Latitudinal distribution of extant fossilizable ...

Marine Micropaleontology 123 (2016) 41–58

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Research paper

Latitudinal distribution of extant fossilizable phytoplankton in the Southern Ocean: Planktonic provinces, hydrographic fronts and palaeoecological perspectives E. Malinverno a,⁎, P. Maffioli a, K. Gariboldi b a b

Department of Earth and Environmental Sciences, University of Milano-Bicocca, Italy Department of Earth Sciences, University of Pisa, Italy

a r t i c l e

i n f o

Article history: Received 9 November 2013 Received in revised form 4 January 2016 Accepted 17 January 2016 Available online 20 January 2016 Keywords: Biogeography Coccolithophores Silicoflagellates Diatoms Dinoflagellates Parmales Archaeomonads Corona-index Southern Ocean

a b s t r a c t We present the combined abundance of all extant fossilizable planktonic groups (Dinoflagellates, Coccolithophores, Silicoflagellates, Diatoms, Parmales, Archaeomonads and micro-zooplankton) from surface waters collected along a latitudinal transect in the western Pacific sector of the Southern Ocean, ranging from ~48°S, offshore New Zealand, to ~70°S in the Ross Sea, Antarctica. Latitudinal shifts in species' distribution correspond with the Antarctic Circumpolar Current fronts and with the seasonal position of the ice-edge. Distinct bioprovinces are defined by clustering samples with a high degree of similarity. Our data confirm the importance of previouslydefined taxa as palaeoceanographic proxies but also reveal some differences: the shift in dominance between the silicoflagellates genera Dictyocha and Stephanocha, used as a proxy of palaeo sea-surface temperatures, occurs slightly north of the Southern Sub-Antarctic Front rather than at the Polar Front; the shifts in abundance between the open-ocean diatom species Fragilariopsis kerguelensis and sea-ice related F. curta and F. cylindrus, as well as the drop in the coccolithophore Emiliania huxleyi, occur at the southern Antarctic Circumpolar Front rather than at the Polar Front. Finally, we introduce the Corona-index, based on the ratio of the coronatid to non-coronatid silicoflagellates species Stephanocha speculum, as a new proxy for sea-ice and we confirm the occurrence of abundant Archaeomonads and Parmales (Triparma laevis subsp. ramispina) in the marginal ice-edge zone. © 2016 Published by Elsevier B.V.

1. Introduction The Southern Ocean (SO) is an important High Nutrient Low Chlorophyll (HNLC) zone of the World's oceans, where phytoplankton growth is thought to be limited by the availability of iron in nutrientreplenished surface waters (De Baar, 1994). The SO plays a key role in the climate system: it influences global ocean circulation through the flow of the Antarctic Circumpolar Current (Orsi et al., 1995) and the formation of bottom waters in the Weddel and Ross Sea; it controls the distribution of nutrients in the upper oceans (Sarmiento et al., 2004) and is a key area for heat and CO2 exchange with the atmosphere. In the last decades, several international and national research projects targeted the SO as a key area to understand the mechanisms and consequences of recent climate change. Surveys of sea-surface temperature, salinity and upper ocean thermal structure, often coupled with surface water sampling through the ship's pump or the Continuous Plankton Recorder (Hosie et al., 2003), are routinely carried out as

⁎ Corresponding author at: Piazza della Scienza, 4, – 20126 Milano, Italy. E-mail address: [email protected] (E. Malinverno).

http://dx.doi.org/10.1016/j.marmicro.2016.01.001 0377-8398/© 2016 Published by Elsevier B.V.

ancillary activities. Several latitudinal transects, located in different areas of the SO, and during different periods of the year, provide information on the lateral variability of physical and biological processes in the different sectors of the SO and contribute to the identification of seasonal patterns and inter-annual variation. Biological zonation of the Southern Ocean is long recognized as one main goal of marine research (Deacon, 1982). Large-scale efforts for a bio-regionalization of the oceans (Grant et al., 2006) showed the importance of the hydrographic control on the patterns of biological production. Light availability, macro- and micro-nutrients are identified as key factors controlling phytoplankton processes in the Southern Ocean (Boyd, 2002), with water column stability, temperature and the extent and variability of sea-ice also playing an important role in structuring the Antarctic marine ecosystems. Grazing pressure is a further factor that controls phytoplankton standing stocks. After the first pioneer work on diatoms (Ehrenberg, 1844), early works (Hart, 1934, 1942; Halldal, 1953; Husted, 1958; Beklemishev, 1964; Hasle, 1969) showed the biogeographic distribution and seasonality of the main phytoplankton species in the Southern Ocean. Although most early and following studies were focused on diatoms (among others Husted, 1958; Fenner et al., 1976; Hasle, 1976; Burckle et al., 1987), several studies targeting all phytoplankton groups (Kopczyńska et al., 1986, 1998, 2007; Froneman et al., 1995; Ehnert and McRoy, 2007; Gomi et al.,

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E. Malinverno et al. / Marine Micropaleontology 123 (2016) 41–58

2007; Chang et al., 2013; Patil et al., 2013) pointed out the importance of latitudinal gradients in defining the biogeographic distribution of single species and the role of the PF as an important hydrographic boundary. The influence of sea-ice on the composition of extant phytoplankton was also highlighted (El-Sayed and Taguchi, 1981; Buck and Garrison, 1983; Bianchi et al., 1992; Garrison et al., 1993; Leventer, 1998; Wright and van den Enden, 2000; Kang et al., 2001). In the long-range geological record, bioprovincialism appears as an important feature, with different species making an important contribution in high- or low-latitude settings, thus requiring the creation of distinct biostratigraphic schemes for different latitudinal provinces. In sub-polar marine settings, the palaeo-biogeography of distinct species or species groups is used as a palaeoenvironmental tool to infer the latitudinal shift in position and/or gradient of hydrographic fronts. In the SO, fluctuations in the abundance of calcareous nannofossil species (Flores et al., 1999; Findlay and Flores, 2000; Findlay and Giraudeau, 2002; Fenner and Di Stefano, 2004), in the proportion of silicoflagellate species (Ciesielski, 1974) and diatom assemblages (among others Burckle, 1984; Pichon et al., 1987, 1992; Labeyrie et al., 1996; Zielinski and Gersonde, 1997; Crosta et al., 1998; Zielinski et al., 1998; Gersonde and Zielinski, 2000; Gersonde et al., 2005) along sedimentary successions are used to the define the latitudinal shifts of the PF and/or the distribution of sea-ice on glacial–interglacial time scales. The latitudinal distribution of extant fossilizable species in relation to frontal hydrography in the SO has been addressed only sectorially, through the analysis of specific groups such as coccolithophores (Hasle, 1960; McIntyre et al., 1970; Nishida, 1986; Findlay and Giraudeau, 2000; Cubillos et al., 2007; Böckel and Baumann, 2008; Gravalosa et al., 2008; Mohan et al., 2008; Saavedra-Pellitero et al., 2014; Malinverno et al., 2016), diatoms (Hasle, 1976; Smetacec et al., 2002; Tremblay et al., 2002; Cefarelli et al., 2010; Olguín and Alder, 2011) and silicoflagellates (van der Spoel and Hallegraeff, 1973; Malinverno, 2010) but detailed comprehensive information on the main fossilizable phytoplankton groups is still scarce (Eynaud et al., 1999; Hinz et al., 2012) and incomplete. The aim of this work is to increase our knowledge on the biogeography of all extant fossilizable phytoplankton species, i.e. Coccolithophores, Silicoflagellates, Diatoms, Dinoflagellates, Parmales and Archaeomonads, through the analysis of their distribution along a latitudinal gradient in surface waters. Identification at the lowest possible taxonomic level and the use of morphotypes allow overcoming the limitation of broadlydistributed species. Although our data are restricted to one transect, one season and the surface layer, statistical analysis of species/morphotype's distribution allowed to define distinct biogeographic provinces, which correlate with upper the ocean thermal structure and hydrographic fronts of the ACC, satellite-detected Chl-a data and published surface macronutrients from the same transect. The palaeoceanographic applications of species/morphotypes' distribution in relation to frontal hydrography are discussed. The analysed transect, located in the western Pacific sector of the SO, crosses the PF at 62.8°S, much to the south than in other circumAntarctic areas, and reaches until ~ 70°S in the Ross Sea, Antarctica, through a corridor in the pack-ice belt. 2. Oceanographic setting 2.1. Physical oceanography The Southern Ocean is characterized by the eastward flow of the ACC, driven by the westerly winds which flow between 45–55°S (Orsi et al., 1995 and references therein). The ACC is bound to the north by the Subtropical Front (STF), which separates it from the warmer and saltier waters of the subtropics, and to the south by the southern boundary (Bdy), which separates it from the coastal circulation driven by the

