Spatial and Temporal Variability of SeaWiFS Derived Chlorophyll Distributions
West of the Antarctic Peninsula
Marina Marrari, Kendra Daly and Chuanmin Hu
Introduction
Antarctic krill Euphausia superba occupies a key role in the Southern Ocean
ecosystem as the major pelagic herbivore and prey for a large variety of whales, seals,
birds and even fish. Krill has been commercially harvested since the 1960s and today is
the subject on an active fishery by several nations (Ichii, 2000). The waters west of the
Antarctic Peninsula support one of the largest concentrations of krill in the Southern
Ocean (Marr, 1962). The population is maintained by occasional strong year classes with
often poor recruitment in the intervening years, and the set of physical-biological
conditions that control krill recruitment remain poorly understood.
As part f the Southern Ocean Global Ocean Ecosystems Dynamics Program (SO
GLOBEC), an extensive field program including survey and process cruises to the
Western Antarctic Peninsula in austral fall and winter of 2001 and 2002 was developed.
Krill populations were observed to vary strongly, with a large recruitment of juvenile krill
in 2002 in the vicinity of Marguerite Bay. Net data also indicated that during the fall of
2001 larval krill were very abundant, with younger stages dominant off-shelf and older
stages mainly on-shelf, and although in fall 2002 larvae were again found in elevated
numbers off-shelf, all stages were scarce on-shelf (Daly, 2004). The suite of conditions
that drive this variability remain unlcear; however, elevated food concentrations during
late spring-summer might induce early reproduction and ensure an adequate supply for
the newly born larvae.
E. superba is mainly herbivorous and feeds on phytoplankton during summer and
fall when conditions are adequate for algal growth. A successful reproduction (December
thru March) and larval survival is linked to an adequate food supply (Ross and Quetin,
1989), and thus the availability of phytoplankton during late spring-summer, and the
timing, extent, and evolution of blooms is critical to krill overwintering survival.
Remote sensing of ocean color is a useful tool to investigate phytoplankton
dynamics and primary production in the ocean, especially in remote and inaccessible
areas such as the Southern Ocean where frequent ship surveys are not possible. The
principle behind it is that the ratio of blue to green light leaving the ocean surface varies
systematically with the amount of chlorophyll present in surface waters, and thus this
relationship can be used to derive chlorophyll concentrations. Phytoplankton blooms are
dynamic meso-scale features that can change considerably over short distances and from
one week to the next, so the synoptic and large-scale aspects of satellite data are essential
to adequately describe their dynamics.
Ocean color remote sensing has been successfully applied in polar regions (e.g.
Sullivan et al., 1993; Moore et al., 1999; Holm-Hansen et al., 2004). Antarctic surface
waters are rich in macronutrients, but phytoplankton distributions are very patchy and
heterogeneous. West of the Antarctic Peninsula, elevated chlorophyll concentrations have
been identified from satellite data and in situ samples on- and off-shelf (e.g. Smith et al.
1998, 2001; Prézelin et al., 2000) for particular periods of time. Nonetheless,
phytoplankton dynamics during krill’s reproductive season and its interannual variability
have not yet been investigated on longer spatial and temporal scales. In addition, most
studies to date focused on the northern area of the Antarctic Peninsula and the adjacent
Scotia Sea, and thus the Marguerite Bay area, an important krill habitat, remains largely
undescribed in terms of chlorophyll dynamics.
We examine general phytoplankton patterns and dynamics west of the Antarctic
Peninsula with special emphasis on the Marguerite Bay region to better understand the
conditions that make it a suitable habitat for krill. We generate a climatology of the
chlorophyll-a field and relate the anomalies corresponding to summer-fall 2001 and 2002
to the interannual variability observed in krill populations during the SO GLOBEC
cruises. We also investigate the effects of other environmental parameters such as sea ice
extent on phytoplankton and krill distributions.
