Report of Activities on the RVIB N.B. Palmer Cruise 02-04

4 September 2002

 

On 4 September, work in the northern sector of the Southern Ocean GLOBEC survey grid was focused on stations 10 and 16. BIOMAPER-II was deployed at the end of the work at station 11 at midnight on 3/4 September and towyoed almost to station 10. The towed body was brought on deck a mile or two from the actual station location because of pack ice ridging that the Palmer had difficulty getting through with the vehicle in the water. Work at station 11 commenced about 0500 with a pair of CTDs and then BIOMAPER-II was returned to the water for the transit to station 16 by 0800.

 

An under-ice SCUBA dive took place in a large lead in the mid-afternoon and a series of CTD casts were done while the divers were in the water. A balky outboard engine made it difficult for the divers in the Zodiac to make it back the last hundred meters; paddles were needed to get the boat to the Palmer's starboard side. In the early evening, ice collections were made on a large floe and the ROV surveyed its underside for krill furcilia and other plankton. Completing the work was a 1-m MOCNESS tow to about 500 m and a Tucker trawl to collect live animals for experimental work. Just before midnight, BIOMAPER-II was again deployed for the towyo to station 15.

 

September 4 was another day that provided excellent working conditions and good viewing. In the morning, low billowy clouds covered most of the sky, but the sun was trying to burn through so it was bright and the visibility was good. Later in the morning, the mountains of Adelaide Island were showing with a band of clouds down at their base and high clouds hiding their tops. There was a real mix of sun and cloud. As we came onto station 16 around 1400 we neared a spectacular iceberg that resembled a wrecked ship that was listing badly to starboard. A dense fog hung in some areas and visibility was very poor in those directions, while the sun shone in other areas and the visibility was excellent. Sea smoke arose from the open water in the leads because of the cold air temperatures (-15C). By 1600, the clouds had completely cleared from the local vicinity and were only present as a low band close to the pack ice out on the horizon. The snow- and ice-covered mountains of Adelaide Island were a stark white some 30 to 40 nm away. The setting sun produced two sun dogs touching the horizon, each with rainbow coloration. Winds during the entire day were <13 kts out of the southwest (200). Air temperature varied between -12C and -16.5C, while barometric pressure climbed from 1001.0 to 1004.5 mb.

 

CTD Group report (Baris Salihoglu, Eileen Hofmann, Bob Beardsley, Chris MacKay, Francisco (Chico) Viddi, Sue Beardsley)

In the morning of 4 September, the last CTD cast on survey transect 2 was completed at station 10. In the afternoon, we started working on transect 3 and completed the first station (survey station 16) on this transect. At this station we first did five consecutive casts to 350 m with the last cast down to the bottom done according to standard CTD procedure. Eighteen Niskin bottles were closed to obtain water samples for nutrients and biological properties. The repeated casts were made to observe the variability in microstructure in the water column (to be described in a future CMiPS report).

 

At station 10, surface waters were below freezing (-1.837C) and the well-mixed Winter Water layer extended to 75 m. Below this, temperature increased to a maximum of 1.49C at 378 m, after which it again decreased to 1.28C at the bottom (446 m). The salinity at the temperature maximum was 34.70 and that at the bottom it was 34.72.

 

At station 16, the surface waters were above freezing at -1.79C. The well-mixed Winter Water extended to 80 m. As observed at station 10, temperature increased to a maximum of 1.43C at 250 m with a corresponding salinity of 34.67. Below this depth, temperature decreased to 1.08C at the bottom. Bottom salinity was 34.71.

 

The vertical temperature section for transect 1 which extended from off-shelf towards the northern side of Adelaide Island showed that modified Upper Circumpolar Deep Water (UCDW) (>1.4C) extended at least 100 km into the shelf.

 

The other water masses that were observed off-shore of the shelf break were UCDW (>1.6C) and Lower Circumpolar Deep Water (~1.5C) around 200 to 500 m and 800 to 1000 respectively. At the offshore end of the transect, the water temperatures were observed to increase to 2C between 200 and 250 m.