cyclonic Ross Sea Gyre (Orsi et al., 1995). Different fronts within the ACC are identified as bands of enhanced latitudinal property gradients in surface waters and by pronounced isopycnal tilt throughout the deep water column. The Subantarctic Front (SAF) and Polar Front (PF) carry out most of the transport of the ACC and are associated with strong surface currents (Orsi et al., 1995; Nowlin et al., 1977). An additional front, the Southern ACC Front (sACCf), is identified on the basis of the subsurface temperature signature (Orsi et al., 1995). Its position is often very close to the Bdy. Although the ACC flow is driven by atmospheric forcing through westerly winds, its path is strongly controlled by bottom topography (Gordon et al. 1978) so that the fronts are located at different latitudes across the different zonal sectors of the SO. Campanelli et al. (2011) described the characteristics and latitudinal position of the ACC fronts along our transect from New Zealand to the Ross Sea (Fig. 1): - the SAF, as defined by a sharp temperature gradient at depths of 300 or 400 m (Belkin and Gordon 1996 and Orsi et al., 1995, respectively) is here split in two branches: the Northern SAF (NSAF) corresponds with a thermal gradient in the range 4–7 °C at 300 m and occurs at 51.7°S, while the Southern SAF (SSAF) is associated with a thermal gradient in the range 3–4 °C at 300 m and occurs at 58.6°S. While the NSAF position is stable in time, over the continental slope at the southern edge of the Campbell Plateau (see also .kml map), the SSAF position is not controlled by bottom topography and is strictly related to the development of meander features (Budillon and Rintoul, 2003; Campanelli et al., 2011), probably influenced by the interaction with the southern tip of the Campbell Plateau (Yaremchuk et al., 2001). - the PF, as defined by the subsurface temperature minimum of 2 °C at depth above 200 m (Orsi et al., 1995) and a 2 °C gradient in SST, is located at 62.8°S, but has been found to oscillate by up to 2° of latitude on a seasonal and interannual basis, due to the lack of a topographic constrain. When the same track has been crossed several times during the same season, the PF has been found southward from November to February (Budillon and Rintoul, 2003; Campanelli et al., 2011). - the sACCf, as defined by temperatures below 0° at the sub-surface (b 150 m) temperature minimum and by temperatures above 1.8 °C in the deeper (N500 m) temperature maximum (Orsi et al., 1995), is located at 63.6°S. Its position is stable in time and strictly constrained, in the area of our transect, by the steep northern flank of the SE Indian/Pacific–Antarctic Ridge.

Within these fronts, the water masses show more uniform characteristics, with more gradual changes in physical properties and can be therefore described as distinct zones (Orsi et al., 1995). From north to south, these are (Fig. 1): - the Subantarctic Zone (SAZ) between the STF and the SSAF: in this sector of the Southern Ocean, it is divided by the NSAF into a northern SAZ (nSAZ) and a southern SAZ (sSAZ); - the Polar Frontal Zone (PFZ) between the SAF and the PF; - the Antarctic Zone (AZ) from the PF to the Antarctic continent. The sACCf and the Bdy lay within the AZ and correspond with the winter and summer position of the sea-ice limit, respectively. South of the Bdy the circulation is dominated by the Ross gyre, a cyclonic clockwise feature driven by the interaction with the westward coastal flow. Within the AZ, the seasonal waxing and waning of sea-ice defines different hydrographic conditions, that we sampled along our transect: - the permanently open ocean zone (POOZ), between the PF and the winter sea-ice (WSI) limit; - the seasonal ice zone (SIZ) including the marginal ice-edge zone (MIZ) at the edge of the retreating pack-ice belt. During the cruise, we crossed the “corridor” of open water that develops seasonally into the belt of pack ice at the entrance of the Ross Sea (PNRA,


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Fig. 1. Map of the Pacific sector of the Southern ocean with location of the oceanographic fronts and sampling stations, plotted with ODV on an orthographic south-polar projection (Schlitzer, 2008). Background colours show monthly sea surface Chlorophyll-a values from SeaWifs data, averaged at 0.1° for January 2005 as obtained from the Giovanni online data system, developed and maintained by the NASA GES DISC (http://disc.sci.gsfc.nasa.gov/techlab/giovanni/). Light-grey area represents sea ice cover on January 5th, as obtained from daily Advanced Microwave Scanning Radiometer (AMSR-E) data, downloaded from http://www.iup.uni-bremen.de:8084/amsr and hand-drawn on the geo-referenced map. Fronts' positions (NSAF, northern Sub-Antarctic Front, SSAF, Sub-Antarctic Front, PF, Polar Front, sACCF. Southern ACC front, Bdy, Southern Boundary) (after (Orsi et al., 1995) and the winter sea-ice (WSI) limit are drawn through the ODV Graphic Object Tool Fronts and adjusted along the cruise track following Campanelli et al. (2011). The different oceanographic zones are indicated at the right side of the map (nSAZ, northern Sub-Antarctic Zone, sSAZ, southern Sub-Antarctic Zone, PFZ, Polar Frontal Zone, AZ, Antarctic Zone, POOZ, permanently open-ocean zone, SIZ, seasonal ice zone, MIZ, marginal ice zone, PI, pack ice, POL, polynya). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

2005). Although this area is seen from satellites as a corridor of icefree water, the abundant occurrence of thin sparse floes defines this region as a latitudinally-extended MIZ; - the Ross-Sea polynya, further into the Ross Sea, was not sampled along this transect.

2.2. Biological oceanography The SO is one of the most important high-nutrient low-chlorophyll zones of the oceans, where macro-nutrients are provided in excess by upwelling and primary production is limited by the external input of micronutrients, iron in particular (Martin et al., 1990; De Baar et al., 1995; Boyd, 1999; Boyd et al., 2000). Remotely-sensed surface Chl-a concentration patterns along the ACC reveal zones of uniform concentrations and seasonality separated by the ACC fronts (Sokolov and Rintoul, 2007): highest values are found north of the STF and south of the sACCf, lower values are found in the vicinity of the PF and the lowest values are found between the northern and middle branches of the SAF. Regions of enhanced Chl-a concentration and primary production are reported locally along the PF where this front interacts with bottom topography (Laubscher et al., 1993; Moore and Abbott, 2002), in coastal areas influenced by shelf sediments and glacial melt (Sullivan et al., 1993), in the vicinity of SO islands where

micronutrients are enriched through upwelling of iron-rich waters (e.g. Kerguelen Islands, Blain et al., 2007; Blain, 2008), in localized areas fertilized by dust input of iron (Erickson and Hernandez, 2003), and in the MIZ (El-Sayed and Taguchi, 1981; Garrison et al., 1987; Smith, 1987; Kang and Fryxel, 1993; Kang et al., 2001) where the input of freshwater from melting ice serves as a source of iron and increases water column stability (Eiken, 1992). Different phytoplankton groups characterize regions of different productivity and Chl-a regimes: while diatoms and the prymnesiophyte Phaeocystis antarctica are the dominant group and the main responsible of blooms in high-productivity areas, nanoplankton (i.e. 2–20 μm, Sieburth et al., 1978) and secondarily picoplankton (b2 μm) play a key role in the majority of open ocean HNLC waters (Fay, 1973; Weber and El-Sayed, 1987; Daly et al., 2001; Kang et al., 2001; El-Sayed, 2005; Wright et al., 2009). Such differences shape the trophic web, with the classic diatom–krill–whales chain in restricted bloom areas, and the nanoplankton–micro–zooplankton microbial chain in the major HNLC areas (El-Sayed, 1987). Early works (Hart, 1934, 1942; Beklemishev, 1964; Hasle, 1969) and subsequent latitudinal analysis of phytoplankton composition from discrete water samples (Kopczyńska et al., 1986; Burckle et al., 1987; Hardy et al., 1996; Eynaud et al., 1999; Kopczyńska et al., 2007; Chang et al., 2013; Patil et al., 2013) and net samples (Froneman et al., 1995), Chl-a concentration and pigment composition (Peeken, 1997; Wright and van den Enden, 2000; Enhert and McRoy, 2007; Takao et al., 2014) and genetic analyses (Wilkins et al., 2013; Wolf et al., 2014) in

