Morphological characteristics and molecular identification of Pseudo-nitzschia meridionalis and Pseudo-nitzschia glacialis (Bacillariophyceae) from Antarctic waters

The carbon dioxide concentration in the atmosphere has progressively risen since pre-industrial times. About one-third of the anthropogenically generated CO2 is absorbed by the waters of the World Ocean, whereas the waters of the Southern Ocean take up about 40% of this CO2. The concentrations of oxygen and carbon dioxide dissolved in seawater are sensitive to climate changes, transferring anthropogenic pressures with consequences for the biogeochemical cycles in the World Ocean. The Southern Ocean is a key region for the exchange of oxygen and carbon between the surface water and the atmosphere and for their transfer with cold water masses to the deep layers of the Ocean. In this paper, we discuss the dynamics of the carbon dioxide partial pressure (pCO2) and dissolved oxygen (O2) in the surface waters of the Atlantic Southern Ocean based on data collected during the 87th cruise of the R/V “Academik Mstislav Keldysh”. The study area includes the Bransfield Strait, Antarctic Sound, the Powell Basin, the Weddell, and Scotia Seas. We have analyzed the spatial distribution of pCO2 and oxygen for the areas of transformation of water masses and changes in biogeochemical processes. In the zone of Scotia and Weddell Seas, we have observed an increase in pCO2 and a decrease in oxygen concentrations at the transect from the Weddell Sea at 56° W to the Powell Basin. From the Antarctic Sound to the Bransfield Strait, a decrease in oxygen saturation and an increase in pCO2 has been traced. The surface waters of the Bransfield Strait have revealed the greatest variability of hydrochemical characteristics due to a complex structure of currents and intrusions of different water masses. In general, this area has been characterized by the maximum pCO2, while the surface waters are undersaturated with oxygen. The variability of the AOU/ΔpCO2 (w-a) ratio has revealed a pCO2 oversaturation and an O2 undersaturation in the waters of the Bransfield Strait. It is evidence of active organic carbon decomposition as the major controlling process. Yet, photosynthesis is the major biogeochemical process in the studied areas of the Weddell and Scotia seas, and their waters have been undersaturated with pCO2 and oversaturated with O2. As it comes from the analysis of the distribution and correlation coefficients of AOU and the sea-air gradient of pCO2 with other physical and biogeochemical properties, the predominance of the biotic processes to the dynamics of O2 and pCO2 in the surface water layer has been demonstrated for the studied areas. Yet, there is evidence of additional sources of CO2 not associated with the production and destruction processes of organic matter.

Speciation of mercury in the waters of the Weddell, Amundsen and Ross Seas (Southern Ocean)

Morphological differences in Lissarca notorcadensis Melvill and Standen, 1907 from the Scotia, Weddell and Ross Seas

Hydrographic control of the marine ecosystem in the South Shetland-Elephant Island and Bransfield Strait region

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The West Antarctic Peninsula region is an important source of Antarctic krill (Euphausia superba) in the Southern Ocean. From 1980–2004 abundance and concentration of phytoplankton and zooplankton, krill reproductive and recruitment success and seasonal sea ice extent here were significantly correlated with the atmospheric Southern Oscillation Index and exhibited three- to five-year frequencies characteristic of El Niño–Southern Oscillation (ENSO) variability. This linkage was associated with movements of the Southern Antarctic Circumpolar Current Front and Boundary, a changing influence of Antarctic Circumpolar Current and Weddell Sea waters, and eastward versus westward flow and mixing processes that are consistent with forcing by the Antarctic Dipole high-latitude climate mode. Identification of hydrographic processes underlying ecosystem variability presented here were derived primarily from multi-disciplinary data collected during 1990–2004, a period with relatively stable year-to-year sea ice conditions. These results differ from the overwhelming importance of seasonal sea ice development previously established using 1980–1996 data, a period marked by a major decrease in sea ice from the Antarctic Peninsula region in the late 1980s. These newer results reveal the more subtle consequences of ENSO variability on biological responses. They highlight the necessity of internally consistent long-term multidisciplinary datasets for understanding ecosystem variability and ultimately for establishing well-founded ecosystem management. Furthermore, natural environmental variability associated with interannual- and decadal-scale changes in ENSO forcing must be considered when assessing impacts of climate warming in the Antarctic Peninsula–Weddell Sea region.