Materials and Methods
SeaWiFS daily Level 2 data between December 1997 and December 2004 were
obtained from the ocean color team at NASA Goddard Space Flight Center
(http://oceancolor.gsfc.nasa.gov). These data were generated from the high-resolution
Level 1 data collected by all HRPT ground stations as well as occasional onboard
recording over the area using the most updated algorithms and software package
(SeaDAS4.8). A total number of 6606 data files were obtained and mapped to a
rectangular projection with approximately 1 km2/pixel for the area between 45 - 75˚S and
50 - 80˚W. Due to extensive cloud and ice coverage, the daily data were further averaged
to generate bi-weekly composite data/images. Chlorophyll concentrations were estimated
with the OC4v4 empirical algorithm (O’Reilly, 2000):
Chla = 10 0.366 - 3.067R + 1.93R2 + 0.649R3 - 1.532R4
where R=log10((Rrs443 > Rrs490 > Rrs510)/Rrs555) and Rrs is the remote sensing
reflectance, a data product after atmospheric correction. Following the results in Marrari
et al. (submitted), we used the standard un-modified OC4v4 algorithm, and not a regionspecific algorithm, as previously recommended for Southern Ocean waters (e.g Dierssen
and Smith, 2000).
We generated a bi-weekly chlorophyll-a climatology by averaging chlorophyll
concentrations for each 14-day period from September 1997 thru December 2004. The
corresponding anomalies (in mg chl/m3 above or below the mean) for each year were also
estimated. Sea ice extent data was obtained from xxxx. Describe data processing, number
of images, geographical extent, etc.
We describe general chlorophyll dynamics and identify anomalous years. We
also discuss possible reproductive patterns for krill in the area and relate the anomalies in
chlorophyll concentrations to the interannual variability observed in krill distributions
during the SO GLOBEC fall cruises. Finally, we analyze the relationship between sea ice
extent and krill and phytoplankton distributions.
Results
The study area is shown in figure 1a. The location of the cruise track and net
stations occupied during SO GLOBEC cruises in fall of 2001 (figure 1b) and 2002 (figure
1c) are also shown. Krill distributions (from Daly, 2004) are shown in figure 2 (should
we include??). Briefly describe patterns observed and differences between the 2 years.
The chlorophyll climatology resulted in 27 average bi-weekly composite
data/images and a total of 191 anomaly images from December 1997-2004. As an
example, figure 3 shows the mean chlorophyll concentrations for January 1-14 19982004. Some interesting patterns are evident upon inspection of the data: the southern tip
of South America, off the coast of Argentina, consistently shows the highest chlorophyll
concentrations of the whole area. The ACC (Antarctic Circumpolar Current) waters of
Drake Passage have the lowest biomass, with values as low as 0.01 mg chl/m3. West of
the Peninsula, in coastal areas, the highest chlorophyll concentrations are always detected
in Marguerite Bay. This area has chlorophyll values of up to xx mg chl/m3, while the
highest values recorded in any sectors of the northern Peninsula shelf are ~ xx mg chl/m3
(explain location and date). This onshelf-offshelf biomass gradient with highest
concentrations always on the shelf, is observed from December through March during all
years analyzed (data not shown). In October and November however, as light becomes
available and ice begins to recede, the first biomass accumulations of the season can be
observed off-shelf, mainly in the southern (Bellinghausen Sea) and northern (Scotia Sea)
sectors of the study area. This indicates that although during most of the summer the
highest biomass is located on the shelf, the growing season starts with high
concentrations off-shelf, that gradually move onshore and reach the coast by midDecember, where they remain until late March when blooms disappear.
Figure 4 shows the anomalies for the period January-February of 2001 and 2002.