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the Southern Ocean revealed a clear biogeographic distribution of phytoplankton species. These studies showed the dominant role of the PF as a clear boundary to the distribution of pico-, nano- and microplankton autotrophs. In particular, the PF marks the southern limit for calcareous nannoplankton and the northern limit for several siliceous taxa. This feature is well reflected in bottom sediments, where a clear shift from calcareous to the North to siliceous to the South is observed at the present PF position. South of the PF, phytoplankton composition in the AZ is controlled by the seasonal distribution of sea-ice, with different species inhabiting the POOZ, SIZ, MIZ and the ice environment itself. 3. Methods 3.1. Hydrography and remotely-detected data The position of the ACC fronts, as derived from Orsi et al. (1995) and adjusted along the cruise track (January 1–5) following Campanelli et al. (2011), is shown in Fig. 1. Sea surface chlorophyll-a data were obtained from NASA's Giovanni web site (http://disc.sci.gsfc.nasa.gov/techlab/ giovanni/), using monthly-averaged SeaWifs chlorophyll-a data from January 2005, averaged at 0.1° cells and used as input to the Ocean Data View software (ODV, Schlitzer, 2008) to obtain the surface distribution map on an orthographic south-polar projection. The same data were sub-sampled to produce a latitudinal profile of Chl-a concentration along the investigated transect (Fig. 2b). Data of sea ice extent were obtained from daily Advanced Microwave Scanning Radiometer (AMSR-E) data, downloaded from http:// www.iup.uni-bremen.de:8084/amsr, for January 5, 2005. Sea ice cover was manually drawn on the ODV map from the geo-referenced AMSRE image.

extensive geologic literature from Deep-Sea Drilling Project and Ocean Drilling Program. Data on Stephanocha speculum morphotypes and groups were partly recalculated from Malinverno (2010). For diatom counts, only intact cells with attached valves were considered. Species' identification follows mainly Schrader and Gersonde (1978); Scott and Thomas (2005). The frequent occurrence of colonies, observed in girdle view, required a major taxonomic effort, since commonly used diagnostic characters are based on valve view. Finally, hard-shelled microzooplankton (small planktonic foraminifera and tintinnids) was enumerated on the slides along with the other plankton groups, often without determination at species level. The taxonomy of Parmales and Archaeomonads follows Booth and Marchant (1987) and Deflandre (1932), respectively. All plankton counts (n) were transformed into cell concentrations (cells/L), through correction for the initial volume of filtered seawater (L), effective filtration area (A) and scanned area (a), using the following equation: cells=L ¼ n A=a 1=L: Cell concentrations of the major species are shown in Fig. 2 and data of all species are provided in Supplementary Table 1. Depending on the relative amount of filtered sea-water and scanned filter area, the detection limit was 25–160 cells/L for coccolithophores, 11–350 cells/L for silicoflagellates, dinoflagellates and microzooplankton, 30–2000 cells/L for diatoms, and 900–2000 cells/L for Parmales and Archaeomonads: for the latter two cases, it is worth to note that the high values of detection limit were obtained in samples where the amount of filtered seawater was lowest but cell abundance was highest.

3.2. Phytoplankton collection and analysis

3.3. Statistics

Water samples were collected from the ship's pump (mounted on the hull at approximately 3 m water depth and always kept running during the transit) on board R/V Italica, from December 31st 2004 to January 5th 2005 along a latitudinal transect (Fig. 1). A variable amount of water (0.5–4 L) was filtered on cellulose acetate filters (0.45 μm pore size, 47 mm diameter). Filters were oven-dried at 40 °C for 4 h and stored in plastic Petri dishes. One portion of the filter was mounted on a semi-permanent glass slide with a drop of microscope oil. The slides were scanned along radial transect through a BX/50 polarized light microscope at 100 × for cell identification and counting. Separate counts were performed on the same slide at crossed nicols for coccolithophores (crossed nicols, 1.9– 12.5 mm2, see Malinverno et al., 2016) and at parallel nicols for silicoflagellates (3.5–28 mm2, partly from Malinverno, 2010), dinoflagellates (3.5–28 mm2) and diatoms (0.3 to 30 mm2). For the analysis of smaller siliceous taxa, the identification of Emiliana huxleyi morphotypes (i.e. (Malinverno et al., 2016) and for high-resolution imaging of the most important species in selected samples (TR001, 003, 006, 007, 013,. 017, 023, 025, 031, 033, 037, 043, 044, 045, 046, 047, 049, 051, 055, 057), one portion of the filter was mounted on an aluminium stub with bi-adhesive graphite tape and sputtercoated with gold for SEM examination. Samples were analysed with a Vega Tescan SEM at the University of Milano-Bicocca. The identification and count of Parmales and Archaeomonads were performed on N100 fields of view at 5000 × (N0.36 mm2). Check counts of total Archaeomonads, without identification at species level, were performed at the light microscope on two samples: the obtained cell numbers were very similar to those obtained through SEM counts. For coccolithophore counts, only intact or slightly collapsed coccospheres were considered; species' identification follows the taxonomic concepts of Young et al. (2003), Jordan et al. (2004), Malinverno et al. (2008). Dinoflagellate species were identified following McMinn and Scott (2005). Silicoflagellates were identified following the

Bray–Curtis similarity was calculated with Primer 7 (v. 7.0.7) software for samples and variables after square-root transformation of the original data sheet. Only the first 50 most significant variables (species) are used by the programme to create the similarity matrix. Data matrices of sample and variable resemblance are provided as supplementary material (Supplementary Tables 2–3). A shade plot of the data was created through the matrix display wizard tool, where real data are shown with samples and variables arranged and clustered according to their similarity (Fig. 3). 4. Results The latitudinal distribution of taxa is described for the different hard-shelled phytoplankton groups considering their total abundance, major species' assemblage (Fig. 2c–h) as well as their relative contribution to the total hard-shelled phytoplankton assemblage (Fig. 2j). The contribution from mineralized micro-zooplankton is also shown (Fig. 2i). Background hydrological data are displayed (Fig. 2a, b) and the locations of the fronts and zones are indicated. The taxonomic composition of each group is described with particular focus on the latitudinal distribution of each significant taxon. 4.1. Dinoflagellates Armoured dinoflagellates (combined photo- and heterotrophic) are consistently present in the northern part of the section until the sACCf, with highest abundance throughout the sSAZ and the PF but never exceeding 1.2 × 103 cells/L (Fig. 2c). Overall, dinoflagellates make a negligible contribution to total hard-shelled phytoplankton throughout the transect, with percentage values from 0.5 to 1.5%. Among autotrophic taxa, Prorocentrum compressum is abundant throughout the transect, while Mesoporus perforatus was recovered in samples from the southern part of the sSAZ and from the PFZ. The