The Weddell-Scotia marginal ice zone: Physical oceanographic conditions, geographical and seasonal variability

The length of the sea ice season summarizes in one number the ice coverage conditions for an individual location for an entire year. It becomes a particularly valuable variable when mapped spatially over a large area and examined for regional and interannual differences, as is done here for the Southern Ocean over the years 1979–1986, using the satellite passive microwave data of the Nimbus 7 scanning multichannel microwave radiometer. Three prominent geographic anomalies in ice season lengths occur consistently in each year of the data set, countering the general tendency toward shorter ice seasons from south to north: (1) In the Weddell Sea the tendency is toward shorter ice seasons from southwest to northeast, reflective of the cyclonic ice/atmosphere/ocean circulations in the Weddell Sea region. (2) Directly north of the Ross Ice Shelf anomalously short ice seasons occur, lasting only 245–270 days, in contrast to the perennial ice coverage at comparable latitudes in the southern Bellingshausen and Amundsen Seas and in the western Weddell Sea. The short ice season off the Ross Ice Shelf reflects the consistently early opening of the ice cover each spring, under the influence of upwelling along the continental slope and shelf and atmospheric forcing from winds blowing off the Antarctic continent. (3) In the southern Amundsen Sea, anomalously short ice seasons occur adjacent to the coast, owing to the frequent existence of coastal polynyas off the many small ice shelves bordering the sea. Least squares trends in the ice season lengths over the 1979–1986 period are highly coherent spatially, with overall trends toward shorter ice seasons in the northern Weddell and Bellingshausen seas and toward longer ice seasons in the Ross Sea, around much of East Antarctica, and in a portion of the south central Weddell Sea.

Somatic growth of pelagic invertebrates is controlled by temperature and food, both of which vary in space and time. Species-specific growth rate responses to environmental variability may affect populations through changes in reproductive potential; therefore, measuring spatial and temporal variability in growth rates of highly abundant zooplankton is critical to predict the impact of climate change on pelagic ecosystems. Here, we used length frequencies from bi-annual surveys conducted 1 month apart to estimate growth rates of one the most abundant euphausiids in the Southern Ocean, Thysanoessa macrura. We analyzed summer data from 4 separate years (1995, 1998, 2001, and 2004) that varied widely in temperature and primary production. Stations within the surveys were grouped by water characteristics: warm, low salinity Antarctic Circumpolar Current (ACC) water, and cold, saline Bransfield Strait and Weddell Sea (MBW) water, to assess inter-annual and spatial variability in cohort growth. Mid-summer cohort growth rates of T. macrura varied between years and water masses, ranging from -0.037 mm d -1 in MBW water in 2004 to 0.081 mm d -1 in ACC water in 1995. Growth rates were faster in ACC water than in MBW water during all years. Growth rates were strongly correlated with temperature (R 2 = 0.82) but weakly correlated with copepod density (R 2 = 0.38), and were not correlated with chl a concentration (R 2 = 0.11). These results suggest that the growth rates of T. macrura may increase in regions exhibiting warming trends, such as the Antarctic Peninsula. This contrasts with published data on the growth rates of Euphausia superba, which is predicted to be impacted negatively by climate warming.

