AMLR 2003 Weekly Report No. 7

23 February 2003

 

1. Our current position is approximately 15 nautical miles south of King George Island, where we are conducting the second of two surveys of bio-oceanographic conditions in the vicinity of the South Shetland Islands. The West and Elephant Island areas of the survey have been completed. An extensive band of pack ice and icebergs pushing northward out of the Weddell Sea forced a change in the conduct of the survey in the Joinville area allowing only minor penetration into the area. During the first survey of this year the heavy ice conditions reached appreciably into the Elephant Island area hindering operations. The fact that we completed the Elephant Island on our second survey illustrates a dramatic retreat on the part of the Weddell Sea ice in the past four weeks. Weather has been mild to moderate allowing the time lost to a patch of heavy weather earlier in the survey to be made up.

 

2. Krill, salps and other zooplankton. Post larval krill were present in 42 of 48 (88%) Elephant Island samples. Mean and median abundance values (95 and 9 per 1000 m3) were ca. 33% those observed here during Survey A. This was associated with decreased numbers of small juveniles (<30 mm lengths) and a modest increase in older mature krill >40 mm in length. Overall proportions of juvenile, immature and mature stages was 21%, 52% and 27% compared to 42%, 39% and 19% the previous month. Reproductively advanced female stages (i.e., gravid and spent) were scarce, comprising <1% of total krill abundance in the area. The gaunt appearance of mature individuals gave the impression that spawning was delayed or minimized due to poor feeding conditions. Larval krill were present in 43% of samples and had a slightly higher mean abundance than last month (6.9 vs. 4.6 pre 1000 m3). While most of these were early calyptopis stages (1 & 2), 12% were furcilia stage 2 indicating a prolonged, possibly sporadic, spawning season.

 

Salpa thompsoni abundance (mean 62 and median 10 per 1000 m3) was essentially the same as during last month. Greatest concentrations were offshore. The aggregate stage comprised 92% of individuals. Only 10% of these were <20 mm in length indicating negligible chain production during the intervening period. The median length (34 mm) was 8 mm larger than during Survey A and conforms to a ca. 0.44 mm per day growth rate. These observations suggest retention of a relatively stable salp population within the Elephant Island area during the 2003 summer. This could be explained by the sluggish circulation indicated by the drifters deployed in outer shelf and offshore areas. Increased abundance of solitaries was due to new production, as indicated by the fact that 75% were <25 mm. Production of this over wintering stage, together with negligible chain production, signals an early end to the summer production cycle.

 

As during Survey A, copepods numerically dominated the Elephant Island zooplankton catches. As in the West area, copepod abundance had a seasonal increase with the mean value 3 and median value 2 those in January; this increase was primarily due to coastally derived Metridia gerlachei (62% of total). Post larvae of the euhpausiid Thysanoessa macrura followed copepods in overall abundance.

 

3. Krill biomass and dispersion. Seven transects were completed over the Elephant Island Area during the week ending 23 February 2003. Highest densities of krill biomass were mapped along the westernmost transect and to the north and north east of Elephant Island, along the shelf break, and again to the east and northeast of Clarence Island. Krill densities for each transect ranged from 0.95-27.72 g/m-2. Mean krill density over the seven transects was 12.1 g/m-2, which is significantly lower than the mean density measured during Survey A of the 2003 cruise (22.1 g/m-2). Compared with Survey D results for 2002 and 2001, krill density is higher then 2002 (6 g/m-2) and on par with 2001 estimates (13 g/m-2). Myctophid mean biomass densities for the West area are estimated to be 49.4 g/m-2 and 40.9 g/m-2 for the Elephant Island area during leg III. These density estimates indicate an increase in myctophid abundance as compared with the West and Elephant Island areas during leg I (35.1 and 16.1 g/m-2 respectively).

 

4. Phytoplankton. Chlorophyll concentrations in the Elephant Island have increased since Leg I. For 48 stations surveyed, 5 m chlorophyll was 0.63 +/- 0.42 mg m-3 and integrated (to 100 m) was 21.44+/- 13.69 mg chl m-2. These values compared to 0.10 +/- 0.09 mg chl m-3 and 16.4 +/- 9.1 mg chl m-2 for 5 m and integrated values, respectively, measured for Leg I. Leg II values are lower than the February average (1990-2002) of 1.13 +/- 1.23 mg chl m-3 for 5 m and 70.91+/- 64.43 mg chl m-2 for integrated (to 100 m) chlorophyll concentrations (572 stations). Highest chlorophyll concentrations were observed in shelf and shelf-break waters (bottom depth less than 1000 m), with 5 meter samples having 0.63 +/- 0.51 mg chl m-3 (16 stations), compared with 5 m samples of pelagic stations having 0.25 +/- 0.31 mg chl m-3.

