Nitrogen oxides, collectively referred to as NOx (NO + NO2), are an important component of atmospheric chemistry involved in the production and destruction of various oxidants that contribute to the oxidative capacity of the troposphere. The primary sink for NOx is atmospheric nitrate, which has an influence on climate and the biogeochemical cycling of reactive nitrogen. NOx sources and NOx-to-NO3- formation pathways remain poorly constrained in the remote marine boundary layer of the Southern Ocean, particularly outside of the more frequently sampled summer months. This study presents seasonally resolved measurements of the isotopic composition (δ15N, δ18O, and Δ17O) of atmospheric nitrate in coarse-mode (> 1 µm) aerosols, collected between South Africa and the sea ice edge in summer, winter, and spring. Similar latitudinal trends in δ15N–NO3- were observed in summer and spring, suggesting similar NOx sources. Based on δ15N–NO3-, the main NOx sources were likely a combination of lightning, biomass burning, and/or soil emissions at the low latitudes, as well as oceanic alkyl nitrates and snowpack emissions from continental Antarctica or the sea ice at the mid-latitudes and high latitudes, respectively. Snowpack emissions associated with photolysis were derived from both the Antarctic snowpack and snow on sea ice. A combination of natural NOx sources, likely transported from the lower-latitude Atlantic, contribute to the background-level NO3- observed in winter, with the potential for a stratospheric NO3- source evidenced by one sample of Antarctic origin. Greater values of δ18O–NO3- in spring and winter compared to summer suggest an increased influence of oxidation pathways that incorporate oxygen atoms from O3 into the end product NO3- (i.e. N2O5, DMS, and halogen oxides (XO)). Significant linear relationships between δ18O and Δ17O suggest isotopic mixing between H2O(v) and O3 in winter and isotopic mixing between H2O(v) and O3/XO in spring. The onset of sunlight in spring, coupled with large sea ice extent, can activate chlorine chemistry with the potential to increase peroxy radical concentrations, contributing to oxidant chemistry in the marine boundary layer. As a result, isotopic mixing with an additional third end-member (atmospheric O2) occurs in spring.

In the southern Benguela ecosystem, regenerated production and high levels of organic matter remineralisation are expected to dominate during periods of relaxation. To study microbial growth and productivity under these remineralising conditions, we measured size fractionated (micro-nanoplankton: 10-200 & mu;m and nano-picoplankton: 0.3-10 & mu;m) net primary production, uptake rates of nitrate, ammonium, and urea, as well as nano-and picoplankton community composition and biomass over five consecutive days in autumn (March 2018). Samples were collected from three depths (1 m, 25 m, and 50 m) at a single station in St Helena Bay and abundances of nanophytoplankton, picophytoplankton and heterotrophic bacteria were determined using flow cytometry. There were differences in productivity among days but depth-differentiation was more apparent in the rates of net primary production and nitrogen uptake, with the highest rates (rate & PLUSMN; SD) (NPP: 3.36 & PLUSMN; 1.82 & mu;mol L-1 d-1; pNO3 -: 0.30 & PLUSMN; 0.18 & mu;mol L-1 d-1; pNH4+: 2.27 & PLUSMN; 0.75 & mu;mol L-1 d-1; pUrea: 1.38 & PLUSMN; 0.02 & mu;mol L-1 d-1) measured at the surface. Ammonium and urea uptake rates were two-to six-fold higher than those of nitrate, indicating that most of the biomass was produced through regenerated production with f-ratios of 0.02-0.21. Nano-picoplankton comprised 67% of the carbon biomass and were responsible for 90% of net primary pro-duction and 79-85% of total nitrogen (i.e., nitrate + ammonium + urea) uptake. Small cell size likely conferred advantages on nano-picoplankton in the nutrient-deplete euphotic zone and the low-oxygen (<89.3 & mu;mol L-1) conditions at 25 m and 50 m. Nitrite oxidation rates were fastest at deeper depths where heterotrophic bacteria were most abundant. Heterotrophic bacteria also contributed the most (95%) carbon biomass at all depths, suggesting a major role for these microorganisms in carbon and nitrogen cycling throughout the water column of St Helena Bay.

