AMLR 2003 Weekly Report No. 4

2 February 2003


1. Our current position is approximately one-third of the distance across Drakes Passage in route from the South Shetland Islands to Punta Arenas, Chile. Operations have been completed for the first leg of the AMLR 2003 cruise. During the two-days in port, some personnel will be exchanged, provisions and fuel will be brought aboard, and the ship will again sail on 7 February.


2. Earlier in the week an extensive band of ice, presumably streaming out of the northwestern Weddell Sea, was encountered in the eastern part of the survey area. Planned stations for the eastern portion of the Elephant Island Area, most of the Joinville Island Area, and all of the northwestern Weddell Sea had to be cancelled. Instead transects were conducted across some of the more interesting features mapped earlier in the survey, including two transects in the vicinity of the Shackleton Fracture Zone. Calls were made at the Admiralty Bay and Cape Shirreff field camps to recover personnel (L. Rektoris and M. Goebel) and garbage.


3. Krill, salps and other zooplankton. Post-larval krill (Euphausia superba) were present in 77 of 83 (93+ACU-) samples collected during the survey.  Mean and median values, respectively, of 193 and 14 per 1000 m3 reflect their broad, fairly homogeneous distribution across the entire South Shetland Island area.  Among the three adequately sampled subareas, catch frequency and size were greatest around Elephant Island (97+ACU- of samples, mean+AD0-319 and median 91 per 1000 m3) vs. the West Area (96+ACU-, 38 and 8 per 1000 m3) and South Area (78+ACU-, 87 and 1 per 1000 m3). Although krill lengths ranged from 13 to 54 mm, 80+ACU- were 22-39 mm and centered around 30-33 mm. These individuals resulted from strong recruitment of the past two year classes (i.e., 2001/2002 and 2000/2001).  Accordingly, juveniles constituted 45+ACU- and immature stages 40+ACU- of the total catch.  Length and maturity stage composition showed modest spatial variation with older individuals occurring in the West (37 mm median length, 45+ACU- immature and 36+ACU- mature stages) and younger individuals in the South (31 mm median length, 48+ACU- juvenile and 49+ACU- immature stages).  Length and maturity stages were more evenly represented in the Elephant Island Area.  Here lengths were centered on 25-28 mm and 35-37 mm modes representing juvenile (42+ACU-) and immature (39+ACU-) stages.  Few (+ADw-5+ACU-) of the 4200 krill examined were reproductively mature. Based on body morphology the general impression is that seasonal spawning activity has been delayed due to poor feeding conditions. However, larval stages (predominantly earliest calyptopis stages) present in 36+ACU- of the West and Elephant Island samples (mean values ca. 4 per 1000 m3) indicate that some spawning has occurred within the past six weeks. This possibly has been by +AD4-50 mm krill that have been notably absent from (but presumably upstream of) the survey area. 

Copepods numerically dominated the zooplankton catch, with mean and median values of 610 and 183 per 1000 m3, respectively.  Oceanic species, Calanoides acutus and Calanus propinquus, were present in 99+ACU- of samples with moderate abundance values (e.g., medians 14-55 per 1000 m3) in all three areas.  The coastal species Metridia gerlachei was less frequent (84+ACU- of samples) but exhibited extreme patchiness, with dense concentrations in Bransfield Strait and over island shelves, and a much higher overall mean value than the oceanic species (340 vs. 70-80 per 1000 m3). Post-larvae of the euphausiid, Thysanoessa macrura, were present in all samples and followed total copepods in overall abundance, with mean and median values of 244 and 123 per 1000 m3. Unlike previous January surveys when post-larval and larval T. macrura demonstrate distinct, diametrically opposed onshore-offshore distribution patterns, the larvae were essentially absent and post-larvae were evenly distributed across the entire survey area. Mean and median abundance of post-larval krill ranked third after T. macrura. The salp, Salpa thompsoni, was the fourth most abundant taxon. It was present in 82+ACU- of samples with overall mean and median values of 63 and 10 per 1000 m3. Aggregate stages comprised 98+ACU- of the total. Lengths ranged from 4-53 mm with 80+ACU- of individuals +ADw-36 mm a median length of 25 mm. These individuals were probably produced since early November following the seasonally late sea ice retreat in the Antarctic Peninsula region.

With respect to the long term AMLR data set, post-larval krill abundance in the Elephant island area during January 2003 was the highest observed over the past 12 years with mean and median values 3× the highs recorded in 1996. This is the direct result of two successive years with good recruitment success. The delayed spawning activity during this month is similar to that observed during 1998 and larval abundance values are similar to those during January 1996.  Neither of those years was noted for good year class success. Relatively small salp catches this year are similar to those in January 1992, 1995 and 1996 and typical of seasons following above average winter sea ice extent and duration.


