AMLR 2005 Weekly Report No. 3

31 January 2005

1. Our current position is the Bransfield Strait en route to Cape Shirreff. The first survey of bio-oceanographic conditions in the vicinity of the South Shetland Islands was completed January 31. An extensive field of large icebergs emanating from the northwestern Weddell Sea allowed only five stations to be occupied in the Joinville Island Area and none of the planned stations in the northwest Weddell Sea were possible. The southern half of the Bransfield Strait also covered heavily with icebergs curtailed our acoustic transects only slightly, the ship managed to work around the ice allowing all stations in the southern area to be occupied. For survey-A, 100 CTD casts, 100 IKMT tows and 24 acoustic transects were conducted over 15 days.

2. Krill, salps and other zooplankton. Composition and abundance of the Elephant Island Area zooplankton assemblage was quite similar to that of the West Area. Both areas exhibited strong numerical dominance by Salpa thompsoni, copepods (primarily Metridia gerlachei) and post larvae and larvae of the euphausiid species Thysanoessa macrura (89% and 92% of mean zooplankton abundance in the Elephant Island and West Areas, respectively). These assemblages are characteristic of the East Wind Drift and reflect Weddell Sea and coastal influences extending across the outer portions of the large survey area. The Elephant Island Area zooplankton differed somewhat in having greater proportions of copepods relative to salps (18% and 62% vs. 15% and 69%) and greater mean abundance and relative contributions of T. macrura and krill larvae than in the West Area.

Largest S. thompsoni concentrations primarily occurred in Zone 1 (Antarctic Circumpolar Current) water adjacent to the Shackleton Fracture zone and may result from retention within the gyre there. Relatively low concentrations occurred in Bransfield Strait. This distribution pattern is unusual as greatest concentrations of this salp typically occur east and south of Elephant Island and reflect input from Weddell Sea source areas. Aggregate stages formed the vast majority of salps collected (94%). These had a size frequency distribution quite similar to that in the West but centered around a 5 mm larger mode (25 vs. 20 mm). Part of this difference may result from somatic growth over the survey period. Copepods exhibited extremely patchy distributions. Relatively large concentrations were associated with the Shackleton Fracture Zone gyre, and gyral or frontal zone areas in Bransfield Strait and the northeast sector; modest concentrations occurred over outer island shelves and slopes. Metridia gerlachei comprised 60% of mean copepod abundance. Small, unidentified species and Calanoides acutus made up 18% and 11%, respectively.

Mean and median concentrations of post-larval krill in the Elephant Island Area (27 and 15 per 1000 m³) were substantially greater than in the West (8 and 2 per 1000 m³). Krill were present in 44 of the 48 samples (92%) with greatest concentrations (100-130 per 1000 m³) offshore in the northeast sector. Elevated concentrations (10-100 per 1000 m³) also occurred over the Elephant Island shelf and between King George and Elephant Islands. These krill were obviously feeding on rich phytoplankton supplies here as evidenced by bright green stomach contents. Krill lengths ranged from 20 to 58 mm but were mostly centered around 47 mm median and modal lengths. Mature individuals dominated (88%) followed by immature (9%) and juvenile (3%) stages. Males and females were more evenly represented than in the West (1.5:1); 50% of the males were reproductively mature; 41% of mature females were gravid and another 9% spent indicating active spawning. Larval krill were present in 56% of the samples, primarily in the eastern portion of the Area. Greatest concentrations (100-500 per 1000 m³) were in deep water areas in the northeast and southeast sectors suggesting the effects of eastward advection and aggregation at frontal zones or in gyres. Moderate concentrations also occurred within the Shackleton Fracture Zone gyre. The vast majority of larvae were early calyptopis (I) stage resulting from December spawning.

3. Krill biomass and dispersion. We have completed acoustic estimates of the krill biomass along the Elephant Island, Joinville and Southern areas of the AMLR survey grid. Preliminary data processing has resulted in several lines that have produced anomalous results and so are excluded from this report. The Elephant Island area exhibited krill abundances of ~36 g per m², while the Joinville and Southern areas produced preliminary values of 9 and 11 g per m², respectively. The preliminary results for the mean biomass for the Elephant Island are about 30% greater than 2004. Both Joinville Island and South area exhibited biomasses that were similar to the 2004 sampling. Over the next week we will reprocess and examine the anomalies to gain a better understanding of the potential reasons for the results we have obtained.

