Spatial and Temporal Variability of SeaWiFS Chlorophyll Distributions West of the
Antarctic Peninsula: Implications for krill development
[the whole purpose of this paper is NOT chl distribution, otherwise it is hard to answer the
“so what” question. So adding this “implication” in the title is important, and should also be
emphasized in the abstract. I am not sure if we shall use “development” or other terms]
[for the same reason, I think it is better to add at least one more figure to show krill
distribution and its change between 2000/2001 and 2001/2002]
Marina Marrari*, Kendra L. Daly and Chuanmin Hu
College of Marine Science
University of South Florida
140 Seventh Avenue South
St. Petersburg, FL 33701, USA
1
* Corresponding author: mmarrari@marine.usf.edu; Tel: 727-553-1207; Fax: 727-553-1186
Abstract
Keywords: Antarctic krill, chlorophyll, climatology, Euphausia superba, SeaWiFS, Southern
Ocean, Antarctic Peninsula, Bellingshausen Sea.
Introduction
The waters west of the Antarctic Peninsula support relatively high densities of
phytoplankton, zooplankton, and upper trophic level predators, and the region is considered to
be one of the most productive of the Southern Ocean (Deibel and Daly, in press). The
Antarctic krill, Euphausia superba, plays a key role in this ecosystem as one of the primary
pelagic herbivores and prey for many predators, including whales, seals, penguins, seabirds
and fish. In addition, krill have been commercially harvested in this region since the 1960s
and are still the subject of an active fishery by several nations (Ichii, 2000). The large krill
population appears to be maintained by occasional strong year classes, with often poor
recruitment in the intervening years (Siegel and Loeb, 1995; Quetin and Ross, 2003). The
suite of physical and biological factors that govern krill reproduction and recruitment,
however, remain poorly known.
Successful krill reproduction (November-March) and larval survival during summer
require an adequate food supply (Ross and Quetin, 1983; 1989). Adult females may require
above-average phytoplankton concentrations (1 - 5 mg chl m-3) to initiate reproduction (Ross
2
and Quetin, 1986) and relatively high chlorophyll concentrations (>0.5 mg chl m-3) to sustain
multiple spawning throughout the summer (Nicol et al., 1995). Early primary production in
polynyas, near ice edges or shelf breaks, and summer blooms onshelf may provide important
food sources for young larvae. Laboratory experiments suggested that first-feeding larvae
may not survive delayed food or when only small flagellates are available as a food source
(Ikeda, 1984; Ross and Quetin, 1989). Furthermore, enhanced food availability allows larvae
to achieve faster growth and developmental rates, thereby obtaining a larger size and being in
better condition to survive over winter (Daly, 2004). Thus, knowledge about differences in
the timing, extent, and evolution of phytoplankton blooms is important for understanding
interannual variability in krill recruitment.
The Southern Ocean Global Ocean Ecosystems Dynamics Program (SO GLOBEC)
investigated the physical and biological factors that influence the growth, recruitment, and
overwintering survival of Antarctic krill in the vicinity of Marguerite Bay, west of the
Antarctic Peninsula, during austral fall and winter of 2001 and 2002. Large differences in
abundances of larval and juvenile krill were observed between these two years (Daly, 2004).
During fall 2001, larvae were very abundant (1 - 19 individuals m-3), with younger stages
dominant off-shelf and older stages dominant on-shelf. Few juveniles were observed
anywhere. On-shelf larvae were in better condition than offshore larvae, suggesting that there
was enhanced food availability near Marguerite Bay during the preceding summer. During
fall 2002, although larvae were again found in elevated numbers off-shelf (0.01 – 110
individuals m-3), overall abundances were an order of magnitude lower than in 2001 and all
stages were scarce in coastal areas. Juveniles, however, were relatively abundant (xxx
individuals m-3), indicating that there was successful recruitment from the 2001 larval
population. These results prompted us to investigate the environmental conditions that
3
contributed to the large krill reproduction and subsequent high larval densities particularly
during austral spring and summer 2000/2001.
Herein, we investigate the interannual chlorophyll patterns and dynamics west of the
Antarctic Peninsula using SeaWiFS ocean color data between 1997 and 2004, 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 surface chlorophyll field and
examine the anomalies corresponding to summer-fall 2000/2001 and 2001/2002 with respect
to the interannual variability in krill abundances during SO GLOBEC. We also investigate
the effects of sea ice extent on the timing and location of phytoplankton blooms west of the
Antarctic Peninsula.
Materials and Methods
The study area consisted of the coastal waters west of the Antarctic Peninsula and
adjacent deep waters in the Drake Passage and Bellingshausen Sea (45 - 75˚ S and 50 - 80˚
W), as chlorophyll in these areas are most likely to influence regional krill populations (Fig.
1).
The chlorophyll dataset includes 6606 SeaWiFS daily Level 2 files (~ 1 km/pixel near nadir)
between September 1997 and April 2004 were obtained from the NASA Goddard Space
Flight Center (http://oceancolor.gsfc.nasa.gov). These data were collected by all ground
stations, as well as occasional satellite onboard recording over the area using the most recent
algorithms and software package (SeaDAS4.8). The Level 2 data were mapped to a
rectangular projection with approximately 1 km2/pixel for the western Antarctic Peninsula
region (Fig. 1). The parameter used in this study is the surface chlorophyll concentration
derived from the OC4v4 empirical band-ratio (blue versus green) algorithm (O’Reilly, 2000).
4
Comparison with in situ chlorophyll a values measured by HPLC showed that for >90% of
the waters in the Southern Ocean (Chl between 0.05 and 1.5 mg m-3) SeaWiFS Chl was
accurate (Marrari et al., in press). For higher concentrations the accuracy is less satisfactory,
but the data are expected to be consistent through time.
