Cross-shelf structure and distribution of mesozooplankton communities in the East-
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Siberian Sea and the adjacent Arctic Ocean
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E. A. Ershova
1,2*
, K. N. Kosobokova
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UiT The Arctic University of Norway, Faculty for Biosciences, Fisheries and Economics, Department for Arctic and Marine
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Biology, 9037 Tromsø, Norway;
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Shirshov Institute of Oceanology, Russian Academy of Sciences, 36 Nakhimova Avenue,
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117997 Moscow, Russian Federation
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*Correspondence:
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Elizaveta Ershova
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elizaveta.ershova@uit.no
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Abstract The East-Siberian Sea (ESS) plays a significant role in circulation of the surface
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water and biological production in the Arctic, yet due to its remote location and historically
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difficult sampling conditions remains the most understudied of all Arctic shelf seas, with
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even baseline information on biological communities absent in literature. We contribute to
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such a baseline by describing the distribution and community structure of
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mesozooplankton in the ESS and the adjacent Arctic Ocean based on recent (September
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2009, 2015) as well as historical (August-September 1946, 1948) data. We found that the
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overall biomass and abundance during our studies were significantly lower than in the
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adjacent Chukchi Sea, but higher than historical estimates from ESS, around 25-35 mg DW
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m
-3
. The diversity was low and characteristic for other Arctic shelf seas, with increasing
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number of species in deeper waters. Biomass was highest at the shelf break, where it
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approached 70 mg DW m
-3
, and was mainly composed of the large copepod Calanus
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glacialis. On the shelf, abundance and biomass were low (10-20 mg DW m
-3
)
and dominated
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by small copepods and chaetognaths. Several distinct assemblages of zooplankton were
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identified and related to the physical properties of the water masses present. A striking
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result was the presence of both Atlantic and Pacific expatriates in offshore waters close to
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the shelf break, but generally not on the shelf. Tracking these advected organisms could be
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a useful tool in determining the pathways, extent and transit time of Atlantic and Pacific
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water entering the Arctic.
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Keywords: Arctic Ocean, zooplankton, pelagic ecosystems, climate change
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Manuscript Click here to
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Introduction
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The East-Siberian Sea (ESS) is located east of the Laptev Sea and west of the Chukchi
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Sea, bordered by the New Siberian Islands on the west and Wrangel Island on the East (Fig.
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1). It is the largest, broadest and shallowest of all Arctic shelf seas, widely open to the Arctic
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Basin. Nearly 70% of the shelf of the ESS is shallower than 50 m, with most of the area
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dominated by depths of 20–25 m (Williams and Carmack 2015). Oceanographically, it
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interacts both with the adjacent Atlantic-influenced Laptev Sea, and the Pacific-influenced
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Chukchi Sea and is heavily influenced by river runoff from large Siberian rivers Kolyma and
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Indigirka (Semiletov et al. 2005). The water exchange between the ESS and neighboring
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Laptev and Chukchi Seas is mostly determined by atmospheric circulation varying
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significantly year to year. Eastward winds keep riverine water from the Laptev Sea close to
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the coast and lead to the development of the Siberian Coastal Current, which carries low
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salinity water eastward through the Long Strait into the Chukchi Sea. In contrast, prevailing
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westward winds cause fresh surface water to be transported off the shelf, and the direction
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of the prevailing currents is reversed, resulting in advection of Pacific-origin water from
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the Chukchi Sea (Weingartner et al. 1999). Historically, the ESS has been the most heavily
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ice-covered shelf within the Eurasian Arctic, characterized by extensive pack ice formation
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that reached 300-500km from the shore (Dobrovolskii and Zalogin 1982).
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Despite much effort being devoted to Arctic research in recent decades, mostly it has
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been concentrated in relatively easily accessible regions within the European and north-
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American sectors (e.g. Kassens et al. 1999; Stein et al. 2003; Flint et al. 2010; Grebmeier
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and Maslowski 2014). The large knowledge gaps remain primarily along the Siberian shelf,
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despite its high significance for sea ice formation and ocean circulation within the Arctic.
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Due to its remote location, shallow depths inaccessible to large research vessels, and
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historically servere ice conditions, the ESS remains the most understudied of all Arctic
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shelves, even compared to other Russian Arctic seas, with the few existing studies in
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western literature limited to oceanography and biogeochemistry (i.e., Münchow et al.
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1999; Semiletov et al. 2005; Anderson et al. 2011; Pipko et al. 2011). Limited information
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on the East Siberian shelf pelagic biological communities collected in the 1940’s, and
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1980’s was published in Russian “grey” literature (Brodsky 1957; Pavshtiks 1994; Pinchuk
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1994). It is not easily accessible even in Russian, and unavailable in English. No published
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zooplankton studies have been conducted in this region since 1986.
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As the sea ice extent, duration and thickness continues to decline in the Arctic, it has
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become common in recent years for the entire East-Siberian shelf to become ice-free
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during the ice minimum (Nghiem et al. 2006; Kwok et al. 2009). Similar to other areas of
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the Arctic (Buchholz et al. 2012; Ershova et al. 2015a; Vihtakari et al. 2018), this is
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expected to result in significant shifts in plankton production patterns and community
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composition. In order to be able to detect the ongoing changes in the pelagic ecosystem of
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this region, within this study we aimed to provide baseline information on the structure of
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the ESS zooplankton communities using net-based data collected in the ESS and adjacent
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Arctic Ocean in September 2015. We also complement our data with other available
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datasets collected on the ESS shelf: 2009, when the area was similarly nearly ice-free, and
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August-September 1946-1948, when it was covered with pack ice (Pavshtiks 1994). This is
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the first study in western literature providing description of the species composition,
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spatial distribution, abundance, biomass estimates and community structure analysis of the
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ESS zooplankton.
