AMLR 2006 Weekly Report No. 3

29 January 2006

 

 

1. Our current position is the Bransfield Strait, South Shetland Islands. The Elephant Island Area was finished 27 January all stations were completed without incident. An extensive field of large icebergs emanating from the northwestern Weddell Sea allowed only five stations to be occupied in the Joinville Island Area and none of the planned stations in the northwest Weddell Sea were possible. The weather and sea state continues to be on our side.

 

2. Krill, salps and other zooplankton. Elephant Island Survey Area Summary. Postlarval krill (Euphausia superba) were present in 39 of 48 (81%) Elephant Island area samples. Their distribution was rather uniform except in the northwest section adjacent to the Shackleton Fracture Zone. Greatest concentrations (100-300 per 1000 m3) were over the outer shelf area northeast of Elephant Island. The overall mean and median abundance values (24 and 11 per 1000 m3) were, respectively, two times and 30% greater than in the West Area. The size and maturity stage composition here were much more diverse than in the West. Although large animals were well represented half of the individuals were smaller than 50 mm and one and two year old length categories (25-35 mm and 36-45 mm) contributed 3% and 25% of the total. The majority of krill were represented by mature stages while juveniles and immature stages made up 7% of the total. Males and females were equally represented and, as in the West area, the majority was actively reproductive with gravid and spent females comprising 27% of the total catch. Relatively large proportions of females (17% of total krill) also evidenced ovarian development and could be undertaking a second spawning bout due to the prevailing elevated primary productivity conditions.

 

Larval krill were substantially more abundant than in the West area and, being predominantly early calyptopis 1 stages, reflected recent ontogenetic migration into the upper water column from spawning initiated ca. three to four weeks earlier (i.e., late December-early January). Greatest larval krill concentrations (10,000-20,000 per 1000 m3) were northeast of Elephant Island, areas characterized by complex fronts and gyres downstream of the Shackleton Fracture Zone. Salpa thompsoni occurred in 68% of samples with mean and median abundance values (63 and 9 per 1000 m3) similar to those in the West. Like the West area, largest concentrations were associated oceanic Antarctic Circumpolar Current (ACC) waters offshore of the island shelf region and adjacent to Shackleton Fracture Zone. These salps were rare or absent south and east of Elephant Island. As in the West area copepods were by far the most abundant zooplankton taxon represented by 3700 and 2300 per 1000 m3 mean and median values, respectively. Dominant species in order of abundance were Metridia gerlachei, Calanoides acutus, Rhincalanus gigas and Calanus propinquus. Greatest concentrations (9000-17000 per 1000 m3) occurred in ACC water adjacent to the Shackleton Fracture Zone. Here the co-occurred with elevated concentrations of chaetognaths, radiolarian, Limacina helicina and larval Thysanoessa macrura in association with presence of the ACC southern front.

 

3. Krill biomass and dispersion. The preliminary estimation of the biomass of krill in the elephant island sub area has completed using the EK-60. Mean krill density averaged ~2.2 gm2 along the first five transects, and averaged 25 gm2 along the final two transects in the Elephant Island area. These numbers are an order of magnitude lower than last year. Consequently, biomass in the elephant island area was concentrated on the southeastern side of the island, and the biomass was only 400K tons. Similar to the situation on the west shelf, biomass in 2006 is a quarter of the biomass (1.5 Mtons) than in 2005. We will estimate krill biomass using the SDWBA approximation in the final report.

 

4. Phytoplankton. Continued high biomass was encountered in the Elephant Island Area. In the region north of Elephant Island and the Loper Channel, 5 meter chlorophyll averaged 1.7 1.5 mg m-3 (29 stations), with integrated values of 73 50 and 48 25 mg chl-a m-2 for integration to 100 m and 1% PAR, respectively. Similar values were also found in the Bransfield Strait portion of this area, with 1.6 0.7 mg chl-a m-3 averaging at the surface (13 stations), and 100 m and 1% PAR integrated values of 82 39 and 45 16 mg m-2, respectfully. Highest values were found in the Loper Channel (shelf area between Elephant and King George Islands), where surface values averaged 4.5 1.7 mg m-3 (7 stations), and 100 m and 1% PAR values were 184 71 and 80 11 mg m-2, respectfully. Highest chl-a concentrations were found in the Loper Channel at Station A07-07 where 7.0 mg m-3 was measured at the surface and 338 mg m-2 measured as integrated to 100 m. The lowest phytoplankton biomass recorded was at station A09-02 (a northern station over the Shackleton Fracture Zone) where 0.10 mg m-3 was measured for the surface. Of 49 stations in the Elephant Island area, only 4 stations were found in which surface chl-a measured below 0.5 mg chl m-3.