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autotrophic/mixotrophic genus Ceratium occurs throughout the transect: sporadic C. furca is present in the northern nSAZ, C. fusus occurs in the nSAZ and sSAZ and C. pentagonum makes an important contribution in the sSAZ and PFZ. The calcareous-walled genus Thoracosphaera is reported among minor species in a few samples from the nSAZ. The heterotrophic/mixotrophic genus Dinophysis occurs throughout the transect, with D. antarctica, D. lenticula and D. rotundata. The heterotrophic genus Protoperidinium occurs sporadically in the sSAZ and is abundant throughout the PFZ. Although it was enumerated at genus level under the light microscope, SEM investigation revealed P. antarcticum, P. incertum, P. incognitum and P. latistriatum. Siliceous endoskeletons of heterotrophic dinoflagellates, reported within minor species, include Achradina pulchra, recovered sporadically throughout the nSAZ, sSAZ and PFZ, and Actiniscus pentasterias found in one single sample at 55.9°S. 4.2. Coccolithophores Coccolithophore abundances are described by Malinverno et al. (2016). They are consistently present in the northern part of the transect, with variable abundance of 5–137 × 103 cells/L. Total coccolithophore abundance is highest in the PFZ and drops to near-zero values south of the sACCf, with sporadic occurrences until 67.7°S. They represent the majority of hard-shelled phytoplankton in the nSAZ and make an important contribution to total hard-shelled phytoplankton throughout the sSAZ and PFZ, where they represent 40–70% and 30–60%, respectively. The coccolithophore assemblages include 14 species/subspecies and 3 morphotypes, whose distribution is described in detail by Malinverno et al. (2016). Here we show (Fig. 2d) the combined abundance of E. huxleyi, Calcidiscus leptoporus subsp. leptoporus and the cumulative abundance of minor species throughout the transect. E. huxleyi is the major species driving total coccolithophore abundance and is represented by 3 morphotypes, whose distribution is strictly related to the location of the ACC fronts (Fig. 3 of Malinverno et al., 2016): type A (Plate 1c) is restricted to the nSAZ, type O (Plate 2c) is the dominant form in the nSAZ and decreases throughout the sSAZ to low abundance in the PFZ, while type B/C (Plate 3e) increases throughout the sSAZ and is dominant in the PFZ. C. leptoporus spp. leptoporus (Plate 1d) makes a significant contribution (3–24%) in the nSAZ and, although present throughout the sSAZ, represent only 1–2% of the total coccolithophore assemblage there. Minor taxa include Syracosphaera species (Plate 1f) and holococcolithophores (Plate 1g) which occur throughout the nSAZ and species with sub-tropical affinity (e.g. Umbellosphaera tenuis type II of Young et al., 2003) (Plate 1e), which only occurs in the northernmost samples. 4.3. Silicoflagellates Silicoflagellates are a minor phytoplankton component, accounting for b1% to 4% of total hard-shelled phytoplankton throughout the transect. They are scarce to absent in the nSAZ, display intermediate values (1–9 × 103 cells/L) throughout the sSAZ, the PFZ and across the PF, peak in the northern part of the AZ (up to 3 × 104 cells/L), then decrease southward. The silicoflagellate assemblages are represented by variable contribution of the genera Dictyocha and Stephanocha (former Distephanus,

Fig. 2. Background hydrographic and plankton data along the investigated latitudinal transect: a) Sea Surface Temperature (from Campanelli et al., 2011), b) satellite-detected sea surface Chlorophyll-a concentration (as in Fig. 1), c–h) abundance of the major hardshelled phytoplankton groups and major species' contribution: c) Dinoflagellates; d) Coccolithophores (redrawn after Malinverno et al., 2016); e) Silicoflagellates; f) Diatoms; g) Parmales; h) Archaeomonads; i) combined abundance of mineralized micro-zooplankton groups; j) combined abundance of all hard-shelled phytoplankton groups. Oceanographic fronts (thick grey lines) and zones and the average position of the WSI limit, as in Fig. 1, are indicated at the top along with SST values (dashed lines).

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Fig. 3. Shade plot of square-root-transformed data showing similarity clustering of samples and variables (first 50 species). Clusters of samples are colour-shaded and the corresponding oceanographic zone is indicated. Sub-clusters are indicated as “a” and “b”. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

i.e. Jordan and McCartney, 2015). Dictyocha is represented by two large (40–50 μm) species, D. aculeata cf. (see discussion in Barron et al., 2005) and to a lesser amount D. stapedia, plotted cumulatively in Fig. 2e. They form the majority (50–100%) of silicoflagellates assemblages in the nSAZ, 43–50% in the sSAZ until 56.9°S, then decrease within the PFZ and disappear just before the PF. Stephanocha speculum is consistently present south of the NSAF and shows peak abundances in the AZ, also with abundant double skeletons which represent the division stage (McCartney et al., 2014). Within S. speculum, Malinverno (2010) distinguished different morphotypes, which display a significant latitudinal distribution pattern. We plot here the combined abundance of pentagonal, hexagonal, heptagonal, open and aberrant forms as S. speculum s.s., the sum of species possessing one or two apical spines, i.e. var. monospicata and var. bispicata, the abundance of large S. speculum (group B of Tsutsui et al., 2009) and the combined abundance of pentagonal, hexagonal, heptagonal, open, aberrant and var. minuta forms possessing coronatid ornamentation as var. coronata. As already pointed out by Malinverno (2010) (but increased sample resolution is provided here), while S. speculum s.s. dominates throughout the sSAZ, PFZ and the northern part of the AZ, var. monospicata and var. bispicata have their maximum concentration and contribution (19–31%) in the northern AZ, while var. coronata clearly dominates (58–88%) in the southern AZ.

northern part of the AZ until the sACCf, while F. curta, and F. cylindrus (maximum values of 2.2 and 6.3 × 105 cells/L, respectively) dominate in the AZ south of the sACCf, along with minor contribution of F. obliquecostata, F. ritscherii, F. rhombica, F. separanda and F. pseudonana. Among major taxa, Chaetoceros colonies (max. 4.4 × 104 cells/L, mainly C. dichaeta) are abundant across the PF and the northern part of the AZ, while Pseudonitzschia heimii (Fig. 2f) and Thalassiothrix antarctica (not plotted) occur with low abundance in the PFZ and peak in the AZ. Minor species also show a clear biogeographic distribution. Actinocyclus exiguus, Corethron spp., Nitzschia bicapitata, N. sicula var. bicuneata, Thalassionema nitzschioides and Thalassiosira oestrupii are present throughout the sSAZ and PFZ; Thalassiosira gracilis (var. gracilis and var. expecta), the ProbosciaRhizosolenia group, Dactyliosolen antarcticus and Asteromphalus hookerii also make a contribution in the PFZ. Finally, Nitzschia leicontei, T. gracilis, T. oestrupii, Corethron spp., the Proboscia-Rhizosolenia group, A. hookerii (northern part) and A. hyalinus (southern part), T. nitzschioides, D. antarcticus, several species of Thalassiosira (T. ambigua, T. angulata, T. eccentrica, T. gravida, T. tumida), Azpeitia spp. (listed with decreasing abundance) and sporadic A. actinochilus and E. antarctica form the “minor species” of the AZ.

4.4. Diatoms

Parmales were recovered exclusively from the AZ, showing increasing abundance southward with peak concentration (3.3 × 105 cells/L) in the southernmost sample. Overall, they contribute from 2 up to 27% (with increased contribution southward) to the total mineralized phytoplankton in the AZ (Fig. 2h, j). The abundance pattern of Parmales is dominated by Triparma laevis subp. ramispina (50–100%) throughout the AZ with a minor contribution (with decreasing abundance) from T. laevis subsp. pinnatilobata, T. laevis s.s., T. laevis subsp. inornata, Tetraparma pelagica and T. columnacea subsp. alata.

Diatoms are the overall most abundant mineralized phytoplankton group throughout the section. They show very low abundance in the nSAZ, then increase in both total and relative abundance (22 up to 60%) throughout the sSAZ and in the PFZ (40–63%) and peak (1.2 × 106 cells/L) south of the sACCf, representing the dominant group (72–98%) of the total hard-shelled phytoplankton assemblage in the AZ (Fig. 2f, j). A total of 50 species, 3 varieties and 3 forms, represented by solitary and chain-forming taxa, were recovered as intact frustules along the transect. The genus Fragilariopsis is by far the most represented and consists of several species. Among them, F. kerguelensis (maximum values of 1.1 × 105 cells/L, Fig. 4d) is dominant in the sSAZ, the PFZ and in the

4.5. Parmales

4.6. Archaeomonads Although Archaeomonads represent the resting stage of some unknown Chrysophytes, we consider their abundance along with that of mineralized phytoplankton (Fig. 2g, j) due to their distinct


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Plate 1. Phytoplankton of the northern Subantarctic Zone: a) b) c) d) e) f) g) h) i) j) k) l)

General sample view with rare Fragilariopsis kerguelensis (Fk) and abundant Emiliania huxleyi (Eh) — TR001; Sparse diatom fragments, E. huxleyi (Eh), Calcidiscus leptoporus (Cl) and abundant free E. huxleyi coccoliths — TR001; E. huxleyi type A — TR001 C. leptoporus spp. leptoporus — TR001 Umbellosphaera tenuis type II of Young et al. (2003) — TR001 E. huxleyi type A (top) and type O (bottom), slightly etched — TR003 Syracosphaera borealis — TR001 Corisphaera strigilis — TR003 Prorocentrum compressum — TR013 Dictyocha stapedia — TR013 D. aculeata cf. — TR013 Actinocyclus exiguus — TR001.

biogeography and high preservation potential. Archaeomonads were encountered exclusively in the Antarctic Zone, with peak values (4 × 104 cysts/L) in the southern part of the transect. The only two

species encountered, Archaeomonas areolata and Litheusphaerella spectabilis, show overlapping distribution and contribute up to 7% to the stock of total hard-shelled phytoplankton.

