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

19 April 2002

On April 19, the N.B. Palmer was working on survey line 4 mostly in the mid-continental shelf region just north of Marguerite Bay where water depths were around 500 m. Weather on 19 April was again very good. Winds during the day were out of the north (000) about 15 kts and the seas had only a moderate swell. The air temperature remained steady at about -0.4ºC and the barometer was up a bit at 991.4 mlb . Sea surface temperature was -0.463ºC. There were high clouds hiding the sun and there was no blue sky. But there was a glint of sunlight at the horizon to the north. The visibility was very good. At mid-morning, the ship passed close by a very large and beautiful iceberg, which was accompanied by patches of brash ice. Ice bergs were seen off in the distance for a good portion of the day.

Work was completed at stations 18, 19, 20, and 21. The CTD was deployed at all of the stations with the microstructure sensor package and the FRRF, with the exception of station 19, which was too deep to deploy the FRRF. A 1-m ring net tow for surface zooplankton was done at Station 19 and a MOCNESS tow was done at station 21. An APOP cast was also done at this station with adolescent krill individuals, which have been kept alive since they were caught at station 7. Underway measurements included the sea bird and mammal surveys and BIOMAPER-II towyos between all of four stations. Two sonobuoys were also deployed along the trackline.

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

The CTD group did four casts today almost completing the 380 line across the shelf. These stations continue to be typical of the shelf without oceanic influence, even station 21 on the outer part of the shelf. The middle/outer shelf stations have high surface chlorophyll compared to most measurements made on this cruise. The Winter Water (WW) layer at all stations has nearly eroded away, likely due to the deepening wind mixed layer that has penetrated to WW layer. Small-scale variability, double diffusive layering, continues to occur in the pycnocline of most stations.

Station 18 (cast 21, 379 m). There was a 20 m thick surface layer with increasing temperature and salinity, while the mixed layer extended to 55 m. Surface chlorophyll was 0.2 ug/l. The WW layer had nearly eroded. Weak layering was occuring in the pycnocline. There was no temperature maximum and bottom temperature was 1.3ºC.

Station 19 (cast 22, 615 m). The Surface layer was uniform to 60 m. Surface chlorophyll was 0.2 ug/l. Thick (10 m) layering was seen in upper pycnocline. There was a thin temperature maximum (1.5ºC) at 300 m.

Station 20 (cast 23, 484 m). The surface layer was uniform to 50 m. Surface chlorophyll was 0.55 ug/l. Small scale variability occurred in pycnocline. Temperature below 200 m was uniform at 1.3ºC.

Station 21 (cast 24, 467 m) The surface layer had uniform temperature and salinity to 50 m. Surface chlorophyll was 0.4 ug/l. Thicker (10 m) layering was occurring in the upper pycnocline. The temperature maximum (1.3ºC) was at 300 m.

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

The hydrography group is currently analyzing the water velocity information collected with the shipboard Acoustic Doppler Current Profiles (ADCP) on the N.B. Palmer. The system was set up by a project from Eric Firing (University of Hawaii) and Teri Chereskin (Scripps Institution of Oceanography, UCSD) and automatically collects, calibrates, and produces a series of graphs and data files with the magnitude and direction of the currents along the ship track. Thanks to this system, any scientist on board can have a graph of the change of the velocity field in the current cruise without the difficulty of manipulating the raw data. More details on how this works can be found at http://peale.

The work of hydrography group is to make sense of the graphs that are automatically produced by the system by further processing this information, plotting the data, and calculating other parameters that help understand the dynamical processes in the West Antarctic Peninsula and their influence on the biology. One example of this is the vertical structure of the currents and its temporal and spatial variation. How significant are the differences between the currents in the upper mixed layer and the deep currents? The ADCP data, as well as those from the casts, will be used to address these and other questions.

Nutrients Status Report (Rob Masserini)

Nutrients are being analyzed from water samples taken from the CTD casts by Yulia Serebrennikoiva and Rob Maserini. To date, the first four survey lines transect of the SO GLOBEC grid, which included 22 stations, have been completed, and nutrient analyses have been run on 492 seawater samples taken from rosette and the data processed. Essentially, every niskin bottle on every cast is sampled. The seawater samples are analyzed for dissolved inorganic nutrients which include nitrate, nitrite, ammonia, phosphate, and silicic acid. Thus, 2460 individual analyses have been completed, not including the approximately 4000 associated standards and other quality control analyses that are run to insure that our data are of the highest possible quality, accuracy, and precision. The analysis of seawater samples have been conducted according to the JGOFS/WOCE suggested protocols for nutrient analysis.

