AMLR 2005 Weekly Report No. 7

27 February 2005

1. Our current position is approximately 55 nautical miles southwest of Elephant Island where we are conducting the second of two surveys of bio-oceanographic conditions in the vicinity of the South Shetland Islands. After a quick passage across Drake Passage the ship fetched Cape Shirreff, Livingston Island in the afternoon of 21 February.

Provisions, propane, fuel and personnel (J. Hinke) were transferred ashore by zodiac. Survey-D began 22 February, but abruptly our progress was brought to a stand still for 24 hours due to a strong gale with easterly winds sweeping into the South Shetland archipelago. The survey was resumed once the weather moderated. Survey operations in the West Area were completed this past week. Despite our coarse beginning recent agreeable weather and sea conditions put us back on schedule.

2. Krill, salps and other zooplankton. West Area Summary. Results from net sampling in the West Area during Survey-D were quite similar to those last month. A generally depauperate zooplankton assemblage numerically dominated by Salpa thompsoni, copepods (notably Metridia gerlachei) and post larval Thysanoessa macrura. Mean salp abundance (817 per 1000 m³) was half of that in January but the median was somewhat greater (635 vs. 424 per 1000 m³) reflecting a more even distribution across the area. This was associated with mixing of oceanic (Zone 1) and coastal waters. Greatest concentrations were northeast of King George Island) and probably result from frontal zones or eddies. The overall aggregate stage length distribution was bimodal centered around 38 mm and 10 mm. These lengths correspond to the ageing population (now ca. 16 mm longer than last month) plus a modest pulse of recent chain production. Since only 20-25 of aggregates were smaller than 20 mm the peak production season has probably ended. This is supported by increased proportions of small solitary stages (6-25 up from 4-25 of total salps) that will form the over wintering population.

Mean and median abundance of copepods (805 and 337 per 1000³) and post-larval T. macrura (179 and 566 per 1000 m³) were about 3 times greater than in January. Largest concentrations of both occurred over the South Shetland Island shelf region. Metridia gerlachei (a coastal species) was dominant in this region while offshore catches were generally dominated by Rhincalanus gigas and Calanoides acutus.

Krill were again sparse with mean and median abundance values (7.8 and 1.4 per 1000 m³) equal to those in January. The majority of individuals were large mature stages representing the 2000/2001 and 2001/2002 year classes. Males and females were more equally represented (1.6.1 ratio) than before with mature males comprising 51-25 of the catch. The majority of females were in early reproductive stages (3a-b) and only 36-25 of mature forms were advanced (3c-e) suggesting a delayed spawning season. This is supported by low concentrations, and early stages, of krill larvae.

3. Krill biomass and dispersion. We have completed acoustic estimates of the krill biomass in the West area of the AMLR survey grid. Preliminary data processing has resulted in an abundance of 13.14 g/m², almost half of the biomass of leg one, 24.68 g/m². The preliminary results for the mean biomass for the West area are about 20-25 greater than 2004 leg 2 results. There was significant noise on two of the transect lines but the cause was later identified and fixed.

4. Phytoplankton. For the West Area lying east of Line 11, surface chlorophyll concentrations averaged 0.93-0.68 mg m^-3, with integrated concentrations being 72 (to 100 m) and 40 (to 1-25 incident light) mg chl-a m^-2. These values are higher than found during January, primarily due to increased phytoplankton concentration for pelagic waters. Offshore surface concentrations (12 stations) were 0.43-0.36 mg chl-a m^-3 as compared with average values of 0.01 mg chl-a m^-3 last month.

Integrated values were approximately 37 and 24 mg chl-a m^-2 for values integrated to 100 meters and 1-25 incident PAR (meter depth), respectfully. Surface concentrations for the shelf and shelf-break (~2000 meter bottom depth, 9 stations) were also found slightly higher than January values, being 1.59-0.32 mg chl-a m^-3 values integrated over 100 m depth averaged 119 mg chl-a m^-2, and integrated over depth to 1-25 incident PAR(~43 m) averaged 62 mg chl-a m^-2. Stations D14-08 and D15-09 (located in the central shelf-break region of the S. Shetland Islands) measured the largest integrated concentrations of phytoplankton biomass, while station D15-05 (due north over the Shackleton Trench) measured the lowest phytoplankton biomass.

5. Oceanography and meteorology. February 22, after the first station on the big survey grid was completed, the wind direction changed from North Westerly to an Easterly direction, this was associated with an increase in wind speeds averaging around 35 knots and gusting up to 47 knots. The strong winds were maintained for a 24-hour period, halting all sampling on the survey. The barometric pressure over this 24-hour period dropped from 996 to 971 millibars. For the rest of the week calm conditions were experienced with winds averaging around 10 knots, mainly from an Easterly direction. The air temperature hovering around 1ºC for the week, dipping to below -1ºC during the Easterly storm. 30 CTD stations were occupied and successfully sampled in the West and Elephant Island areas. According to the Water Zone Classification table, typical Water Zone 1 (ACW) was found at the offshore stations of each transect in the West area. This was, with exception of Station D18-08 that was classified as Water Zone 2. “Coastal” waters (Water Zone 2) usually present within the shelf and shelf-break north of the Shetland Islands were observed for the rest of the stations sampled in the area, except for station D12-08 which was classified as Water Zone 4 (Bransfield Strait).

During the week some problems occurred with the CTD system and data acquisition computer. The Datasonics Altimeter stopped logging data and was removed from the CTD system and replaced with a spare unit. The Seatech Transmissometer was found to be giving suspect data and was removed for repairs. The data acquisition computer “froze” during certain CTD casts and had to be rebooted on occasion, with loss of some SCS data during these periods. Problem with dirty contacts on graphics card was also experienced.

6. Predator diet studies. Scat samples 2361-80 (Weeks 7 and 8) have been processed and the data entered. Scat samples 2381-90 (Week 9) as well as 12 raw and duplicate milk samples were transferred to the ship from Cape Shirreff. Thirty-three individual prey samples (species: E. antarctica,P. antarcticum, G. brauri and G. nicholsi) were prepared for fatty acid analysis.

7. Bird and marine mammal observations. The crossing of the Drake Passage (20-21 February) and the AMLR West area surveys, northwest of the South Shetland Islands (22-26 February), featured possibly unprecedented numbers of petrels of the genus Pterodroma. To date, we have encountered five Kerguelen Petrels, P. (Aphrodroma) brevirostris, eight Mottled Petrels, P. inexpectata, and more than 580 Soft-plumaged Petrels, P. mollis. Many of the latter have shown signs of wing molt, while others have not, suggesting the presence of two age classes. The overwhelming majority of these petrels have been located in the deeper water sections of the survey transects, with the largest concentrations noted near the Shackleton Fracture Zone. Most appear to be traveling to the west. The nearest known breeding colonies of P. mollis are on Tristan de Cunha and Gough islands in the South Atlantic. Another highlight was the ship following, for two days (23-24 February), of a probable non-exulans great albatross, possible either gibsoni (Gibson's) or dabbenena (Tristan). The taxonomy and identification of these forms remains uncertain, however. No notable feeding assemblages of seabirds were noted during the period, and most expected species were encountered in low to moderate densities. Marine mammal sightings have included three probable Strap-toothed Whales in the Drake near 57 30ºS, numerous pods of Fin Whales (maximum 20+ individuals/day), and several pods of Hourglass Dolphins (maximum of 25 individuals) in the West area survey.

Submitted by A. Jenkins.