Upon inspecting the images, it becomes evident that 2001 was a much more productive
year than 2002. Most of the shelf area, particularly outer-Marguerite Bay and the coast
along the Peninsula had chlorophyll concentrations well above average, with anomaly
values of up to 5 mg chl/m3. Nonetheless, it is interesting to note that inner Marguerite
Bay consistently had negative anomaly values of up to xx mg chl/m3. Summer of 2002
showed negative anomalies on the shelf area throughout most of the growing season. The
only exceptions are a bloom in outer-Marguerite Bay in late January. On the other hand,
the off-shelf area showed above average chlorophyll concentrations from November
through early March 2002. Unusually high values were detected in Drake Passage, with
positive anomalies of up to xx mg chl/m3. Later in March, the chlorophyll accumulations
observed off-shelf during most of the summer reached the coastal waters of Marguerite
Bay and persisted along the Peninsula reaching Crystal Sound in late March. Figure 5
shows mean chlorophyll concentrations for the periods January 1-14 and Feburary 12-25
1998-2004 at 30 locations along the peninsula (figures 5a and b). Figure 5c shows the
location of the 30 samples. The x-axis on figures 5a and 5b represents the location
number, from west to east along the Peninsula. It is again evident that 2001 had
extremely elevated chlorophyll concentrations when compared to any other year
analyzed.
Discussion
West of the Antarctic Peninsula, E. superba reproduces during austral summer,
from December to March. During the spawning season, developmental stages are
spatially separated along the Peninsula. Adult females migrate off-shelf to oceanic waters
where they spawn, while juveniles are found in coastal regions. Several processes have to
take place in order for krill to recruit successfully. First, reproduction off-shelf the
previous summer has to be successful. Then, larvae must be advected onto the shelf, and
finally these larvae must be retained on the shelf, where food and environmental
conditions are adequate for a successful development. Daly (2004) found that in fall 2001
(May-June) larval krill were very abundant on and off-shelf, with concentrations reaching
the highest numbers ever recorded for the area (xx ind/m3). Larvae on-shelf were older
than the ones identified off-shelf, indicating that they were probably born in oceanic
waters earlier in the season and were transported to coastal areas where food is more
abundant. The larvae found offshelf were most frequently of stage calyptopis III (CIII).
Following Ikeda (1984), CIIIs are 63 days old on average. If we assume surface currents
of the ACC move in a general northeastern direction at a mean speed of 0.04 m/s (ref),
these larvae had to be born xx km to the southwest, approximately off the shelf shelf of
xxx (see solid-line box in Figure 2). If we assume a mean current speed of 0.1 m/s (ref)
the spawning area could have been located farther southwest, in the Bellinghausen Sea
(see dashed box in figure 2).
In any of these 2 possible scenarios, E. superba’s spawning grounds would be
located in one of the most productive areas of the whole Antarctic Peninsula, where
maximum surface chlorophyll concentrations can reach xx mg/m3 in DATES. This area
is also the first to sustain elevated chlorophyll concentrations. The first phytoplankton
blooms, which start to form in late November off the shelf, are identified in this region
from satellite data.
By spawning in the oceanic waters of the Bellinghausen Sea starting in early
December, krill ensure their larvae will have adequate food supply throughout their
journey to the northeast first in waters of the ACC, and onto the Antarctic Peninsula shelf
later in the summer. By fall, when food in the water column becomes scarce and sea ice
starts to cover the shelf, older larvae and juveniles will have refuge from predators under
the sea ice and an alternate food source in the biota that grows in the undersurface of the
ice (Daly, 1990).
MORE ON MARGUETE BAY, NORTHERN SHELF AREAS, DRAKE
PASSAGE, ETC…
Figures and Tables
Figure 1a. Map of the study area describing some geographic features and bathymetry.
Isobaths corresponding to 50, 100, 500, 1000 and 2000m are shown. 1b and 1c. Location
of sampling stations during SO GLOBEC fall cruises 2001 and 2002, respectively.
Figure 2. Larval krill distributions in surface waters (0-50m) from net data during fall
2001 and 2002. Figure from Daly (2004).
Figure 3. Climatology of surface chlorophyll-a for the period January 1-14.
SEE ATTACHED .DOC FILE
Figure 4. Anomalies for chlorophyll concentrations for the periods December-March
2001 (first column of images) and 2002 (second column). Anomalies are mg chl/m3
above or below the 8-year mean.
