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

26 April 2002

 

On April 26, the N.B. Palmer was again out in the deep water off the Western Antarctic Peninsulas continental shelf working at the ends of survey lines 6 and 7. As the survey work moves steadily south and as austral winter solstice approaches, the light of the day has been noticeably diminishing. On this day, the sun rose around 0900 and set about 1600.

 

The steam from the outer shelf station 44 to 45 and then 46 in the deep offshore waters of the Antarctic Circumpolar Current was done in with increasing wind and seas. By the time, we arrived at station 45 in the late evening, the winds were in the 35 to 40 kt range, there was snow blowing across the decks, and seas were too rough to either bring BIOMAPER-II on board or to deploy the CTD. Instead, an Expendable CTD (XCTD) was deployed while continuing to steam on to station 46. Upon reaching station 46 in the early morning, the winds were diminishing, but the seas remained too rough to work, so the ship was put onto a northerly course into the wind and seas, and Sea Beam bathymetric data and BIOMAPER-II data were collected while waiting for the conditions to improve. By the time, the Palmer arrived back at station 46 about 0830, the seas still had a large swell running, but the wind had dropped to the low teens and the sea surface was beginning to calm. The barometer was still low (961.7 mlb), the air temperature just above freezing (0.3C) and snow was falling lightly. During the day, the skies cleared a bit and sporadically there was some blue sky showing. But most of the time, there was a persistent fog limiting visibility. In the evening, at station 27, the winds were still a light 10-12 kts out of the south, the air temperature had dropped to -2.7C, and the barometer was up to 969.3 mlb.

 

This was another day in which only a couple of stations were completed because of the long steaming time between stations and the long times needed to deploy the equipment. Work was completed at stations 45, 46 (depth 2086 m), and started at 47 (depth 2845 m) including 3 CTD casts (two to the seafloor) and an XCTD, a 1-m ring net surface zooplankton tow at station 46, and a deep 0-1000 m MOCNESS tow at station 47. Seabird and mammal observations were made during daylight under marginal visibility conditions and BIOMPAPER-II was towyod between all the stations.

 

CTD Group report (John Klinck, Tim Boyer, Chris Mackay, Julian Ashford, Andres Sepulveda, Kristin Cobb)

The CTD group did three casts at two stations and one XCTD at a third station. One station was on the outer shelf, while the others were offshore in the Antarctic Circumpolar Current (ACC). Because of sea state, the CTD cast at station 45 was replaced with the first XCTD cast of the cruise. The shelf station showed no indication of oceanic water, and deep cold temperatures mean that this water has been on the shelf for a while. The two oceanic stations showed typical features of the ACC; there are some differences in near surface properties (different surface layer depth, strength of the Winter Water (WW), chlorophyll levels).

 

During the long cast at station 46, I again looked at the ADCP display. Currents below about 50 m were southwestward (about 20 cm/s), opposite to the expected direction of the ACC. Further, the upper 50 m waters were moving WNWward, consistent with the moderate (15 kt) southerly winds at the time. The reversal of the ACC is consistent with an offshore deflection of the main current and creating of a reversed flow at the shelf break (like Gulf Stream meanders in the South Atlantic Bight). The dynamic topography (0 to 400 m) for the stations so far is consistent with a reversal of ACC flow along the shelf break. These flow reversals seem to be associated with projections of the shelf break topography, but facts are in short supply making any dynamical explanation at this point premature.

 

Station 45 (XCTD cast 1) There was a uniform mixed layer (-01.1C, 33.65 psu) to 55 m. The temperature maximum (1.3C) was at 300 m. Weak small-scale variability was detected.

 

Station 46 (cast 49, 50, 2086 m). The first cast was to 100 m with the FRRF. There was a uniform mixed layer (-0.7C, 33.8 psu) to 55 m. Surface chlorophyll was 0.7 μg/l. There was a slight WW layer centered at 75 m. Strong layering was seen from the base of the mixed layer to about 150 m. Temperature maximum (1.9C) was at 250 m. There was a relative temperature minimum within 100 m of the bottom where there appears to be a layer with near uniform properties.

