AMLR 2005 Weekly Report No. 8

6 March 2005

 

1. Our current position is the eastern end of the Bransfield Strait. The Elephant Island area of the large area survey was completed yesterday. An extensive field of large tabular icebergs emanating from the northwestern Weddell Sea allowed only five stations to be occupied in the Joinville Island Area, the same number as completed during Survey-A. Once again due to concentrated ice none of the planned stations in the northwest Weddell Sea were reachable. A spiteful cold/flu virus has been taking a toll on both the AMLR crew and Russian crew the past week as well.

 

2. Krill, salps and other zooplankton. Elephant Island Area summary: Results from 48 samples collected in the Elephant Island Area were similar to those in the West Area surveyed last week and in both of these areas during the large-scale Survey A: numerical dominance by Salpa thompsoni, copepods (particularly Metridia gerlachei) and post larval Thysanoessa macrura. Greatest differences between these and January results occurred in the distributions and absolute and relative abundance of dominant taxa. Mean and median abundance of S. thompsoni (1211 and 671 per 1000 m3) was reduced by ca. 30% over January values. Greatest concentrations were no longer associated with Zone 1 (ACC) water but with mixed waters associated with gyral circulation and fronts adjacent to the Shackleton Fracture Zone and to the north and east of the Elephant Island shelf. Based on the increased proportions of small solitaries (recently extruded over- wintering forms) and small recently budded aggregates (25% of total mean numbers) the decreased abundance probably results from a variety of factors: advection of the dominant aggregate stage away from the area; their seasonal vertical migration to deeper waters; and/or post reproductive mortality.

 

While median copepod abundance (343 per 1000 m3) was lower than that of salps the mean (1023 per 1000 m3) was greater and overall copepods contributed a greater proportion of total mean zooplankton abundance than salps (37% vs. 32%). Copepod abundance increases ca. 3.8 times those in January reflect seasonal ontogenetic processes rather than introduction of the abundant copepod-rich zooplankton assemblage of the ACC Southern Front. Metridia gerlachei and Calanoides acutus dominated the copepods and respectively contributed 26% and 5% of total mean zooplankton abundance. These dominant copepods demonstrated elevated concentrations in an oscillating band, primarily located over the northern island shelf areas, demarcating likely frontal areas between Bransfield Strait and offshore waters. A second area with elevated concentrations of these copepods was located east of the Shackleton Fracture zone gyre. Post larval Thysanoessa macrura comprised the third most abundant zooplankton taxa and contributed 16% of the total mean abundance. Mean and median numbers of this amphipod (441 and 275 per 1000 m3) exhibited a 2.5 fold increase over the previous survey. Distributions were rather monotonous with elevated concentrations limited to four stations over the north Elephant Island shelf. The larval form was rarely collected.

 

Changes in mean and median post larval krill abundance over the January survey (mean increased from 27 to 48 per 1000 m3, median decreased from 15 to 3 per 1000 m3) reflected primarily distributional factors associated with increased patchiness. Greatest concentrations (to 1121 per 1000 m3) occurred across the northern Elephant Island shelf and offshore to the east of the Shackleton Fracture zone. Large mature forms continued to dominate, with length modes of 45 and 48 mm and another very modest mode at 37 mm. Juveniles contributed <1% and immature stages 13% of total krill. Males and females were equally represented. Advanced reproductive stages (mostly 3c, with developing ovaries) represented 71% of the mature females while reproductively mature males contributed 26% of the total. Concentration of these maturity stages over the northern island shelf area suggests active mating aggregations.

 

Mean and median concentrations of larval krill (195 and 5 per 1000 m3) exhibited, respectively, order of magnitude and five fold increases over the previous month. Five of the greatest concentrations (100-6300 per 1000 m3) occurred in frontal areas north and east of Elephant Island two others were east of the Shackleton Fracture zone. Distribution and abundance of Ihlea racovitzai, a salp characteristic of input of Weddell Sea (polar slope) water into the area, were quite similar to those in January. Greatest concentrations occurred east of Clarence Island, suggesting its westward intrusion there. This was also an area of highest larval krill concentrations.

 

3. Krill biomass and dispersion. We have completed the second acoustic estimates of the krill biomass along the Elephant Island area, and are beginning to sample the Joinville and Southern areas of the AMLR survey grid. During the first leg, the Elephant Island area exhibited krill abundances of ~36 g per m2, and this biomass has declined significantly to just 9 g per m2. This decline in biomass over the second leg is the same order of magnitude as for other years and other surveys in the area.