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Plate 2. Phytoplankton of the southern Subantarctic Zone: a) b) c) d) e) f) g) h) i) j) k) l)

General sample view with Chaetoceros spp. (Ch), Fragilariopsis kerguelensis (Fk), Calcidiscus leptoporus and a radiolarian — TR023 Dactyliosolen antarcticus (Dan), F. kerguelensis (Fk), Stephanocha speculum (Ss), Emiliania huxleyi (Eh) — TR023 E. huxleyi type O, bilayered coccosphere — TR015 Ceratium fusus (Cfus) with Chaetoceros sp. (Ch), F. kerguelensis (Fk), D. antarcticus (Dan), S. speculum (Ss) — TR017 Ceratium furca — TR015 C. pentagonum — TR025 S. speculum (Ss) and E. huxleyi (Eh) — TR025 T. tumida T. perpusilla T. perpusilla F. rhombica T. oestrupii.


Plate 3. Phytoplankton of the Polar Frontal Zone: a) b) c) d) e) f) g) h) i) j) k) l)

General sample view with Chaetoceros spp. (Ch), Fragilariopsis kerguelensis (Fk), Asteromphalus sp. (Ast), Dactyliosolen antarcticus (Dan), Corethron sp. (Cor) — TR044 General sample view with Chaetoceros spp. (Ch), F. kerguelensis (Fk), Proboscia inermis (Pi), Asteromphalus sp. (Ast), D. antarcticus (Dan) — TR043 Azpeitia tabularis (At), F. kerguelensis (Fk), Chaetoceros spp. (Ch), Thalassiotrix sp. (Tst), N. leicontei (Nl), S. speculum (Ss) — TR043 Chaetoceros (Ch) and F. kerguelensis (Fk) colonies — TR044 E. huxleyi type B/C — TR043 Codonellopsis pusilla (Cp) and (part of) D. antarcticus (Da) — TR037 Stephanocha speculum (Ss), F. Kerguelensis (Fk), Chaetoceros spp. (Ch), Nitzschia spp. (N) — TR043 Protoperidinium sp. (P) with Chaetoceros spp. (Ch) and S. speculum (Ss) — TR033 Protoperidinium latistriatum — TR043 Proboscia inermis (Pi), Nitschia sicula var. bicuneata (Nsb) — TR044 T. lentiginosa (Th), A. heptactis (Ast), A. tabularis (At), F. kerguelensis (Fk).

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4.7. Hard-shelled micro-zooplankton Planktonic foraminifera were observed in our filter samples throughout the transect, showing increased abundance in the PFZ. The tintinnid species Codonellopsis pusilla, whose agglutinated lorica is typically made up of coccoliths, was found in the PFZ and across the PF. The sampling method, designed for phytoplankton study, is clearly unsuitable for the enumeration of micro-zooplankton, which occurs at lower cell numbers and is often most abundant at the sub-surface. Our counts thus result in a likely underestimation and possibly lack of detection of micro-zooplankton. Keeping this limitation in mind, we report here the combined occurrence of planktonic foraminifera and the agglutinated tintinnid species (Fig. 2i) to show their match with the pattern of mineralized phytoplankton abundance. 4.8. Statistical analyses Samples were grouped by their Bray–Curtis similarity values (Supplementary Table 1) into four clusters separated at N 55% similarity. The four clusters are geographically consistent and identify the nSAZ, sSAZ, PFZ and AZ, with the boundary in coincidence with the ACC fronts. Sub-clusters within the nSAZ and the AZ, showing similarity values N60%, are geographically separated. Clustering of species by similarity (Supplementary Table 2) returns their latitudinal distribution and reflects their affinity for the ecological factors associated with each zone. The contribution of each of the first 50 species within each cluster is shown in the shade plot (Fig. 3), where samples and variables are arranged according to their similarity percent values. 5. Discussion Statistical sample clustering into distinct bioprovinces corresponding to hydrographic zones confirms, at least along the investigated transect, the role of ACC fronts as biological boundaries, as also shown by the pattern of remotely-sensed chlorophyll-a data (Sokolov and Rintoul, 2007). As shown by early (e.g. Hart, 1942; Hasle, 1969) and following studies, different phytoplankton assemblages characterize different water masses across the ACC. While the PF is long known as the main boundary separating a carbonate-dominated Subantarctic Zone from a silica-dominated AZ, with a distinct signature in bottom sediments (Bohaty and Harwood, 1998 among others), we found all the ACC fronts to exert a strong control on the distribution of extant mineralized species. The palaeoceanographic potential of mineralized plankton made them the target of investigations focusing on their extant latitudinal distribution in relation to frontal hydrography. However, most such studies focused on a single taxonomic group, such as coccolithophores, i.e. (Hasle, 1960; Nishida, 1986; Findlay and Giraudeau, 2000; Cubillos et al., 2007; Mohan et al., 2008; Malinverno et al., 2016), diatoms (Smetacec et al., 2002; Tremblay et al., 2002; Olguín and Alder, 2011) or silicoflagellates (Malinverno, 2010), while only a few surveys (Eynaud et al., 1999; Hinz et al., 2012) dealt with more than one fossilizable phytoplankton group. Early studies did not focus on morphological variations at sub-species level within broadly-distributed phytoplankton species, which are instead often correlated with gradients in physico-chemical water properties (see, among others, Findlay and Giraudeau, 2000; Cubillos et al., 2007; Henderiks et al., 2012; Malinverno et al., 2016 for E. huxleyi, Malinverno, 2010 for S. speculum). Such morphotypes, either genetically-based (i.e. Young and Westbroek, 1991; Hagino et al., 2011) or phenotypically controlled, provide a further tool for palaeoceanographic reconstructions and will

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be discussed below. Moreover, siliceous nanoplankton (Parmales) has been often overlooked in previous phytoplankton literature due to the limitation of their detection under the LM. Although specific work has addressed their diversity and abundance in the SO (Silver et al., 1980; Booth and Marchant, 1987; Kosman et al., 1993), their contribution to the rest of mineralized plankton is rarely reported (Nishida, 1986; Hedoin and Couté, 1992; Hinz et al., 2012), even though they are known to be preserved in bottom sediments (Franklin and Marchant, 1995; Zielinski, 1997). Similarly, the distribution of siliceous chrysophyte stomatocysts (Archaeomonads) is rarely reported in wholeplankton studies (Riaux-Gobin et al., 2011), although their restricted geographic distribution and their preservation in bottom sediments make them a strong palaeoceanographic proxy (Mitchell and Silver, 1982). The hydrographic boundaries, species assemblage composition and likely ecological forcing of each bioprovince are discussed in the following paragraphs. 5.1. The northern and southern Subantarctic Zones and the Subantarctic Front Bray–Curtis similarity values clearly separate the SAZ into a northern and a southern bioprovince, divided by the NSAF. The surface expressions of this front roughly correspond to the 8 °C isotherm which appears to be a limit for cold-water diatom species (e.g. F. kerguelensis). Overall, the absolute abundance of siliceous taxa and their contribution to the mineralized plankton stock is negligible in the nSAZ (Plate 1), which appears to be dominated by coccolithophores, and slightly higher in the sSAZ, with the transition occurring at the NSAF location. Temperature represents a limiting factor also for coccolithophores: species of sub-tropical affinity: U. tenuis type II and Acanthoica quattrospina occur only in the northernmost samples, as also reported by Eynaud et al. (1999) for a transect in the Atlantic sector. The range of other coccolithophore species with affinity for colder waters is strongly controlled by the NSAF: Syracosphaera borealis and Corisphaera strigilis are restricted to the nSAZ (Malinverno et al., 2016: see Syracosphaera spp. and holococcolithophores in Supplementary Table 1) and C. leptoporus subsp. leptoporus, although persistent southward, displays very low abundance in the sSAZ. Given the low abundance and low preservation potential of minor coccolithophore species, we plot the relative abundance of C. leptoporus subsp. leptoporus to E. huxleyi as a potential palaeo-indicator of the NSAF position: values N5% up to 30% are distinctive of the nSAZ. Within E. huxleyi, the NSAF also marks the southern limit for the distribution of type A along this transect (Fig. 3 of Malinverno et al., 2016) as reported before (Hiramatsu and De Dekker, 1996; Findlay and Giraudeau, 2000; Cubillos et al., 2007; Mohan et al., 2008) at similar latitudes. Careful identification of E. huxleyi morphotypes in sediments could thus contribute to indicate the southern extension of the nSAZ. Within the nSAZ, two sub-clusters are identified on the basis of % similarity. These separate northern and southern samples, with the exception of sample TR005 which is grouped with southern samples and samples TR006-7 which are grouped with northern samples. Such grouping is due to a higher abundance of major coccolithophore species (E. huxleyi and C. leptoporus subsp. leptoporus) in sub-cluster a than in sub-cluster b. Notably, this difference is also detected in the pattern of sea surface Chl-a (Fig. 1), although the footprint of satellite-detected chlorophyll is much wider than our discrete samples: sample TR005 lies in a patch of low-Chl-a while samples TR006-7 show higher Chl-a values, as the other samples of sub-cluster a. This feature is probably related to the entrainment of nutrient-rich waters in eddies that develop

