Near-field zooplankton, ice-face biota and proximal hydrography of
free-drifting Antarctic icebergs
R.E. Sherlock a,n, K.R. Reisenbichler a, S.L. Bush a,b, K.J. Osborn c, B.H. Robison a
a Monterey Bay Aquarium Research Institute, 7700 Sandholdt Rd, Moss Landing, CA 95039, USA
b Department of Biological Sciences, College of Environment and Life Sciences, University of Rhode Island, Kingston, RI 02881, USA
c Scripps Institution of Oceanography, 8615 Discovery Way, La Jolla, CA 92037, USA
a r t i c l e i n f o
Article history:
Received 12 November 2010
Accepted 12 November 2010
Keywords:
Southern Ocean
Weddell Sea
Iceberg
Ice
Plankton
ROV
a b s t r a c t
A small ROV was used to collect plankton, make video surveys and take hydrographic measurements in
close proximity to six free-drifting, Antarctic icebergs. The icebergs studied ranged in size from o0.5 to
432 km in length. Large icebergs have a greater scale of influence than do smaller ones and iceberg-
mediated differences in the hydrographic characteristics of their surrounding water depend on the scale
sampled. Irrespective of size, temperature generally decreased in close proximity to an iceberg while
salinity increased. Chlorophyll a was often lower in the surface waters near the iceberg, relative to the
surfacewaters further away. Tabular icebergs typically had 3 distinct underwater features: shelf, side and
bottom. Ablation pockets were a common feature of subsurface ice. The ice itself is a dynamic and
seemingly harsh environment with relatively few macrofauna living on it. Those that do inhabit the ice
face are either highly specialized or highlymobile. Species composition of zooplankton within 40 m of an
iceberg did not change relative to distance. However, biomass was generally greater within 5 m of an
iceberg than it was 15 to 40 m distant.
& 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Tabular icebergs formas glaciers flow from land into the sea and
calve or break off. Historically about 5000 icebergs a year calve into
Antarctic waters from the Amery, Filchner and Ross ice shelves
(Wadhams, 1986). Fewer in number thanArctic icebergs but larger,
tabular Antarctic icebergs typically have a depth of ca 250 m, the
mean thickness of the ice shelf at thepoint of calving. Large icebergs
are prone to ablation by fracture and calving. Smaller icebergs can
roll and the smallest icebergs are often rife with complex physical
features like caves or ice spires, wrought by erosion and wastage.
These dynamic aspectsmake getting close to icebergs both difficult
and dangerous. Consequently, there have been few field observa-
tions carried out in close proximity to large, free-drifting icebergs
(but see Pisarevskaya and Popov, 1990, as well as Ohshima et al.,
1994, for a study done on fast ice).
The occurrence of large icebergs (418.5 km long) originating
from ice shelves in the Ross, Bellingshausen and Weddell Seas has
increased during the last decade (Scambos et al., 2000;
Bindschadler and Rignot, 2001; Long et al., 2002; Bindschadler,
2006; Jacobs, 2006). Recent evidence suggests that one large
iceberg (Lazzara et al., 1999) grounded in the Ross Sea drastically
restricted the flow of pack ice, thus reducing the area of openwater
and resulting in a 440% decrease in primary production of the
region (Arrigo et al., 2002). A reduction of this magnitude impacts
all trophic levels including top predators. Large icebergs may have
large-scale effects but they are rare compared to the much greater
abundance of smaller icebergs. Smaller icebergs have amuch larger
surface area to volume ratio, provide relatively more substrate for
organisms andwhen considered in total, could impact a vast region
of the Southern Ocean. There are an estimated 200,000 icebergs in
the SouthernOceanwith linear dimensions of tens ofmeters to tens
of kilometers (Orheim, 1988).
Icebergsmay have important structuring effects on local pelagic
communities (Smith et al., 2007). Increased concentrations of Fe
and chlorophyll a accompanied by increased abundance of nano-
plankton were measured in the wake of a drifting iceberg in the
Southern Indian Ocean (de Baar et al., 1995; Raiswell et al., 2008;
Schwarz and Schodlok, 2009). At higher trophic levels, the diversity
of acoustically-reflective targets, believed to be zooplankton and
micronekton,were twice as highunder a free-drifting icebergwhen
compared to surrounding open water in the Weddell Sea
(Kaufmann et al., 1995). Fish have been observed in small caves
within the walls of a coastal iceberg off Greenland (Holmquist,
1958) and grounded icebergs in the Ross Sea (Line, 2000; Stone,
2003). In addition, fish have been observed attached to the smooth
surface of a grounded tabular iceberg in theDavis Sea (Gruzov et al.,
1967) and to the subsurface of a marginal ice shelf in the SE
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/dsr2
Deep-Sea Research II
0967-0645/$ - see front matter & 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dsr2.2010.11.025
n Corresponding author. Tel.: +1831 775 1763.
E-mail address: robs@mbari.org (R.E. Sherlock).
Please cite this article as: Sherlock, R.E., et al., Near-field zooplankton, ice-face biota and proximal hydrography of free-drifting
Antarctic icebergs. Deep-Sea Research II (2011), doi:10.1016/j.dsr2.2010.11.025
Deep-Sea Research II ] (]]]]) ]]]–]]]
Weddell Sea (Gutt, 2002). Top predators such as Chinstrap pen-
guins and Antarctic fur seals are known to associate with icebergs
in the NW Weddell Sea (Joiris, 1991; Rubic et al., 1991). Commu-
nities of seabirds, including Southern Fulmars, Wilson’s Storm
Petrels, Antarctic Petrels and Mottled Petrels have been associated
with icebergs in the Ross Sea (Ainley et al., 1984) as well as in the
NW Weddell Sea (Smith et al., 2007; Ruhl et al., this issue).
Recruitment of these predators may be an active response to
shelter or enhanced productivity, or it may be the result of passive
entrapment and entrainment of prey.
Little is known about the organisms associatedwith icebergs. In
this study we used a small, remotely operated vehicle (ROV) to
survey, sample and characterize the macro-organisms living in
close proximity to and on the submerged faces of free-drifting,
tabular icebergs in theWeddell Sea in the late austral spring of 2005
(December) and late summer (March/April) of 2009. At the same
time, we recorded salinity, temperature and chlorophyll data to
determine how those parameters varied with distance from the
iceberg and how that variability affected the associated organisms.
