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

2 May 2002

 

On 2 May, the SO GLOBEC broad-scale survey nearly reached the seaward end of line 8. A large topographic feature off the continental shelf that has raised bottom depths lies centered just to the northwest of this survey line. The feature is thought to contribute to the meandering in the Antarctic Circumpolar Current in this region and perhaps to the development of the intrusions of oceanic water onto the shelf that make it into Marguerite Bay. To assist in understanding the dynamics of the currents in this area, the spacing of stations 64 to 70, which run from the edge of the continental shelf out to the deep ocean, was reduced to between 5 and 8 nm instead of the more usual 21 nm. On 2 May, sampling was done at five of these stations63, 64, 65, 66, and 67.

 

The day was dark and gray, with intermittent snow and fog in the morning. The afternoon was clearer with light winds continuing to be the norm (about 10 kts out of the northwest) and a calm sea. The barometer climbed during the day to 1002.7 mlb, the highest reading yet since leaving Punta Arenas. Air temperatures varied between -0.6 and -1.8C. There has been remarkably little fluctuation in the air temperatures since arriving in the study site.

 

The work at the stations included 4 CTDs one each at stations 63, 64, 65, 66, and a drops of XBTs at station 67 (attempts to deploy XCTDs failed because of electrical problems with the probes and cabling). An APOP cast was conducted at station 66 and a 1-m ring net surface tow was taken at stations 63 and 66. During the transits between stations, BIOMAPER-II was towyoed to below 200 m, and seabird and marine mammal observations were made during daylight when the visibility permitted. Two sonobuoys were deployed along the trackline.

 

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

The CTD group did four CTD casts and two XBT drops over the outer shelf and oceanic part of the 300 line. The stations span the shelf break and show deep temperatures from shelf-like 1.3C to oceanic 1.8C. The mixed layer was mainly 50 m thick, but Station 65 had a thin (20 m) mixed layer for unknown reasons. The most offshore station was clearly within the southern edge of the ACC.

 

The deep CTD casts, starting with station 66 (cast 70) to station 70, use double oxygen sensors to investigate the oxygen hysteresis issue. On cast 70, the oxygen sensors were rather different (by almost 1.0 ug/l) on the downcast, but agreed to with 0.1 ug/l on the upcast. These sensors need some time to soak, so no judgement is made at this time about the relative behavior of these sensors.

 

An attempt was made to do XCTD and XBT at station 67, as a replacement for a lengthy CTD cast. After trying 3 XCTD probes, we could not get a load indicator. Two T5 XBTs were thrown with the deepest lasting to 800 m. After some effort, an XCTD probe was loaded, but we had long passed the station, so only the temperature record was obtained.

 

Station 63 (cast 67, 323 m). There was a uniform mixed layer to 50 m (-1.3C, 33.6 psu, 0.2 ug/l chlorophyll). Energetic layers (5-8 m) occurred to 120 m with thinner layers to 200 m. The deep temperature was 1.3C with no temperature maximum.

 

Station 64 (cast 68, 426 m). There was a uniform mixed layer to 50 m (-1.4C, 33.55 psu, 0.15 ug/l chlorophyll). A warm layer at 60 m was above a weak Winter Water (WW) layer centered at 80 m and weak small-scale variability was in the pycnocline. The deep temperature below 320 m was 1.3C with no temperature maximum.

 

Station 65 (cast 69, 427 m). A uniform mixed layer occurred to 20 m (-1.4C, 33.55 psu, 0.2 ug/l chlorophyll). There was a gradual temperature and salinity increase from the mixed layer to 300 m, with a weak WW layer at 90 m to break the monotonicity of temperature. There were numerous temperature reversals (3-5 m) in the pycnocline. The deep temperature maximum (1.6C) was at 300 m.

 

Station 66 (cast 70, 1136 m). On this cast, there were double oxygen sensors on the CTD, using the par channel. A uniform mixed layer occurred to 60 m (-1.2C, 33.7 psu, 0.3 ug/l chlorophyll) with a weak WW layer at 100 m. There was clear layering between 100 and 200 m. The temperature maximum (1.6C) was at 300 m and there was a temperature minimum (0.9C) at 1100 m.