 

The vertical temperature section for transect 2, which is located 40 km to the south of transect 1, showed similar intrusions of modified UCDW and of UCDW. In this case UCDW was not as shallow as was observed on transect 1 but it was below 300 m (whereas at transect 1 it extended up to 200 m at the shelf edge). This structure also pushed the isohalines upward towards off-shelf on transect 1. The isohalines were tilted downwards on transect 2. This may be an indication that the UCDW is intruding onto the shelf from the northern side of the study region.

 

Nutrients (Yulia Serebrennikova and Steve Bell)

The nutrient hydrography for GLOBEC survey grid lines 1 (stations 1-4) and 2 (stations 10-13) was essentially the same as for the transects completed in the southern and central parts of the grid. Overall, it was very similar to that found during GLOBEC II (last year's winter cruise - NBP01-04).

 

The concentrations of nitrate and phosphate were 29 and 2.0 micromolar, respectively, in the upper mixed layer (50-100 m) and 33 and 2.3 micromolar, respectively, in the deep water. Silicic acid concentration increased monotonically with depth from 80 micromolar right below the mixed layer to 140 micromolar at 3000 m. The deep water on the shelf exhibits elevated silicic acid concentrations relatively to those at the same depth off shore. And again 110 micromolar concentration of silicic acid was found both on the shelf bottom and at 900-m depth off the shelf break. The mixed layer silica concentrations were 55 micromolar at station 1 and 75 micromolar elsewhere. A silica minimum (60 micromolar) was observed at station 2 at 75-100 m.

 

Nitrite mixed layer concentrations were 0.1 micromolar for most locations and 0.14 for station 1. A subsurface maximum, 0.16 micromolar, was found at station 2 below the mixed layer and coincided with silica minimum. Ammonium concentrations were 0.2 micromolar at station 1 and 0.4-0.5 micromolar elsewhere.

 

Seabirds (Chris Ribic and Erik Chapman)

Seabird and Crabeater seal surveys were conducted for almost six hours on 4 September as the ship moved south from station 10 to station 16. The survey took place about 35 miles off Adelaide Island, over the continental shelf and relatively far from the ice edge. Ice type varied between small, medium and vast floes in 9 to 10/10ths concentration.

 

Very few birds were observed today compared to the numbers recorded off the continental shelf where there was more open water. The Snow Petrel was the most abundant species, followed by the Adlie Penguin. Snow Petrels were observed flying low over cracks between ice floes, apparently looking for fish or zooplankton. In general, we have not seen many birds over the continental shelf, midway between shore and the shelf-break. In previous studies in the Antarctic, birds have been shown to concentrate near the ice-edge. This appears to be the case during this cruise as well. We continue to see few Crabeater Seals in the northern sector of the grid and no seals were recorded today in the survey transect.

 

A summary of the birds and marine mammals observed on 4 September (YD 247) during 5 hours, 42 minutes of survey time as the ship traveled between stations 10 and 16 is the following:

Species (common name)

Species (scientific name)

Number observed

Snow Petrel

Pagodroma nivea

13

Antarctic Petrel

Thalassoica antarctica

2

Southern Giant Petrel

Macronectes giganteus

1

Adelie Penguin

Pygoscelis adelii

8

 

 

Krill distribution, physiology, and predation (Kendra Daly, Kerri Scolardi, Emily Yam and Jason Zimmerman)

We have continued to use tandem deployments of the 1-m Reeve and 1-m ring nets at the last rendezvous site with the Gould and at stations 4, 13, and 11 to collect live animals for shipboard experiments. The depths of the nets were generally about 5 and 10 m below the surface.

 

The zooplankton composition was similar at most of the sites. Euphausia superba furcilia and juveniles and E. crystallorophias furcilia, juveniles and some adults occurred in all nets tows. Ctenophores, adult males and females of the copepod, Calanus propinquus, fish larvae, chains of salps, and amphipods also were common. At station 16, we completed another SCUBA dive, where we collected some E. superba furciliae, 4 ctenophores, and several samples of sea ice biota from the ice-water interface where furcilia feed.

 

Ingestion and egestion rates and assimilation efficiencies were measured for the furcilia collected in the Reeve and Ring nets. We also routinely measure and stage E. superba from net samples, as well as freezing individuals for dry weight and carbon and nitrogen content.

