AMLR 2005 Weekly Report No. 2

AMLR 2005 Weekly Report No. 2

23 January 2005

1. Our current position is approximately 20 miles northwest of Elephant Island where we are conducting the first of two surveys of bio-oceanographic conditions in the vicinity of the South Shetland Islands. The survey is divided into four strata: the West Area north of King George and Livingston Islands; the Elephant Island Area encompassing the northern portion of the South Shetland archipelago; the Joinville Island Area in the western portion of Bransfield Strait; and the South Area in the central portion of Bransfield Strait south of King George and Livingston Islands. Survey operations in the West Area and half of the Elephant Island Area were completed this past week. Agreeable weather and sea conditions enhanced progress during the week.

2. Krill, salps and other zooplankton. The 25 IKMT samples collected in the West Area revealed a relatively depauperate zooplankton assemblage clearly dominated by Salpa thompsoni, which occurred in all samples and accounted for 69% of total mean abundance. These were primarily small aggregate forms, half of which were <22 mm in length and, based on an estimated 0.44 mm/day growth rate, resulted from late November-early December production. This roughly corresponds to the time that winter sea ice retreat was observed in this portion of the Antarctic Peninsula. Copepods were the next most abundant taxon, comprising 15% of mean abundance. Over half of copepod individuals were Metridia gerlachei a species characterized by a coastal affiliation. Post-larvae of the euphausiid Thysanoessa macrura followed copepods in relative abundance.

Modest numbers of post larval krill (Euphausia superba) were present in 17 (68%) samples. The largest catches (100 and 120 individuals of 507 total) were over the shelf area north of Cape Shirreff. Lengths were mostly between 45 and 55 mm, centered around a 50 mm mode, and represent the successful year classes from the 1999/2000, 2000/2001 and 2001/2002 spawning seasons. Due to ontogenetic migrations and spatial separation of age/maturity stages it is not unusual to find predominantly large, mature krill in the West area at this time of year. Virtually all individuals were mature stages, mostly reproductively active male 3b and female 3b stages. Males outnumbered females by almost 3:1. These observations, along with sparse numbers of early calpytopis stage larvae, suggest a relatively recent onset of spawning activity (e.g., mid-December), mostly located outside of the West Area. With the exception of Salpa thompsoni, which demonstrated an order of magnitude increase in mean and median abundance over January 2004, the overall abundance and composition of zooplankton in the West Area was quite similar to that observed Survey A last year. These depauperate assemblages dominated by salps, post larval krill and T. macrura are typical of the coastal influence of the East Wind Drift. In this context, it will be quite interesting to see if, like last year, there is a significant faunal change between the two surveys associated with seasonal southward movement of the Antarctic Circumpolar Current Southern Front.

3. Krill biomass and dispersion. The ER-60 software has been working without complication. Initial problems with noise were eliminated by the removal of two resistors in the signal generating circuit of the 120 and 200 kHz transducers. This necessitated deriving independent equations within Echoview to derive acoustic estimates of krill biomass. The removal of the two resistors has resulted in much cleaner data with few noise issues. We have completed acoustic estimates of the krill biomass along the western shelf of the Shetland Islands. Krill biomass estimates ranged from 16 to 39 g /m² along transects and averaged 28 g/m² across the entire western area. This value is essentially the same as inferred in 2003-2004 sampling year. The preliminary nature of the data preclude strong conclusions, but the values coupled with the paucity of krill collected in concurrent net tows, coupled with the consistent collection of salps, and the presence of type 1 waters through out the western shelf suggests that biomass along the western shelf is low this year.

4. Phytoplankton. Stark contrast in chlorophyll concentrations were found between pelagic (23 stations) and shelf to shelf-break waters (12 stations) for the West Area and northeastern portion of the Elephant Island Area. Extremely blue water conditions were found in pelagic waters, averaging 0.083 ± 0.084 mg CHL-a m^-3 for 5-meter water samples. Rather normal January concentrations were found in the shelf and shelf-break region north of the South Shetland Islands and Loper Channel, averaging 0.91 ± 0.31 mg CHL-a m^-3 for 5-meter water samples. The highest surface concentrations were located at Station A0908 (north of the eastern tip of King George Island), being 1.4 mg CHL-a m^-3. For this station, Chl was 70 mg m^-2 as integrated to 100 m, or 44 mg m^-2 as integrated to 1% PAR. In contrast, the pelagic station A0906 had 12 mg Chl-a m^-2 and 8 mg m^-2 integrated to 100 m and 1% PAR, respectfully. In addition to chlorophyll analysis from water bottle samples, 10 stations were selected for collecting water (to 750 m) for examination of inorganic nutrients (N. Silva, Chile), dissolved organic carbon (L. Aluwihare, SIO), and (surface samples) phytoplankton composition, biomass (D-L. Iriarte, Chile) and size class.