The Southern Ocean (SO) is one of the most pristine regions of our Planet, characterised by high levels of biodiversity (5% of the global diversity) (David and Saucède 2015) and hosting a unique fauna (up to 90% of SO species are endemic) (De Broyer and Danis 2011; Chown et al. 2015). Yet, the knowledge on SO biodiversity is still far from being completed. In addition, the knowledge on the impact that changing environments have on SO species-richness is very little and for some groups, it is still totally unknown. For instance, most of studies generally focus on one single species such as Antarctic krill (Kawaguchi et al. 2011), Clio pyramidata Linnaeus, 1767 (Orr et al. 2005), Globigerina bulloides d'Orbigny, 1826 (Moy et al. 2009), or only on a high taxonomic level (e.g. phylum, class): Echinodermata, Crustacea, Mollusca, Porifera, Bryozoa, Brachiopoda, Hydrozoa, Ascidiacea, Holoturoidea (Barnes 1999; Rowden et al. 2015; Post et al. 2017; Gutt et al. 2019; Vause et al. 2019; Pineda-Metz et al. 2020). Ultimately, the influence of sea-ice coverage on benthic species diversity was totally unknown prior to this study. In light of this, the objectives of the thesis are: 1. To expand the knowledge on shelf and deep-sea peracarid assemblage structure and abundance on a small regional (Weddell Sea) and on a large regional (Atlantic sector of the SO and South Atlantic Ocean) geographic scale. 2. To assess the environmental variables driving peracarid assemblage structure and abundance from the above mentioned areas. 3. To investigate SO benthic isopod species diversity from the Atlantic sector of the SO and assess the influence of environmental variables on their species-richness and composition. 4. To describe new possible peracarid species by means of integrative taxonomy, using morphological descriptions and whole genome sequencing analyses to support the species identification. Objective outcomes: The present thesis provides new information on the abundance and assemblage structure based on 64766 peracarid crustaceans from different 28 locations within the Atlantic sector of the SO continental shelf and deep sea (Chapters I-II). These locations are characterised by different environmental conditions, for instance different sea-ice concentrations. Results from Chapters I-II confirmed the dominance of peracarid assemblages in the benthos, with amphipods being the most abundant group, followed by isopods. Sea ice was identified as the main driver shaping benthic peracarid assemblage structure (Chapter I). On a larger geographic scale and wider bathymetric range (e.g. including sampling locations from previous studies performed in the South Atlantic Ocean and at a depth range from 160 to ~6000 m), depth was the main physical variable driving peracarid assemblage structure (Chapter III). In addition, 16157 isopod specimens from the Atlantic sector of the SO were identified to species level at a smaller scale (Chapter IV). In this case, sea ice was identified as the main physical driver affecting isopod diversity and composition among sampling locations (Chapter IV). Reduced concentration of sea ice causes a decrease in isopod biodiversity, thus climate change was identified as a huge threat for this taxon and for SO benthos in general. During the identification process, two new isopod species were discovered (Chapter V). The two new species (Notopais sp.1 n. sp. and Notopais sp.2 n. sp.) were accurately described and identified by means of integrative taxonomy. This provided the first whole genome sequencing of benthic isopods from the SO and the first complete mitochondrial genome of the genus Notopais (Chapter V). Thanks to the collaboration with the University of Genoa (Dipartimento di Scienze della Terra dell'Ambiente e della Vita, DISTAV, Italy) and the National Antarctic Museum (MNA) in Genoa, two new SO species of the suborder Valvifera G. O. Sars, 1883 were described by means of classical taxonomy. In this case, a molecular approach could not be used because both new species were represented by a single specimen, therefore it was important to preserve the integrity of the holotypes (Chapters VI-VII).