A bi-weekly time-series and bi-weekly 7-year climatology of chlorophyll a (mg m-3)
distributions were generated from the mapped Level 2 data, between 1997 - 2004 (Fig. 2). In
total, we generated 191 bi-weekly composite images, 27 climatology bi-weekly composite
images, and 191 corresponding anomaly images for September 1997 through December 2004.
Fourteen regions were defined within the study area, each representing different
geographic locations and oceanographic conditions (Table 1, Fig 3a). The average (median)
chlorophyll concentration for each region (mg m-3) was estimated during the 2000/2001 and
2001/2002 productive seasons (September - March) and plotted over time in relation to the
climatology (Fig. 3b). In addition, the anomalies (mg chl m-3 above or below the
climatology) within the entire study area were mapped during the spring-summer of
2000/2001 and 2001/2002 (November - February) (Fig. 4).
The mean monthly location of the ice edge within the study area during October,
November and December of 2000 and 2001 was determined using a two-dimensional linear
interpolation of monthly ice-concentration on a 25 km resolution grid. Monthly averaged
gridded ice concentrations, generated using the NASA Team algorithm and Nimbus-7 SMMR
and DMSP SSM/I passive microwave data, were obtained from the National Snow and Ice
Data Center (Cavalieri et al., 2005). The ice edge was considered to be the location where sea
ice concentration was equal to or less than 15% (Gloersen et al., 1992) [what’s the meaning of
15%? Proportion of ice versus melted water? Need to clarify]. The mean location of the ice
edge during these months was superimposed over the concurrent biweekly SeaWiFS
5
chlorophyll images. In addition, the location of the ice edge during the preceding month was
also overlaid on the image in order to evaluate whether chlorophyll concentrations increase
significantly within the region of the ice edge retreat and the time scale associated with those
changes (Fig. 5).
The daily area free of sea ice (km2) in the northern and southern sections of
Marguerite Bay was estimated from satellite data following the methods described in Arrigo
and van Dijken (2003) (Fig 6a). A daily climatology from 1997 through 2004 was calculated
as the mean daily ice-free area within each sub-region for the 8 years analyzed. A running
average was applied to the data to reduce the daily variability. The climatology was plotted in
relation to the daily ice-free areas for 2001 and 2002 in Figure 6.
Results
Biweekly climatology patterns of chlorophyll in waters west of the Antarctic
Peninsula between September and March 1997 - 2004 (Fig. 2) indicate that offshore waters in
the Antarctic Circumpolar Current (ACC) typically had relatively low chlorophyll
concentrations (0.1 - 0.2 mg m-3). In contrast, the highest chlorophyll concentrations
consistently occurred in coastal waters in the vicinity of Marguerite Bay and to the south of
the Bellingshausen Sea. These phytoplankton blooms persisted throughout the summer
(December - March), with mean values in January of xx ± SD mg m-3 and in March of xx ±
SD mg m-3 for 1997 - 2004. A wide range of intermediate values (mean = xx ± SD) were
observed over the more northern continental shelf regions west of the Antarctic Peninsula and
downstream in the Scotia Sea. The spatial and temporal changes in chlorophyll patterns
suggest that the biomass accumulations initially occur during October and November in
offshelf waters, mainly in the Bellingshausen Sea and to a lesser extent near the shelf break in
6
the vicinity of the Shetland Islands at the northern end of the Antarctic Peinsula (Fig. 2). As
the season progresses (mid-December), phytoplankton blooms develop onshore especially in
the vicinity of Marguerite Bay, where they remain well established until early April. Thus,
oceanic areas in the Bellingshausen Sea and coastal Marguerite Bay waters showed
particularly high chlorophyll concentrations during spring and summer in comparison with
any other area west of the Antarctic Peninsula.
Of the 14 pre-defined regions (Table 1, Fig. 3a), regions 1 - 6 represent offshelf
oceanic regimes with depths greater than 2000 m, while regions 7 and 8 are located over the
continental shelf slope, defined as the area between 500 and 2000 m. Regions 9, 10 and 11
represent coastal waters along the Antarctic Peninsula shelf and regions 13 and 14 are located
in Marguerite Bay. Region 12 includes both coastal and oceanic waters in the Scotia-Weddell
confluence area.
Both the median chlorophyll distributions in these areas and the anomaly data indicate
that, overall, 2000/2001 had higher chlorophyll concentrations than 2001/2002, particularly in
the Bellingshausen Sea, Marguerite Bay, and other coastal areas along the Peninsula (Figs 3
and 4). The median chlorophyll concentrations for the 14 regions reveal relatively small
variations from the climatology in offshelf regions of the ACC, Drake Passage, and the Scotia
Sea (regions 1 - 6), although chlorophyll concentrations during 2000/2001 in offshore waters
of the Bellingshausen Sea (regions 1 and 4) showed a moderate increase with respect to the
typical (i.e., climatologic) values (Fig. 3b). Shelf break and coastal regions showed increased
production relative to the climatology with median values in some regions exceeding the
average by up to 5 mg m-3. In the Bellingshausen Sea (regions 7 and 9), chlorophyll
concentrations were generally above average during both seasons, with high variability
observed both between and within years. The summer of 2000/2001 showed the highest
7
median concentrations with values reaching 1.7 mg chl m-3 in February, whereas values
during 2001/2002 were generally lower and closer to the climatology. Shelf break and coastal
regions along the Antarctic Peninsula (regions 8, 10 and 11) also had elevated chlorophyll
concentrations in relation to offshore areas, although the variations with respect to the
climatology were less evident than in the Bellingshausen Sea and median values never
exceeded 0.63 mg chl m-3. Marguerite Bay (regions 13 and 14) had the largest anomalies in
comparison with any other region analyzed. In September, average chlorophyll
concentrations in northern Marguerite Bay were 0.3 - 0.63 mg m-3, but during September
2000, these values reached 5.62 mg m-3. For the same region, 2002 values were consistently
near or below the 8-year mean. Sea ice prevented satellite data collection during most of the
2001/2002 summer in Southern Marguerite Bay, but 2000/2001 showed high median values
of up to 3.21 mg chl m-3 from late December through February, a 270% increase with respect
to the average conditions (i.e. 1.19 mg m-3).