Fig. 4. Plot of selected species and indices of palaeoceanographic significance along the investigated latitudinal transect: a) ratio of C. leptoporus spp. leptoporus to E. huxleyi; b) ratio of Dictyocha to Stephanocha; c) Diatom ecological groups (following Armand et al., 2005; Crosta et al., 2005; Romero et al., 2005); d) absolute and relative abundance of F. kerguelensis within the diatom assemblage; e) ratio of F. curta + F. cylindrus (following Gersonde and Zielinski, 2000); f) Corona-index; g) ratio of Archaeomonads to diatoms (excluding major thinly silicified species, F. cylindrus and F. curta). Oceanographic fronts (thick grey lines) and zones, SST values and average position of the WSI limit as in Fig. 2.

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south of the Subtropical Front. Although no nutrient measurements are available in this portion of the transect, the increase in E. huxleyi and C. leptoporus subsp. leptoporus, which are known to respond to nutrient increase under suitable light and temperature conditions, supports this inference. The sSAZ (Plate 2) is characterized by comparable abundances of coccolithophores and diatoms and increased contributions of silicoflagellates and dinoflagellates to the mineralized phytoplankton. This zone displays rather uniform characteristics, with no distinct geographic grouping of sub-clusters. Among diatoms, species with temperate as well as cold-water affinity (Fig. 4c, grouping after Crosta et al., 2005; Romero et al., 2005) characterize the sSAZ. Among coccolithophores a variable contribution of E. huxleyi type O and B/C forms an almost monospecific assemblage (Fig. 3 of Malinverno et al., 2016), with negligible contribution of C. leptoporus subsp. leptoporus. Silicoflagellates show a slight increase in abundance within this zone and their assemblage is formed by both the cold-water species S. speculum and the temperate species D. aculeata and D. messanensis. The ratio of the two genera Dictyocha to Stephanocha has been applied in the past as a palaeotemperature proxy (Ciesielski, 1974; DeFelice and Wise, 1981; Pichon et al., 1987) corresponding to a main shift observed at the PF in surface bottom sediments. Along our transect, this ratio (Fig. 4b) shows a step-like pattern from very high but discontinuous values in the nSAZ (very low abundance of silicoflagellates there), stable values around 45% throughout the sSAZ, a drop to 5% in the southern part of the sSAZ and a further drop at the SSAF. Notably, although the southern distribution of Dictyocha is constrained by the PF (Supplementary Table 1), the most significant shift in the Dictyocha to Stephanocha ratio is recorded further north, within the nSAZ. Although a temperature control is a likely explanation, water column stability can also play a role in controlling the shifts in the two silicoflagellates genera. S. speculum is typical of nutrient-rich upwelling settings, while D. messanensis is adapted to low-nutrients conditions and D. aculeata prefers well-stratified water masses (Barron and Bukry, 2007a). The upper ocean thermal structure along our transect (Fig. 3c of Campanelli et al., 2011) shows the emergence of the 5 °C deep (N 300 m) isotherm at the SSAF but a tilt of isotherms is observed further north, consistent with the observed switch in silicoflagellates genera. To the south, the SSAF marks the transition to the PFZ. Although this front is not a circumpolar feature but forms typically south of New Zealand as a branch of the SAF, due to the interaction with bottom topography, it also corresponds to important floristic changes. A shift to higher total phytoplankton concentrations corresponds, among diatoms, with the persistence of low abundance of temperate taxa and the increase in cold-water taxa (Fig. 4c). Among coccolithophores, minor taxa nearly disappear while E. huxleyi peaks. 5.2. The Polar Frontal Zone and the Polar Front The Polar Frontal Zone (Plate 3) is characterized by overall high stocks of mineralized plankton as compared to the sSAZ and by peak values of heterotrophic micro-zooplankton, including dinoflagellates of the genus Protoperidinium, planktonic foraminifera and the tintinnid species C. pusilla. The statistical analysis showed a high (N70%) percentage of similarity among all samples of this zone. Coccolithophores are represented by a monospecific assemblage of E. huxleyi (Fig. 2) and Malinverno et al. (2016) showed that populations are dominated by type B/C with a small contribution of type O. Notably, the lorica of C. pusilla recovered from this zone is formed by coccoliths of the former type (Plate 3f). Silicoflagellates are featured by an almost monospecific assemblage of S. speculum (Dictyocha to Stephanocha ratio b1) but still contribute little to the total stock of mineralized phytoplankton. The diatom assemblage shows the persistence of taxa with sub-tropical affinity and an increase of cold-open ocean taxa (ecological grouping following Crosta et al., 2005; Romero et al., 2005). Large colonies of Chaetoceros spp. and F. kerguelensis along with other large taxa (Proboscia-Rhizosolenia group, D. antarcticus and A. hookerii) are the

most evident feature in this zone, as also pointed out by Tremblay et al. (2002). In particular, F. kerguelensis is the main contributor to the open-ocean SO diatom ooze belt (Burckle and Cirilli, 1987) in the underlying bottom sediments. Although the PFZ is indicated as a region of enhanced primary production and Chl-a concentration at some places (Moore and Abbott, 2002), the floral assemblages along our transect indicate indeed higher but not really eutrophic conditions. The latitudinal pattern in surface macro-nutrients concentration along the transect (Campanelli et al., 2011) evidences that nitrite–nitrate and orthophosphates are available, but orthosilicates are possibly limiting diatom growth: Campanelli et al. (2011) comment that comparison with nutrient measurements from early and late November supports the progressive utilization of the silicate pool during the preceding bloom season. Peak values of E. huxleyi in this zone support diatom bloom limitation by either silica or micronutrients. Typical features like low percent of bilayered coccospheres and a low percentage of free coccoliths/ coccosphere indicate that E. huxleyi populations are actively growing in this zone (Malinverno et al., 2016). Nonetheless, the increased micro-zooplankton abundance throughout this zone suggests that grazing pressure can be another factor that keeps phytoplankton standing stocks relatively low. The PF coincides along this transect with a SST of about 3 °C and a sharp SST gradient. Its location at 62.8°S places it very close to the sACCf. Although the PF is defined as the front that separates the Subantarctic and Antarctic zones (Orsi et al., 1995) and major shifts in extant diatom and total phytoplankton assemblages have been previously observed at this location, our assemblages indicate that a major transition corresponds with the location of the sACCf. Notably, samples TR042, 043, 044, at the PF and within the POOZ, share common characters with samples of the PFZ, so that they cluster together with a similarity N 70% (Fig. 3). Malinverno et al. (2016) reviewed previous data on the southernmost extent of E. huxleyi in the SO and showed that, irrespective of the latitudinal front position in the different zonal sectors of the SO, this species is constrained, since it was first reported (e.g. Hasle, 1960), by the location of the sACCf. Upwelling of circumpolar deep water occurs at the sACCf, where the 2 °C isotherm surfaces from deep (N500 m) layers, so that an increase in macronutrients is expected in this region. Indeed, Campanelli et al. (2011) showed increased silicate values south of this front, although iron is often reported as a limiting micronutrient in upwelled waters and no micronutrients are likely provided in this area by shallowbottom-influenced upwelling waters or atmospheric input. 5.3. The Antarctic Zone The most striking feature of the AZ is the strong dominance of Fragilariopsis curta and F. cylindrus in the diatom assemblage, which corresponds to a significant increase in total diatom abundance and their prevalence over other phytoplankton groups (Fig. 2f, j). The switch in dominance between F. kerguelensis and F. curta + F. cylindrus occurs at the sACCf (Fig. 4d, e). Data from surface sediments (Armand et al., 2005; Crosta et al., 2005; Gersonde et al., 2005; Zielinski and Gersonde, 1997) showed that the latter species are typically associated with the presence of seasonal sea-ice, in contrast with the open ocean character of F. kerguelensis. High concentrations of extant F. curta and especially F. cylindrus are found in fact within sea-ice and in adjacent open waters of the MIZ (Kang and Fryxel, 1992; Cefarelli et al., 2010; Hinz et al., 2012). Sediment-trap work (Gersonde and Zielinski, 2000) identified the % abundance of F. curta and F. cylindrus as a possible proxy for the reconstruction of winter sea-ice extension, based on the observation that maximum fluxes of these species were observed during summer blooms in areas that were covered with sea-ice during winter and thus implying a possible seeding effect from sea-ice to the open ocean (see also Garrison et al., 1987; Garrison et al., 1993). The abundance of sea-ice diatoms (Fig. 4c, ecological grouping following Armand et al., 2005) is driven by F. curta + F. cylindrus, as shown by the curve of percentage values of these species in the total diatom abundance (Fig. 4e). A contribution of