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Methods
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Zooplankton collection and processing
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Zooplankton samples were collected in September 2015 from the R/V “Akademik
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Tryoshnikov” at 16 stations in the northern ESS, on two transects extending from the shelf
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(~35m depth) onto the basin (>2000m) (Fig. 1). Mesozooplankton was collected using a
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closing Juday net with a mesh size of 180 μm and opening diameter of 37 cm. At each
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station, stratified samples were taken at depth intervals of ~0-25, 25-65, 65-130, 130-260,
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and 260-450 m. No samples were collected deeper than 450 m due to the limitations of the
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research vessel. The net was towed vertically with a wire speed of 0.5 m/sec, and closed at
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each designated depth with a messenger, which was propelled down the wire as the net
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ascended. The volume of the water sampled was calculated from the height of each tow;
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100% filtering efficiency was assumed, as there were no observed cases of clogging of the
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nets. Zooplankton samples were preserved using 10% formalin (4% formaldehyde) for
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later processing in the laboratory.
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In the laboratory, each sample was scanned under stereomicroscope for large and
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uncommon species, which were identified to the lowest taxonomic level and measured. The
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rest of the sample was split using a Folsom splitter until there were ~100 individuals of the
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most common species in the terminal split. Increasingly larger splits were scanned to
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obtain counts for rarer taxa; a total of 400-600 individuals were examined from each
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sample. All organisms were measured using a computer measurement system (ZoopBiom
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software, Roff and Hopcroft 1986) and the DW of each specimen was predicted from a
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length-weight regression relationship known for the same species, or a morphologically
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similar organism (Ershova et al. 2015b). Copepods were staged and identified to species;
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copepodite stages within some genera, which are morphologically undistinguishable (i.e.
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Pseudocalanus spp.) were pooled together by stage. Meroplankton was grouped to the
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macrotaxa or to the family level (in the case of shrimp larvae).
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Sea ice conditions, oceanography and Chlorophyll-a
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Sea ice concentrations were obtained for each sampling location from the Nimbus-7
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SMMR and DMSP SSM/ISSMIS Passive Microwave Data set, available through the NSIDC
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archives (Cavalieri et al. 1996). In addition to sea ice concentration, the distance to the
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nearest ice edge, defined as 15% ice concentration, was calculated for each station (with
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positive values indicating open water stations, and negative values indicating ice-covered
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stations). Temperature and salinity data were collected with a Seabird SBE911plus CTD
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system (SeaBirdTM Electronics Inc.) equipped with a dissolved oxygen sensor,
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transmissometer, fluorometer, and turbidity sensor with data binned into 1-m intervals
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during post processing. The water column was divided into water masses based on the
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definitions for the Arctic Ocean in Rudels (2008). Chlorophyll samples were collected using
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Niskin bottles attached to the CTD casts at depths approximately corresponding to 3, 10,
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20, 30, 40 and 50 meters. Typically, 500 ml of sample water was filtered onto GF/F glass
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fiber filters, extracted in 90% acetone and analyzed fluorometrically. All samples were
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processed at sea.
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Comparison to other datasets
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Our data on zooplankton distribution was compared to published and unpublished data
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from other expeditions collected during the same time period (September) in 2009
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(Ershova et al. 2015b), as well as 1946 and 1948. The 1946 data, collected from the Soviet
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ice-breaking vessel “Temp”, is found in a brief publication about ESS zooplankton by
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Pavshtiks (1994); the 1948 data, sampled from the ice-breaker “Severnyj Poljus” in the
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Chukchi and East-Siberian Sea, is available from an archive compiled for the Arctic regions
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by Markhaseva et al. (2005). To our knowledge, this joint dataset represents all publically
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available zooplankton data for this region, with the exception of the expedition to Chaun
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Bay in 1986 (Pinchuk, 1994). The latter was excluded due to being restricted only to the
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inner bay and not extending onto the shelf. The listed expeditions have little spatial
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overlap (Fig. 1), but together cover a wide area of the ESS shelf. The samples during the
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2009 expedition were collected by a 150-µm double ring net of 60-cm mouth diameter,
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with flowmeters attached at the mouth, towed vertically from ~5 m off the seafloor to the
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surface at 0.5 m/sec. While the wider mouth of the net may have affected the sampling
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efficiency, the similar mesh size makes the datasets partially comparable. During the two
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historical cruises, 1946 and 1948, zooplankton were sampled with a closing 168-µm Juday
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net (mouth diameter 37 cm), in a manner identical to ours. The species lists produced for
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the two historical datasets are very detailed for some groups (i.e. copepods), with
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identification done to species and stage level, but very coarse for others, with just the broad
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taxonomic categories (i.e. cnidarians, amphipods) listed. The taxonomy during all years
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was cross-checked using the Arctic Register of Marine species (Sirenko et al. 2019), and
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World Register of Marine Species (WORMS Editorial Board, 2019) in order to remove
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synonyms and suspicious identifications. For comparing years, taxonomic assignments
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within each dataset were adjusted to the highest common denominator. Abundance data
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from 1946 and 1948 was converted to biomass by using average dry weights for each taxa
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based on our own results.
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Data analysis
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All analyses were carried out in R (R Core Computing Team 2017). Differences in biomass
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and abundance of key groups were compared between transects (2015) and years using a
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one-way ANOVA, with values log-transformed to meet ANOVA assumptions. Within the
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