Similar patterns can be observed from a temporal perspective (Fig. 4). During late
November, both seasons [which seasons? Do you mean both years?] showed strong positive
anomalies west of the Antarctic Peninsula. In 2000/2001, however, these above-average
chlorophyll concentrations were observed over the continental shelf, while in 2001/2002
positive anomalies were observed in oceanic waters and to the southwest in the
Bellingshausen Sea. By December, 2000/2001 showed positive anomalies in Marguerite Bay
and in coastal areas along the entire Peninsula, while offshore areas had negative values. In
contrast, 2001/2002 had below-average chlorophyll concentrations in the Peninsula coastal
waters and above-average concentrations in oceanic areas. In January and February 2001,
widespread and strong positive anomalies were still present in the Bellingshausen Sea,
Marguerite Bay, and along the Peninsula shelf, whereas ACC waters showed average
8
conditions. In January and February 2002, overall conditions had progressed toward a mean
state, and strong positive values were only observed in a narrow band along the ice edge
(black area) in the Bellingshausen Sea during January. It is important to note, however, that
due to extensive cloud cover, the number of valid data points available for late February 2002
is considerably lower than that for 2001. In summary, the 2001/2002 season started with
higher than normal chlorophyll concentrations offshelf, but by January, conditions had
progressed toward an average state. Even though the 2000/2001 spring-summer began with
weaker positive anomalies than 2001/2002, by January many areas showed increased
production. Throughout the rest of the summer, 2000/2001 showed very widespread positive
anomalies, particularly in the Bellingshausen Sea, Marguerite Bay, and along the continental
shelf.
The location, timing, and extent of sea ice were examined in relation to chlorophyll
concentrations to better understand the relationship between sea ice and phytoplankton
blooms (Fig. 5). Chlorophyll concentrations were highly variable in relation to the receding
ice edge in our study area during 2000/2001 and 2001/2002. In offshelf waters during spring,
phytoplankton blooms occurred both adjacent to the ice edge and/or in waters previously
covered by sea ice 2 - 4 weeks earlier. For example, during September 2000 and 2001, the ice
edge was located in oceanic waters of the ACC. By October, the ice margin had retreated
considerably and occurred closer to the coast. Chlorophyll had not increased significantly at
the September ice edge locations by October (Fig. 5a and 5b, top 2 panels), suggesting that on
average, October was too early in the productive season for any significant chlorophyll
accumulations to occur within the ice edge zone. During November, however, chlorophyll
concentrations had increased significantly in this region, reaching ~ 5 mg m-3, but presumably
were too far from the ice edge to be influenced by ice processes (Fig. 5a and 5b, center
9
panels). During November, the ice edge receded onshelf in the mid-Antarctic Peninsula, but
remained offshore of the southern Peninsula in both 2000 and 2001. Subsequently during
December, enhanced chlorophyll concentrations occurred within the region of ice retreat. In
some regions the 2 phenomena seem to coincide in space after a periodic time lag of 2 to 4
weeks. In addition, areas that were never influenced by sea ice also showed increased
production. For example in November and December of 2001 (Fig. 5b), the ice edge
occupied coastal areas of the Bellingshausen Sea and along the Antarctic Peninsula to Anvers
Island (Fig. 1). Even though ice never occupied the northern end of the Peninsula, elevated
chlorophyll concentrations were observed along the shelf break and in coastal areas. Thus,
processes other than the retreat of the ice edge likely influenced phytoplankton dynamics in
this area.
Sea ice coverage in Marguerite Bay also showed strong differences between the
spring-summer of 2000/2001 and 2001/2002. During summer and early fall (January - April),
typical values of ice-free areas range from approximately 9,000 to 11,000 km2 in northern
Marguerite Bay and from ~ 4,500 to 7,500 km2 in southern Marguerite Bay (Fig. 6a). A
comparison of the climatology and 2001 and 2002 daily ice-free areas (km2) indicates that
2002 had above average sea ice in both the northern and southern sectors throughout the
spring, summer, and fall (Fig 6b and 6c). In addition, ice formed earlier in 2002 than in 2001.
In contrast, 2001 had sea ice values significantly below the 8-year mean, particularly from
January through July, as indicated by the unusually large ice-free areas observed both in the
northern and southern sectors. In 2001 these values reached approximately 12,000 and
11,500 km2 in the northern and southern regions respectively. On the other hand, the areas
free of ice only reached 6,000 - 9,000 km2 in the northern and 0 - 2,000 km2 in southern
sectors during the same months in 2002, suggesting an especially extensive sea ice cover.
10
During winter (starting in mid-July or Julian Day ~ 200), sea ice conditions were similar for
both years, although, as mentioned above, sea ice occupied both the northern and southern
sectors considerably earlier in 2002.