 

Station 67 (XCTD, XBT). We tried three XCTDs, but could not get any to load. Two T5 XBTs were run, but only a cast to 800 m was obtained. The temperature maximum (1.8C) was at 300 m. There were clear temperature layers from 100 to 350 m.

 

Marine Mammal report (Debra Glasgow)

May 2 was another day of fog, some snow, and variable visibility, although sightability did improve in the afternoon. The Beaufort sea state was 2. We passed a few icebergs and 21 fur seals were counted, some resting at the surface, but most were active. At 1039 ( 68 00.66S; 73 35.25W), a like humpback whale surfaced twice, 600 meters to port, bearing 260 and was swimming fast towards 140. Matt Becker and Erik Chapman, the bird observers, saw this whale and by radio informed me a few minutes later. Although a careful watch was kept, I did not see this whale resurface. Ana Sirovic later picked up some humpback sound on sonobuoy #36 at 1135. This was the only cetacean sighting for the day. Survey began at 0921 on transit to station 64 and ended in poor light at 1649 on transit to station 66.

 

Sea Birds (Erik Chapman and Matthew Becker)

When the sun came up on 2 May, there was a thick fog and calm winds as we traveled to station 65 about mid-shelf at the southern edge of Marguerite Bay. The fog quickly lifted and visibility was excellent for most of the day. In support of the seabird survey, two net tows were done, one at station 63 and a second at station 66. The tow at station 63 caught no diatoms and a good number of amphipods and copepods in the top 50 m of the water column. The tow at station 66, on the other hand, did catch diatoms, and a few amphipods and copepods in the top 50 m. BIOMAPER-II indicated very little zooplankton biomass in the water column in the survey area with occasional patches below 200 m.

 

Bird observations during today's survey were consistent with the open water habitat and limited food availability indicated by BIOMAPER-II acoustics. We observed open water species in low abundance during the survey, with a relatively large number of Antarctic Petrels in the mix.

 

Snow Petrels distribution is believed to be closely associated with ice cover, and this has been generally the case during the SO GLOBEC cruises. However, today, as during last year's cruises, Snow Petrels were observed in small numbers in open water far away from sea ice. This is an interesting finding that suggests prey found in association with ice that is available at the surface is at times limited. During these times, Snow Petrels may supplement their diet by foraging in open water, outside of their preferred ice habitat.

 

A summary of the species and number of individuals of birds and seals within the 300 m transect during 4 hours, 8 minutes of daytime surveys between consecutive stations 63 and 66 is the following:

 

Species (common name)

Species (scientific name)

Number observed

Cape Petrel (Pintado Petrel)

Daption capense

5

Southern Fulmar

Fulmarus glacialoides

4

Antarctic Petrel

Thalassoica antarctica

28

Blue Petrel

Halobaena caeulea

7

Southern Giant Petrel

Macronectes gfiganteus

4

Snow Petrel

Pagodroma nivea

7

 

 

Water Sampling for Microzooplankton (Phil Alatalo)

Survey transect 7 ran inshore to some deep troughs inside Marguerite Bay. Particle abundance and chlorophyll concentration in surface waters decreased steadily from deep water, along the shelf into the Bay until the very inner station, No.55. At that station, the underside of the ice was green and small diatoms were plentiful in the surface and 50 m samples. This transect showed some compositional differences. First, thin, spiky diatoms which looked like Nitzschiaa, were prevalent at stations 47, 51, and 55. Secondly, several tintinnid ciliates were seen at station 52 and 53. Tintinnids are a major class of ciliate protozoans, that secrete a test or shell shaped like a small bell or ice-cream cone. Because of this hard body and distinctive shape, tintinnids are easier to identify and are often observed in the guts of organisms that feed on them, such as fish larvae and krill furcilia. Ciliates, in general, have become more common along this transect, being present at the surface and in the upper pycnocline. The distinctive diatom Corethron was present throughout the outer stations and the ciliate Mesodinium continued to be present in surface waters everywhere except Station 49 and the inshore Station 55.