2. Materials and methods
ROV dives for this study were made in December of 2005 and
March/April of 2009 from the ARSV L.M. Gould and RVIB N.B. Palmer,
respectively (Smith, this issue). The ROV was equipped differently
during each of those years. The 2005 expedition was primarily a
reconnaissance mission made on a limited budget and the ROV
(Deep Ocean Engineering Phantom HD2) initially had no hydro-
graphic or biological sampling capability, only a video camera and
incandescent lights. The ROV was subsequently outfitted with a
modified 0.25-m2 plankton net during the cruise and with it we
sampled tufts of attached diatoms, as well as a rock lying on the
shallow shelf of a tabular iceberg. Although the ROV was rated to a
depth of 600 m, it was limited to depths that could be reachedwith
its 300-m tether (Hobson et al., this issue). Over the course of
7 dives made in the NWWeddell Sea in the late Austral spring, we
obtainedvideo footage of fish anddiatomson the ice, andkrill, salps
and other gelatinous zooplankton nearby (Smith et al., 2007;
Table 1).
Based upon what we learned during our 2005 cruise, the ROV
was upgraded to provide more power and was outfitted to collect
High Definition (HD) video imagery, biological samples and
hydrographic data in 2009. A Sony HJDR-HC5 camcorder trans-
mitted live video to the surface where it was recorded on mini-DV
tape. HD video images were captured and stored to the camera’s
flash memory. The ROV was equipped with a 29-chamber suction
sampler carousel and a fluorinert-filled, pressure-compensated
Rule 24 VDC suction pump. The suction sampler carousel served as
a repository for individual samples drawn into the carousel through
either a 0.14-m2 plankton net (202 mm) mounted on the forward
portion of the ROV toolsled, or a suction nozzlemounted in front of
the ROV on the bumper bar. The plankton net was used for
collecting zooplankton during transecting operations. The suction
nozzle was used to collect attached diatoms and macroplankton
from the ice face and water column. The plankton net and suction
nozzle could not both be used on the same dive. An individual
suction sampler chamber consisted of a 5.1 cm diameter by 7.6 cm
long, open-ended acrylic tube fitted with 202-mm mesh nylon
sock to retain the sample as water was drawn through by the
suction pump.
Conductivity, temperature and depth datawere collectedwith a
Falmouth Scientific Micro-CTD MCTD III, while chlorophyll a and
turbidity were measured with a WETLabs ECO FLNTU fluorometer
and turbidity sensor attached to the CTD. Because these instru-
ments were mounted near the bottom of the ROV, measurements
made while descending were more likely to represent the undis-
turbed water column; however, in some cases only ascending data
were obtained (Table 1). For calibration, the ROV CTD/fluorometer
was removed and mounted alongside the CTD/fluorometer used
aboard the RVIB Nathaniel B. Palmer’s CTD rosette. Measurements of
chlorophyll amadewith the ROVwere thus cross-calibrated over a
cast to 600 m and corrected via bottle samples analyzed in the
shipboard lab. Speed through water and distance traveled was
determined with a General Oceanics Model 2031 H electronic
flowmeter with a low-speed impeller. Distance and orientation
to the iceberg face were determined and recorded for later analysis
with a Kongsberg-Simrad Mesotech 1000 scanning sonar. See
Hobson et al. (this issue) formore detail on the ROV’s specifications
and capabilities.
In 2009 the better-equipped ROV made 7 complete biology
dives on 3 icebergs that ranged in size from o0.5 to 32 km in
greatest dimension. All the icebergswe examined in 2009 occurred
over the Powell Basin of the Weddell Sea (Fig. 1). During a typical
dive the ship usually moved 1-2 km (Table 1) due to linear or
rotational movement of the iceberg.
Most dives were made on the lee side of an iceberg. Choosing a
dive site depended on several factors, among them were iceberg
surface topography and the apparent likelihoodof calving aswell as
the current relative to the iceberg and other icebergs in the vicinity.
The ROV was deployed with the ship’s starboard A-frame. Once in
the water, the vehicle transited to the iceberg on the surface and
then dove upon reaching the ice face. Occasionally, conditions like
brash ice in the water required that we make the last part of the
approach to the ice face submerged. A floating tether configuration
was used for these missions due to the long stand-off distance
required between the ship and iceberg. This allowed the tether to
be continuously visible to the ship’s bridge crew and tether
handlers as it streamed out 100 to 300 m to the iceberg. However,
this left it vulnerable to being snagged by passing growlers or
brash ice.
The position of the ROV relative to the movement of the iceberg
was calculated from the ship’s track, heading, and the fact that the
ROV was always deployed off the starboard side. Shipboard GPS
was used to calculate the distance and direction traveled (Table 1).
ROV speed varied greatly based on prevailing conditions. Current
effects on the tether were by far the largest factor in determining
speed. Whether the ROV dove up or downstream of an iceberg
varied and, in some cases, changed during a dive.
Two types of ROV missions were performed for this study: ice
face surveys and vertical transects. The primary purpose of the
surveys was to explore the ice face from close to the surface
waterline down to the apparent bottom of the iceberg. Detailed
observations of the ice structure and associatedbiotaweremade on
these dives, and targeted specimens were collected for identifica-
tion in the lab using the 0.25-m2 plankton net as a scraper/collector
in 2005, or with the suction sampler in 2009. In addition to the
dedicated survey dives, a single descending ice-face survey was
also conducted on each of the transecting dives prior to initiating
those activities. The relative abundance of diatom tufts was
estimated during ice face surveys and scored on a scale of 0 (no
diatoms visible) to 5 (4 6 tufts/ablation cup). In total, ice-face
surveys were completed on 5 dives in 2005 and 7 dives in 2009.
Vertical transecting missions were run to quantify and compare
organisms living on and very close to the iceberg with those living
up to 45 m away. No dives were repeated in the same place on any
iceberg.