4. Krill biomass and dispersion. Highest densities of krill were acoustically mapped in the vicinity of the shelf break north of Cape Shirreff, Livingston Island, along the shelf break between King George and Elephant Islands, and offshore at the northern margin of the survey grid near the Shackleton Fracture Zone. Krill were relatively scarce in the latter area when it was re-surveyed. Mean biomass densities ranged from 11 to 27 g m-2, highest in the West Area, lowest in the South Area, and indicated a modest but increasing trend from densities observed in January 2001 and January 2002.


5. Phytoplankton. Phytoplankton biomass measures continue to be low, with 5 m chlorophyll values of 0.2-0.1 mg m-3 (3 stations) in the Joinville Island Area, and 0.38-0.14 mg m-3 (7 stations) along the 06 transect in the Elephant Island area. For stations 06-04 through 06-08, chlorophyll concentrations were slightly higher than values obtained for neighboring stations of 05.5-04 through 05.5-08 with 0.19 mg chl m-3 (Jan 24) and 07-04 through 07-08 with 0.26 mg chl m-3 (Jan 23). Although these differences are slight, SeaWiFS chlorophyll images indicate slight increases in phytoplankton biomass in this vicinity during 2nd and 3rd weeks of January. Integrated (to 100 m) chlorophyll for the Elephant Island transect was 26.5-6.8 mg m-2 and for the Joinville Island Area was 24.65-4.27 mg m-2. These values are much lower than the 12-year mean for January. Satellite images of the Drake Passage (January 20-24) show a broadening of a blue water zone having exceptionally low surface chlorophyll concentrations (+ADwAPA-0.3 mg m-3) extending to near the shelf region of the Shetland Islands and Elephant Islands. The eddy-like structure of enhanced chlorophyll east of the Shackelton Fracture Zone (SFZ) has dissipated much over the previous weeks. Chlorophyll concentrations measured at 5 stations along a transect at 59.75ēS, and due east of the SFZ, averaged 0.12  0.08 mg chl m-3 at 5 m, with an integrated (to 100 m) value of 13.96-4.62 mg chl m-2. Ocean currents, as measured by 11 drifter buoys released this Leg, show southwesterly flow along the continental shelf of King George and Livingston islands, a complex +ACI-circular+ACI- flow in the shelf/break region between Elephant and King George Islands, and southerly flow along the western SFZ that abruptly turns easterly north of the shelf break for Elephant Island.


6. Oceanography and meteorology. Water zone mapping was inhibited by extensive ice cover over the eastern portion of the survey grid. The overall picture clearly shows the Zone I ACC water following the continental shelf edge north of the islands, Bransfield Zone IV in the western Bransfield Strait east to King George Island with a tongue extending north of the islands. A small area of Zone II Transition water was identified in the southwest and northwest corners of the survey area. The easterly portion of the survey area was dominated by Zone V Weddell Sea water, accompanied by an extended field of icebergs. Evidence of an intrusion of Weddell Sea water across Bransfield Strait and extending into the coastal waters north of the South Shetland Islands was observed. Field classification of this water zone using previous criteria identify it as Zone V+ADs- however, further analysis of other hydrographic parameters may be required to confirm these tentative conclusions as some of these stations may include Zone III and IV waters.

Excellent weather was experienced with some warm, sunny periods on Wednesday, Friday and Saturday+ADs- maximum air temperature reached 7.2ēC on Friday. Moderate temperatures between 1-4ēC were experienced during the rest of the week. Winds were predominantly from the east, mostly between 10-25 kt (maximum of 32 kt), dropping later in the week as the wind swung south and southwest. Air pressure was variable, peaking at a high of 1007 mb on Tuesday, dropping steeply on Wednesday with stronger winds and settling to a Saturday low of 989 mb before rising again on Sunday.


7. Predator diet studies. 30 fur seal scats and 1 enema were processed. All samples contained krill, 11 samples contained fish (otoliths present) and 3 samples contained squid (squid beaks present). 8 myctophids (Electrona antarctica, E. carlsbergi and Gymnoscopelus braui) were dissected for stomach content and fullness, sexed, length and weight measured and otoliths were removed and saved. These myctophids were also homogenized for lipid extraction. In addition, krill were collected at 17 stations which will be homogenized as well for lipid extraction. 127 fresh krill have been sexed, measured for total length and carapace length and width.


8. Bird and marine mammal observations. Cape petrels, Antarctic fulmars, and chinstrap penguins were conspicuous in the pelagic waters north-west of Elephant Island, particularly in the vicinity of dense krill swarms near the insular shelf, and the north-west boundary of the survey. The largest aggregation of birds was observed in northeastern Bransfield Strait, which was dominated by Antarctic fulmars, chinstrap penguins, and Wilson's storm petrels. During the first leg of the AMLR 2003 cruise (excluding only shore visits to the South Shetland Islands and the northbound crossing of the Drake Passage), approximately 140 hours of observations were conducted, and 29 bird species were observed. Cetacean sightings during the first leg included humpback, minke, fin, long-finned pilot and southern bottlenose whales.