AMLR 2005 Weekly Report No. 9

13 March 2005

1. Our current position has the ship anchored in Ezcurra Inlet, Admiralty Bay, King George Island. The large area survey-D was completed on 10 March, the final two stations in the South area due were dropped due to extended storm conditions. The past three days the ship has been involved in the closure of three Antarctic field camps. Cape Shirreff, Livingston Island, the Chilean INACH camp, Livingston Island and Copacabana Field station, King George Island. Presently all AMLR and Chilean camp personnel are safely aboard the ship and all retrograde equipment and materials were recovered to the vessel via zodiac. The ship's echosounder was calibrated over the night in Ezcurra Inlet. The ship will be departing for Punta Arenas, Chile this afternoon with ETA of 16 March.

2. Krill, salps and other zooplankton. Krill abundance estimates from the January and February-March Elephant Island Area surveys (means 27-48 per 1000 m³, medians 3-15 per 1000 m³) were comparable to those from last year. Seasonal abundance changes across the large survey area reflected distributional shifts with increased patchiness and dense concentrations localized over the north shelf of Elephant Island and within Bransfield Strait during the latter period. Like last year the length/maturity composition (i.e., primarily large, mature individuals three years and older) represented the highly successful 1999-2000, 2000-2001 and 2001-2002 year classes. However, the presence of <40 mm individuals each survey indicates modest recruitment success from the past two years. During January juveniles comprised ca. 3% of total krill in the Elephant Island and South Areas suggesting an approximate level of proportional recruitment success for the 2003-2004 year class. This is substantially lower than had been predicted based on the prolonged and apparently successful spawning period and presumably favorable over wintering conditions presented last year. However, localized recruitment success may be strongly impacted by advective regimes (e.g., transport away from the area) and/or behavior (possible migration to coastal refuges south of the survey area). In contrast to 2003-2004, reproductive activity appeared to be temporally limited this year. About 81% of mature females in the Elephant Island Area were in advanced reproductive stages during January (i.e., with developing ovaries and spent) but less than 10% were in advanced stages in February-March. This coincided with apparent seasonal regression of male maturity stages and southward ontogenetic migration.

Despite an apparently brief spawning period the larval krill concentrations were relatively high, with mean and median values in the Elephant Island Area during February-March (195 and 5 per 1000 m³, respectively) comparable to those in 2001. The larval stage composition (primarily calyptopis 2 and 3 and furcilia 1 stages) supports a mid-December to mid-January spawning peak. Interestingly, significantly elevated concentrations of these larvae were within coastal (Zone 5) water (ANOVA, P<0.001) in western Bransfield Strait and south and east of Elephant Island suggesting source areas to the southwest (e.g., Gerlache Strait) and east (e.g., Weddell Sea).

The zooplankton assemblage remained relatively depauperate and numerically dominated by Salpa thompsoni, Metridia gerlachei and post-larval Thysanoessa macrura across both large-area surveys. During January mean and median abundance values of S. thompsoni (1210 and 671 per 1000 m³) rivaled the highs of 1993 and this species alone accounted for 61% of the total mean zooplankton abundance within the Elephant Island Area. This salp demonstrated a substantial seasonal abundance decrease with subsequent values (861 and 493 per 1000 m³) comparable with those of February-March 1998. This overall abundance decrease occurred despite a modest pulse of late-season aggregate chain production suggesting net loss through advection, post-spawning vertical migration to deeper water and/or mortality of the summer dominant aggregate stage. As a consequence S. thompsoni contributed only 31% of mean zooplankton abundance, and ranked second to copepods, during the latter survey. In contrast to previous years, S. thompsoni appeared to be advected into the area from western vs. eastern (Weddell Sea) source regions. During January its distribution was significantly associated with ACC (Zone 1) water; during February-March significantly higher concentrations occurred within mixed (Zone 2) water than more coastal zones. The high-latitude salp Ihlea racovitzai remained relatively rare, its distribution limited to Zone 5 water in the eastern portion of the area suggesting minimal Weddell Sea water influence across the survey period.

January copepod abundance values demonstrated a continued decline from the highs observed in 2002. As with the 2003 surveys mean and median values exhibited modest two to three fold increases that contrasted markedly with order of magnitude seasonal increases in 2001, 2002 and 2004. This, along with persistent numerical dominance by coastal species Metridia gerlachei and a generally depauperate zooplankton assemblage, suggests that southern movement of the Southern Front of the Antarctic Circumpolar Current did not extend into the survey area during 2003 and 2005, both of which represent El Niño periods.

3. Krill biomass and dispersion. We have completed acoustic biomass estimates for both the Joinville and South areas of the AMLR sampling grid. Biomass in the Joinville area declined between legs from 250,520 metric tons in February to just 35, 495 metric tons in March. Likewise, the biomass of krill decline din the southern area from 345,511 metric tons to 141,689 metric tons. Such declines area similar to those observed in previous years. Average densities of krill were 13.98 and 1.96 g per m² in the Joinville and South areas, respectively. For the entire cruise period, and the entire AMLR survey area, krill densities declined from 27 to 9 g per m², between the two cruise legs.

4. Phytoplankton. For the Joinville Island area, surface chlorophyll concentrations averaged 0.37 ± 0.13 mg m^-3, with concentrations of 30 ± 12 and 18 ± 5 mg chl-a m^-2 as integrated to 100 m and 1% PAR, respectfully. Depth of 1% PAR was estimated ~66 m. The South Area was found slightly richer in phytoplankton, with surface chl concentrations of 0.60 ± 0.19 mg m^-3, and concentrations of 44 ± 9 and 27 ± 6 mg chl-a m^-2 as integrated to 100 m and 1% PAR, respectfully. Depth of 1% PAR was estimated ~58 m. For both survey areas, phytoplankton stocks were found below average (~1.9 mg chl m^-3, 11 year mean surface concentration) for this time of the season.

5. Oceanography and meteorology. During the week winds were mainly from a South Easterly direction, averaging around 15-20 knots. On Thursday the wind speeds increased to around 35 knots, gusting up to 47 knots. The increase in wind speeds was associated with a drop in the barometric pressure from 991 to 976 millibars. For the duration of the week the barometric pressure averaged around 990 millibars. Also on Thursday, the coldest temperatures for the survey were recorded, with the minimum temperature reaching -5.9°C. The average temperature for the beginning of the week hovered just above 0°C and then dropped to an average of around -4°C, after Thursday.

18 CTD stations were occupied and successfully sampled in the South Area, with 2 stations at the end of the survey grid being lost due to bad weather. No stations were lost due to ice, although some of the southern station was moved owing to the proximity of icebergs. According to the Water Zone Classification table, mainly Water Zone 4 (Bransfield Strait) was found in the area, although some Water Zone 5 (Weddell Sea) waters were found in the South Western part of the area. With 5 stations still to be occupied before the end of the survey, the CTD underwater unit malfunctioned and had to be replaced with the spare unit. During the latter part of the week when the colder air temperatures were experienced, seawater had to be continuously run over the CTD sensors to prevent the instruments from freezing up.

Submitted by A. Jenkins.