AMLR 2006 Weekly Report No. 2

22 January 2006



1. Our current position is approximately 60 nautical miles northwest of Elephant Island where we are conducting a survey 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. Post larval krill (Euphausia superba) were present in 19 of 25 West Area samples (76%) with relatively high mean and median abundance values of, respectively, 10 and 8 per 1000 m3.  Greatest concentrations were along the inner shelves of the South Shetland Islands where they were obviously grazing on ample phytoplankton resources.  Virtually all individuals were large reproductively mature stages, 76% of which were males and 22% were gravid and spent females.   The vast majority of mature females were in advanced reproductive stages (96%) indicating a favorable early spawning season.  Most of the krill were between 49 and 57 mm in length with a 52 mm mode.  These individuals represent the successful 1999/2000, 2000/01 and 2001/02 year classes now four to six years of age.


Copepods were the most abundant taxonomic category (14500 per 1000 m3 mean, 1230 per 1000 m3 median) and numerically dominated the zooplankton.  Oceanic species Calanoides acutus, Calanus propinquus and Rhincalanus gigas were particularly abundant with their distribution pattern reflecting the presence of the Antarctic Circumpolar Current Southern Front (SACCF) along the outer island shelf area.  Other zooplankton taxa associated with the SACCF (e.g., larvae of the euphausiid Thysanoessa macrura, chaetognaths and the pteropod Limacina helicina) were also relatively abundant.  In contrast, the salp Salpa thompsoni was comparatively rare, with mean and median concentrations of 72 and 7 per 1000 m3.  Its distribution was in Drake Passage north of the SACCF where postlarval krill were scarce.


3. Krill biomass and dispersion. We have completed preliminary estimation of the biomass of krill along the west shelf of the South Shetland Islands using the EK-60. Mean krill density along each line was very low ranging from 1.3 to 6.8 g/m2, and averaging just 2 g/m2. These numbers are an order of magnitude lower than last year. Consequently, biomass along the west shelf was also low, just ~100K tons. Likewise krill biomass in 2006 is an order of magnitude lower than last year. We will estimate krill biomass using the SDWBA approximation in the final report.


4. Phytoplankton (C.D. Hewes, reporting). High concentrations of chl-a were found along the entire northern coastal margin of the South Shetland Islands. Shelf and shelf-break waters averaged 2.8 ± 1.2 mg m-3 chl-a at 5 m, with 102 ± 47 and 59 ± 17 mg m-2 for chl-a integrated to 100 meter and the euphotic zone (1% incident irradiance), respectively (15 stations). Highest values were at stations A23-07 and A14-08 where chl-a concentrations were measured over 4 mg chl-a m-3 in surface waters. Pelagic waters (>2000 meter bottom depth; 10 stations) had lower surface concentrations, averaging 0.5 ± 0.4 mg m-3, with integrated values averaging 30 ± 15 and 23 ± 13 mg m-2 (100 meter and euphotic zone, respectively). Although these are relatively low biomass values, they are considerably higher than the ~15 year average of ~0.2 mg m-3 found of the area. Pelagic Drake Passage waters are associated with low surface chlorophyll concentrations, but a chlorophyll maximum that typically occurs at ~75 meter depth. This season, only 3 of 10 pelagic Drake Passage stations (A1505, A1808, and A1909) as well as two stations above the Shackleton Fracture Zone (A0902 and A1101) had a deep chlorophyll maximum. However, the deep chlorophyll maximums this year are occurring at 30-75 meters, and therefore are much shallower than has been observed during past years. This corresponds with a much shallower winter water remnant than found for average in Drake Passage waters.  This year, in collaboration with AMLR, NSF- and NASA-sponsored phytoplankton studies are also being conducted. Members and projects include:


1) Murat Řzturk,  NTNU-TBS, Norway, has collected 60 samples (Stations A09-02, A15-05, and A 20-10) for measurements of particulate and dissolved iron as well as determination of Fe speciation and organic ligands.


2) Henrieta Dulaiova and Paul Henderson (Charette group, WHOI) will estimate vertical particle export from thorium-234 measurements, and horizontal water transport in surface water from measurements of four naturally occurring radium isotopes. The first stations were occupied to optimize sampling and measurement conditions. 42 radium samples have thus far been analyzed, and 77 thorium-234 samples have been filtered for analysis in the near future. Preliminary beta counter results indicate a classic disequilibrium in the surface mixed layer for waters north of the South Shetland Islands.


3) Nigel Delaney and Brian Seegers (Mitchell group, SIO) have acquired primary productivity verses irradiance data (30 station/depths), particulate organic carbon/nitrogen (CHN) and photosynthetic pigment (HPLC) samples (26 station/depths), and particulate and soluble absorbance spectra (27 and 18 station/depths, respectively). Additionally, in-situ radiometric, backscatter light and fast repetition rate fluorometry profile data from 6 stations have been obtained.


5. Oceanography and meteorology. Predominantly overcast skies with periods of rain, saw air temperatures ranging between 1 and 6°C during the week, with only short periods of sunshine. Gradual barometer swings between 983 and 1007 millibars produced a northwesterly wind speed averaging between 10 and 15 knots. The average wind speed increased to 18 knots when the wind swung around to the southwest on Friday.  Three of the five wind powered autonomous instrument buoys off Cape Shirreff continued to transmit data during the week, but two of the buoys have switched into power saving standby mode, due to the low average wind speeds experienced. 46 CTD casts were completed in the West and Elephant Island Areas and an initial field water typing assessment was done according to the Water Zone Classification table. This showed classical, well defined Water Zone 1 (ACW) zones at the deep offshore stations, Water Zone 2 stations along the shelf break, and on the shelf itself, and north of the islands. The western end of the survey area produced stations of transition between Water Zone 1 and 2. One Water Zone 3 station (11-07) was found on the shelf on the eastern end of the West Area.


6. Predator diet studies.  Predator diet studies have been initiated for Antarctic fur seals (Arctocephalus gazella). Lipids have been extracted from 28 milk samples using the FOLCH method. A total of 15 scats have been processed; 5 scats from week one, 6 scats from week two and 4 scats from week three. So far, only one myctophid otolith from Electrona antarctica has been discovered from week two of the scat collections. The scats are dominated by large krill (Euphausia superba) ranging from 35-55 mm in length. No squid beaks have been seen to date.


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 West area.  In total, 18 transits between stations were collected equaling approximately 360 nautical miles of sampling effort (75% of the West area).  Feeding activity by seabirds was fairly common throughout the shelf break region, especially between the 500 and 1500 m isobaths.  Aggregations of Antarctic Fulmars, and Cape Petrels were especially abundant on the westernmost transects of the survey.  Black-browed and Gray-headed Albatrosses were common, and on a few occasions they were observed in loose aggregations surface-seizing for prey.  As in past AMLR surveys, Blue Petrels and Antarctic Prions were abundant along the South Shetland Trough, and were observed flying predominantly into the headwind.  There were 5 sightings of Humpback Whales, and one sighting of a Southern Bottlenose Whale.  The most exciting sighting of the survey period was a pod of Killer Whales on the morning of 17 January while positioned at AMLR station 17-09.  Approximately 8 adults with at least 2 calves were traveling in a southeast direction.  Photographs and video footage was collected.


A.  Jenkins sends.