Discussion
Chlorophyll concentrations in waters west of the Antarctic Peninsula show great
temporal and spatial variability during spring and summer between 1997 and 2004. Although
chlorophyll can reach elevated values over the continental shelf along the Peninsula during
January - February, concentrations in Marguerite Bay and the Bellingshausen Sea are
consistently higher than in any other areas, making these southern sectors significantly more
important in terms of phytoplankton production throughout the spring and summer. In
addition, chlorophyll accumulations occur earlier in the spring and persist longer throughout
the summer in these southern areas than elsewhere along the Antarctic Peninsula, further
contributing to their higher overall production. There have been numerous reports of high
chlorophyll concentrations over the continental shelf along the Peninsula with some values
occasionally in excess of 30 mg m-3 (e.g., Holm-Hansen et al., 1989; Smith et al., 1998;
2001). This [what? These previous reports or your own result?] contributed to the belief that
the continental shelf is the most productive area in the western Antarctic Peninsula region.
However, very few studies to date included the Bellingshausen Sea (Smith et al., 1992;
Savidge et al., 1995), and data from Marguerite Bay are scarce. [deleted, because no data
doesn’t mean people ignored them] Our results reveal the importance of these areas in
primary productivity and in the foodweb.
West of the Antarctic Peninsula, E. superba typically reproduces during late spring
and summer, from November through March (Siegel, 1986). During the spawning season,
11
developmental stages may be spatially separated: adult females migrate near the vicinity of
the shelf-break in spring where they spawn in oceanic waters, while juveniles are found in
coastal regions (Siegel, 1986). Several processes have to take place in order for krill to
achieve a successful recruitment (Daly 2004). First, reproduction during spring and summer
has to occur, for which female krill need above-average food concentrations (Ross and
Quetin, 1986; Nicol et al, 1995). Eggs hatch at depth and the newly hatched larvae (naupliar
stages) swim to the surface before metamorphizing into the first feeding stage, calyptopis I
(CI). It is then crucial for these larvae to encounter an adequate food supply in the euphotic
zone within about two weeks; otherwise they will not survive (Ross and Quetin, 1986). In
addition, offshore larvae that are advected onto the shelf will find an enhanced food supply
and more favorable environmental conditions. Larvae that are not advected onto the shelf are
transported eastward away from the Antarctic Peninsula and into the Scotia Sea by the
Antarctic Circumpolar Current. Of those larvae advected onto the shelf, retention
mechanisms are important to keep larvae in that favorable environment. Lastly, larval krill
must survive the long unproductive winter in order to recruit to the juvenile stage in spring.
Field evidence indicates that during 2000/2001, krill reproduction started relatively
early and continued for an extended period; thus environmental conditions were conducive to
support successful reproduction. A wide range of larval stages were observed in off-shelf
waters during fall 2001, including older larval stages (furcilia VI), a dominant furcilia I (FI)
mode, and significant numbers of calyptopis III (C3) (Daly 2004). Following Ikeda (1984),
FI’s are estimated to be about 63 days old and, therefore, likely originated from a late
February – early March reproductive event. Assuming that the general eastward current
velocity of the Antarctic Circumpolar Current is approximately 10 cm sec-1 (refs), E.
superba’s spawning would have occurred in oceanic waters of the Bellingshausen Sea,
12
presumably in the area bounded by 85 – 80W, and 65 – 70S. Mesoscale meanders and
eddies may act to reduce this transport rate. Nevertheless, on average, the earliest and highest
chlorophyll concentrations of the spring - summer season are consistently seen in this region,
suggesting that offshore waters of the Bellingshausen Sea and Marguerite Bay can provide an
adequate and sustained food supply for both reproducing females and young larvae.
Additional evidence that there was successful krill reproduction in 2001 is that fall
(April - June) larvae [“fall varvae”? reads awkward] were very abundant on and off-shelf,
with concentrations reaching up to ~ 343 ind m-3, the highest numbers ever recorded for the
area (Daly 2004). There was a wide range of larval stages offshelf, whereas only older larvae
were observed near the coast, suggesting that most larvae were hatched in oceanic waters and
later transported into coastal areas. During fall 2002, relatively high concentrations of young
larvae were again detected in oceanic waters, although abundances were significantly lower
(41 individuals m-3). Offshelf larvae were primarily calyptopis, onshelf they were calyptopis
and FI and II stages. In 2002, juveniles also were relatively abundant, indicating a successful
recruitment from the 2001 larval population. Ekman transport and Upper Circumpolar Deep
Water intrusions may advect larvae onto the shelf (Dinniman and Klinck, 2004; Klink et al.,
2004).
A cyclonic circulation and relatively long residence times (up to seven months) in
Marguerite Bay (Hofmann et al., 1996; Beardsley et al., 2004) act to retain larval krill in the
region. [the description of this krill ecology is too long, even after I changed the title]
Variability in the recruitment of larval krill populations west of the Antarctic
Peninsula appears to be related to differences in food supply during the preceding spring summer. During SO GLOBEC, 2000/2001 was more productive than 2001/2002, with strong
widespread positive anomalies during summer in the Bellingshausen Sea, Marguerite Bay and
other coastal areas. Chlorophyll concentrations higher than typical values by 2 - 5 mg m-3
13
were observed during 2000/2001 in both oceanic and coastal waters [you sure it is 2-5? This
is very high for oceanic water, not to mention that it is an anomaly]. These elevated
chlorophyll levels likely provided krill with the food required for a successful reproduction
and recruitment, which resulted in the elevated larval abundances observed during fall 2001
and the high juvenile abundances recorded in coastal waters during fall 2002. In contrast,
even though 2001/2002 showed above average chlorophyll concentrations early in the season,
conditions shifted toward a mean state by February – March. The lack of older stage larvae
and relatively lower larval densities in 2001/2002, suggests that there was either delayed
reproduction of adult females in summer and/or a lower larval survival. Thus, the presence of
above-average chlorophyll concentrations in offshore waters of the Bellingshausen Sea and
Marguerite Bay during summer seems critical for the successful reproduction of the Antarctic
krill population that will later inhabit coastal waters along the western Antarctic Peninsula.