 

4. Phytoplankton. For the Elephant Island survey, surface chlorophyll concentrations averaged 0.62 0.38 mg m-3, with concentrations integrated as 49 27 and 28 14 mg chl-a m-2 for 100 m and 1% PAR respectfully. These values of phytoplankton biomass are lower than our 15-year time-series of the area (~0.8 0.5 mg m-3 and ~55 30 mg m-2 for surface and 100 m integrated chlorophyll values, respectively). The shelf and shelf-break areas surrounding Elephant Island (and including the Loper Channel) had higher chl-a than pelagic regions, being 0.78 0.36 mg m-3 for surface chlorophyll concentrations (61 31 and 31 14 mg chl-a m-2 for 100 m and 1% PAR respectfully). The highest phytoplankton stocks were found at the Loper Channel, with station D09-08 having 1.5 mg chl-a m-3. The lowest concentrations of chlorophyll were found to the west of the Shackleton Fracture Zone (0.07 mg chl-a m-3 at the surface for stations D08-04 and D09-04) and also at Station D05-03 (surface chlorophyll at 0.07 mg m-3). Higher chlorophyll concentrations often associated with an eddy that develops in February for most years and lying east of the Shackleton Fracture Zone (in the north-east corner of the Elephant Island Survey Area) were not realized this year.

 

5. Oceanography and meteorology. During the week winds were mainly from the North West, but changed to an Easterly direction on Friday. The average wind speeds during the week were around 20 knots, but with the shift in wind direction, to the East, wind speeds increased to over 30 knots for a period of over 12 hours, halting sampling for this period. A significant drop in air temperature was observed on Thursday, with a 6C drop in air temperature over a period of 4 hours. Readings obtained from the Weatherpak/SCS system were compared with the bridge and were found to be similar. The air temperature dropped from 5.1C to -1.7C, this was associated with a decrease in air pressure from 996 to 979 millibars just before the sharp drop in air temperature was observed. After this event the pressure sharply increased to 1004 millibars. During the strong Easterly winds the air pressure dropped again to around 980 millibars. 48 CTD stations were occupied and successfully sampled in the Elephant Island and Northern Joinville Island areas. The southern Joinville Island area could not be sampled due to icebergs in the area. According to the Water Zone Classification table, Water Zone 1 was found in the western part of the Elephant Island area and mainly Water Zone 2 waters found in the North and Northeast. The Southern stations were found to be mainly Water Zone 4 (Bransfield Strait), with the South Eastern stations, below Elephant Island, being Water Zone 5 (Weddell Sea) waters.

 

6. Predator diet studies. All scat samples for weeks 8 and 9 have been processed and the data entered

 

7. Bird and marine mammal observations. Seabird and marine mammal observations continued during between-station transects in the West and Elephant survey areas. Avian highlights included three sightings of Snow Petrels Pagrodroma nivea, one each on 27 & 28 February and 1 March, the first bird staying with the ship for three hours or more. Four Pale-faced Sheathbills Chionis alba were recorded, one on 1 March and three south of Elephant Island on 3 March. Soft-plumaged Petrels Pterodroma mollis were numerous again this week, though in diminishing numbers as we proceeded eastward; a total of 288 were recorded during the period, most in the deeper waters at the northern edge of the survey areas. A sample of 82 individuals seen at close range found 56% of the birds showing evidence of wing molt (adults?) and 44% in fresh plumage (juveniles?). An additional five Kerguelen Petrels Pterodroma (Aphrodroma) breivirostris were recorded during the period, bringing the trip total to 10 birds. Other species of note included a Slender-billed Prion Pachyptila belcheri on 27 February and a total of five Light-mantled Sooty Albatrosses Phoebetria palpebrata during the period. Once again, no substantial feeding aggregations of birds were found, though high daily totals of several species were recorded, all involving mostly unidirectional flights, perhaps of birds in transit between breeding sites and feeding areas offshore. Among these were daily totals of 425+ Antarctic Prions Pachyptila desolata north of Clarence Island on 4 March; 360+ Black-bellied Storm-Petrels Fregatta tropica southwest of Elephant Island on 1 March; and 660+ Antarctic Fulmars Fulmarus glacialoides southwest of Elephant Island on 1 March and another 840+ southeast of Elephant Island on 3 March.

 

Marine mammal observations were highlighted by two sightings of porpoising Southern Bottlenose Whales, two individuals on 28 February and two more on 4 March. A large pod of Long-finned Pilot Whales, numbering 70+ individuals, was along side the boat for 20 minutes or more on 28 February, accompanied by a few Hourglass Dolphins. Fin Whales continued to be numerous, with as many as 28+ individuals noted per day, and at least one or two Minke Whales were sighted.

 

 

Submitted by A. Jenkins.