AMLR 2005 Weekly Report No. 5

13 February 2005

 

1. Our current position is north bound in the Drake Passage in transit to Punta Arenas, Chile from the South Shetland Islands. The joint NSF/U.S AMLR supported near-shore acoustical survey off Cape Shirreff, Livingston Island was completed February 10.  Personnel Y. Trembly, J. Warren, S. Sessions, M. Patterson, D. Needham, A. Jenkins and equipment were recovered to the ship along with three zodiac loads of materials to be retrograded. In the morning February 11, the ship fetched Copacabana field camp. Due to high winds and swell generated from a deep low-pressure system moving in from the west zodiac operations were delayed until the evening of the 12 February. Four Zodiac loads of retrograde material and propane tanks were recovered to the ship from Copacabana.

 

2. Nearshore Survey. The 2005 nearshore survey occurred from 1-10 February 2005. The goal of the nearshore survey of the area just north of Cape Shirreff, Livingston Island is to map the distribution and abundance of the krill population and to better understand the physical and biological factors that control this ecosystem. Penguin and fur seal colonies are found on Livingston Island and these nearshore waters serve as the main foraging area for these animals. The broadscale AMLR acoustic and net surveys conducted over the past decades do not survey the nearshore regions due to the inability of vessels to navigate these waters.

 

In order to survey in a shallow-water environment, a modified 19’ inflatable Zodiac boat, the RV Ernest II, was constructed. It contains numerous scientific equipment including: a two frequency (38 and 200 kHz) scientific echosounder, GPS, VHF radio, meteorological station, and a surface temperature and conductivity sensor (SeaBird MicroCAT). It is also capable of deploying small nets or a video camera system for ground truthing the acoustic data. Two modified waterproof cases provide power (12 V battery bank), 15” LCD screen, laptop computer and several other systems. There is also a stainless steel insert with a canvas and vinyl cover to protect the equipment and personnel from the elements.

 

A previous version of the Ernest was used to conduct surveys of the waters north of Livingston Island in 2002 and 2004. These surveys are undertaken in conjunction with the RV Yuzhmorgeologiya which conducts multiple-frequency acoustic transects, IKMT net tows, and CTD casts.  The transect lines for the Yuzhmorgeologiya and Ernest are designed to investigate the hypothesis that two submarine canyons (located to the west and east of Cape Shirreff) provide nutrient rich water to the nearshore region which supports the phytoplankton and krill populations.  A calibration of the acoustic systems was performed using a 38.1 mm Tungsten Carbide sphere. Surveys were conducted on 7 of the 8 following days. On 10 February a post-survey calibration of the acoustic system was performed and the RV Ernest II was brought back aboard the Yuzhmorgeologiya.  During the nearshore survey, the Ernest collected data over 275 n. mi. of transects. On average 5.5 hours each day were spent on the water conducting the survey. Weather conditions were often quite poor with ubiquitous rain, wind speeds typically 15-25 knots, and sea states ranging from 2 to 4 m.

 

All equipment and data acquisition systems performed without error or breakdown, continuing the excellent track record of the 2004 survey.  Initial results from the Ernest survey are similar to previous years.   There were large aggregations of scatterers at the edges of the canyons often in waters between 100 and 150 m in depth. From video observations and net tow data from the Yuzhmorgeologiya, these scatterers are believed to be krill. The presence of these patches was often indicated by the observation of penguins, fur seals, and humpback whales during the survey.  During this same time period, the RV Yuzhmorgeologiya conducted several transects covering the region from the 500 m isobath from the western canyon to the eastern canyon. Hydrographic and net tow stations were occupied several times in the western canyon, mid-canyon rise, and eastern canyon. Forty CTD stations and over thirty net tows were conducted.

 

3. Nearshore AUV operations. Fetch1 AUV operations were conducted in the vicinity of Livingston Island, South Shetlands, 1-9 February, AUV operations were conducted every day during this period, except February 6 when weather allowed the hull to be opened up and the analog Hi 8 mm videotape retrieved and replaced. The Li Ion battery for the internal Sony VTR was recharged.  Fetch1 conducted 55 dives to depths as great as 70 m, and traveled an estimated 12.1 nm, while collecting 260 Mb of 600 kHz side scan sonar data, 4 hours of underwater video, and simultaneously logging C, T, depth, bathymetry, and dissolved oxygen data at 2 Hz. AUV operations generally lasted 4 hours per day because of battery life limitations of the vehicle. AUV operations were conducted from a 19’ Zodiac. A. Jenkins served as boat driver using dead reckoning navigation based on its initial course, speed, and planned dive duration to track the AUV. D. Needham served as electronics engineer and tracked and located the vehicle using the ultrasonic pinger locator. The vehicle carried tags transmitting on 38 and 50 kHz. Most surveys were conducted with 1-2 nm of the Cape Shirreff camp in 50-85 m of water, in areas of dramatically changing topography. Penguins and seals and whales were observed diving in these areas, and one krill swarm was successfully imaged during the first 2 hours of video, along with some salps. The AUV also filmed a kelp covered underwater pinnacle that it collided with inadvertently.  Detailed analysis of side scan imagery correlated with video imagery is ongoing.  It appears that krill are seen as patches in the side scan images. They appear in a manner similar to swarms of smaller fishes seen in other ecosystems and thus it appears that krill can be detected at 600 kHz using side scan from an AUV.