Whenever possible, vertical transects were made at 3 distances
from the iceberg: 0-5, 10-15 and 30-45 m(Table 1). The operational
goal for these transectswas tomove diagonally up or down the face
of the iceberg and/orwater column at about a 451 angle tomaintain
movement forward into undisturbed water, as well as to maintain
R.E. Sherlock et al. / Deep-Sea Research II ] (]]]]) ]]]–]]]2
Please cite this article as: Sherlock, R.E., et al., Near-field zooplankton, ice-face biota and proximal hydrography of free-drifting
Antarctic icebergs. Deep-Sea Research II (2011), doi:10.1016/j.dsr2.2010.11.025
Table 1
ROV dives made in the Weddell Sea during the late fall of 2009 and late summer of 2005. ea¼Eusirus antarctica; f¼fish; ts¼diatom tufts (Thalassioneis signyensis).
Dive # Date (mm/dd/yyyy)
duration (hh:mm)
Iceberg Depth iceberg
bottom (m)
"Dist. fr.
ice (m)
Location Mission Epifauna Dive
direction
Ship’s heading
(deg.)
"Dist. Travelled
(km)
Iceberg edge
sampled
4 03/17/2009 C-18a 190 1 m NW Powell Basin survey/collect epifauna ts,f N 270-310 1.3 side/trailing
02:18
6 03/20/2009 C-18a 148 1 m NW Powell Basin survey/transects f NE 279-290 1.4 trailing
02:59 15 m
40 m
8 03/23/2009 C-18a 195 1 m NW Powell Basin survey/collect epifauna f N 334-357 0.7 leading
03:06 (x 2 de-scents)
12 04/02/2009 C-18a 128 1 m N Powell Basin survey/transects ts,f S 340-5 2.6 side/leading
03:10 15 m
40 m
13 04/02/2009 C-18a 176 1 m N Powell Basin survey ts,f S 340-10 1.9 side/" leading
03:01
17 04/06/2009 IA-4 167 1 m S Powell Basin survey/transects ts,f NW 290-313 1.1 leading
02:00 30 m
18 04/10/2009 IA-5 190 1 m W Powell Basin survey/transects ea,f NE 30-70 1.6 leading
03:24 15 m
40 m
2 12/11/2005 W-86 208 survey ts,f
03:13
3 12/15/2005 W-86 210 survey ts,f
02:46
4 12/18/2005 A-52 survey
03:06
5 12/19/2005
02:23 A-52 survey ts,f
6 12/20/2005 A-52 Survey
01:31
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flow through the plankton net as we changed depth. While
transecting, plankton samples from discrete depth intervals were
collected in individual suction sampler chambers. The volume of
water filtered on each segment of a transect was estimated by
calculating vector distance traveled (using horizontal distance
travelled and vertical change in depth) and multiplying by the
open area of the plankton net mouth. Three complete transect
series of 3 transects each ("1 m, 15 m, and 40 m) were made in
total. Two of those occurred at iceberg C-18a and the third at
iceberg IA-5, in the western Powell Basin. A fourth transect series
made at IA-4 omitted the middle distance because of time
constraints (Table 1).
The method for ice face transects was standardized in order to
achieve consistent sampling. The vehicle was oriented at about a 301
angle fromparallelwith theplaneof the ice faceand then it proceeded
forward, and either upward or downward, as described above for the
water-column transects. This angle gave us the best lighting and
contrast to recognize animals close to the ice or to the ROV. Using the
angle of approach to the ice face and the viewing angle of theHD lens,
an average viewed surface area of 1.6 m2 was calculated. Organisms
seen and sampling events were annotated with the MBARI video
annotation and reference system – VARS (Bush and Robison, 2007).
Video was further annotated in greater detail in the lab, in order to
describe iceberg structure and associated organisms. Video quanti-
fication of zooplankton was precluded by changing lighting condi-
tions close to the ice and by image dropouts when the ROV bumped
the ice and the camera stopped writing to its hard drive.
Aboard the RVIB Nathaniel B. Palmer, each plankton sample was
photographed then divided with a plankton splitter. One half of each
sample was frozen (#80 1C) for subsequent analysis (biomass
and/or CHN) and the other half was roughly sorted into taxonomic
groups. Once ashore after the cruise, taxa weremore carefully sorted.
Copepods, while very abundant, were not identified to species and
thus were excluded from abundance analyses. However, copepods
were retained as a component of calculated biomass. Frozen samples
were dried at 30 1C for 48 hours on pre-weighed filters and overall
biomasswas estimated fromeach half sample by doubling itsweight.
Multi-dimensional scaling (MDS) was used to examine links
between community composition, the iceberg sampled, and
sampling distance from the ice. Because of sampling constraints
imposed by operating near icebergs, we transformed abundance
data to presence/absence to account for speed-induced sampling
bias. Presence/absence weights equally those taxa less frequently
sampled. The presence/absence of invertebrate fecal pellets, fish
fecal pellets, andfish scaleswere included in the list of taxa because
the ROV moved too slowly to effectively sample animals like fish
and adult euphausiids, but the presence of their fecal pellets was
considered to be a proxy. The null hypotheses of no differences in
community composition with: (1) distance from the iceberg
sampled, and (2) overall distance sampled (spanning the Powell
Basin); were tested using analysis of similarity (ANOSIM) and the
RELATE function in the multivariate statistical software, Primer
(Clarke and Gorley, 2006). A data matrix composed of the taxa
sampled by the ROV was constructed. In order to test the first
hypothesis, a model distance matrix was made that grouped
samples collected during each dive by their respective distance
from the ice. The model matrix for overall distance sampled
was constructed based on the metric latitude and longitude for
each dive.
3. Results
In 2005, five ROV dives were completed on two separate
icebergs that ranged in size from 2 to 21 km in greatest dimension.
These dives were exploratory in nature and consisted of ice face
surveys and the collection of attached diatoms. These surveys
yielded information on the subsurface ice face structure, and the
vertical distribution of cryopelagic fish and attached diatoms, as
well as, krill, salps and other near-ice zooplankton (Smith et al.,
2007). Samples of attached diatoms were collected for identifica-
tion (Robison et al., this issue). No quantitative data were collected
on these dives.