The lowest biogenic silica production rates in the Southern Ocean (average of 2.59 mm01 Si m-2 d-l) have been recorded in an area of heavy ice cover along a transect through the Weddell Sea from Joinville Island to Cap Norvegia (November-December 1990).The associated biomass was also very low (concentrations 50.6 pm01 1-I for biogenic silica and S0.8 pg I-' for chlorophyll a ) .Based upon these direct measurements of biogenic silica production rates and other data available from the marginal ice zone and open ocean areas, we estimated the annual production of biogenlc silica in the northern Weddell Sea to be 810 to 870 rnrnol m-2 yrrl.Thls leads to a revised estimate of the total annual biogenic s d x a production in the Southern Ocean of between 11 and 32 Tmol Si yr-l.Comparing our annual production estimate to previous estimates of vertical flux of opal in the Weddell Sea, we conclude that no more than 1 % of the silica produced annually by phytoplankton in the upper water column reaches a depth of 800 m.This is consistent with the general distribution of high accumulation rates of opal in Southern Ocean sedirnents which evidence an unexplained gap in the Weddell Sea.Thus, regarding the cycling of biogenic s h c a in the Southern Ocean, the Weddell Sea appears to be atypical.

Abstract. An upgraded version of the biogeochemical model SWAMCO is coupled to the ocean-sea-ice model NEMO-LIM to explore processes governing the spatial distribution of the iron supply to phytoplankton in the Southern Ocean. The 3-D NEMO-LIM-SWAMCO model is implemented in the ocean domain south of latitude 30° S and runs are performed over September 1989–December 2000. Model scenarios include potential iron sources (atmospheric deposition, iceberg calving/melting and continental sediments) as well as iron storage within sea ice, all formulated based on a literature review. When all these processes are included, the simulated iron profiles and phytoplankton bloom distributions show satisfactory agreement with observations. Analyses of simulations and sensitivity tests point to the key role played by continental sediments as a primary source for iron. Iceberg calving and melting contribute by up to 25% of Chl-a simulated in areas influenced by icebergs while atmospheric deposition has little effect at high latitudes. Activating sea ice-ocean iron exchanges redistribute iron geographically. Stored in the ice during winter formation, iron is then transported due to ice motion and is released and made available to phytoplankton during summer melt, in the vicinity of the marginal ice zones. Transient iron storage and transport associated with sea ice dynamics stimulate summer phytoplankton blooming (up to 3 mg Chl-a m-3 in the Weddell Sea and off East Antarctica but not in the Ross, Bellingshausen and Amundsen Seas. This contrasted feature results from the simulated variable content of iron in sea ice and release of melting ice showing higher ice-ocean iron fluxes in the continental shelves of the Weddell and Ross Seas than in the Eastern Weddell Sea and the Bellingshausen-Amundsen Seas. This study confirms that iron sources and transport in the Southern Ocean likely provide important mechanisms in the geographical development of phytoplankton blooms and associated ecosystems.

Open-ocean polynyas effectively couple the ocean and atmosphere through large ice-free areas within the sea-ice cover, release vast quantities of oceanic heat, and impact deep ocean ventilation. Changes in polynya activity, particularly in the Weddell Sea, may be key to longer time-scale climate fluctuations, feedbacks and abrupt change. While changes in the occurrence of Weddell Sea polynyas are generally attributed to changes in the atmospheric surface forcing, the role of internal ocean dynamics for polynya variability is not well-resolved. In this study we employ a global coupled ocean-sea ice model with a repeating annual atmospheric cycle to explore changes in Weddell Sea water mass properties, stratification and ocean circulation driven by open-ocean polynyas. During the 1300-year long simulation, two large polynyas occur in the central Weddell Sea. Our results suggest that Weddell polynyas may be triggered without inter-annual changes in the atmospheric forcing. This highlights the role of ocean processes in preconditioning and triggering open-ocean polynyas on multi-centennial time-scales. The simulated polynyas form due to internal ocean-sea ice dynamics associated with a slow build-up and subsequent release of subsurface heat. A strong stratification and weak vertical mixing is necessary for building the subsurface heat reservoir. Once the water column turns unstable, enhanced vertical mixing of warm and saline waters into the surface layer causes efficient sea ice melt and the polynya appears. Subsequent, vigorous deep convection is maintained through upwelling of warm deep water leading to enhanced bottom water formation. We find a cessation of simulated deep convection and polynya activity due to long-term cooling and freshening of the subsurface heat reservoir. As subsurface waters in the Southern Ocean are now becoming warmer and saltier, we speculate that larger and more persistent Weddell polynyas could become more frequent in the future.