Nitrate and phosphate exhibit the classic distribution of these nutrients, being of higher concentration at depth increasing slightly just below the mixed layer and then decreasing substantially within the euphotic zone. The approximate concentrations (all concentrations are given in micromolar units) for nitrate and phosphate in these regions are as follows: deepwater 32.8 and 2.32, just below the mixed layer 35.3 and 2.47, euphotic zone 22.5 and 1.6, respectively. Nitrite and ammonia concentrations are essentially zero below the mixed layer (zero here defined as less than the detection limit of the chemistries employed). Within the mixed layer the average nitrite and ammonia concentrations are approximately 0.23 and 1.6, respectively. Within the mixed layer, ammonia exhibits an enrichment in the near shore stations at a depth of approximately 50 meters to 1.8 micromolar. The ammonia concentrations in the mixed layer seen thus far are comparable with those of the Fall SO GLOBEC I cruise last year for the same region, which are higher than the Winter SO GLOBEC II cruise values. The nitrite concentrations exhibit a subsurface maxima near the bottom of the mixed layer increasing to roughly 0.33 micromolar. Also, approximately twenty nautical miles from the furthest offshore station, a region of even higher nitrite concentrations have been sampled between approximately 70 and 100 meters, with nitrite reaching a concentration of roughly 0.4 micromolar. Silicic acid, like ammonia, exhibits an increase in concentration near shore within the mixed layer. In general, it also exhibits a classic nutrient structure with an average concentrations below the mixed layer of roughly 110 micromolar decreasing to 50 micromolar within the euphotic zone. One feature of note in the silicic acid data is the presence of a bolus of depleted silicic acid at a depth of approximately 260 meters associated with 1.8º water seen at station 9 on transect 2. This water mass had a silicic acid concentration of approximately 90 micromolar, 10 micromolar less than the water surrounding it.

Marine Mammal report (Debra Glasgow)

April 19 was the best day of observations so far on this voyage with 31 fur seals counted (the majority were resting at the surface) and 7 whale sightings with a total of 21 whales. These were 2 minke, 3 unidentified baleen whales, 2 humpback-like whales and 14 humpbacks, including one juvenile humpback. Surveying began at 0841 as we started our transit at -67 12.30ºS; -70 14.29ºW to Station 20 and ended as we reached Station 21 in fading light at 1730. Though overcast, good visibility and a sea state of Beaufort 3 persisted throughout the day creating good conditions for cetacean observations. Most of the whales seemed to be surface feeding or were socializing and three groups approached the ship closer than 200 meters. Identification photographs were taken of two groups of humpbacks.

Sea Birds (Erik Chapman and Matthew Becker)

The sea bird survey in the mid-shelf region between stations 19 and 21 lasted almost 7 hours on 19 April and once again, we saw a dramatic shift in species assemblage and densities today. There were few birds overall for the transit distance and for the first time, Snow Petrel was the most common species observed. This species is typically associated with sea ice, though we did not travel through ice today. The Snow Petrels were either following the ship or flying relatively high in groups of 5 or more, probably in transit to some other place. We did not see much feeding by any birds today. Once again both Cape Petrel and Southern Fulmar were seen in similar numbers. More birds in general, and more Cape Petrels specifically are being observed off the shelf, and it will be interesting to see if the same pattern is observed during tomorrow’s survey in deep water.

A surface tow was taken at station 19 and the catch was again dominated by a lot of green diatom sludge, with amphipods and copepods also in the mix. The BIOMAPER-II crew directed us toward some regurgitated stomach contents from an unknown bird that appeared overnight on the helicopter deck. The bird was gone, but we found 8 or 9, fresh 35 to 45 mm Euphausia superba on the deck. Because Southern Fulmars are rarely found on the ship, this bird was probably a Cape or Snow Petrel that appeared to have eaten mostly crustaceans and had obviously had a pretty good meal of krill very recently. The bird probably landed on the ship overnight and may have been feeding on adult krill that migrated toward the surface in the evening. A summary of the 6 hours, 45 minutes of daytime survey results is the following:

Species (common name)	Species (scientific name)	Number observed
Cape Petrel (“Pintado Petrel”)	Daption capense	30
Southern Fulmar	Fulmarus glacialoides	25
Antarctic Petrel	Thalassoica antarctica	0
Blue Petrel	Halobaena caerulea	0
Unidentified Prion		0
Grey-headed Albatross	Diomedea chrysostoma	4
Wilsons Storm-petrel	Oceanites oceanicus	3
Unidentified Skua		0
Snow Petrel	Pagodroma nivea	41
Southern Giant Petrel	Macronectes giganteus	5
Antarctic Tern	Sterna vittata	0
Antarctic Fur Seal	Arctocephalus gazella	26

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

In the evening after the MOCNESS tow at station 21, the second APOP cast of the cruise was conducted in the same way as reported earlier (at 20 m depth intervals down to 200 m). Since attempts to capture more live krill over the past two days with the Reeve net have not been successful, the juvenile krill caught on the 15 April were used. These krill had been kept alive in the aquarium. This cast was intended to compare the material properties of the juveniles to those acquired for the adults a couple of days ago. The exoskeleton of these smaller juvenile krill feels softer than larger adult krill and they are more fragile than adult krill. More than 40 juvenile krill were put into the APOP acoustic chamber. The average size of these krill was 26.9 mm, with a standard deviation of 6.1 mm. After the cast, all animals, but one, were alive and a few of them seemed sluggish, with visible color change on part of their body (from almost transparent to pale). After putting them into a bigger container, the color changed back to transparent. The preliminary results showed that the sound speed contrast of these juvenile krill was smaller than that of the adult krill. There was slightly an increase in sound speed contrast as a function of depth (less than 1%). There was a small difference between the down cast and up cast. The mean sound speed contrasts were 1.0145 and 1.0212 for down and up casts, respectively, and the overall mean value was 1.0179, slightly lower than that of the adult krill (1.0238).

About a half hour after the sound speed measurements, the density of the same krill were measured. Their density contrast was 1.007 +- 0.005, which is much lower than the adult krill.

Combining the results from sound speed and density measurements, it appears that the juvenile krill are less efficient scatterers than adult krill. This result, however, is preliminary and more measurements are needed.

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

MOCNESS #6 was conducted in the middle portion of the continental shelf along transect line #4 at Station 21. This station is situated off of Adelaide Island and north of the mouth of Marguerite Bay proper. The tow was conducted from 0-440 m of depth (water depth about 490 m) and was done in the early evening, after dark. Euphausiids (krill) were found from 100-440 m. Species of the genera Thysanopoda and Thysanoessa were among those present. Gelatinous organisms were found in the 200-440 m range. Krill furcilia may have been present from 150-200 m, but were absent otherwise. Copepods were quite numerous from 200-440 m and then again from 0-100 m. During the sample processing, blue flashes of luminescence were observed in some of the near surface samples indicating the presence of Metrida. Green phytoplankton matter was found from 25-75 m, originating likely from dead and decomposing diatom chains. Black amphipods were numerous, along with copepods, in the upper 25 m.

The towyoing with BIOMAPER-II went smoothly during 19 April, as we steamed between station 18, 19, 20 and 21. At station 21, the towed body was brought onto the deck to make way for a MOCNESS tow at this station. During the late night transit between stations 18 and 19, a number of interesting backscattering targets were observed just below the surface turbulence zone created by the ship in the 15 to 30 m depth zone. They were very intense and had fairly small vertical and horizontal dimensions. These features were observed infrequently on earlier survey lines, but now they seem to be occurring more persistently. In the daylight period of steaming between stations 19 and 20, the backscattering on all of the frequencies was relatively light and there were no major patches of scatterers evident, except for a few right at the surface. At many locations along the transect line especially in mid- and outer shelf areas, the 1.0 MHz transducers had high backscattering levels in the 0-60 m depth interval presumably from the very high diatom concentrations that can be observed on the Video Plankton Recorder. A diatom bloom of significant proportions has been occurring in the northern portion of the SO GLOBEC survey grid. Upon leaving station 21 around 2300, backscattering levels in the surface layer were moderately strong on the 120 to 1000 MHz ducers, but very little backscattering was evident on the 43 kHz echogram. The midwater zone for the 43 kHz had a now familiar faint scattering layer 240 to 300 m below the surface. This coincided with where the deep scattering layer started on the 120 kHz echogram. The latter intensified with increasing depth to the bottom, which was greater than 500 meters.

Cheers, Peter