3.1. Iceberg structure
The icebergs we studied varied in size, but typically they shared
some above-water features like vertical walls and an undercut
Fig. 1. Location of ROV dive sites in the Weddell Sea. Icebergs are distinguished by different colors. Individual dive tracks from 2009 are shown at similar scales to give an
indication of iceberg movement during the 2–4 hour long dives. IA¼Iceberg Alley. All icebergs were tabular except for IA-1.
R.E. Sherlock et al. / Deep-Sea Research II ] (]]]]) ]]]–]]]4
Please cite this article as: Sherlock, R.E., et al., Near-field zooplankton, ice-face biota and proximal hydrography of free-drifting
Antarctic icebergs. Deep-Sea Research II (2011), doi:10.1016/j.dsr2.2010.11.025
carved by the sea surface. Underwater, tabular icebergs typically
had 3 distinct features: shelf, side and bottom. The shelf began just
below the sea surface and sloped gradually deeper. Side-scan sonar
images of one tabular iceberg measured a shelf width of ca 55 m
(S. Rock, pers com). However, shelf width varied greatly on and
between icebergs. The shelf break, which occurred between 20-40 m,
was blunt and rounded. There the ice face sloped vertically 901 or
more. Below the break, the face of the iceberg usually sloped
slightly inward, generally 5 to 101, and varied with depth. This
nearly vertical wall extended to the bottom of the iceberg, which
was also rounded and led to a seemingly flat bottom (Fig. 2A) that
occurred between 120–200 m for the icebergs we examined in
2009 (Table 1). Though the size of their shelves varied considerably,
these featureswere consistent for icebergs A-52,W-86, C-18a and a
smaller tabular iceberg, IA-4. The tabular iceberg IA-5 – similar in
size to IA-4 -was ca1.5 km long and 190 mdeep and did not exhibit
a shelfwherewedove. The lack of a shelf on this iceberg could be an
indication that IA-5 had recently calved off another iceberg.
We did not intentionally dive under any iceberg to observe
bottom topography. However, on dive #4 we were pulled under
iceberg C-18a by a current strong enough that the vehicle could not
makewayunder its ownpower andwehad tohaul in tether topull it
clear. Fortunately, the bottom was flat and without ice projections.
Forward-looking sonar found no projections off the bottom of any
iceberg toat least50 munder the iceberg. Theapparentlyflatbottom
on iceberg IA-5 (dive 18) extended to a radius of at least 300 m.
Ablation pockets formed the most consistent visual, subsurface
feature on icebergs. Themajority of the subsurface icewe observed
bore cupped, golf ball-like depressions (ablation pockets) that
varied in diameter from ca 10 to 20 cm and were usually a few
centimeters deep. The size and spacing of the pocketswas generally
consistent within a given region. With the outer edges of the
pockets overlapping those of their nearest neighbors, a hexagonal
patternwas producedby the ice ridges that formed the rims around
the pockets (Fig. 2A–D). At different depths and at different stages
of ice dissolution, the clarity of the hexagonal shape varied, but
upon close inspection, the general shape could be discerned.
Ablation pockets were present irrespective of ice orientation and
were observed on vertical as well as horizontal surfaces; however,
they appeared to be much less regular the few times we ventured
under an iceberg.
Vertical striations were sometimes seen on ice walls of tabular
bergs. In some cases they were tens of centimeters deep and
appeared to run the extent of the wall, from bottom to shelf
(Fig. 2C). More rarely, we encountered deep crevasses or caves that
broke the vertical surface of large icebergs. Twice in 2005 we
encountered small holes where gas bubbles seeped from the ice.
Huppert and Josberger (1980) postulated that bubbles emanating
from the ice could have caused the vertical striations they saw in
laboratory experiments; however, no striations were associated
with these bubble streams. No bubbles were associated with the
larger vertical striations we observed in 2009. In one instance, the
cleft continued over the shelf shoulder and ran along the horizontal
ice shelf, indicating that bubbles could not be the sole cause of such
features.
In the late austral spring of 2005 we also observed streams of
fresh water cascading off the large tabular iceberg, A-52. Nothing
similar was seen in the late summer (March/April) of 2009.
All large, tabular icebergs we studied calved while we observed
them irrespective of the season, occasionally at inconvenient
times.
3.2. Ice-face diatoms
Tufts of the diatom, Thalassioneis signyensis, (Robison et al., this
issue) were found growing on 4 tabular icebergs: A-52 and W-86
(2005), C-18a and IA-4 (2009). Distribution was patchy, although
whenpresent, they always occurred on the rims of ablation pockets
(Fig. 2D). Ice-face diatom distribution and abundance was notably
different between our December (late spring) 2005 and March/
April (late summer) 2009 studies. The diatoms were seen on 3 of
5 dives in 2005 and tuft densitywas highest on the shelf, at a depth
of 25 m (Fig. 3). The maximum depth of occurrence was 95 m, but
density generally decreased quickly below 50 m. During the 2009
expedition, diatom tufts were seen on 3 of 7 dives. On the open-
ocean iceberg, C-18a, diatom tufts were only seen below the shelf
break, with the highest density between 38 and 55 m depth, but
they occurred in patches as deep as 100 m. East of the Antarctic
Peninsula, the small, tabular iceberg IA-4, had its highest density of
diatom tufts between 20 and 25 m depth, with a number of less
dense patches found as deep as 125 to 145 m.
Fig. 2. Tabular iceberg C-18a. (A) Rounded and apparently flat bottom at 187 m. (B) Ablation pockets on the almost horizontal surface at 25 m. (C) Vertical striations (the
camera angle is skewed, lighter blue is the sea surface) running the extent of the ice wall on C-18a. (D) Thinly distributed tufts of the diatom, Thalassioneis signeyensis, on the
wall of C-18a, 27 m.
R.E. Sherlock et al. / Deep-Sea Research II ] (]]]]) ]]]–]]] 5
Please cite this article as: Sherlock, R.E., et al., Near-field zooplankton, ice-face biota and proximal hydrography of free-drifting
Antarctic icebergs. Deep-Sea Research II (2011), doi:10.1016/j.dsr2.2010.11.025
3.3. Ice-face epifauna
ROV observations were limited to organisms approaching
0.5 cm in size and up. In this size-range, relatively few ice-face
epifauna were observed. Fish were the most notable exceptions.
