Research vessels equipped with fibre optic and copper-cored coaxial cables support the live onboard inspection of high-bandwidth
marine data in real time. This allows for towed still-image and video sleds to
be equipped with latest-generation higher-resolution digital camera systems
and additional sensors. During RV Polarstern expedition PS118 in February–April 2019,
the recently developed Ocean Floor Observation and Bathymetry System (OFOBS)
of the Alfred Wegener Institute was used to collect still-image and video
data from the seafloor at a total of 11 predominantly ice-covered locations
in the northern Weddell Sea and Powell Basin. Still images of 26-megapixel
resolution and HD (high-definition) quality video data were recorded throughout each
deployment. In addition to downward-facing video and still-image cameras,
the OFOBS also mounted side-scanning and forward-facing acoustic systems, which
facilitated safe deployment in areas of high topographic complexity, such as
above the steep flanks of the Powell Basin and the rapidly shallowing,
iceberg-scoured Nachtigaller Shoal. To localise collected data, the OFOBS
system was equipped with a Posidonia transponder for ultra-short baseline
triangulation of OFOBS positions. All images are available from: 10.1594/PANGAEA.911904 (Purser et
al., 2020).
Introduction
Recent studies indicate that climate change processes in the maritime
Antarctic are accelerating, leading to the increasing retreat of the ice
sheets of the eastern coast of the Antarctic Peninsula and the near
disintegration of the ice shelves Larsen A in 1995 and Larsen B in 2002
(Rott et al., 1996; Scambos et
al., 2013; Shepherd et al., 2003), as well as the break-off of giant
icebergs from Larsen C
(Han et al.,
2019; Shepherd et al., 2003; Skvarca, 1993). This change in ice coverage has
resulted in spatially extensive habitat change both above and below water,
with many processes from light penetration, ocean stratification, surface
productivity and food transportation pathways being affected
(Barnes
and Tarling, 2017; Chaabani et al., 2019; Fillinger et al., 2013; Griffiths
et al., 2017; Gutt et al., 2019). By assessing the seafloor habitats and
associated benthic fauna communities in these areas recently free of
permanent ice cover with the Ocean Floor Observation and Bathymetry System
(OFOBS) of the Alfred Wegener Institute (AWI), Helmholtz Centre for Polar
and Marine Research (Germany) (Fig. 1), the
ecological effects of climate-driven ice shelf loss can be studied. The OFOBS is
a towed platform capable of deployment in moderately ice-covered conditions and
capable of concurrently collecting acoustic, video and still-image data from
the seafloor (Purser et al., 2018).
The Ocean Floor Observation and Bathymetry System (OFOBS)
of the Alfred Wegener Institute (AWI), Helmholtz Centre for Polar and Marine
Research, deployed from the RV Polarstern during cruise PS118 in the
waters east of the Antarctic Peninsula and on the flanks of the Powell Basin
(February–April 2019).
During RV Polarstern expedition PS118 (9 February–10 April 2019)
(Dorschel, 2019b) the OFOBS was deployed 11 times,
conducting concurrent high-resolution still-image, video and acoustic
surveys across diverse and contrasting regions of the Antarctic seafloor
(Fig. 2). Ice conditions during the cruise were harsh, though a
north–south transect from the Weddell Sea continental shelf to the northwestern
continental shelf edge of the Powell Basin was completed, with stations on
the Weddell Sea plateau, Nachtigaller Shoal
(Dorschel et al., 2014), and flanks and
rim of the Powell Basin investigated (Table 1). The forward-facing
acoustic camera allows for the towed OFOBS operator to be aware of steep
structures or rising seafloor ahead of the device and allows for ample time to
winch the system to a safer height. This allowed for use even in heavy ice, with a
minimal ability for ship manoeuvre, and close to the steep flanks of the
Powell Basin, allowing for collection of this novel dataset.
Regional map showing the positions of OFOBS deployments
made during RV Polarstern cruise PS118.
Locations of OFOBS deployments during PS118. Start and end
position coordinates and times are given, in addition to the numbers of
timer and hotkey images collected during each deployment.