Although sea ice extent and length of the sea ice season in the Bellingshausen Sea and
along the Antarctic Peninsula have shown a clear tendency to decrease over the past 25 years
(Parkinson, 2002; Ducklow et al., in press), high interannual variability can still be observed
in this region. The summer - fall of 2001 showed an unusually low sea ice extent in
Marguerite Bay, while 2002 was characterized by particularly extensive and persistent sea ice
cover, a pattern that has also been recorded in other areas west of the Antarctic Peninsula.
Ducklow et al. (in press) analyzed 14 years of sea ice extent data near Palmer Station in the
vicinity of Anvers Island (1991 - 2004), and found that 2001 had the lowest (69,932 km2)
winter sea ice extent of all years analyzed, while 2002 had the highest (109,936 km2) (mean =
91,112 km2). The presence of sea ice during late spring and/or summer results in reduced
light penetration into the water column and prevents phytoplankton blooms from developing,
which in turn leads to lower food concentrations available for herbivores. The early retreat of
14
sea ice in Marguerite Bay in spring of 2001 and subsequent presence of large phytoplankton
blooms in ice-free waters during summer probably contributed to the elevated krill
concentrations observed in the area during the following fall. In contrast, the persistent
presence of sea ice in Marguerite Bay during summer - fall 2002, resulted in overall lower
chlorophyll concentrations in coastal surface waters, which probably contributed to lower
krill abundances.
Sea ice also has been repeatedly linked to larval krill overwintering survival and
recruitment (Daly and Macaulay, 1988; Marschall, 1988; Daly, 1990; Kawaguchi and Satake,
1994). Elevated numbers of first year juvenile krill have been observed following years with
heavy ice cover, while juvenile abundances are generally lower following years with low sea
ice extent, (Siegel and Loeb, 1995). An extensive sea ice cover may provide refuge from
predators and sea ice biota as an alternate food source for larvae (Daly, 1990). [But not
during SO GLOBEC. I’LL ADD MORE ON THIS]
In the Southern Ocean, several factors have been proposed as influencing
phytoplankton processes with varying magnitude, including micro- and macronutrient
availability (Prézelin et al., 2000; Boyd, 2002; Holm-Hansen et al., 2004), grazing pressure
(Walsh et al., 2001; Garibotti et al., 2003; Korb et al., 2004; more refs grazing) and stability
of the water column (Holm-Hansen et al., 1989; Moline and Prézelin, 1996; Garibotti et al.,
2003). In addition, sea ice often is cited as tightly coupled to spring phytoplankton blooms in
Antarctic waters (Sullivan et al., 1988; Arrigo and McClain, 1994; Garibotti et al., 2005).
Previous studies relate the retreat of sea ice to the development of phytoplankton blooms a
few weeks later at the location of the receding ice edge. They argue that as ice melts the
addition of a fresher surface layer increases water column stability, which results in a
shallower mixed layer in which cells are retained and light availability increased. These
15
processes, coupled with the availability of nutrients in surface waters, affect phytoplankton
production. Our results, however, suggest that the formation of spring phytoplankton blooms
west of the Antarctic Peninsula is not coupled to the retreat of the ice edge, but rather that
other processes taking place at the shelf-break influence phytoplankton in the area. Even
though the location of the receding ice edge and chlorophyll accumulations coincide spatially
in the southern sector of the study area (with a 2 - 4 week lag), most sectors north of Anvers
Island are never influenced by sea ice and still support significant phytoplankton blooms
during spring. In these northern sectors, blooms first appear at the location of the shelf break
and gradually progress to more coastal areas, suggesting that shelf-break processes could be
fueling phytoplankton growth. Previous results indicate that even in the southernmost sectors
where sea ice could have a major influence, spring blooms are not related to the ice edge.