 

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

The animals used in the combined shipboard and APOP cast measurements on May 2 were copepods (more than 98 % were Calanus). These small crustaceans, very different in body form from the euphausiids, can be very abundant throughout the water column in the SO GLOBEC study site and can be important contributors to the acoustic backscattering at the higher frequencies. These animals are much smaller than krill and have a large portion of lipids in their body. The mean length and standard deviation of the copepods used in the measurements were 4.1 mm and 0.3 mm, respectively. The cast was made at station 66 (67 49.870S; 74 13.350W). In contrast to krill, the sound speed and density contrasts of these copepods were both less than unity. The accurate density measurement was difficult using the current dual-density method, since the density of the seawater and distilled water mixture contained too many micro-particles that altered the density of pure fluid mixture significantly. To obtain a reasonable estimate of the density contrast, error analysis and numerical simulations were performed. Even with the values by taking into account the extreme errors that were reasonable from the measurements, the density contrast was still less than unity. The best estimate from our analysis for the density contrast of the Calanus was 0.995, a number that was almost equal to unity. This result was consistent with what we observed before the density measurement: most animals floated on the surface of the beaker, indicating that these animals were positively buoyant.

 

The mean value of the sound speed contrast from the shipboard measurement was 0.959 and the mean value from the cast was 0.949, with a standard deviation of 0.013. The difference in mean value between down and up casts was 0.005, which was a reasonable value. There was no obvious bias between the down and up casts. However, there was a distinct pattern observed in the sound speed contrast for both down and up casts. The sound speed contrast was more or less a constant from the surface to about 100 m, corresponding to a basically constant sound speed in water, or temperature, within the same depth range. Below 100 m, the sound speed in water increased as a function of depth, with a gradient of 0.12 m/s/m, or 0.08% per meter, while the corresponding sound speed contrast of copepods decreased (negative gradient) with a rate of 0.03% per meter. This negative gradient was very clear and definitive, especially for the down cast (monotonic). This result is consistent with and also confirms what we ob served on the Calanus finmarchicus from a different cruise to Wilkinson Basin, Gulf of Maine in August 1999. The sound speed contrast of these lipid-rich creatures may indeed depend on temperature and depth!

 

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

There was no MOCNESS tow taken during 2 May.

 

The acoustic, video, and environmental sensor survey with BIOMAPER-II continued without interruption between stations 63 to 67. During the course of the day, it was finally determined that the cable between the upper 200 kHz transducer and the echosounder in the towed body was causing the intermittency on that frequency. Careful inspection of the cable for electrical continuity and water leakage did not reveal a cause, but because this cable needed cleaning earlier in the cruise, it is suspected that one or more of the pins on the male connecter were thinned by corrosion and although clean, were not making good contact in one of the female sockets. When a spare cable was used in its place, the transducer began working properly again.

 

On transit across the mid-shelf region between 62 and 63, backscattering was generally low. A few small intense patches of backscattering were seen around 40 m on the 420, 120, and 43 kHz echograms. Further out on the shelf between station 63 and 64, a bottom layer 90 m thick was very well defined on the 120 echogram. Small krill-like patches occurred occasionally just above the top of the bottom layer. There was a near surface layer starting about 50 m that fluctuated from being a weak single layer to a series of thin layers of more somewhat more intense scattering. The latter tended to each be 7 to 10 meters thick, similar in placement and dimensions to those observed in the CTD profiles. There was heavier scattering on the 1 MHz echograms as BIOMAPER-II went through the pycnocline and bottom of the surface mixed layer, which again correlated with the presence of diatoms and radiolarians in the VPR images and the green goo that came up in the 1-m surface net tow at station 66. At the shelf break and beyond, there was low scattering in most of the water column, except for some small-scale surface patches and a very light surface backscattering below the ships turbulence layer, which is about 8 meters thick. A very diffuse mid-depth scattering layer started about 260 m and went down to the bottom of the echograms on the 43 and 120 kHz.

 

Cheers, Peter