 

Three additional (and final) drifter buoys to measure ocean currents were deployed this week, thus 8 total drifters were deployed during Leg II. Signals are still being obtained from the 6 of 12 drifters deployed during Leg I. Preliminary inspections indicate that drifters deployed west of the Shackleton Fracture Zone (SFZ) are deflected in both northerly and southerly directions to suggest that the easterly flow of the Antarctic Circumpolar Current is interrupted by the SFZ. CTD casts made in the Elephant Island Area have been of sufficient coverage to resolve a region of possible down-welling parallel to and approximately 50 km east of the SFZ, ranging from at least 60.0S, 57.5W to 60.75S, 56.5W. This down welling condition was best observed by a plume of higher concentrations of oxygen and lower temperatures relative to surrounding waters to 750 m. As 60S is our most northerly survey boundary, it is not known how much further north this region of possible down welling extended.

 

With aid of Local Area Coverage from SeaWiFS satellite, a small patch of phytoplankton bloom (near Station D11-07) surrounded by a large area of water having low chlorophyll concentrations was observed north of the eastern end of King George Island. As this bloom was near our current position, an extra CTD station (Station X10-07) outside the determined survey pattern was made. 5 m chlorophyll was 1.73 and 2.31 mg chl m-3 and integrated chlorophyll (to 100 m) was 32.2 and 55.9 mg chl-2 at stations X10-07 and D11-07, respectively. The high chlorophyll concentrations at these two stations corresponded to a possible upwelling condition at these two stations as evidenced by cooler, less dense, and more oxygenated water column profiles than for surrounding stations. Water column properties (Type III water) of these two stations suggest that they had been influenced by Bransfield Strait water that had pushed northward over the sill that separates King George and Elephant Islands.

 

5. Oceanography and meteorology. Northwesterly winds, averaging twenty knots with maximum speeds reaching 35knts, were experienced for most of the week. Tuesday and Wednesday saw the winds swing to northeast, with average speeds of fifteen knots. Atmospheric pressure showed a gradual increase until midnight on Thursday, reaching ~1010 mbar, then a steady decrease to 980 mbar on Sunday. Air temperatures ranged between 1 and 5C with most days being overcast.

 

48 CTD stations were occupied in the Elephant Island area with two stations (D09-08 & D08-08) only done at the end of EI07 transect, due to bad weather at the beginning of the week. An extra CTD station X10-07, identified as a high chlorophyll concentration area by a SeaWiFS satellite image, was added between the West and Elephant Island areas. This station and the adjacent station (D11-07) showed higher concentrations of chlorophyll and possible upwelling in the CTD traces than the surrounding stations in the area. As per the Water Zone classification tables, Type 1 (ACW) water was found in the Western side of the area and just below the Shackleton Fracture Zone. Mixed waters of Type II & III are found on the upper eastern side of the fracture zone, with the ACW becoming diminished. This can be seen in the CTD traces in the area which shows lots of mixing occurring and some evidence of down-welling taking place at some stations. Type V (Weddell Sea) water being found on the extreme lower eastern side of the area. The lower part of the area is mainly influenced by Type IV (Bransfield Field Strait) water in the west and around Elephant Island shelf area.

 

6. Predator diet studies. This week 10 additional scats were sifted and processed. All samples but one contained krill and 8 of the 10 contained fish otoliths. Week 5 scats continue to show a reversal of otolith dominance from the previous week, with G. nicholsi becoming much more prevalent than E. antarctica. In addition, week 5 scats contained a large number of small, unidentified otoliths, most likely eroded G. brauri. Five and a half squid beaks were discovered and stored in ethanol. Twenty-four fur seal milk samples had lipid extracted, and an additional 25 fresh krill were gendered and measured for total length, carapace length, and width. Opportunistically, extra myctophids and krill are collected from the net tows for later homogenization and lipid extraction.

 

7. Bird and marine mammal observations. This report includes information regarding the Elephant Island area survey. Systematic observations of vertebrate abundance and behavior were conducted during daylight hours in transit between stations. A total of 34 transits were sampled and 24 species of seabirds were recorded. Highlights included Thin-billed prion, Soft-plumaged petrel, White-headed petrel, Common diving petrel, and Arctic tern. Chinstrap penguins were highly conspicuous in the pelagic waters north of Elephant Island and were the most abundant species. A strikingly large aggregation of Black-browed (300+) and Grey-headed (40+) albatrosses were observed sitting on the water on transect line E4. Such a dense aggregation of albatrosses may represent an abundant and predictable prey resource. On transect line E6, we encountered hundreds of Antarctic prions every hour, and the majority of which were flying in a westward direction. Great numbers of Wilson and Black-bellied storm petrels were observed on transit south of Clarence Island. On transit to station 5.5-08, thousands of Antarctic fulmars were observed flying north; most likely towards Gibbs, Aspland, Eddie, and O'Brien Islands, where there may be nesting colonies for this species. A total of 22 humpback whales were counted during transit to 5.5-08 as well. Other marine mammal observations included: fur seal, fin whale, humpback whale, minke whale, southern bottlenose whale, and hourglass dolphin.

 

 

 

A. Jenkins sends