 

On the last day of AUV operations, the Ernest II rendezvoused with Zodiac I and conducted several runs where the AUV was sent on a heading for 8 minutes, the Ernest followed behind. These dives were an attempt to see whether there is any avoidance reaction by krill to the ER60 echosounder and small boat, vs. the passage of the AUV.  Other targets are also seen on the side scan images. Many of these targets appear to be about a meter in length and the suspicion is that these are penguins. On one occasion, it was observed that a penguin near the surfaced appeared to follow the AUV as it dove.  Several dives were devoted inshore to attempt to make usable images of the bottom, that is, benthic mapping, using the underwater camera. It was determined that the Chilean nautical chart of Cape Shirreff, made in the 50's, has positional errors approaching 1/4 nm. An offset was measured using the Fetch1 and Zodiac I GPS units, so that latitude and longitude offsets can be applied to portraying AUV missions near the Cape Shirreff coastline from this image.

 

4. Krill, salps and other zooplankton. Survey A Summary. The 99 IKMT net samples collected during Survey A were numerically dominated by the salp Salpa thompsoni which alone contributed half of the total mean zooplankton abundance.  The distribution of this salp was significantly correlated with Zone 1 water that extended across the offshore (Drake Passage) portion of the area.  Greatest concentrations were adjacent to the Shackleton Fracture Zone and gyre.  Salp abundance in Bransfield Strait was an order of magnitude, and significantly lower, than to the north.  This distribution and water zone affiliation differs greatly from previous years when greatest concentrations occurred in Bransfield Strait, apparently advected there from eastern Weddell Sea source areas. This dramatic change may in some way be related to conditions associated with, or resulting from, the prolonged extensive sea ice development observed in spring 2004.  Another salp species, Ihlea racovitzai, was relatively uncommon and limited to areas in southern Bransfield Strait and east of Elephant Island influenced by Weddell Sea water.  Other relatively abundant taxa were copepods (mostly Metridia gerlachei, Calanoides acutus and small unidentified forms) and post larval and larval stages of the euphausiid Thysanoessa macrura.  These taxa dominated in Bransfield Strait and areas adjacent to South Shetland and Elephant Island shelves.  As previously stated, this low diversity, relatively depauperate assemblage is characteristic of coastal waters (East Wind Drift), despite the widespread presence of Zone 1 water across the northern portion.  It will be interesting to see if, like last year, a major faunal change will occur next month due to seasonal southward movement of the Southern Antarctic Current Front into the survey area.

 

Post-larval krill were distributed across the entire area but had maximum frequency of occurrence (92% of samples) and abundance within the Elephant Island Area.  These moderate values were similar to those of January 2001 and 2003.  Largest concentrations occurred in areas characterized by frontal zones and eddies, as indicated by dynamic height plots.   Large, mature forms >40 mm in length predominated across the entire area and represented the strong 2000/2001 and 2001/2003 year classes.  Within the Elephant Island Area juveniles made up 2.6% and immature stages 8.7% of the total catch, suggesting two successive years of poor recruitment success.  This contradicts last year's predictions for extremely strong recruitment success based on a prolonged and apparently successful spawning effort.  As suggested by elevated concentrations of juveniles and small immature forms in eastern Bransfield Strait, the younger krill may have been advected out of the area.  However, other factors affecting their distribution and abundance in the survey area may be involved and become apparent during the second survey effort.

 

Larval krill were most abundant in Southern Bransfield Strait, east of Elephant Island and adjacent to the Shackleton Fracture Zone.  Overwhelming predominance of early calyptopis stages indicate a mid to late-December spawning effort.  Mean and median concentrations of larvae within the Elephant Island Area (22 and 1.1 per 1000 m3) are higher than the previous two years.  However, like the juveniles, their distribution patterns suggest that they are also being advected away from the area. 

 

5. Krill biomass and dispersion.  Surveying was completed February 9.

 

6. Phytoplankton. For near shore survey, Line Y5 had highest overall surface concentrations of chlorophyll, averaging 2.1 ± 0.3 mg chl m-3, compared with 1.8 ± 0.3 mg chl m-3 for Lines Y2 and Y8. Lines Y2 and Y5 had highest chlorophyll integrated to 100 m, averaging 133 mg-2, while Line Y8 averaged 103 mg chl m-2. Due to presence of a chlorophyll maximum, surface values where highest offshore, whereas integrated values tended to be highest inshore. However, since there was very high variability between replicate stations, any trends are regarded with caution. Chlorophyll concentrations were generally high above the pycnocline that occurred at ~50 meters depth, whereas the 1% PAR was estimated slightly shallower at ~40 meters.

 

7. Oceanography and meteorology. Clear skies were experienced on Wednesday and Friday. The 20 knot average westerly to northwesterly winds of the week dropped to 10 knots during the shore operations at Cape Shirreff at the end of the Near Shore Survey, but a barometer drop from 1000 to 965millibars during Saturday saw the average wind speed shift from 20 to 35 knots, and peak at 50 knots.   An additional 13 CTD stations were completed as part of the Near Shore Survey of Cape Shirreff, with the 3 transects lines (Y2, Y5 and Y8) being sampled for the third time. This brought the total number of CTD stations completed during Leg 1 of the cruise to 140 casts. All CTD cast data files, of the Main and Near Shore Surveys, have been processed and the results converted for presentation in Ocean Data View. The dynamic height plots of the survey area were also plotted on request from the zooplankton group.

 

8. Bird and marine mammal observations. This report includes information including observations collected during the nearshore survey at Livingston Island.  The most numerous Chinstrap Penguin aggregations were located at the head of the east canyon.  Also, observations of  penguins in the east survey area were primarily porpoising in a southeast direction towards the canyon head.  Cape Petrel feeding aggregations (numbering 50-200 birds per 5 nautical mile) continued to be encountered on the edges of the canyons and at the head of each canyon.  Black-browed Albatrosses were also found in high numbers at the canyon heads.  Fur seals were highly conspicuous in locations northwest of Cape Sherriff.  Humpback Whales were primarily located in the east canyon.  A King Penguin was observed at the Polish research base Arctowski, on King George Island.

 

Submitted by A. Jenkins.