Savidge et al. (1995) analyzed chlorophyll distributions in relation to the ice edge in the
Bellingshausen Sea between 65.5 - 69°S and 83.5 - 88°W during spring 1992 and found no
relation between phytoplankton increases and the retreat of the sea ice. Antarctic surface
waters are rich in macronutrients and thus nitrate, phosphate or silica are usually not
considered limiting for phytoplankton growth, at least south of the Polar Front (e.g., Daly et
al., 2001). However, deficiencies in micronutrients, particularly iron, have been proposed as
a possible factor controlling bloom formation (De Baar et al., 1995; Holm-Hansen et al.,
2004; 2005). Iron concentrations are generally low in surface waters of the ACC (~ 0.05 to
0.5 nM) (Holm-Hansen et al., 2005; Löscher et al., 1997) and the addition of this trace
element has been shown to result in localized increased production (e.g., Martin et al., 1990;
Boyd et al., 2000). On the other hand, surface iron concentrations at some frontal regions of
the ACC such as the Polar Front and the Southern ACC Front can be elevated and it is
believed that this iron is supplied from deep waters rather than by atmospheric input (De Baar
16
et al., 1995). The ACC flows eastward at relatively high velocities, which can reach 25 cm
sec-1 at these frontal areas. The strong current interacts with the bathymetry when it
encounters the shelf-break generating meanders that can usually be detected at the surface,
and this interaction of the ACC with bottom features has the potential of bringing iron-rich
deep waters to the surface and supply the necessary nutrients to sustain the development of
phytoplankton blooms. The upwelling of iron-rich deep ACC waters has been described for
other regions of the Southern Ocean, including the Scotia Sea, the Polar Frontal region
downstream of South Georgia and the Ross Sea (De Baar et al., 1995; Measures and Vink,
2001; Holm-Hansen et al., 2005). We hypothesize that the upwelling of iron-rich deep waters
at the shelf-break, rather than the retreat of sea ice may be a major factor controlling the
formation of spring phytoplankton blooms in offshore waters of the Bellingshausen Sea and
along the Antarctic Peninsula. Nevertheless, it is likely that sea ice also plays an important
role in maintaining these blooms throughout the summer as they progress toward coastal areas
by supplying a fresher surface layer that increases water column stability and light
availability. [this is a long paragraph – break up]
Summary
The analysis of typical chlorophyll distributions west of the Antarctic Peninsula from 1997
through 2004 revealed that the southernmost sectors of the region, such as the eastern
Bellingshausen Sea and Marguerite Bay, are considerably more productive in terms of
phytoplankton growth than any other areas, including the continental shelf along the
Peninsula and the western Scotia Sea. These southern sectors support elevated chlorophyll
concentrations from October until March, providing the food levels required by Antarctic krill
for a successful reproduction and larval survival. In particular, the variability observed in
17
phytoplankton distributions during the spring - summer 2000/2001 and 2001/2002 can be
related to the strong differences observed in krill abundances during fall 2001 and 2002,
respectively. The 2000/2001 season was more productive than 2001/2002. High chlorophyll
concentrations in the vicinity of Marguerite Bay and the Bellingshausen Sea could be
observed from January through March, and coincided with very high krill abundances
recorded in oceanic and coastal waters the following fall. In contrast, 2001/2002 showed
reduced chlorophyll concentrations during summer and lower overall krill numbers during fall
throughout the study area. Differences in sea ice conditions in Marguerite Bay likely
contributed to the variable krill numbers observed in this area. Although ice conditions were
similar during both winters, sea ice persisted throughout the spring - summer 2001/2002,
preventing the formation of diatom blooms which are the main food source for krill in coastal
waters during the productive months. In contrast, sea ice melted early in 2000/2001, allowing
elevated chlorophyll concentrations to develop within the Bay by December - January. Both
the presence of above average chlorophyll concentrations in the Bellingshausen Sea and
outside of Marguerite Bay during krill’s reproductive season (November - March) and
favorable sea ice conditions in coastal areas during summer - fall likely facilitated the
increased krill abundances observed during 2001. The formation of phytoplankton blooms in
offshore waters during spring was not related to the retreat of the ice edge in our study area.
It is likely that other processes taking place at the shelf break, such as the upwelling of ironrich deep waters, are influencing the development of these blooms.
Acknowledgements
This study was supported by the US National Science Foundation (NSF) grants OPP-9910610
and OPP-196489 to K. Daly, and by US NASA grant NNS04AB59G to C. Hu. We are
18
grateful to K. Arrigo for providing the daily sea ice data for Marguerite Bay and to E.
Chapman for making the monthly ice edge data available. We also thank Brock Murch of
USF/IMaRS for his assistance in obtaining and processing the SeaWiFS satellite data. This is
GLOBEC contribution number xxx.
References
Arrigo, K.R., McClain, C.R., 1994. Spring phytoplankton production in the western Ross Sea.
Science 266, 261-263.
Arrigo, K.R., van Dijken, G.L., 2003. Phytoplankton dynamics within 37 Antarctic coastal
polynya systems. Journal of Geophysical Research 108, xxxx
Beardsley, R.C., Limeburner, R., Owens, W.B., 2004. Drifter measurements of surface
currents near Marguerite Bay on the western Antarctic Peninsula shelf during austral summer
and fall, 2001 and 2002. Deep-Sea Research II 51, 1947-1964.
Boyd, P.W. 2002. Environmental factors controlling phytoplankton processes in the Southern
Ocean. Journal of Phycology 38, 844-861.
Boyd, P.W., Watson, A.J., Law, C.S., Abraham, C.R., 31 others, 2000. A mesoscale
phytoplankton bloom in the polar Southern Ocean simulated by iron fertilization. Nature 407,
695-702.
Cavalieri, D., Parkinson, C., Gloerson, P., Zwally, H.J., 1996, updated 2005. Sea ice
concentrations from Nimbus-7 SMMR and DMSP SSM/I passive microwave data, June to
September 2001. Boulder, CO, USA: National Snow and Ice Data Center. Digital media.
Daly, K.L., 1990. Overwintering development, growth, and feeding of larval Euphausia
superba in the Antarctic marginal ice zone. Limnology and Oceanography 35, 1564-1576.
19
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 II 51, 2139-2168.
Daly, K.L., Macaulay, 1988.
Daly, K.L., Smith, W.O., Johnson, G.C., DiTullio, G.R., Jones, D.R., Mordy, C.W., Feely,
R.A., Hansell, D.A., Zhang, J., 2001. Hydrography, nutrients, and carbon pools in the Pacific
sector of the Southern Ocean: Implications for carbon flux. Journal of Geophysical Research
106, 7107-7124.
De Baar, H.J., De Jong, J.T.M., Bakker, D.C.E., Löscher, B.M., Veth, C., Bathmann, U.,
Smetacek, V., 1995. Importance of iron for plankton blooms and carbon dioxide drawdown in
the Southern Ocean. Nature 373, 412-415.
Deibel, D., K.L. Daly. Zooplankton Processes. In: Smith, W.O., Jr. and D. Barber (Eds),
Polynyas: Windows into Polar Oceans. Elsevier Oceanography Series (in press).
