AMLR 2002 Weekly Report No. 7

 

24 February 2002

 

1. Current position is approximately 9 nautical miles west of Snow Island, conducting an ecosystem assessment of the Southern Ocean in the vicinity of the South Shetland Islands. The current survey is the second of the AMLR large area survey grid for the 2002 field season. With good weather the survey should take 14 days to complete. The weather is stable, with overcasts skies and wind at 15 knots from the west.

 

2. Cape Shirreff Inshore survey. During February 17-23, the R/V Yuzhmorgeologiya maintained a safety watch over the Cape Shirreff inshore survey vessel, the 19-foot R/V Ernest. While keeping and eye on the Ernest, acoustic data was collected consistently by ship's echo sounder during the near shore survey. At night, a total of 10 transects were completed, with five lines containing acoustic data only and the remaining five including four to five stations consisting of a CTD cast and IKMT net tow. During the day, the ship completed small transects perpendicular to the nighttime survey lines while remaining within 10 nautical miles of the R/V Ernest. The purpose of this survey was to collect bathymetry data and perform targeted tows through acoustic targets. A week of manageable weather during the survey allowed for more than 60 percent of the planned high resolution acoustical survey to be covered by Jenkins and Warren aboard the R/V Ernest. Most krill aggregations were mapped to the east of the Cape. However, there was much less krill in the near-shore area than in 2000, the year of the of the first small-craft survey. All of the small boat systems and equipment functioned properly.

 

On the 17th of February, D. Demer and D. Needham deployed an instrumented spar-buoy fitted with an acoustic Doppler current profiler, which recorded a 0.7 kt current from north-northwest and flowing up the canyon head to the east of Cape Shirreff. Krill swarms are frequently mapped in this area and satellite tags on krill predators indicate that it may be a common foraging location. The Buoy's electronics and data collection was successfully telemetered to the shore camp via radio modem for the entire six days of the Cape Shirreff survey. The morning of the last day of the survey, the ADCP buoy was recovered by the ship and replaced with a similar buoy instrumented with a 38 and 200 kHz echo sounder. The latter buoy will record a two week time-series of acoustical backscatter data at the canyon head while the R/V Yuzhmorgeologiya is conducting the large area survey around the South Shetland archipelago. The light-weight, low-cost spar buoy, built by D. Needham and associates, is proving to be an effective tool for remote monitoring of the near-shore areas of the Southern Ocean. Also during the week, total target strengths (TTS) of krill were measured for the first time over the frequency range of 36 to 202 kHz. Mean TTS values were recorded from groups of 300, 100, 60, 50, and 30 live krill. Zooplankton specialists aboard the ship provided support in this effort by providing live zooplankton from night time net tows. The afternoon of 23 February, Demer, Conti, Jenkins and Warren moved back aboard the ship and Holt moved back to Cape Shirreff. All planned operations were successfully completed.

 

3.Oceanography and meteorology. Predominantly north westerly to south westerly winds averaging 15 knots, with a maximum of 36 knots recorded on Thursday. Gradual pressure fluctuations ranging between 977 and 1004 mb were experienced. Air temperature fluctuated between 0.1 and 3.8C with occasional rain and light snowfalls. Conditions were foggy and overcast until Thursday, clearing for the latter half of the week. As observed during Leg I, water zones were again not clearly distinguished. Well defined temperature minima were found from the furthest offshore stations on all the transects, with the minima increasing in temperature towards shore. The depth of the temperature minima increased from 100m along the most southern transect to about 50 m at the most northern transect. All stations beyond the 500 m shelf break had a U-shaped T/S curve with the salinity at temperature minimum 34.1-34.3 0/00. The waters within the 500 m shelf did not fit the true classification of Water Zone III, possibly because these were shallow casts with bottom depths ranging 70-300 meters.

 

4. Phytoplankton. A Blue Water Zone Station at 62.8W 58.9S made on transect south to Cape Shirrefff found Antarctic Circumpolar Current having temperature minimum of -0.27C with salinity 34.02o/oo at 160 meter. Chlorophyll of 0.25-0.30 mg m-3 was distributed to 50 meters and then tapered off in deeper waters. For the near-shore survey, chlorophyll samples were taken every hour from the continuous flow system (n = 78) in addition to bottle samples obtained with the 21 CTD casts (n = 197). Near surface chlorophyll concentrations ranged 0.14-1.82 mg m-3, with waters nearest to shore having lower concentrations than those off shore. Only Stations C016 and C023 demonstrated chlorophyll maxima at 40-50 meters, while all other stations had generally uniform distributions to the thermocline.

 

5. Predator diet studies. Lipids have been extracted from 26 fur seal milk samples and frozen. During the inshore survey one myctophid fish was caught during a net tow, and was frozen for later lipid extraction. In addition, ten fur seal scats were processed. Up to 30 krill carapaces per scat were measured for standard length and width and stored in ethanol, and fish otoliths were sorted to species and saved. The majority of the scats contained large amounts of krill chitin, with few to medium amounts of myctophid otoliths, with one exception. The otoliths came primarily from E. carlsbergi, and to a lesser degree E. antarctica. Only a few otoliths from G. nicholsi were present. In one case, only a small amount of chitin was present, however 712 otoliths were counted. One squid beak was found and preserved in ethanol for later identification to species.

 

A. Jenkins sends