NEED to ADD THIS
Figure 5. Mean chlorophyll concentrations (in mg/m3) at 30 locations along the Antarctic
Peninsula for the periods January 1-14 (5a) and February 12-25 (5b) 1998-2004. The
locations are also shown (5c).
Literature cited:
 Bianchi, T.S., Lambert, C., and Biggs, D. 1995. Distribution of chlorophyll a and
phaeopigments in the northwestern Gulf of Mexico: A comparison between
fluorometric and high-performance liquid chromatography measurements. Bulletin
of Marine Science, 56(1): 25-32.
 Daly, K.L. 1990. Overwintering development, growth and feeding of larval
Euphausia superba in the Antarctic marginal ice zone. Limnology and
Oceanography, 35(7): 1564-1576.
 Daly, K.L. 2004. Overwintering growth and development of larval Euphausia
superba: an interannual comparison under varying environmental conditions west
of the Antarctic Peninsula. Deep Sea Research Part II, 51(17-19): 2139-2168.
 Haberman, K.L., Ross, R.M., and Quetin, L.B. 2003. Diet of the Antarctic krill
(Euphausia superba Dana): II. Selective grazing in mixed phytoplankton
assemblages. Journal of Experimental Marine Biology and Ecology, 283: 97-113.
 Ichii, T. 2000. Krill harvesting. In: Everson, I. (Ed.). Krill, biology, ecology and
fisheries. Blackwell Science, Berlin, pp 228-261.
 Kawaguchi, S. and Satake, M. 1994. Relationship between recruitment of the
Antarctic krill and the degree of ice cover near the South Shetland Islands. Fisheries
Science, 60(1): 123-124.
 Marr, J.W.S. 1962. The natural history and geography of the Antarctic krill
(Euphausia superba Dana). Discovery Report 32: 33-464.
 Moore, J.K., Abbott, M.R., Richman, J.G., Smith, W.O., Cowles, T.J., Coale, K.H.,
Gardner, W.D., and Barber, R.T. 1999. SeaWiFS satellite ocean color data from the
Southern Ocean. Geophysical Research Letters, 26(10): 1465-1468.
 Prézelin, B.B., Hofmann, E.E., Mengelt, C., and Klinck, J.M. 2000. The linkage
between Upper Circumpolar Deep Water (UCDW) and phytoplankton assemblages
on the west Antarctic Peninsula continental shelf. Journal of Marine Research, 58:
165-202.
 Ross, R.M. and Quetin, L.B. 1989. Energetic cost to develop to the first feeding
stage of Euphausia superba Dana and the effect of delays in food availability.
Journal of Experimental Marine Biology and Ecology, 133: 103-127.
 Ross, R.M., Quetin, L.B., Baker, K.S., Vernet, M., and Smith, R.C. 2000. Growth
limitation in young Euphausia superba under field conditions. Limnology and
Oceanography, 45(1): 31-43.
 Siegel, V., and Loeb, V. 1995. Recruitment of Antarctic krill Euphausia superba
and possible causes for its variability. Marine Ecology Progress Series, 123: 45-56.
 Smith, R.C., Baker, K.S., and Vernet, M. 1998. Seasonal and interannual variability
of phytoplankton biomass west of the Antarctic Peninsula. Journal of Marine
Systems, 17: 229-243.
 Smith, R.C., Baker. K.S., Dierssen, H.M., Stammerjohn, S.E., and Vernet, M. 2001.
Variability of primary production in an Antarctic marine ecosystem as estimated
using a multi-scale sampling strategy. American Zoologist, 41: 40-56.
 Sullivan, C.W., Arrigo, K.R., McClain, C.R., Comiso, J.C., and Firestone, J. 1993.
Distributions of phytoplankton blooms in the Southern Ocean. Science, 262: 18321837.
 Trees, C.C., Kennicutt, M.C., and Brooks, J.M. 1985. Errors associated with the
standard fluorometric determination of chlorophylls and phaeopigments. Marine
Chemistry, 17: 1-12.
Acknowledgements