 

ROV report (Scott Gallager, Phil Alatalo, Alec Scott)

ROV 20 was a late afternoon deployment at 1730 on 4 September at Station 16 (-66 44.746′, -70 10.327′). Ice conditions were 9/10 ice cover with 10 to 50 m diameter floes and compressed ridge lines. The ice team was working on a large floe next to the ship as the ROV was deployed and began a transect out to a ridge line about 100 m to starboard. As the ROV traversed the undersurface of the floe, we observed a smooth undersurface and no furcilia larvae. Just as the edge of the floe and junction with the ridge line was approached, swarms of furcilia appeared in view. The larvae were small, possibly stage 4 or 5 and formed dense aggregations in direct contact with the ice. The ice itself was different in that the light from the ROV was absorbed and not reflected back to the cameras as observed under most conditions. Internal reflectance is common in ice in the process of melting and forming brine channels.

 

The ROV continued down under the ridge and began a transect to the south along the axis of the ridge. Throughout this transect, no furcilia were observed. It was as if they had disappeared. The ROV turned again 90 to starboard on a bearing taking it back across the open floe surface. As the junction between the ridge line and flow was approached, again there were masses of furcilia forming just below the ice surface. The ROV continued back to the stern of the ship and began a fourth transect at a bearing heading for where the ice team was working and had established a transect line of bore holes. The first bore hole was encountered with help from the bridge calling out positions of the ROV strobe light seen through the 1 m thick ice. A pole was inserted from above as the ROV documented depth penetration and characteristics of the ice surrounding the bore hole. The bridge again called out a bearing to the next bore hole on the transect and the ROV began the search. This was repeated five times until the last bore hole was located about 90 m from the ship at the floe-ridge junction. Chris Fristen had an auger extending through the last hole, which was drilled through multiple layers of rafted ice. Krill furcilia were abundant in the area around the hole as the auger was repeatedly removed and replaced in an attempt to suck some furcilia through the hole. Ctenophores and a few larval fish were also present where furcilia were abundant.

 

These observations support the idea of a combination of biophysical processes which control the under ice distribution of larvae. It is likely that as the water flow across the flat surface is disturbed by the ridge line, small eddies and counter currents develop extending 1 meter or more from the ice. The swimming behavior of the larvae interacts with the physical flows to produce aggregations at certain positions. Furcilia larvae are rheotactic, meaning that they orient into and swim against a water current just as fish do in a stream. As the furcilia swim against the local water flow generated by rapid changes in ice topology, their retention time in that particular part of the flow increases causing a localized increase in larval concentration. This increase in local concentration is what we refer to as an aggregation. The larvae become concentrated in the same area of the flow simply because they have similar swimming behaviors. Whether there are other biological cues involved in initiating or maintaining such aggregations are unknown. Aggregations of larvae are in contrast to the large masses of adult krill referred to as swarms. Biophysical processes similar to those described for the larvae may initiate a swarm, but intensification and persistence of the swarm is most likely augmented by both chemical and mechanical signals generated as a result of swimming and for reproduction.

 

MOCNESS Report (Phil Alatalo, Peter Wiebe, Dicky Allison, Ryan Dorland, Scott Gallager, Gareth Lawson)

The MOCNESS tow at station 11 (-66 13.5′S; -70 23.6′W) on 3 September produced an interesting set of samples. The tow position was mid-shelf in the northern sector of the sample grid in continuous pack ice. Here small copepods and small chaetognaths were abundant at depth, decreasing in abundance nearing the surface. Euphausiids, salps, and large chaetognaths commanded the bulk of the tow at depths closer to the surface. A surprising number of fish were caught at all depths. Blown fuses and low batteries caused some technical difficulties at the start of this deployment. Unfortunately, the strobe, which is used to blind krill and fast-swimming organisms, was not operational on this night tow.