4. Phytoplankton. Since the last weekly report, we have completed the eastern half of the Elephant Island Area. Chlorophyll concentrations at 5 meters averaged 0.89 ± 0.64 mg m^-3 (38 stations) in this subsection, with station A0306 having the highest surface value of 2.42 mg Chl m^-3 (95 mg m^-2 and 64 mg m^-2 chlorophyll integrated to 100 m and 1% light, respectfully). 5 m values for the entire Elephant Island Area averaged 0.77 ± 0.65 mg chl m^-3, being about average with respect to a 13 year mean of 0.75 ± 0.31 mg chl m^-3. The Joinville Island Area surface concentrations averaged 1.16 ± 0.51 mg chl m^-3 (6 stations), and was considerably higher than last years standing stock (<1 mg chl m^-3) measured during January. Although the South Area chlorophyll concentrations have not been completely processed at this time, 5-meter phytoplankton stocks averaged 1.55 ± 0.70 mg chl m^-3 (9 stations), with highest concentrations measured at Stations A0909 and A0810, both measuring 2.4 mg chl m^-3 (78 mg m^-2 and 46 mg m^-2 chlorophyll integrated to 100 m and 1% light, respectfully). A 13-year average for surface phytoplankton stocks in the South Area is 1.27 mg chl m^-3; therefore values for this year appear to fit into the “average” year category. With respect to geographical features, blooms are occurring west and south of Elephant Island (surface values of 1-1.5 mg chl m^-3), as well as within and south of the Loper Channel (surface values 1.5 - 2.4 mg chl m^-3). As described in last weeks report, high chlorophyll concentrations were also measured north and northeast of the eastern portion of King George Island, and it is not known whether this is an independent process, or “spillover” from blooms occurring to the south.

5. Oceanography and meteorology. Two slow, unspectacular cycles of the barometer during the week, spanning 982 and 996 millibars, and winds from all points of the compass (speeds averaging 15, peaking 35 knots), saw weather and sea conditions conducive to incrementing the “successful-CTD-station” count up to one hundred and to complete the large scale CTD survey. No stations were missed due to adverse weather, and the ship was only required to heave-to once, before the third last station. The CTD instrumentation performed well, with no breakdowns or the usual cable/connector problems. The Biospherical 4p PAR sensor that was mounted on the ship was calibrated and is logging comparable SCS data with the Weatherpak's 2p Licor PAR sensor. The underway SCS data Matlab averaging routine was modified to generate a one minute data average routine as well as the normal five minute average, on request from the data users. All one hundred CTD stations' data were processed and a first “in-field” water-typing attempt, across the Elephant Island, Joinville Island and South Areas, was made. These areas produced the classical scenario of Water Zone 1 to the west of, and around, the Shackleton Fracture Zone with mixing occurring to the west of Elephant Island. Weddell Sea (Water Zone 5) influence was found in the southeast of the Elephant Island Area, as well as extending across southern Bransfield Strait, terminating south of Livingston Island. Typical Water Zone 4 was observed in the remainder of the Bransfield Strait.

6. Predator diet studies. All scats samples have been processed from weeks three and four. Fish otoliths from myctophid fish have been present in the diet of Antarctic fur seals in week four only (week four scats were collected on 1/13/05) to date. Otoliths from myctophid Gymnoscopelus nicholsi are the most abundant in four scats with Electrona antarctica present in 3 of the scats. However overall abundance of fish otoliths is low compared to previous years during the same time. No squid beaks have been observed. All 37 milk samples have been processed. A total of 20 more scats will be when the ship arrives at Cape Shirreff from weeks five and six and will be processed during the near shore survey. Additionally, six to eight more milks will also be processed during the near shore survey.

7. Bird and marine mammal observations. This report includes observations of seabirds and mammals for the Elephant Island, Joinville Island, and South-Bransfield Strait strata. A total of 74 transits between stations have been collected representing approximately 1480 nautical miles of continuous observations. In total, 26 species of seabirds have been recorded. Highlights in the Elephant Island area include numerous observations of feeding flocks, consisting primarily of: Cape Petrels, Antarctic Fulmars, White-chinned Petrels, Black-browed Albatross, Gray-headed Albatross, Light-mantled Sooty Albatross, Antarctic Prions. These feeding frenzies were observed on all transits north of Elephant Island, Wilson's and Black-bellied Storm Petrels. These feeding aggregations were primarily located beyond the shelf break zone. Soft-plumaged Petrels continued to be common (100+) near Elephant Island. On transit southwest of Deception Island, thousands of Cape Petrels and Antarctic Fulmars were recorded feeding and sitting in large groups on the water. Within these dense flocks, an Antarctic Petrel was observed. Brown Skuas were observed at the beach at Deception Island near the whaling station. Some of these birds were fitted with a plastic white leg band inscribed with LOV on the left leg with a metal band on the right leg. Humpback whales were the numerically dominant cetacean in the Bransfield Strait, and 12-15 animals were regularly observed during transit between stations. On several occasions, groups were observed feeding, as well breaching. Also of interest, many groups of 3-5 animals were observed traveling in a northeast direction. Southern Bottlenose Whales were observed at the shelf break north of Elephant Island.

Submitted by Adam Jenkins.