other. None of the whales showed any vessel attraction or avoidance, rather they seemed content in what they were doing. One group of two humpback whales was seen frequently at the surface, and then fluked up on a dive. Sonabuoys were deployed, and good, distinct, humpback sounds were recorded. Had the seas been more cooperative, these would have been excellent animals to attempt to biopsy.
BIOMAPER-II/MOCNESS report (P. Wiebe, C. Ashjian, S. Gallager and
Following the gale during which BIOMAPER-II was strapped to the deck of the R/V Palmer, it was again deployed about 0600 on 15 May at the end of the work at Station #64. Towyo's were conducted on the transits to Stations 65 and 66. At Station 66, the towed body was retrieved to allow for a MOCNESS tow and then deployed again at the end of the station late in the afternoon. The towyoing continued to the end of the line at Station 68, which we reached about 0800 on 16 May.
The volume backscattering along the outer section this transect line showed low intensity in the 43 echograms and increasing by diffuse scattering throughout the water column on the 120, 200 and 420 echograms. Occasional small patches of intense backscattering occurred in the upper 60 meters. As the bottom shoaled following Station 67, the backscattering on the upper three frequencies increased. Just as we came to Station 68, a very large "krill" patch appeared, the most intense observed on this cruise. It was situated between 70 m and 170 meters in an extremely variable topography with a center at about 140 meters. The scattering intensities were on the order of -50 db. At this location, the bottom was about 200 m and the patch tapered off towards the bottom. The highest backscattering intensities were in a 30 meter thick zone about the center. During the CTD cast, we were hoping to see the CTD profile on the echograms and any response by the krill to its presence, but no luck. Neither were observed. So after the CTD was on board, a profile was made with BIOMAPER-II down through the layer hoping to view the krill on VPR. But it seemed that the towed body caused an avoidance reaction and BIOMAPER-II went through the layer providing images of some, but not many large adult krill. Profiling with the towed body may have caused some holes to appear in the echograms, however, which were evident when it came back up to the surface. The presence of large number of seals and the presence of minke and humpback whales described above suggest that they knew the krill patch was present. On the transit between Stations 68 and 69, more krill patches were observed similar to the one at Station 68.
At Station 69, BIOMAPER-II was brought on board for minor servicing and to allow for the MOCNESS tow. The topography during the transit to Station 70 during the evening of the 16th mirrored that experienced between stations 68 and 69; it was highly variable. Dips to 500 to 600 meters were followed closely by rapid rises to 150 meters or so. As a result, we mostly kept the towyo's restricted to the upper 100 m. On a number the towyo legs, the bottom came to within 40 meters of the fish. There was a near bottom layer almost continuously along the trackline, which was probably dominated by krill. A couple of times we tried to dip BIOMAPER -II into it, but only once did it get down far enough to penetrate the top if it and the VPR did see some individual krill. Most of the time, the towed body descent was stopped short of the layer because the bottom was coming up too fast.
At Station 64 on 15 May, a MOCNESS tow was done to 500 m. Biomass was very low for all depths at this outer shelf station. Very little was caught in the deep nets (below 100 m) although there was a 3" fish in net 1 (500-350 m). Larval krill were observed only in the upper 50 m. Copepods also were seen in the upper 100 m. The MOCNESS tow to 550 meters at station 66 had higher abundances than previous tow further offshore. Copepods were found throughout. Euphausiid calyptopes were found at around 75-100 m with furcilia in the upper 25-50 m (as well as calyptopes). Also pteropods were observed at mid-depths (75-100 m). The presence of calyptopes so late may indicate a very late spawning.
The MOCNESS tow at station 69 on May 16th was conducted to 80 m total depth, with the eight nets each sampling ten meter intervals. The water column was well mixed over the depth of the tow. Krill furcilia were observed throughout the water column, with greater abundances at depth than near the surface. Pteropods were observed at around 40 m depth. Small fish also were seen, especially near the surface (0-10). Two types were seen: one approx. 2" long, almost clear with black dots along the dorsal and ventral sides (photophores), the second more opaque and whitish, about 1" long, with a very dark gut. Both types had eyes. A very few juvenile krill were observed sporadically through the water column.