AMLR 2005 Weekly Report No. 9
1. Our current position has the
ship anchored in Ezcurra Inlet,
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
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.,
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 (
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 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
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
Submitted by A. Jenkins.