 

Ancillary projects have measured in-situ radiometric, backscattering and fast repetition rate fluorometry data from 7 stations, with samples taken for 35 CHN, 49 HPLC pigment, and 32 photosynthesis verses irradiance curves. Four stations with 60 samples have been taken for particulate and dissolved iron, iron speciation and organic ligands. Also, 22 radium samples have been analyzed, plus 163 thorium-234 for vertical transport analysis, and 10 deep water (2500 meter) samples taken for calibration. A total of 18 drifters have been deployed, including 12 around Elephant Island and 6 around King George Island.

 

5. Oceanography and meteorology. The CTD instrumentation worked reliably and the onboard Guildline salinometer salinity verifications, done on the Niskin bottle samples, showed consistent agreement, with an average difference of -0.0057 ppt. An additional 2500 m deep CTD cast was done at Station A03-02 to obtain water samples for the thorium work. The 97 processed CTD casts data were used for in-field water-zoning, across the Elephant Island and Joinville Island Areas. These areas produced the classical scenario of Water Zone 1 types off the shelf break to the west of, and around, the Shackleton Fracture Zone with some Water Zone 2 stations being found off the shelf, to the north of Elephant Island. Complex mixing patterns on the shelf and shelf break in the Elephant Island Area produced Water Zone 1/2, 2, 3 and 3/4 types with Water Zone 4 stations to the east and south of Elephant Island and in the Joinville Island Area. Water Zone 5 stations were sampled at the southernmost ends of line 02, 04 & 06, before ship progress along these lines had to be prematurely terminated due to icebergs. The first half of the week saw a consistent Northwesterly averaging between 10 and 15 knots. A wind direction swing to the southeast, and a barometer drop from 996 to 977 millibars during Thursday was accompanied by rising wind speeds reaching periods of 35 knots on Saturday.

 

6. Predator diet studies. Lipids have been extracted from 36 Antarctic fur seal milk samples. At the commencement of the nearshore survey, more milk samples will be arriving onboard on 2 February 2006. Additionally, 30 more scat samples will be arriving from weeks 5-7. A total of 31 scat samples from weeks 1-4 have been processed to date. Only three scats individually contained one single otolith showing a low occurrence of fish in the diet in the first four weeks of scat collection. Krill total length, calculated from the carapace length and width, ranged between 36-60 mm with the majority of total lengths averaging between 50-55 mm. This shows a dominance of large krill taken by Antarctic fur seals at Cape Shirreff, Livingston Island. This is also reflected in the zooplankton net tows where more than 50% of the krill were greater than 50 mm in total length for the West Area (per. comms. V. Loeb) where the fur seals are foraging. In order to continue with developing regression equations and discriminate functions for calculating krill total length and sex respectively from the scats, 150 krill will be measured over the next week (collected from the net tows). There have been 18 myctophid fish and one juvenile ice fish that have been caught in the net tows that will be measured for total length, weighed, sexed and otoliths removed for the otolith library. They will be homogenized according to species for lipid extraction and fatty acid signature analysis (at the SWFSC).

 

7. Bird and marine mammal observations. Data on the distribution, abundance and behavior of seabirds and mammals was collected during underway ship operations in the Elephant Island area. In total, 37 transits between stations were collected for approximately 685 nautical miles of sampling effort (78% of the stratum). Feeding activity by seabirds was relatively uncommon throughout the shelf break region and offshore waters, and there were substantially fewer multi-species feeding aggregations observed in comparison to AMLR 2005. Interestingly, seabird diversity and feeding intensity was greatest along transits between stations on lines 2 and 3 (eastern most transects) where the greatest estimated krill density was recorded by the hydroacoustic system. Soft-plumaged petrels, Antarctic Prions, and Blue Petrels were especially abundant on route between lines 2 and 3, and were predominantly flying into the headwind. Black-browed and Gray-headed Albatrosses were also common along the northernmost part of the survey area, and the majority of Gray-headed Albatross sightings were of juvenile and sub-adult birds. As in past AMLR surveys, numerous sightings of Chinstrap Penguins, Black-bellied Storm Petrels and Antarctic Fulmars were the most common birds recorded on transits in the vicinity of Aspland, Gibbs, and Clarence Islands. The largest Chinstrap penguin aggregations occurred in the southeast section of the stratum. For at least 6 days, numerous observations of Wandering Albatrosses (maximum of 7) and Royal Albatrosses were observed as ship followers. There were two Wandering Albatrosses with red breast marks that were observed ship following for at least 3.5 days. On transit to station 02-01, three Killer Whales, two Southern Bottlenose Whales, and two Fin Whales were sighted within one 5 minute interval. Humpback Whales were common (1-3 individuals) on transects in the southern portion of the survey area. Minke Whales were also common during the survey of waters at and north of the shelf break zone. Antarctic Fur seals were almost inconspicuous during the survey period, with approximately 10 sightings altogether.

 

 

A. Jenkins sends.