 

Station 47 (cast 51, 2845 m). There was a uniform mixed layer (-0.7C, 33.8 psu) to 80 m. Surface chlorophyll was 0.6 μg/l. There was a strong WW layer (-1.3C) centered at 100 m. Energetic layering was seen from the base of the mixed layer to about 300 m. The temperature maximum (1.7C) was at 300 m.

 

Phytoplankton Primary Production (Wendy Kozlowski)

During the second week of sampling (21 April 26 April), the primary production group has completed six more simulated in situ (SIS) experiments, at stations 24, 28, 34, 39, 41 and 46. Preliminary results through station 39 show generally higher levels of production off shore than on, with highest levels seen at the northern end of the grid. The fast repetition rate fluorometer (FRRF) has been deployed at another thirteen stations, with two casts declined due to rough weather. At stations 37 and 46, a separate cast to 100 m allowed collection of data at deeper water stations as well as those under 500 m, both off the shelf and inside Marguerite Bay.

 

Chlorophyll samples continue to be taken at all stations, and samples for particulate carbon have been preserved at twelve more stations. Chlorophyll measurements (through station 34) show mixed layer levels ranging between 0.2 and 1.7 μg/l, with stations 20 and 28 being as high as 2.0 μg/l. Station 28 also showed slightly higher levels of production.

 

The first new ice of the cruise was encountered in the vicinity of station 36, and newly forming pancakes (<50 cm diameter) were collected at station 37. Slightly older pancakes (~1-2 m diameter) were sampled in the area of station 40, and after melting, the ice water was sampled for chlorophyll, particulate carbon, and production.

 

Marine Mammal report (Debra Glasgow)

The marine mammal survey began on 26 April at 1210 in dense fog after the work this morning at station 46 which included two CTD casts and a 1-m ring net tow. Visibility was poor, but variable with light snow at times, but mostly around 500 meters or less all day. The sea state was Beaufort 2-3 with a large following swell. No cetaceans were sighted and only 1 fur seal was seen at 1249 porpoising beside the ship to starboard of the bow at 67 08.06S; 73 30.11W. Not many birds were around throughout the day. The effort ended at 1700 in visibility less than 300 meters and fading light.

 

Sea Birds (Erik Chapman and Matthew Becker)

Once again, the barometer dropped overnight and the Palmer rocked in high seas only to have winds die and seas calm as daylight arrived. This time snow and fog remained throughout the day and visibility was poor. We were able to sneak about 4 hours of observations in with visibility between 400 m to 600 m and only briefly had to shut down when visibility dropped below our 300 m transect limit. The transit between stations 46 and 47 was the fourth survey off the continental shelf, and for the third time, very few birds were seen there. Only during the first, northern-most transect that went from off the shelf onto the continental shelf were birds seen in abundance. This was also where physical oceanographers found an intrusion of Circumpolar Deep Water, so there could be a correlation here, but any comment on a relationship between these findings would be pure speculation.

 

A surface 1-m net tow was done this morning at station 46, which revealed very little zooplankton biomass at the surface. This agreed with BIOMAPER-II results that reportedly indicated very little biomass in the water column in general and the surface layers in particular between stations 46 and 47. So, on 26 April, few birds were seen in an area that appeared to have little biomass at the surface.

 

Eventually, we'll be looking at results for both years of cruises in more detail to consider all possible sources of variability and to test hypotheses based on what is known (or predicted) to be factors that influence seabird distribution. Physical or biological variables will be used that are being measured by other groups during these cruises. In this way, we hope to identify key linkages between physical and biological variables, from primary production on up to zooplankton 'predators.' If the extent that these linkages are found, a new understanding will be developed of how the marine system functions during winter in Marguerite Bay.