The Weddell Sea is known to feed recently formed deep and bottom water into the Antarctic circumpolar water belt, from whence it spreads into the basins of the world ocean. The rates are still a matter of debate. To quantify the flow of bottom water in the northwestern Weddell Sea data obtained during five cruises with R/V Polarstern between October 1989 and May 1998 were used. During the cruises in the Weddell Sea, five hydrographic surveys were carried out to measure water mass properties, and moored instruments were deployed over a time period of 8.5 years to obtain quasi‐continuous time series. The average flow in the bottom water plume in the northwestern Weddell Sea deduced from the combined conductivity‐temperature‐depth and moored observations is 1.3±0.4 Sv. Intensive fluctuations of a wide range of timescales including annual and interannual variations are superimposed. The variations are partly induced by fluctuations in the formation rates and partly by current velocity fluctuations related to the large‐scale circulation. Taking into account entrainment of modified Warm Deep Water and Weddell Sea Deep Water during the descent of the plume along the slope, between 0.5 Sv and 1.3 Sv of surface‐ventilated water is supplied to the deep sea. This is significantly less than the widely accepted ventilation rates of the deep sea. If there are no other significant sources of newly ventilated water in the Weddell Sea, either the dominant role of Weddell Sea Bottom Water in the Southern Ocean or the global ventilation rates have to be reconsidered.

Bio-essential trace elements, such as iron (Fe) and zinc (Zn), also known as micronutrients, are critical for all life because of their biochemical roles in various metabolic processes. They are also pivotal for marine phytoplankton growth, the base of the marine food web. In the oceans, High-Nutrient Low-Chlorophyll (HNLC) surface regions are characterized by sufficient macronutrient (e.g., phosphate, nitrite/nitrate, sillicate) supply but limited primary productivity due to the lack of Fe and light. As the largest HNLC region, the Southern Ocean is suggested to have a great potential to absorb a large amount of atmospheric CO2 if the phytoplankton requirements for Fe and light are fulfilled. Given the importance of the Southern Ocean to the global climate and ocean circulation, it is crucial to understand its marine biogeochemistry, notably the cycling of trace elements in coastal Antarctic regions, and their influence on globally relevant processes. In this thesis, two bio-essential trace elements, Fe and Zn, together with Cd, and their isotopic compositions, are studied in two distinct coastal Antarctic regions – the Amundsen Sea (AS) and the Weddell Sea (WS) to expand our understanding of these crucial bioactive elements in the dynamic Southern Ocean. The AS is characterized by the extensive intrusion of warm modified Circumpolar Deep Water (mCDW) onto the continental shelf through glacial troughs that lead to rapid ice sheet melting as it flows under ice shelves (i.e., Dotson Ice Shelf, DIS). The upwelling of mCDW also accelerates sea ice melting, creating polynyas (i.e., open areas surrounded by sea ice) that harbour productive and long-lasting phytoplankton blooms in spring-summer – the Amundsen Sea Polynya (ASP) shows the highest annual net primary production rate per unit area among Antarctica polynyas. The WS is part of the wind-driven Weddell Gyre, which is a crucial component of the global oceanic circulation as a primary formation region of deep-water masses. In the southern and western parts of the WS, dense, saline, cold shelf water forms through sea ice formation and brine rejection in winter, which subsequently descends along the continental slope, eventually exiting the WS through the Scotia Sea, contributing to AABW and the global ocean conveyor belt. Both the AS and the WS hold the potential to influence or be influenced by the global biogeochemical cycles of trace metals. Chapters 2, 3, and 4 delve into the biogeochemical aspects of these elements in the AS, while Chapters 4 and 5 focus on the WS. These regions hold particular significance due to their potential regional biogeochemical dynamics, which may have global repercussions. I utilized stable isotopic compositions as a tool to unravel the complex dynamics of these elements in these rapid changing environments.