Although not seen on every dive, cryopelagic fishwere found on all
tabular icebergs in 2005 and 2009. Unfortunately, we were only
able to obtain video footage of the fish despite numerous attempts
to catch themwith the plankton net and suction sampler. The video
images alone were inadequate to provide positive identification.
All but two of the fish appeared to be juvenile nototheniids
ranging in size from 34 to 45 mm and similar in physical morphol-
ogy. There were two general color morphs with some intermediate
variations that may be artifacts of video angles and illumination.
The most abundant type had no skin pigment but had silvery
reflective tissue lining the abdominal cavity and some exterior
surfaces on the eye (Fig. 4A). The second type differedmost notably
from the first by having dark pigment in the formof vertical bars on
the caudal portion of the body and patches on the surface of the
abdomen. (Fig. 4B).
Based upon published descriptions (Eastman and DeVries,
1985; Eastman, 1993) and similarities with the juvenile cryopela-
gic fish observed by Gutt (2002), it is likely that the unpigmented
fish is Pagothenia borchgrevinki. Of the other three known cryope-
lagic fish species, only Trematomus nicolai has a barred pigmenta-
tion pattern similar to the fish we observed. Both of these species
have been reported to live on icebergs: P. borchgrevinki was
observed living in holes (Line, 2000) and T. Nicolai was observed
at 30 m depth ‘‘attached to’’ the smooth surface of an iceberg by
Gruzov et al. (1967; see Andriashev, 1970).
The behavior and distribution of fish types we observed were
similar. They were generally found adhering to the nearly vertical
ice, facing up or down, along one of the rising slopes of the ablation
pockets (Fig. 4C). Due to their reflective abdominal pigmentation,
they were often difficult to discern against the substrate, demon-
strating that they are well adapted to blend in with this habitat.
Fig. 3. Vertical distribution and relative abundance of the attached diatom, Thalassioneis signyensis on the submerged flanks of free-drifting tabular icebergs during:
(A)December, 2005 and (B) April, 2009. Ablation cupswithout visible diatom tufts scored a 0 and thosewith46 tufts received a 5. Bars to the left of zero indicate depthswhere
surveys were conducted, but no diatoms seen.
R.E. Sherlock et al. / Deep-Sea Research II ] (]]]]) ]]]–]]]6
Please cite this article as: Sherlock, R.E., et al., Near-field zooplankton, ice-face biota and proximal hydrography of free-drifting
Antarctic icebergs. Deep-Sea Research II (2011), doi:10.1016/j.dsr2.2010.11.025
Their escape response was minimal and usually consisted of a kick
and glide to an adjacent pocket.
With one exception, all 464 fish observed in this study occurred
below the shelf break at depths between 42 and 206 m. Their lower
limit was generally defined by the bottom of the iceberg. One
pigmented fishwas seen on top of the ledge at approximately 25 m
depth in a dense diatom field in 2005. All 3 ice-face video transects
carried out in 2009 showed a bi-modal distribution with the upper
node between 40 and 95 m and the lower node covering the lower
40 mof the ice face. (Fig. 5). Thehighest density of fish seenduring a
transect segment was 3.2 fish/m2 between 164-171 m depth with
eight fish seen in a single 1.6 m2 video frame taken a few meters
above the transition to the flat bottom of the iceberg. Cryopelagic
fish were observed resting under the iceberg, as well as on the
nearly vertical faces of the icebergs.
3.4. Transecting - biological data and observations
Vertical transects were challenging because of the short, 600 m
tether, iceberg movement, calving, currents, and potential tether-
tangling ice at the sea surface. Consequently, ROV transecting dives
varied in time from 2.5–4 hours. During that time the iceberg
typically travelled 1-1.5 km (Table 1).
Zooplankton observed in the water surrounding the icebergs
were similar to samples collected nearby in the 10 m2 MOCNESS
trawls (Kaufmann et al., this issue). The most easily recognized
macrozooplankton seen on video included Salpa thompsoni, the
medusae Periphylla periphylla and Calycopsis borchgrevinki, the
Fig. 4. Cryopelagic nototheniid fish found on the submerged sides and bottoms of
free-drifting tabular icebergs: (A) unpigmented (B) pigmented and (C) typical
resting position.
Fig. 5. Vertical distribution and frequency of occurrence of cryopelagic nototheniid fish determined from three ice-face transects conducted inMarch/April, 2009. Frequency
of occurrence bars to the left of zero indicated depths where transects were conducted, but no fish seen.
Fig. 6. Comparison of total biomass for ROV plankton samples collected (A) 1, 15
and 40 m from the ice face and (B) above, below and across the thermocline of two
tabular icebergs, C-18a and IA-5. ‘Bottom’ distinguishes the samples collected at the
bottom of the iceberg, adjacent to the ice face.
R.E. Sherlock et al. / Deep-Sea Research II ] (]]]]) ]]]–]]] 7
Please cite this article as: Sherlock, R.E., et al., Near-field zooplankton, ice-face biota and proximal hydrography of free-drifting
Antarctic icebergs. Deep-Sea Research II (2011), doi:10.1016/j.dsr2.2010.11.025
ctenophores Callianira antarctica and Beroe spp., as well as the
calycophoran siphonophore, Diphyes antarctica. Small pteropods,
probably Spongiobranchia australis,were seen less often. Schools of
krill, Euphausia superba and Thysanoessa macrura, were seen on
most dives but their occurrence was patchy. Even on dives when
few krill were seen, their fecal pellets were often present in high
numbers. Krill seemed more abundant in the December of 2005
than in March/April of 2009 and peak density occurred in a large,
deep cave, which the ROV was pulled into by a strong current.
The amphipod Eusirus antarctica was another commonly seen
crustacean. On the smaller tabular berg, IA-5, we observed large
numbers of them clinging to the ice wall from ca 45–75 m. They
Fig. 7. Multidimensional scaling plots (MDS) based on presence/absence of 50 taxa sampled by the ROV. (A) Community compositionwas not based on distance sampled from
the ice, however (B) taxa showed a higher degree of similarity around a given iceberg, and particularlywithin a single dive. (C) The taxonomic groups sampled 20% of the time
or more are shown. The direction of the vector approximates sample location and the length corresponds to abundance.