The PS118 OFOBS data were collected with the same cameras, illumination
regime and deployment protocols as mounted on the previous AWI towed camera
sled system (Ocean Floor Observation System (OFOS)), used to survey the
Antarctic seafloor during previous recent RV Polarstern expeditions
(Piepenburg et
al., 2017), such as PS81, also to the western Weddell Sea area in 2013
(Gutt, 2013; Gutt et al., 2016), and
PS96, to the southeastern Weddell Sea in 2015/16
(Schröder et al., 2016). By continuing to
mount the same camera systems, observations made during PS118 can be most
readily compared with those made during the previous expeditions,
uncomplicated by methodological problems relating to variabilities in camera
performance, flight height or illumination
(Schoening et al., 2020).
The OFOBS data collected during PS118 and presented here are of use for a
range of scientific studies, such as:
assessment of epibenthic megafauna communities observed at several sites
along a south–north transect from the Weddell Sea Antarctic Peninsula
continental shelf to the Powell Basin
spatial comparison of these observed shelf and basin megafauna
communities with those observed with the very similar OFOS towed device sled
during recent cruises
a temporal picture of seafloor communities occupying these seafloor regions
in 2019, for comparison with future studies following continued ice loss
an extensive set of seafloor images from several locations on the Powell
Basin flank, a region of southern seafloor sparsely surveyed to date.
Typical seafloor images collected from each of the OFOBS
surveys made of the Weddell Sea seafloor during RV Polarstern cruise PS118. (a) Station
PS118_6, hotkey_2019_03_06 at 05_07_31. (b) Station
PS118_7, hotkey_2019_03_06 at 19_36_28. (c) Station
PS118_8, hotkey_2019_03_11 at 14_33_24. (d) Station
PS118_9, timer_2019_03_12 at 06_01_06. (e) Station
PS118_11, timer_2019_03_13 at 20_14_04. (f) Station
PS118_11, timer_2019_03_13 at 22_18_47. (g) Station
PS118_12, timer_2019_03_15 at 01_29_36. (h) Station
PS118_38, hotkey_2019_03_23 at 06_27_00. All images
are presented here and in the dataset with no manipulation or colour
correction.
Typical seafloor and cliff escarpment images collected
during OFOBS surveys of the Powell Basin flanks during RV Polarstern cruise PS118. (a) Station PS118_39, hotkey_2019_03_23 at 23_ 21_46. (b) Station
PS118_39, timer_2019_03_25 at 18_50_09. (c) Station
PS118_69, hotkey_2019_03_31 at 12_20_55. (d) Station
PS118_69, hotkey_2019_03_31 at 14_10_56. (e) Station
PS118_77, timer_2019_04_01 at 20_39_51. (f) Station
PS118_77, timer_2019_04_01 at 20_11_32. (g) Station
PS118_81, timer_2019_04_04 at 00_25_31. (h) Station
PS118_81, timer_2019_04_04 at 02_27_33. All images
are presented here and in the dataset with no manipulation or colour
correction.
Materials and methodsOcean Floor Observation and Bathymetry System (OFOBS)
The OFOBS is a state-of-the-art towed camera and acoustic survey sled
recently developed by the Deep-Sea Ecology and Technology group of AWI for
benthic polar research in ice-covered environments
(Purser et al., 2018).
The device was deployed during PS118 as described in
Purser et al. (2018),
taking images under comparable illumination conditions and flight heights and
with the same camera systems as were mounted on the OFOS sled during the
PS86 and PS96 cruises in 2013 and 2015/16, respectively
(Piepenburg et
al., 2017). OFOBS positioning during deployments was carried out with the
iXBlue Posidonia ultra-short base line (USBL) system used by RV
Polarstern, localising the relative position of the OFOBS to the vessel (itself
deriving its position from a satellite-based global navigation satellite
system (GNSS). Every few seconds (depending on deployment depth) the OFOBS
received a new position fix, which was used to position stamp each collected
image against a UTC timestamp. During PS118, a stable position fix was
attained with an accuracy of approximately 0.2 % of the slant range from
the ship to the subsea unit. Environmental and operational factors, such as
ice coverage and vessel speed could result in a slanted tether cable, though
for the deployment depths made during PS118 a positional accuracy of within
∼ 20 m was likely maintained throughout. Images were taken at
26-megapixel resolution with the camera system (iSiTEC, Canon EOS 5D Mark
III) with an automated timer (for the majority of images, these were taken
with a frequency of about one image every 20 s). Additional
images taken at the discretion of the operator. These two image categories
are distinguished in the dataset as “hotkey” and “timer” images –
designations automatically incorporated into the timestamped filenames.
