AMLR 2005 Weekly Report No. 4

6 February 2005


1.  Our current position is off Cape Shirreff, Livingston Island. We are conducting bio-oceanographic operations in support of NSF and AMLR funded research to describe and quantify the importance of local bathymetric canyons to the local abundance of krill. The mechanisms responsible for the local abundances will be used to compare feeding success of seals and seabirds in the Cape Shirreff Vicinity. Operations will continue for an additional two days.


2. Krill, salps and other zooplankton.  Zooplankton assemblages in the Joinville Island and South Areas differed greatly from those of the West and Elephant Island Areas in that they were numerically dominated by copepods and post larval Thysanoessa macrura.   Here the mean and median abundance values for Salpa thompsoni (107-185 and 19-38 per 1000 m3) were ca. an order of magnitude lower in the northern areas, apparently due to limited presence of Zone 1 water.  Ihlea racovitzai, another salp species that is a marker for Weddell Sea and polar slope water, occurred mostly in the eastern portion of Bransfield Strait, with greatest concentrations east and south east of Elephant Island.  Copepod concentrations in the two areas were nearly five times greater than to the north due primarily to dense aggregations of Metridia gerlachei and Calanoides acutus.  These were associated with frontal areas within Bransfield Strait.  Post larval T. macrura were broadly distributed across northern Bransfield Strait while the larval stages were concentrated to the south.


Elevated concentrations of post larval krill across the Joinville Island Area (28 and 2 per 1000 m3 mean and median) were similar to those of the Elephant Island Area.  Within the South Area aggregations were primarily encountered to the southeast of King George Island resulting in 50% lower mean and median values.  Krill length and maturity stage composition in the South was similar to that in the Elephant Island Area:  median and modal lengths were 46-48 mm; juveniles made up ca. 3% of individuals; and males outnumbered females by 50%.    Greatest differences occurred in proportions of immature stages which constituted 20% in the South vs. 9% in the Elephant Island Area.  Krill within the Joinville Island Area exhibited a bimodal length distribution centered around 27 mm and 45 mm lengths representing 1 and 3+ age classes.  Juvenile and immature stages contributed ca. 22% and 27% of the total.  This differed greatly from the other areas which were strongly dominated by the larger, mature age classes.  Mean and median larval krill abundance values in the Joinville Island Area (31 and 14 per 1000 m3) were also higher than in the other areas.  Distribution patterns of these larvae and small (<40 mm) post larval krill suggest  eastward advection through Bransfield Strait, concentration at frontal features within the Joinville Island area and subsequent northward advection out of the survey area. 


3. Acoustics.  With the completion of the first broadscale survey encompassing the South Shetland Islands, the acoustics research has focused on providing acoustic data in support of an NSF and AMLR funded grant to Dave Demer and Joe Warren. We have conducted more than 70 hours of acoustic sampling along 8 twenty nautical mile length transects in the vicinity of the Cape Shirreff field camp. While no analysis of these data has yet been conducted, large areas of backscatter, within the frequencies used to target krill were detected. Over the 7 days of the surveys, patterns seem to be consistent across the area, suggesting that patterns of abundance have a residence time of at least a week.


4. Phytoplankton. South Area chlorophyll concentrations have been completely processed at this time, 5 meter phytoplankton stocks averaged 1.67 ± 0.58 mg chl m-3 (slightly higher than reported last week with only 9 stations processed). The 13 year average for surface phytoplankton stocks in the South Area is 1.27 mg chl m-3, therefore values for this season are slightly higher than on “average”. With respect to the general distribution of chlorophyll for the entire AMLR Survey for Leg 1, pelagic waters (bottom depth >2000 meters) of highest surface phytoplankton concentrations were found in the Joinville Island Area with one station having 1.8 mg CHL m-3, intermediate concentrations were found for the Elephant Island area averaging 0.45 ± 0.45 mg chl m-3, and lowest in the West Area with 0.12 ± 0.18 mg chl m-3. With regard to shelf and shelf/break areas (bottom depth < 2000 meters), the lowest phytoplankton stocks were found in the West area with 0.68 ± 0.44 mg chl m-3, intermediate concentrations of 1.13 ± 0.66 mg chl m-3 and 1.04  ± 0.45 mg chl m-3 in the Elephant Island and Joinville Areas, respectfully, and the South area having the highest values of 1.67 ± 0.58 mg chl m-3.


Although not all samples have been processed for the near-shore survey, general trends are found, with chlorophyll concentrations highest off shore (~1.8 mg CHL m-3) and slightly decreasing near-shore (~1.4 mg CHL m-3). Both extracted chlorophyll and chlorophyll fluorescence profiles (nighttime measures) indicate a slight chlorophyll maximum occurring 20-50 m for the outermost stations. The outermost stations also roughly correspond to stations A17-11 and A16-10, that measured ~0.8 mg CHL m-3 two weeks previous, and indicate an approximate doubling of phytoplankton biomass during this time.


5. Oceanography and Meteorology.  Predominantly Westerly to North Westerly winds, averaging 20 knots, were experienced during the week, with winds peaking at 40 knots on Friday.  The constant strong winds experienced during the week brought with it increase swell size towards the latter part of the week. Barometric pressure showed a steady decline from 995 to 965 millibars through the week, rising steeply on Saturday to end the week on 990 millibars. Air temperature during the week remained around 1.5°C, rising to a maximum of 3.5°C on Saturday.  27 CTD stations were completed during the week as part of the Near Shore Survey being conducted off Cape Sherriff. Four transect, of 5 CTD stations each, were completed on the eastern (Y8) and western (Y2) ends of the survey area. The central transect (Y5) was cancelled due to rough seas, with only 2 CTD stations being completed.


6. Predator diet studies.  All scats samples have been processed from weeks three through six (collection dates of 1/6-1/27/05). Fish otoliths from myctophid fish have been present in the diet of Antarctic fur seals from week four through week six. Otoliths from myctophid Gymnoscopelus nicholsi are the most abundant in ten scats out of 42 collected with Electrona antarctica present in eight of the scats. However overall abundance of fish otoliths is low compared to previous years during the same time. Squid beaks have been observed in two scats. All 47 milk samples have been processed for lipid extraction and will been taken back to the SWFSC for final fatty acid signature determination.  To date a total of 250 live krill have been measure for total length, carapace length and width and sexed. Additionally, 24 various species of Antarctic fish have been caught in net tows and will be measured for total length, weighed, dissected for stomach content and fullness, sexed and have their otoliths removed. These fish will be homogenized for lipid extraction in order to match the fatty acid signatures to the milk samples collected and processed.


7. Bird and marine mammal observations. This report includes information on observations collected during the nearshore survey north of Livingston Island.  Approximately 600 nautical miles were surveyed along transect lines, which represent two samples each for the west and east hydrographic canyons.  Cape Petrels were the numerically dominant feeding seabird recorded, and were primarily observed feeding by plunge-diving for prey.  Preliminary mapping of seabird feeding aggregations suggest that they are forming along the edges, and between the canyons.  Fur seals were primarily observed traveling in a northwest direction and subsequently the greatest number (18) of animals recorded (per hour of transect surveyed) was located at the northwest head of the west canyon.  Humpback and Minke whales were commonly encountered inshore and appeared to be feeding.  Five species of Albatross were recorded throughout the survey period; in decreasing order of abundance: Black-browed, Gray-headed, Wandering, Royal, and Light-mantled Sooty Albatrosses.  Blue Petrels, and Antarctic Prions were patchy in distribution, and were relatively restricted to the west survey lines.  Other avifauna highlights include: one Common Diving Petrel and four Arctic Terns.


Submitted by Christian Reiss.