 

A summary of the species and number of individuals of birds and seals observed during 3 hours, 58 minutes of daytime surveys between consecutive stations 46 to 47 is the following:

 

Species (common name)

Species (scientific name)

Number observed

Cape Petrel (Pintado Petrel)

Daption capense

16

Southern Fulmar

Fulmarus glacialoides

9

Antarctic Petrel

Thalassoica antarctica

2

Wilsons Storm Petrel

Oceanites oceanicus

1

Blue Petrel

Halobaena caeulea

7

Antarctic Fur Seal

Arctocephalus gazella

2

 

 

Material Properties of Zooplankton Report (Dezang Chu, Peter Wiebe)

On April 26, we repeated the shipboard measurements on mysids (Arctomysis) to see whether consistent results could be obtained. The mean length of the animals (total of 15) used in the experiment was 48.3 mm and the standard deviation was 8.0 mm. To avoid influence of temperature variation on the sound speed measurement, we conducted the measurements in a bigger tank (~1 m3) with running sea surface water. The temperature of the tank water was 0.4 to 0.5C. It was extremely difficult to put the live animals into the animal compartment (mid-section of the acoustic chamber) of the APOP without any air bubbles trapped in the acoustic chamber as well as attached to the animals. We didnt see any visible bubbles attached to the exoskeleton of the animals during the measurements. However, we did notice that after the animals were removed from animal compartment, there were two or three of the mysids that had bubbles under their carapaces. The animals were all alive before the acoustic measurement, but a few died after the acoustic measurement. The sound speed contrast was 1.085, similar to those obtained the day before. The density contrast from density measurement on the same animals was 1.024, lighter than the measured density contrast on the same species obtained on April 25.

 

In the afternoon, shipboard measurements of sound speed and density contrasts on E. superba were conducted on animals that were collected three days ago by Kendra Daly. The experimental protocols were the same as those for the mysids. The mean length and the standard deviation of the animals were 36.6 mm and 5.4 mm, respectively. It took us more than 5 hours to complete both sound speed and density measurements. The most difficult part of the experiment was to prevent air bubbles from entering into the animal chamber while we were transferring animals into it. All of the animals were still alive after the sound speed measurement. The sound speed contrast and density contrast were 1.048 and 1.027, respectively. The sound speed contrast was higher than and the density contrast was in the same range as those measured previously in a number of APOP casts.

 

Zooplankton (MOCNESS/BIOMAPER-II) report (Carin Ashjian, Peter Wiebe)

MOCNESS #12 was conducted off the shelf at station 47 in over 3000 m of water on 26 April. The tow was successful. Very low abundances of zooplankton were found at most depths. Chaetognaths were collected from 200-1000 m. Copepods were dominant at all depths, except for the upper 50 m where phytoplankton overwhelmed all other taxa in abundance. Myctophid fish were collected in the 800-1000 m and 400-600 m depth intervals; one fish from each interval was sampled for otoliths prior to preservation by Julian Ashford. Few euphausiids were observed at any depth. The greatest abundance of zooplankton appeared to be in the 50-100 m interval, although zooplankton abundances in the depth strata above that were difficult to assess because of the overwhelming abundance of phytoplankton.

 

BIOMAPER-II was towed through the high winds and seas in the early morning hours of 26 April past station 45, because of the impossibility of bringing the towed body back on deck. It was brought on board about 0850 at the start of station 46 and parked on deck, so that it was ready to go after the station work ended. It was deployed for the long run to station 47 and again recovered for the station work.

 

Just before arriving at station 46, a strong scattering layer appeared on the 43, and 120 kHz echograms about 170 m below the surface and extending down to 320 m. It was only present for a short time. There was very little scattering at the surface or anywhere else in the water column. On the transit between stations 46 and 47 in the afternoon, several krill-like patches were observed at about 200 m depth and visible on the lower four frequencies. Estimated sizes of the patches ranged vertically from 60 to 80 m and horizontally from 400 to 1000 m. Except for these occurrences, volume backscattering throughout water column off the continental shelf has been quite low on the lower frequencies. The 1 MHz echograms consistently showed high backscattering when in the mixed layer or pycnocline where the VPR was capturing images of diatoms and radiolarians in high numbers.

 

Cheers, Peter