Fig. 8. Water column profiles taken by the ROV during 4 divesmade in the Powell Basin in theMarch/April of 2009. Note the shallower pycnocline near C-18a (A) and (B) and
low surface values for chlorophyll a. (C) Thewater columnnear IA-4 began towarmwith depth beginning around 70 m, perhaps due tomixing of AntarcticWWby the iceberg.
(D) Temperature and salinity near the smaller IA-5 differed from C-18a, although chlorophyll was slightly reduced nearer the iceberg.
R.E. Sherlock et al. / Deep-Sea Research II ] (]]]]) ]]]–]]]8
Please cite this article as: Sherlock, R.E., et al., Near-field zooplankton, ice-face biota and proximal hydrography of free-drifting
Antarctic icebergs. Deep-Sea Research II (2011), doi:10.1016/j.dsr2.2010.11.025
were easily disturbed and readily swamoff the icewhen startled by
the lights, noise, or vibrations of the ROV. Previously we had seen
E. antarctica on the ice but only in small numbers. Theywere readily
identified on video, but were also captured by the suction sampler
and the plankton net as we transected close to the iceberg.
Notably absent from the ice as well as the water column
nearby were other amphipods, tomopterid and alciopid worms,
animals commonly encountered in nearby 10 m2 MOCNESS trawls
(Kaufmann et al., this issue).
Because of the small net opening and 202 mmmesh size, plankton
collected by the plankton net during transects tended to be small. In
approximate order ofmost to least abundant these included: diatoms
(Corethron sp.), calanoid copepods, small chaetognaths (Pseudosagitta
sp), dinoflagellates (Chaetocerus sp.), radiolarians, tintinnids, phyllo-
docid polychaetes, cladocerans and appendicularians. In all, 50 taxa
collectedby theplanktonnetwere identified for community analyses.
While transecting vertically, theROVsampledbetween18.6 and
52.3 m3 of seawater during each profile (at distances ca 1, 15 and
40 m from the ice). Samples were taken above, below, and across
the thermocline and sampling depth varied with the dive and the
distance from the iceberg. No proximity-related change was
apparent in plankton community composition within 40 m of an
iceberg, but biomass increased within 5 m of the ice face. Samples
collected near the ice face of icebergs C-18a and IA-5 averaged
10.8 mg/m3 dry weight (range: 8.2–13.4 mg/m3), compared with
2.5 mg/m3 dry weight (range: 0.6–4.7 mg/m3) for samples taken at
15 and 40 maway from the ice face (Fig. 6A). The spatial patterns of
biomass and composition are similar to those reported by
Kaufmann et al. (this issue) albeit at a larger scale; they found
macrozooplankton and micronekton biomass was highest at
trawling sites closest to the icebergs, while no change in commu-
nity composition was apparent out to a distance of 18.5 km.
Samples taken across and below the thermocline generally
had higher biomass (mg/m3 dry weight) than samples from above
it. The exception was the sample taken near the bottom of iceberg
IA-5, which yielded the lowest biomass we collected (Fig. 6B).
We saw a high degree of similarity in the taxa sampled with the
plankton net. An MDS plot showed little distinction between the
sampled invertebrates based on distance from the ice face (Fig. 7A).
Instead, the plot indicated a greater degree of similarity in
community composition around a given iceberg andwithin a given
dive than within any distance grouping from the ice (Fig. 7B).
Ultimately ANOSIM confirmed that no significant differences
existed in community composition based on distance from the
ice (p¼0.50). The null hypothesis of no change in community
composition with distance sampled across the Powell Basin was
tested and rejected (p¼0.019, Rho¼0.114), indicating that taxa did
change with distance sampled over the Powell Basin.
Fig. 9. Changes in temperature, salinity and chlorophyll a measured while the ROV held station on iceberg C-18a. Fluctuation in ROV depth over time (middle panel) is
indicative of the dynamic environment. Distance to the ice (right panel) is not representative of ROV proximity to the ice face above 12 m because the ROV was over the
horizontal shelf. Changes in temperature as the ROV descended (right panel) may indicate horizontal flow of seawater away from the iceberg.
R.E. Sherlock et al. / Deep-Sea Research II ] (]]]]) ]]]–]]] 9
Please cite this article as: Sherlock, R.E., et al., Near-field zooplankton, ice-face biota and proximal hydrography of free-drifting
Antarctic icebergs. Deep-Sea Research II (2011), doi:10.1016/j.dsr2.2010.11.025
3.5. Transecting - hydrographic data and observations
The ROV typically dove to depths between 150–200 m in order
to reach the bottom of an iceberg. Over the Powell Basin this depth
range encompassed two water masses: Antarctic Surface Water
(AASW) and Winter Water (WW) (Smith et al., 1999; Stephenson
et al., this issue). At 1 m from C-18a the pycnocline was conspic-
uous and fairly flat. At 40 m from the ice face, the thermocline was
typically less sharply defined and occurred deeper (Fig. 8A,B).
However, therewere exceptions to these generalities; temperature
and salinity varied, sometimes appreciably, over distances of tens
of meters from an iceberg.
Hydrographic data takenwhile theROVmadewayon the surface,
from the ship to an iceberg, showed relatively fresh water of 33.6–
34.0 psu that usually and somewhat surprisingly became more
saline as the ROVdove close to the iceberg (Fig. 8A—C). Temperature
decreased as the ROV approached the iceberg ca #0.3 to #1.0 1C,
and usually continued to decrease with depth (Fig. 8A,B,D). The
hydrography of IA-4 stood out in that temperature began to increase
at about 100 m (Fig. 8C). Although not usually observed, this
phenomenon was also seen at iceberg C-18a for dive #8, with
warming beginning at about 150 m. Chlorophyll awas typically low
in the surface water and lower near the iceberg than farther away
(Fig. 8A, B, D). The water column near IA-4 (dive #17) was an
exception and chlorophyll was slightly higher farther away from the
ice face (Fig. 8C). Lower chlorophyll, temperature and salinity values
near the surface are likely the result of dilution by meltwater.