 

Biomass was high in the oblique tow to 425 m (net 0). Copepods were by far the most numerous, with chaetognaths, euphausiids, and a medium sized fish, Electrona, making up the rest of the sample. The deepest sample (425-350 m) was surprisingly low in biomass, with no euphausiids present. A few more organisms were caught between 350 m and 250 m, consisting of another Electrona fish, many small copepods, and radiolarians. Again, slightly more biomass was present at the next interval (250-150 m), with salps, euphausiids, and chaetognaths together with copepods comprising the main taxa. This mixture of taxa was maintained until about 75 m, where copepods decreased in abundance dramatically. Salps, euphausiids, and chaetognaths continued to be the main taxa present. Pteropods, including gymnosome larvae of Clione were notable between 150 and 25 m. Ostracods continued to be found in nearly all samples above 150 m. An unusual-looking icefish was caught between 50-25 m.

 

Large amounts of water volume filtered during 1-m MOCNESS tow 11 contributed to a good OPC data set. OPC data showed relatively high counts densities between 500 and 1200 individuals/m3 below the approximate mixed layer depth of 100 m, with highest densities between 300 and 350 m. Net 4, which sampled from 150-100 m was unique, dominated by small count-sizes below 0.8 mm with very few large individuals present in comparison to other nets. The surface nets had fewer counts, but contained predominantly larger individuals between 1.0 and 2.25 mm. Biovolume values ranged between 20 and 100 mm3/m3 with high deviation below 300 m. Acoustics showed moderate scattering above 100 m from the 153 kHz ADCP and high scattering between 300 and 350 m on the 38 kHz Simrad Echosounder. Currents from the ADCP were weak during the tow ranging between 5-10 cm/s to the east-southeast.

 

BIOMAPER-II group report (Gareth Lawson, Peter Wiebe, Scott Gallager, Phil Alatalo, Dicky Allison, Alec Scott)

Our first towyo of September 4 was between stations 11 and 10 along broad-scale survey transect 2. Later in the day, we had the BIOMAPER-II in the water for a second towyo as we steamed from station 10 on transect 2 to station 16 on transect 3. A shallow scattering layer was evident for most of both tows, generally centered at 25 m and approximately 50 m in vertical extent. VPR observations indicated that the plankton community between 25 and 100 m depth again consisted of a diverse species assemblage that included krill, copepods, radiolarians, and diatoms. We also observed a few pteropods.

 

As was the case yesterday, the shallow layer was often partitioned into vertical bands, alternating between high and low levels of backscatter at intervals of about 200 m. In order to test whether these bands relate to density-driven circulation cells (as we speculated in yesterday's report), at one point in the second towyo, we kept the towbody at a fixed depth to look for any fluctuations in density. If circulation cells were present, we might have expected to see distinct variations in density as we passed in and out of the cells. Unfortunately, no such fluctuations were evident, although they may have been beyond the sensitivity of our environmental sensors. During this short experiment, the VPR observed numerous large copepods. Further analysis of the VPR data will indicate whether the abundance of particular taxa showed alternations between highs and lows as we passed through these possible cells.

 

Over the course of the two tows, we observed a number of very dense (up to -55 dB) krill-like patches within the mixed layer, often just below the surface. These patches varied from 25 to 50 m in height and 75 to 170 m in length. The BIOMAPER-II never passed directly through any of the patches to make VPR observations of their species composition. At one point, however, the towbody passed five meters under one patch and did capture an image of a krill.

 

We also observed some enhancement in scattering levels below 300 m. A 1-m2 MOCNESS tow conducted in the vicinity of station 11 sampled many small copepods, one myctophid fish (Electrona), and five large krill (Thyssanoessa) in the 250 to 350 m depth range. These latter two groups of organisms are relatively large, and likely are responsible for the observed elevated backscatter. On the one occasion that the bottom was within range (350 m depth), a dense bottom layer was present.

 

Current Position and Conditions

The N.B. Palmer is currently a station 23, the deep offshore station at the seaward end of SO GLOBEC survey line 4. Work started at the station about 2130 and will continue for about 8 hours. Our position at 2334 on 5 September is -66 35.876′S; -72 06.545′W. The air temperature is -9.8C and the barometric pressure is 1008.5 mb and holding steady. Winds are light at around 10-12 kts out of the east (107). Skies are cloudy and a light snow has been falling off and on during the evening, but visibility if good. The pack ice is loosely aggregated with mostly small floes and newly formed pancakes. An ocean swell is propagating through the area.

 

 

Cheers, Peter