5. Oceanography and meteorology. Predominantly Westerly to North Westerly winds, averaging 15 knots, were experienced early in the week, with periods of 20 to 30 knot North Easterlies, associated with a barometer drop from 1005 to 980 millibars, on Thursday and Friday. Clear skies on Monday saw the air temperature peak at 5°C and drop to a weekly low of 0.5°C on Tuesday, ranging between 1 and 4°C for the remainder of the week.47 CTD stations were occupied and successfully sampled in the West and Elephant Island areas. According to the Water Zone Classification table, predominantly typical Water Zone 1 (ACW) was found at the stations off the shelf break (deeper than 2000 m). This was, with exception of Station A07-01 that had a slightly higher temperature minimum (-0.3ºC), some evidence of mixing in deeper waters, and a deep chlorophyll maximum having 2-3 times more chlorophyll-a fluorescence than found for other typical Water Zone 1 stations. “Coastal” waters (Water Zone 2 and 3) usually present within the shelf and shelf-break north of the Shetland Islands were observed. Water Zone 4 (Bransfield Strait) stations were observed at the southern ends of lines EI09 and EI08 (between King George and Elephant Island).

6. Predator diet studies. Diet studies of Antarctic fur seals consist of processing scats samples and lipid extraction of milk. To date, seven scat samples from week 3 (all collected on 1/6/05 from Livingston Island) have been processed. All samples have contained krill, while none of the scats have contained any fish otoliths or squid beaks. A regression analysis of krill carapace lengths and widths from scat samples yielded a range between 40 and 60 mm. Similarly, this has been observed in the krill caught in the net tows. Out of the 37 milks samples collected, 30 have been processed for lipid extraction. There have not been any problems with any of the samples processed.

7. Bird and marine mammal observations. This report includes observations of seabirds and mammals for the West and Elephant Island strata. A total of 42 transits between stations havee been collected representing approximately 840 nautical miles of continuous observations. In total, 24 of species of seabirds have been recorded. Highlights in the West strata include numerous feeding Cape Petrels and herds of Fur Seals north of Cape Sherrif. There were few observations of Chinstrap Penguins in the West area. Blue Petrels, Thin-billed Prions, and Antarctic Prions were highly conspicuous in the offshore waters of the northeastern section of the West strata. These species were predominantly observed flying in a westward direction, and were likely associated with the southern boundary of the Antarctic Convergence Zone. Soft-plumaged Petrels (Pterodroma mollis) were common (30) in the north-east survey area of the Elephant Island area, and a White-headed Petrel (Pterodroma lessonii) was observed there as well. Five species of Albatrosses were recorded in the Elephant Island, which included Black-browed, Gray-headed, Light-mantled, Wandering, and Royal. Two Wandering Albatross were observed with painted chest marks, one with blue and another with pink. An exceptionally large feeding frenzy was observed on transit to the northwest of Elephant Island, just west of the Seal Islets. The feeding aggregation lasted approximately 10 km, and consisted primarily of thousands of Cape Petrels, Antarctic Fulmars, and hundreds of White-chinned Petrels observed feeding by duck-diving in the surface layer. During these observations, a thin layer of Euphausiids were recorded by the hydroacoustic system. Hundreds of Antarctic Fulmars and Chinstrap Penguins were observed on transit in Loper Passage, between Gibs and Aspland Islands. On an offshore transit in the West area, two Killer Whales accompanied with nine White-chinned Petrels were observed in pursuit of a mother-calf pair of Minke Whales. A lone Long-finned Pilot Whale was recorded at the northwest corner of the Elephant Island strata. On transit to station to 05.5-08 in the Elephant Island strata, dozens of Humpback, Minke and Fin whales were observed surfacing and feeding, which corresponded with a substantial number of krill caught in a net sample. Also, five Southern Bottlenose Whales were observed along the shelf break north of Elephant Island.

Submitted by Adam Jenkins.