Measured near the ice, temperature, salinity and chlorophyll a
varied even while station-keeping at the same depth. Water-
sampling ROV dives (not part of this study) were typically spent
pumpingwater from several depth stations adjacent to the ice. This
dive plan allowed the ROV to sample water from a particular depth
over time and demonstrated the variability in temperature and
salinity close to the ice while depth remained relatively constant
(Fig. 9). The clearest indication ofmeltwater, concentrated near the
surface, came from these dives as well.
4. Discussion
Complex interactions between physical and biological processes
are important in structuring marine ecosystems (Daly and Smith,
1993). ROVs provide a unique opportunity to make integrated
biological, chemical and physical measurements adjacent to ice-
bergs. However, small ROVs lack the motive power to overcome the
constraints of currents, ship movement, and tether management.
The precision with which station-keeping, profiling, and sampling
can be accomplished are all limited by power in this dynamic
environment. Nevertheless we were able to make, for the first time,
measurements in close proximity to free drifting icebergs;measure-
ments not afforded by more traditional oceanographic methods.
4.1. Hydrography
Icebergs cool, freshen, and disturb the water through which they
pass. A number of laboratory investigations have attempted to
replicate how icebergs influence the environment in which they
melt (Gade, 1979; Huppert and Turner, 1980; Neshyba and Josberger,
1980; Huppert and Josberger, 1980; Jacobs et al., 1981; Josberger and
Martin, 1981). These studies revealed a millimeters-thin, upward
flowing boundary flanked by a thicker, more turbulent, downward
flowing layer that was warmer and saltier than the boundary (near-
ice) layer and colder than the far-field water. As this outer layer of
water flowed down, it entrained meltwater from the inner boundary
layer. Depth, salinity and thereby density, increased until reaching
neutral buoyancy at which point the water flowed away from the ice
in nearly horizontal layers (Huppert and Josberger, 1980). Such
horizontal flows may have been the cause of the stepped decreases
seen in some temperature profiles, e.g., dive 7 on C-18a (Fig. 9).
An interesting incongruity in our CTD data was the apparent
lack of meltwater as the ROV dove on an iceberg. Stephenson et al.,
(this issue) measured relatively low density water in hydrocasts
made outside our sampling range, at C-18a.We expected thewater
to be less saline close to the iceberg and were surprised when the
CTD usually showed just the opposite (Fig. 8). Because the CTD
could never be closer to the ice than ca 0.5 m, it is possible that the
instrument was just too far away from the ice and that it remained
outside the extent of the innermost, upward-flowing boundary
layer described by Huppert and Josberger (1980) and Huppert and
Turner (1980).
At times an upward flowing layer adjacent to the ice was
apparent in the HD video. Gelatinous zooplankton were good
current indicators and Salpa thompsoni were particularly conspic-
uous as they moved vertically, telltales in the current (online
supporting video). At times there were currents emanating from
under the iceberg strong enough to prevent the ROV frommaking a
descent (e.g., Dive 6). Conversely, there were also currents that
carried the ROV under the ice (e.g., Dive 4). The ROV dove most
often on the leading edge of icebergs. This was unintentional and
likely a consequence of diving in the lee of the wind. Only dives
4 and 6 (C-18a) occurred on the trailing edge, yet the current flow
under the iceberg was quite different between those two dives.
Shear forces like these are thought to be due to the interactions of
wind and current, the shape and extent of the iceberg’s sides, its
translocations and movement through the water. They are distinct
from the turbulent flow of meltwater described by Huppert and
Turner (1980) and Huppert and Josberger (1980).
Zooplanktonmaybe concentrated by biological aswell as physical
factors.DotyandOguri (1956) found increasedproductivityupstream
of islands surrounded by otherwise nutrient-poor oceanic water.
Alldredge and Hamner (1980) discovered that shear zones formed by
tidal currents around islands served to concentrate zooplankton up to
40 times that of water outside such zones. Like islands, icebergs often
have complex topography and may be quite large. Unlike islands,
icebergs are usually free of the seafloor and are moved by winds and
currents. If those forces are complementary, the iceberg will move
faster than the current. If wind and currents oppose each other, the
entire flow regime around the iceberg changes.
Thus, icebergs undoubtedly share the ‘‘bewildering array of
variables’’ Hamner and Hauri (1981) observed around islands and
reefs; variables that are compounded by an iceberg’s movement
through the water. The complex currents, gyres and eddys that
formed around a reef can be of great importance to the local
distribution of fishes and plankton (Hamner and Hauri, 1981) and
it is reasonable to expect that the same holds true for icebergs.
However, the dynamic nature of the wake and water flow surround-
ing icebergs was particularly apparent at the scales sampled by the
ROVascurrents changed, oftenover the courseof a singledive. Factors
such as whether the ROV dove on the leading or trailing edge of the
iceberg (Table 1) likely had little bearing onwhether itwas actually in
the wake of the iceberg or not. Turbulent flows near the ice face may
have been more consistently important in aggregating zooplankton.
4.2. Biological oceanography
Microbial andmicroalgal communitiesproliferateonand in sea ice
as well as in nearby waters that are modified by the presence of that
ice (Brierley and Thomas, 2002). Micrometazoans like flagellates and
ciliates also inhabit the ice where they are grazed on by zooplankton
(Garrison, 1991). The colonization of sea ice and the growth of sea-ice
biota depend on the state of the ice as well as the physical processes
R.E. Sherlock et al. / Deep-Sea Research II ] (]]]]) ]]]–]]]10
Please cite this article as: Sherlock, R.E., et al., Near-field zooplankton, ice-face biota and proximal hydrography of free-drifting
Antarctic icebergs. Deep-Sea Research II (2011), doi:10.1016/j.dsr2.2010.11.025
that form and deteriorate it. Its microstructure helps define how ice
biota colonize pack ice (Ackley and Sullivan, 1994). The glacial ice of
large, tabular bergs is very different from pack ice; however, the
surface of icebergs is subject to constant change due to currents,
calving, melting, and erosion. Krill and amphipods that temporarily
inhabit icebergs likely do so because there are algae and micro-
metazoanspresent for forage. Like thediatomtufts,which growat the
periphery of ablation pockets, their patchiness may be due to the
changing quality of the ice surface itself.