Dinniman, M.S., Klinck, J.M., 2004. A model study of circulation and cross-shelf exchange
on the west Antarctic Peninsula continental shelf. Deep-Sea Research II 51, 2003-2022.
Ducklow, H.W., Fraser, W., Karl, D.M., Quetin, L.B., Ross, R.M., Smith, R.C.,
Stammerjohn, S.E., Vernet, M., Daniels, R.M. Water-column processes in the West Antarctic
Peninsula and the Ross Sea: Interannual variations and foodweb structure. Deep-Sea Research
II, in press.
El-Sayed, S.Z., 1988. Seasonal and interannual variabilities in Antarctic phytoplankton with
reference to krill distribution. In: Sahrhage, D. (Ed.), Antarctic Ocean and resources
variability. Springer, Berlin, pp. 101-119.
El-Sayed, S.Z., 2005. History and evolution of primary productivity studies of the Southern
Ocean. Polar Biology 28, 423-438. RE-READ
20
Garibotti, I.A., Vernet, M., Ferrario, M.E., 2005. Annually recurrent phytoplanktonic
assemblages during summer in the seasonal ice zone west of the Antarctic Peninsula
(Southern Ocean). Deep-Sea Research I 52, 1823-1841.
Garibotti, I.A., Vernet, M., Ferrario, M.E., Smith, R.C., Ross, R.M., Quetin, L.B., 2003.
Phytoplankton spatial distribution patterns along the Western Antarctic Peninsula (Southern
Ocean). Marine Ecology Progress Series 261, 21-39.
Gloersen, P., Campbell, W.J., Cavalieri, D.J., Comiso, J.C., Parkinson, C.L., Zwally, H.J.,
1992. Arctic and Antarctic Sea ice, 1978-1987: Satellite Passive Microwave Observations
and Analysis. NASA Special Publication 511.
Haberman, K.L., Ross, R.M., 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.
Hofmann et al. 1996 Ant. Res. Ser. 70
Holm-Hansen, O., Kahru, M., Hewes, C.D., 2005. Deep chlorophyll a maxima (DCMs) in
pelagic Antarctic waters. II. Relation to bathymetric features and dissolved iron
concentrations. Marine Ecology Progress Series 297, 71-81.
Holm-Hansen, O., Mitchell, B.G., Hewes, C.D., Karl, D.M., 1989. Phytoplankton blooms in
the vicinity of Palmer Station, Antarctica. Polar Biology 10, 49-57.
Holm-Hansen, O., Naganobu, M., Kawaguchi, S., Kameda, T., Krasovski, I., Tchernyshkov,
P., Priddle, J., Korb, R., Brandon, M., Demer, D., Hewitt, R.P., Kahru, M., Hewes, C.D.,
2004. Factors influencing the distribution, biomass, and productivity of phytoplankton in the
Scotia Sea and adjoining waters. Deep-Sea Research II 51, 1333-1350.
Ichii, T., 2000. Krill harvesting. In: Everson, I. (Ed.), Krill, biology, ecology and fisheries.
Blackwell Science, Berlin, pp. 228-261.
21
Ikeda, T., 1984. Development of the larvae of the Antarctic krill (Euphausia superba Dana)
observed in the laboratory. Journal of Experimental Marina Biology and Ecology 75, 107-117
Kawaguchi, S., 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.
Klink, J.M., Hofmann, E.E., Beardsley, R.C., Salihoglu, B., Howard, S., 2004. Water mass
properties and circulation on the west Antarctic Peninsula continental shelf in austral fall and
winter 2001. Deep-Sea Research II 51, 1925-1946.
Korb, R.E., Whitehouse, M.J., 2004. Contrasting primary production regimes around South
Georgia, Southern Ocean: large blooms versus high nutrient, low chlorophyll waters. DeepSea Research II 51, 721-738.
Löscher, B.M., De Baar, H.J.W., De Jong, J.T.M., Veth, C., Dehairs, F., 1997. The
distribution of Fe in the Antarctic Circumpolar Current. Deep-Sea Research II 44, 143-187.
Marrari, M., Hu, C., Daly, K.L. Validation of SeaWiFS chlorophyll a concentrations in the
Southern Ocean: A revisit. Remote Sensing of Environment, in press.
Marschall, H., 1988. The overwintering strategy of Antarctic krill under the pack-ice of the
Weddell Sea. Polar Biology 9, 129-135.
Martin, J.H., Fitzwater, S.E., Gordon, R.M., 1990. Iron deficiency limits phytoplankton
growth in Antarctic waters. Global Biogeochemical Cycles 4, 5-12.
Measures, C.I., Vink, S., 2001. Dissolved Fe in the upper waters of the Pacific sector of the
Southern Ocean. Deep-Sea Research II 48, 3913-3941.
Moline, M.A., Prézelin, B.B., 1996. Long-term monitoring and analyses of physical factors
regulating variability in coastal Antarctic phytoplankton biomass, in situ productivity and
taxonomic composition over subseasonal, seasonal and interannual time scales. Marine
Ecology Progress Series 145, 143-160.
22
Nicol, S., De la Mare, W.K., Stolp, M., 1995. The energetic cost of egg production in
Antarctic krill (Euphusia superba Dana). Antarctic Science 7(1), 25-30.