other Fragilariopsis species (F. obliquecostata, F. ritscheri, F. rhombica, F. separanda, F. pseudonana), a minor contribution of Thalassiosira species (T. tumida, T. gravida, indicated as typical of the SIZ in Bianchi et al., 1992) and Asteromphalus spp., and rare occurrences of A. actinochilus and E. antarctica add to the sea-ice diatom group south of the sACCf, confirming, along our transect, the coincidence of the average location of the WSI limit with the position of the sACCf. The seasonallyretreating pack-ice is thus a likely source of iron in the SIZ. Indeed, satellite Chl-a values show peak concentrations in the region south of the sACCf-WSI. For the application to palaeo-records, selective opal dissolution has to be considered. Burckle et al. (1987) and Abelmann and Gersonde (1991) report that the thinly-silicified sea-ice species are poorly preserved in silica-undersaturated bottom waters, where a high dominance of F. kerguelensis in the sediment under the MIZ badly compares with its low abundance in the plankton (Zielinski and Gersonde, 1997; Zielinski et al., 1998). The switch in dominance between Fragilariopsis species, appearing as the most striking signal in the plankton, is thus biased by the preservation potential in the sediments. The importance of other proxies of sea-ice is discussed in the following paragraphs. Within the AZ, Bray–Curtis similarity values separated two subclusters. Sub-cluster a (Plate 4) corresponds to the SIZ, while subcluster b (Plate 5) can be interpreted as a latitudinally-extended MIZ proper, cutting through the belt of pack ice and corresponding to the northern branch of the Ross Sea gyre (south of the Bdy). Although F. curta and F. cylindrus are abundant throughout the whole AZ south of the sACCf, indicating their control by winter sea-ice over ice-free summer waters, minor species contribute to the sample sub-clustering. Peak concentrations of the silicoflagellate S. speculum (up to 4% of total mineralized phytoplankton) are observed in AZ sub-cluster a and support upwelling conditions there (Barron and Bukry, 2007b). Notably, two morphotypes, characterized by the presence of one or two apical spines, S. speculum var. monospicata and var. bispicata, although not restricted to this zone, show peak values here (Fig. 2e). Within AZ subcluster b, S. speculum is dominated by var. coronata, a morphotype with a crown of apical spines (complete or incomplete, e.g. 4 to 6 spines in hexagonal forms; Plate 5j). The importance of this morphotype in extant silicoflagellate populations of the Southern Ocean was first pointed out by Malinverno (2010), who showed a distinct increase of S. speculum var. coronata southward of 67°S, coinciding with the Bdy and with temperatures b−1 °C. It is worth to note that coronatid ornamentation, although often overlooked, is a recurring bipolar feature: S. speculum populations of the subarctic Pacific (as shown in Plate 1 of Onodera and Takahashi, 2012) and of the Ross Sea/Terra Nova Bay polynyas (Malinverno, pers. obs.) display such a characteristic trait. We speculate therefore that coronatid ornamentation is a typical character in open waters influenced by sea-ice. The Corona-index (Fig. 4f), defined by the ratio of coronatid to non-coronatid S. speculum, is thus proposed as a new proxy for the MIZ. Such proxy has the advantage that coronatid and non-coronatid forms have the same preservation potential, as their skeletons are similarly silicified, so that their ratio is not affected by selective dissolution. A survey in surface sediments should however confirm the applicability of this proxy to the palaeo-records. Simultaneous variations in multiple morphological characters characterize S. speculum towards high-latitudes, such as an increase of heptagonal (Takahashi et al., 2009), open and aberrant forms (van der Spoel and Hallegraeff, 1973), as well as forms of var. minuta with a wide apical ring (Barron et al., 2009; Bukry, 1981). Coronatid ornamentation occurs in all such forms poleward along the investigated transect (Malinverno, 2010) and is thus identified as the most consistent feature. Whether this character is phenotypic or genotypic could be addressed by genetic analyses. However, morphometric analyses by Malinverno (2010) indicated that coronatid forms are generally larger and/or possess a wider apical ring as compared to specimens of S. speculum s.s, var. monospicata and var. bispicata, although some overlap in size and apical ring width

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occurs and rare forms with three apical ring spines have been recovered, thus suggesting that also coronatid ornamentation is a gradational (phenotypic) character. Within the MIZ, Parmales and Archaeomonads show peak values, contributing significantly to the total stock of mineralizing plankton (up to 27% and 7%, respectively). These groups are detected also in AZ sub-cluster a, immediately south of the sACCf, but in very low concentrations. Parmales are a group of small silicified autotrophs (Taniguchi et al., 1995) belonging to the Bolidophycea (Ichinomiya et al., 2011, 2013), a close sister group to diatoms (Bacillariophycaea). Their importance within high-latitude SO small-sized nanoplankton has been highlighted by Nishida (1986), Wright et al. (2009) and Hinz et al. (2012). Their morphological variability in both northern and southern high latitudes has been described (Silver et al., 1980; Booth et al., 1981) and their taxonomy has been formally established by Booth and Marchant (1987). Observations from bag cultures of Parmales (Taniguchi et al., 1995), in-situ populations and isolates of T. laevis (Ichinomiya et al., 2013; Yamada et al., 2014) indicate that Parmales respond positively to increased silicate levels. Although several species of Parmales have been described from tropical and subtropical settings (Kosman et al., 1993; Bravo-Sierra and Hernández-Becerill, 2003), T. pelagica occurs in high-latitudes of both hemispheres (Hedoin and Couté, 1992; Komuro et al., 2005; Hinz et al., 2012) and T. laevis is typically associated with sea-ice environments (Silver et al., 1980; Buck and Garrison, 1983; Nishida, 1986; Kosman et al., 1993). Different subspecies are recognized within T. laevis, some of which are specific to either hemisphere (e.g. Booth and Marchant, 1987; Konno et al., 2007). The siliceous plates of Parmales are reported in sinking fluxes from the Ross Sea (Accornero et al., 2003) and in bottom sediments close to the Antarctic continent (Franklin and Marchant, 1995; Zielinski, 1997) so that their use as indicators of cold-waters and past sea-ice environment has been proposed. In our samples, T. laevis subsp. ramispina represents the majority of the recovered species, increasing throughout the SIZ and peaking in the MIZ. T. laevis subsp. pinnatilobata shows the same trend but with significantly lower abundance. T. pelagica makes a minor contribution to the overall standing stock of Parmales and has been recovered sporadically throughout the whole AZ south of the sACCf. Although the recovery of Parmales plates in sediments is limited by the use of electron microscopy and information of their preservation potential is lacking, our data confirm that Parmales and in particular the abundance of T. laevis s.l. (subspecies are mostly defined by ornamentation of girdle plates, while shield plates are hardly diagnostic at subspecies level) can be an efficient proxy to trace the extension of the MIZ. Archaeomonads, the resting stages of unknown Chrysophytes, are also known to be associated with sea-ice (Mitchell and Silver, 1982). The life cycle of Archaeomonads is unknown and the factors triggering cyst formation have not been assessed. Whether cysts form as a response to enclosure in sea-ice (Mitchell and Silver, 1982) or are harvested from the open water by growing ice crystals (Mitchell and Silver, 1986), their occurrence in the plankton is always associated with the ice-edge zone and sea-ice (Krebs et al., 1987; Garrison et al., 1993; Roberts et al., 2007). In such areas, they make a significant contribution to the sediment flux (Abelmann and Gersonde, 1991; Riaux-Gobin et al., 2011) and are recorded in bottom sediments. The siliceous wall of Archaeomonads is rather thick and thus has a high preservation potential (Malinverno, pers. obs. from sediment cores). We chose to plot their abundance versus total concentration of highly-silicified diatoms (i.e. excluding the dominant F. curta and F. cylindrus as well as minor lightly-silicified diatom species, Fig. 4g) as a proxy to apply to the palaeo-records. The curve shows peak values (15–50%) in the MIZ, but some contribution (2–3%) is evident throughout the SIZ within the southern portion of the ACC. 6. Conclusions Our latitudinal survey showed that significant species' shifts occur across the frontal boundaries of the ACC and in correspondence with