Tufts and mats of diatoms, fish, and amphipods, were the most
commonorganisms found on the submerged ice of icebergs.Where
they occurred, diatoms were densest above the associated pycno-
cline, although diatom tufts were confirmed down to depths of
about 100 m. The shallower node of the nototheniid distribution
overlapped with the distribution of the diatoms but was indepen-
dent of diatom presence. Fish living on the icebergs were typically
more common below the pycnocline. However, O’Grady and
DeVries (1982) demonstrated that P. borchgrevinki and T. nicolai
can effectively osmoregulate over a range of 500 to 1750 m, which
would indicate that both species should tolerate greater salinity
gradients than we saw along the entire ice face. Stone (2003)
observed an undetermined species of cryopelagic fish living in
freshwater pools at the edge of icebergs, further evidence of their
adaptability in this dynamic environment.
As none of these fish were seen away from the ice face, or
collected in trawls conducted nearby our dive sites (Kaufmann
et al., this issue), we assume that icebergs are a preferred habitat for
this stage of their life cycle. Given that their vertical distribution
begins at about 40 m and that none were seen in ice caves or
burrows, it may be that the upper range of their depth distribution
is limited by predation from shallow-diving birds. Free-drifting
icebergs provide mobile habitats with ablation pockets as hiding
and resting surfaces, access to a rich zooplankton food source, and
the potential for long-range dispersal within the polar front.
The community composition of the plankton we collected was
about 70% similar between the two tabular icebergs, C-18a and IA-5
(Fig. 7C). Larvaceans were not collected at IA-5 but were relatively
common, if exceedingly small, at C-18a. Euphausiid calytopsis
larvae were found at both icebergs, but only once on C-18a. For
iceberg IA-5, the samples with the highest biomass were largely
due to collections of calytopsis larvae and the relatively large
amphipod, Eusirus antarctica, in the near-ice samples.
Biomass varied with depth and with distance from an iceberg.
This could result from physical factors at the leading edge of the
iceberg, where we usually sampled, or be a response by grazers to
the ice-attached algal communities. The range of these values from
far to near the ice face ("3–11 mg/m3, Fig. 6A) is similar to data
from plankton trawls made in the same area at similar depths and
time of year (Boysen-Ennen et al., 1991). For iceberg C-18a as well
as for IA-5, the near-ice samples had more organisms than did
samples collected from further away (Fig. 6A). With one exception,
the highest planktonic biomass collected occurred across or below
the pycnocline (Fig. 6B). However, the depth of the pycnocline was
shallower nearer an iceberg and the Antarctic Winter Water mass
extended vertically, even in comparison with water ca 40 m away
(Fig. 8, and Stephenson et al., this issue). Thus while the pycnocline
was a prominent physical feature and a potential biological
gradient, it was not the only one. This depth range also generally
includes the chlorophyll maximum (Vernet et al., this issue) and it
is likely an interaction of factors that led to more organisms being
collected across and below the thermocline.
We found no evidence that invertebrate communities were
entrained or entrapped by icebergs. In fact, the macroplankton we
collected varied only slightly over the entire cruise in 2009. The taxa
from a given dive were fairly similar in spite of being sampled at
three different distances from the ice (Fig. 7A,B), indicating that
geography and different water types or, possibly time, better
explained the changes in the invertebrate communities we
observed. The organisms found on or adjacent to the ice face are
either highly mobile (e.g., euphausiids, amphipods, and fish), or are
specially adapted (e.g., fish and diatoms) to live in that dynamic
environment. Plankton like Corethron sp., Chaetocerus sp., radiolar-
ians and copepods may be able to quickly benefit in the wake of a
large iceberg; however, macrozooplankton present nearby seem to
be the consequence of being concentrated in the turbulent flows
adjacent to an iceberg and/or stirred up to the shallower pycnocline.
5. Summary and conclusions
$ Small ROVs can be useful tools to study icebergs but lowmotive
power and small payloads limit their efficacy.
$ The five tabular icebergs we examined shared these structural
features: a horizontal shelf, nearly vertical sides, extensive
ablation pockets and an apparently flat bottom.
$ Plankton biomass was higher within 5 m of the icebergs we
sampled than it was between 15 and 40 m distant.
$ The community composition adjacent to icebergs varied over
the geographical scale of the Powell Basin, but did not varywith
distance out to 40 m from any iceberg we examined.
$ Diatom communities associated with ablation pockets were
found at shallower depths and higher densities in the late spring
than in late summer.
$ Cryopelagic nototheniid fish were common and conspicuous on
the submerged flanks of the icebergs we examined.
Acknowledgments
This researchwas funded by a National Science Foundation grant
(ANT-0636813) to K. Smith and B. Robison and by the David and
Lucile Packard Foundation. We thank L. Bird, B. Hobson, P. McGill,
and A. Sherman for herculean efforts to modify and maintain the
ROV. Craig Dawe skillfully piloted the ROV to an equal number of
launches and recoveries. Shipboard support and tethermanagement
was provided by the aforementioned as well as K. Smith, J. Ellena,
C. Hexel, A. Kahn, D. Nail-Meyer and the very capable Raytheon Polar
Services support group. Without the coordination of Captain
M. Watson and all crew on the RVIB N.B. Palmer, as well as Captain
R. Verret and the crew on the ARSV L.M Gould, the ROV work would
not have beenpossible. K.Walz identified and sortedmany plankton
andassistedwithfigures. B. Schlininghelpedwithhydrography. This
ms benefitted from the critique of 3 reviewers and we thank them.
R. Wilson taught something to everyone, not the least of which was
how to sharpen a knife on a coffee cup.
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R.E. Sherlock et al. / Deep-Sea Research II ] (]]]]) ]]]–]]]12
Please cite this article as: Sherlock, R.E., et al., Near-field zooplankton, ice-face biota and proximal hydrography of free-drifting
Antarctic icebergs. Deep-Sea Research II (2011), doi:10.1016/j.dsr2.2010.11.025