O’Reilly J.E., Maritorena, S., O’Brien, M.C., Siegel, D.A., Toole, D., Menzies, D., Smith,
R.C., Mueller, J.L., Mitchell, B.G., Kahru, M., Chavez, F.P., Strutton, P., Cota, C.F., Hooker,
F.B., McClain, C.R., Carder, K.L., Müller-Karger, F.E., Harding, L., Magnuson, A., Phinney,
D., Moore, G.F., Aiken, J., Arrigo, K.R., Letelier, R., Culver, M., 2000. SeaWiFS postlaunch
calibration and validation analyses: Part 3. SeaWiFS postlaunch technical report series, 11. In:
Hooker, S.B., and Firestone, E.R. (Eds.), NASA Technical Memorandum 2000-206892, 49pp.
Parkinson, C.L., 2002. Trends in the length of the Southern Ocean sea-ice season, 1979-1999.
Annals of Glaciology 34, 435-440.
Prézelin, B.B., Hofmann, E.E., Mengelt, C., 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.
Quetin, L.B, Ross, R.M., 2003. Episodic recruitment in Antarctic krill, Euphausia superba, in
the Palmer LTER study region. Marine Ecology Progress Series 259, 185-200.
Ross, R.M., Quetin, L.B., 1983. Spawning frequency and fecundity of the Antarctic krill
Euphausia superba. Marine Biology 77, 201-205.
Ross, R.M., Quetin, L.B., 1986. How productive are Antarctic krill? BioScience 36: 264-269.
Ross, R.M., 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.
Savidge, G., Harbour, D., Gilpin, L.C., Boyd, P.W., 1995. Phytoplankton distributions and
production in the Bellingshausen Sea, Austral spring 1992. Deep-Sea Research II 42, 12011224.
23
Siegel, V., 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., Vernet, M., 1998. Seasonal and interannual variability of
phytoplankton biomass west of the Antarctic Peninsula. Journal of Marine Systems 17, 229243.
Smith, R.C., Baker. K.S., Dierssen, H.M., Stammerjohn, S.E., 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.
Smith, R.C., Prézelin, B.B., Baker, K.S., and others, 1992. Ozone depletion: Ultraviolet
radiation and phytoplankton biology in Antarctic waters. Science 255, 952-957.
Sullivan, C.W., Arrigo, K.R., McClain, C.R., Comiso, J.C., Firestone, J., 1993. Distributions
of phytoplankton blooms in the Southern Ocean. Science 262, 1832-1837.
Walsh, J.J., Dieterle, D.A., Lenes, J., 2001. A numerical analysis of carbon dynamics of the
Southern Ocean phytoplankton community: the roles of light and grazing in effecting both
sequestration of atmospheric CO2 and food availability to larval krill. Deep-Sea Research I
48, 1-48.
24
Tables
Table 1. Information on the 14 regions defined in the study area. ACC: Antarctic Circumpolar
Current.
Region Location
System type
Depth
1
ACC - Bellingshausen Sea
Oceanic
> 2000 m
2
ACC – Drake Passage
Oceanic
> 2000 m
3
ACC - Scotia Sea
Oceanic
> 2000 m
4
Bellingshausen Sea - Offshelf
Oceanic
> 2000 m
5
Antarctic Peninsula - Offshelf
Oceanic
> 2000 m
6
Scotia Sea - Offshelf
Oceanic
> 2000 m
7
Bellingshausen Sea
Shelf break
500-2000 m
8
Antarctic Peninsula
Shelf break
500-2000 m
9
Bellingshausen Sea - Southern Ant. Pen. Continental shelf
< 500 m
10
Mid-Antarctic Peninsula
Continental shelf
< 500 m
11
Northern Antarctic Peninsula
Continental shelf
< 500 m
12
Scotia Sea – Weddell Sea
Cont. shelf & shelf break ~ < 2000 m
13
Northern Marguerite Bay
Continental shelf
< 500 m
14
Southern Marguerite Bay
Continental shelf
< 200 m
25
Figures
Figure 1. Location of the study area and geographic references. The dashed line represents
the 1000 m isobath. [this figure is identical to the RSE figure – you may revise just a little bit
to avoid the copyright issue]
26
27
28
Figure 2. Bi-weekly 7-year climatology (1997-2004) of SeaWiFS chlorophyll concentrations
(mg m-3) between September. White areas indicate no available data.
(a)
29
Figure 3. (a) Location of the 14 regions along the western Antarctic Peninsula, overlaid over
the 7-year climatology (1998 - 2004) of SeaWiFS chlorophyll concentrations during January
1 - 14. (b) The median chlorophyll concentration at each region for each 2-week period
during the 2000/2001 (black dots) and 2001/2002 (grey squares) spring – summer seasons
(Southern Hemisphere). Concentrations lower than 0.01 mg m-3 and greater than 20 mg m-3
were excluded from the calculations. The 7-year climatology (median) was also shown
(black solid line) as a reference.
30
31
Figure 4. Biweekly anomalies of SeaWiFS chlorophyll concentrations (mg chl above or
below the 7-year climatology) for late November, December, January and February of 2001
(first column of images) and 2002 (second column). Black regions indicate no data is
available due to the presence of clouds and/or sea ice.
(a)
32
(b)
Figure 5. Biweekly SeaWiFS chlorophyll concentrations (Chl, mg m-3) in October (Oct),
November (Nov), and December (Dec) of (a) 2000 and (b) 2001. The mean location of the
ice edge is also shown: red line for the previous month and yellow line for the current month.
White line: 1000 m isobath.
33
Figure 6. (a) Location of the regions analyzed. Ice-free area (km2) in (b) Northern and (c)
Southern Marguerite Bay during 2001 (red solid line) and 2002 (blue broken/dotted line).
The daily 8-year climatology (1997-2004) also is shown (black broken line). [why not rotate
(a) to make it the same orietation as other figures?]
34