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Plate 4. Phytoplankton from the Antarctic Zone–Seasonal Ice Zone a) b) c) d) e) f) g) h) i) j) k) l)

General sample view with Chaetoceros spp. (Ch), Fragilariopsis curta (Fcu), Thalassiosira gravida (Th), Proboscia inermis (Pi), Stephanocha speculum (Ss) — TR049 General sample view with Chaetoceros spp. (Ch), Corethron pennatum (Cp), F. kerguelensis (Fk), S. speculum (Ss) — TR049 T. frenguellii (Th) and Nitschia sicula var. bicuneata (Nsb) T. gravida F. rhombica T. dichotomica T. gracilis S. speculum var. monospicata; T. gravida (Th), Chaetoceros sp. (Ch) — TR045 S. speculum var. bispicata, double skeleton, Proboscia inermis (Pi), F. curta (Fcu), F. cylindrus (Fcy) — TR045 Tetraparma pelagica — TR049 Triparma laevis subsp. inornata — TR049 Triparma laevis subsp. pinnatilobata cf. — tr047.

the winter sea-ice limit. Sample clustering by their percent similarity defined four bioprovinces that coincide with the oceanographic zones as defined by physical property changes, namely the nSAZ,

sSAZ, PFZ, and AZ. One significant difference from previous records is the important role of the sACCf, as opposed to the PF, as a boundary in species' distribution: samples from the permanently open-


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Plate 5. Phytoplankton from the Antarctic Zone–Marginal Ice-edge Zone a) b) c) d) e) f) g) h) i) j) k) l)

General sample view with Asteromphalus hookeri (Aho), Thalassiosira sp. (Th), Stephanocha speculum var. coronata (Ssc) — TR055 General sample view with A. hookerii (Aho), A. heptactis (Ahe), Proboscia inermis (Pi), Dactyliosolen antarcticus (Dan), Fragilariopsis curta (Fcu); S. speculum (ss) — TR053 F. curta (Fcu), F. cylindrus (Fcy), Archaeomonas areolata (Aa), Triparma laevis subsp. ramispina (Tlr) — TR051 F. curta colony T. frenguellii T. gravida T. poroseriata Triparma laevis subsp. ramispina (distal view) — TR051 T. laevis subsp. ramispina (detail of ventral view) — TR051 Stephanocha speculum var. coronata; F. cylindrus (Fcy) and T. laevis subsp. ramispina (Tlr) — TR053 Archaeomonas areolata — TR051 Lithuesphaerella spectabilis — TR051.

ocean AZ cluster together with samples from the PFZ at 70% similarity values and are clearly separated from the other samples of the AZ.

Overall, as general phytoplankton studies underline the importance of nano- and picoplankton to total phytoplankton stocks, our combined abundance data show that, also within mineralized phytoplankton, an

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important contribution is due to small-sized species. This functional role is carried out, across the ACC and the AZ, in a sort of latitudinal relay from E. huxleyi to F. cylindrus to T. laevis. Although our data are limited in time and space, they provide an insight into previously-defined proxies and allowed to propose a new proxy for the MIZ, the Corona-index. Along sediment successions, selective dissolution of carbonate vs. siliceous skeletons or thickly- vs. thinlysilicified skeletons requires the use of a multi-proxy approach for the correct identification of the palaeo-position of ACC fronts and the winter sea-ice limit where bottom waters are undersaturated. The increase in F. kerguelensis abundance, the drop in C. leptoporus subsp. leptoporus to. E. huxleyi ratio and the disappearance of E. huxleyi type A identify the NSAF. The shift in dominance between Dictyocha and Stephanocha characterizes the SSAF rather than the PF and is accompanied by an increase of the cold open-ocean compared to temperate diatom species. The shift in abundance between Fragilariopsis species occurs at the sACCf, where a drop of E. huxleyi and a slight increase of sea-ice proxies occurs. The Corona-index, likely unaffected by selective dissolution, applied in combination with the abundance of Archaeomonads and Parmales, potentially provides a robust constrain of the MIZ position. Author contributions E.M. collected the samples, analysed Coccolithophores, Silicoflagellates, Dinoflagellates, Parmales and Archaeomonads; P.M. and K.G. analysed the diatoms; E.M. led the writing. Acknowledgements Water samples analysed for this study were collected during the XX Italian Antarctic Expedition on board R/V Italica (Austral Summer 2004-2005) within ABIOCLEAR Project (Antartic BIOgeochemical cycles-Climatic and palEoclimAtic Recostructions), funded by the Italian PNRA (Piano Nazionale di Ricerca in Antartide, project 2004/8.06). Comments from the Editor and two anonymous reviewers strongly contributed to the improvement of the manuscript. E.M. is grateful to Helge Thomsen and Ric Jordan for stimulating discussion on Southern Ocean phytoplankton. Thanks to R. Cristina-Reggiani and P. Gentile for technical assistance with SEM operations in Milano-Biccoca. Map. KMZ file containing the Google map of the most important areas described in this article. Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.marmicro.2016.01.001. These data include the Google map of the most important areas described in this article. References Abelmann, A., Gersonde, R. 1991. Biosiliceous particle flux in the Southern Ocean. Mar. Chem. 35 (503–536). Accornero, A., Manno, C., Esposito, F., Gambi, M.C. 2003. The vertical flux of particulate matter in the polynya of Terra Nova Bay. Part II. Biological components. Antarct. Sci. 15 (2), 175–188. Armand, L.K., Crosta, X., Romero, O., Pichon, J.-J. 2005. The biogeography of major diatom taxa in Southern Ocean sediments: 1. Sea ice related species. Palaeogeogr. Palaeoclimatol. Palaeoecol. 223 (1–2), 93–126. Barron, J.A., Bukry, D. 2007a. Development of the California Current during the past 12,000 yr based on diatoms and silicoflagellates. Palaeogeogr. Palaeoclimatol. Palaeoecol. 248, 313–338. Barron, J.A., Bukry, D. 2007b. Solar forcing of Gulf of California climate during the past 2000 yr suggested by diatoms and silicoflagellates. Mar. Micropaleontol. 62, 115–139. Barron, J.A., Bukry, D., Dean, W.E. 2005. Paleoceanographic history of the Guaymas Basin, Gulf of California, during the past 15,000 years based on diatoms, silicoflagellates, and biogenic sediments. Mar. Micropaleontol. 56, 81–102.

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Parmales species (siliceous marine nanoplankton) in surface sediments of the Weddel Sea, Southern Ocean: indicators for sea ice environment? Mar. Micropaleontol. 32, 387–395. Zielinski, U., Gersonde, R. 1997. Diatom distribution in Southern Ocean surface sediments (Atlantic sector): implications for paleoenvironmental reconstructions. Palaeogeogr. Palaeoclimatol. Palaeoecol. 129, 213–250. Zielinski, U., Gersonde, R., Sieger, R., Fűtterer, D.K. 1998. Quaternary surface water temperature estimations: calibration of a diatom transfer function for the Southern Ocean. Paleoceanography 13, 365–383. Elisa Malinverno is a Doctor in Geology. Her main research interests are related to the ecology, and biogeography of extant coccolithophores and silicoflagellates in different oceanographic settings (Mediterranean Sea, eastern Pacific, Southern Ocean), their contribution to (palaeo)fluxes and their application in paleoceanography. Paola Maffioli is a Doctor in Natural Sciences. Her research interest is in to the study of marine diatoms, focusing on biostratigraphic and palaeoenvironmental reconstructions in different oceanographic settings (Mediterranean Sea, Southern Ocean). Karen Gariboldi is a PhD student in Geology. Her main goals are to study the role of diatoms in past upwelling settings, with particular regard to the Indian Ocean monsoon system and the Peru upwelling system, where an exceptional preservation of marine fossil vertebrates occurs.

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