Hotkey images were commonly taken on the first observation of a particular
fauna species or to record a feature of interest, such as a whale fall or
interesting geological structure. No processing stages were applied to the maximum-resolution .jpeg data collected from the camera, which are provided here at the maximum acquired resolution.
Throughout all of these deployments the OFOS and OFOBS systems were equipped
with three red sizing lasers (FLEXPOINT)
(Purser
et al., 2018; Purser and Sablotny, 2020), arrayed in an equilateral triangle
with 50 cm spacing around the still-image camera housing. This laser array ensures
that each image recorded has three red dots near the centre of the image,
each spaced by 50 cm. These dots can be used in subsequent analysis to
determine accurately the area covered by a particular image. Throughout
PS118 the OFOBS was deployed approximately 1.5–2 m above the seafloor,
giving a coverage within each collected image of 4–6 m2. Illumination
of the seafloor was provided by four downward-facing SeaLight Sphere 3150 LED lights positioned in the corners of the main OFOBS frame, with two
additional strobe lights (iSiTEC UW-Blitz 250, through the lens (TTL) driven) firing
concurrently with image collection. Throughout all deployments, HD (high-definition) video data
were recorded by the OFOBS for the duration of each dive with an HD video
recorder (iSiTEC, Sony FCB-H11)
(Purser et al., 2018).
Field samplingWeddell Sea sampling
OFOBS surveys were carried out in a roughly south–north transect from the
Weddell Sea continental shelf of the Antarctic Peninsula to the northern
Powell Basin. In total, seven OFOBS deployments were made between 65 and
62∘ S (Table 1 and Fig. 2). With the exception of an
OFOBS deployment across the Nachtigaller Shoal
(Dorschel et al., 2014) (station
PS118/11-2), the seafloor in these regions was observed to be predominantly
made up of soft material with occasional drop stones present (Fig. 3).
Powell Basin sampling
Four successful OFOBS deployments were made on the flanks and rim of the
Powell Basin (Table 1 and Fig. 2). Three of these deployments,
PS118/39-1, PS118/69-1 and PS118/81-1, benefitted from the equipping of the
OFOBS sled with a forward-looking sonar
(Purser et al., 2018).
This sensor allowed for the OFOBS to be used over very steep terrain with minimum
risk, by giving advance warning of any hard structures ∼ 30 m
ahead of the sled. The majority of towed sleds, such as the OFOS, are less
capable in high-relief regions, where snagging on tall structures such as
cliffs or vent structures can occur. With the OFOBS, the ∼ 30 m
warning of approach is sufficient to allow the operators to commence
winching of the system in good time to minimise the risk of impact of the
sled with the seafloor whilst still collecting usable image data. These datasets each cover many hundreds of metres of the Powell Basin flank walls,
visually surveying these traditionally difficult-to-investigate regions of
seafloor (Fig. 4).
Data availability
All seafloor images collected with the OFOBS system are available from the
data publisher PANGAEA. No preprocessing or processing stages were applied
prior to upload, with no colour correction or light-vignetting algorithms
applied. These images are provided with georeferenced positions for each
image, as derived from the Posidonia system (10.1594/PANGAEA.911904; Purser et
al., 2020). The full cruise track is also available via PANGAEA (10.1594/PANGAEA.901319; Dorschel,
2019a) with information on additional environmental and scientific data
collected during the cruise given in the PS118 cruise report
(Dorschel, 2019b). Video data collected via the
OFOBS system are available from the authors on request. In addition to the
image data presented in this paper, the multibeam data concurrently
collected by the OFOBS device has also been uploaded to PANGAEA and will be
available from April 2021 for open-access download or on request from the
authors.
Author contributions
AP applied for the secondary user time for the PS118 cruise, conceived of
the investigation and ran the data collection campaign. BD was the chief
scientist for the PS118 expedition. SD, LH, HS, HG and AN helped run the
OFOBS platform. AP, HG, KJ, DP, CR and BD determined sampling strategies for
the OFOBS and aided in data collection. AP prepared the paper with
contributions from all co-authors.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
The captain and crew of RV Polarstern expedition PS118 are thanked for their support and
interest in the OFOBS deployments conducted during the cruise. Ulrich Hoge is
thanked for his assistance in installing the OFOBS system prior to cruise
commencement.
Review statement
This paper was edited by David Carlson and reviewed by Katrien Van Landeghem and one anonymous referee.
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