The Benguela Upwelling System (BUS) is one of the world’s most productive ecosystems, supporting globally relevant pelagic fisheries. BUS marine community can change as a function of nutrients and omega-3 long chain polyunsaturated fatty acids (hereafter, omega-3) availability. Phytoplankton growth is supported by upwelled nitrate, a new source of nitrogen (N), or by recycled N forms such as ammonium. Preferential assimilation of one N form over another may lead to differences in omega-3 production between high and low food-quality species. This study evaluates how upwelling and the N source(s) used by phytoplankton influence omega-3 production. Sampling was conducted in the BUS at an anchor station sampled daily for 10 consecutive days. An upwelling event on days 5-6-7 supplied high concentrations of nutrients to surface waters, while pre- and post- upwelling the water column was well-stratified with low nutrient concentrations. Omega-3 and phytoplankton concentrations declined to ⁓zero during the upwelling event. Nanoplankton (2.7-10µm) were responsible for most of the productivity (30-95%) and relied on nitrate for their growth. Omega-3 concentrations at the surface reached peaks of 215.5 and 175.3µgL-1 pre- and post-upwelling, which were up to 10-times higher than previous measurements from the BUS. Pre-upwelling, non-diatom trophic markers were dominant, with a rapid switch (over just two days) to diatom trophic markers post-upwelling. This study defines the key role of upwelling in promoting phytoplankton omega-3 production, which is tightly coupled to the introduction of new-N during upwelling. The high concentrations of omega-3 reported suggest that global omega-3 production is largely underestimated.

Across the Southern Ocean, large (≥20 μm) diatoms are generally assumed to be the primary vector for carbon export, although this assumption derives mainly from summertime observations. Here, we investigated carbon production and export potential during the Atlantic Southern Ocean's spring bloom from size-fractionated measurements of net primary production (NPP), nitrogen (nitrate, ammonium, urea) and iron (labile inorganic iron, organically complexed iron) uptake, and a high-resolution characterization of phytoplankton community composition. The nanoplankton-sized (2.7 to 20 μm) diatom, Chaetoceros spp., dominated the biomass, NPP, and nitrate uptake across the basin (40°S to 56°S), which we attribute to their low iron requirement, rapid response to increased light, and ability to escape grazing when aggregated into chains. We estimate that the spring Chaetoceros bloom accounted for >25% of annual export production across the Atlantic Southern Ocean, a finding consistent with recent observations from other regions highlighting the central role of the phytoplankton "middle class" in carbon export.

Abstract. Across the Southern Ocean in winter, nitrification is the dominant mixed-layer nitrogen cycle process, with some of the nitrate produced therefrom persisting to fuel productivity during the subsequent growing season, potentially weakening the spring/summer biological CO2 sink. To better understand the controls on Southern Ocean nitrification, we conducted nitrite oxidation kinetics experiments in surface waters across the western Indian sector in winter. While all experiments (seven in total) yielded a Michaelis-Menten relationship with substrate concentration, the nitrite oxidation rates only increased substantially once the nitrite concentration exceeded 115±2.3 to 245±18 nM, suggesting that nitrite oxidizing bacteria (NOB) require a minimum (i.e., "threshold") nitrite concentration to produce nitrate. The half-saturation constant ranged from 134±8 to 403±24 nM, indicating a relatively high affinity of Southern Ocean NOB for nitrite, in contrast to results from culture experiments. Despite the high affinity of NOB for nitrite, its concentration rarely declines below 150 nM in the Southern Ocean's mixed layer, regardless of season. In the upper mixed layer, we measured ammonium oxidation rates that were two- to seven-fold higher than the coincident rates of nitrite oxidation, indicating that nitrite oxidation is the rate-limiting step for nitrification in the winter Southern Ocean. The decoupling of ammonium and nitrite oxidation, combined with a possible nitrite concentration threshold for NOB, may explain the non-zero nitrite that persists throughout the Southern Ocean's mixed layer year-round. We hypothesize that the apparent threshold nitrite requirement of NOB indicates nitrite undersaturation of the heme-rich nitrite oxidoreductase enzyme, perhaps driven by the limited availability of iron in surface waters.

Atmospheric nitrate originates from the oxidation of nitrogen oxides (NOx=NO+NO2) and impacts both tropospheric chemistry and climate. NOx sources, cycling and NOx to nitrate formation pathways are poorly constrained in remote marine regions, especially the Southern Ocean, where pristine conditions serve as a useful proxy for the pre-industrial atmosphere. Here, we measured the isotopic composition (δ15N and δ18O) of atmospheric nitrate in coarse-mode (>1 µm) aerosols collected in the summertime marine boundary layer of the Atlantic Southern Ocean from 34.5 to 70∘ S and across the northern edge of the Weddell Sea. The δ15N–NO3- decreased with latitude from −2.7 ‰ to −42.9 ‰. The decline in δ15N with latitude is attributed to changes in the dominant NOx sources: lightning at the low latitudes, oceanic alkyl nitrates at the mid-latitudes and photolysis of nitrate in snow at the high latitudes. There is no evidence of any influence from anthropogenic NOx sources or equilibrium isotope fractionation. Using air mass back trajectories and an isotope mixing model, we calculate that oceanic alkyl nitrate emissions have a δ15N signature of -21.8±7.6 ‰. Given that measurements of alkyl nitrate contributions to remote nitrogen budgets are scarce, this may be a useful tracer for detecting their contribution in other oceanic regions. The δ18O–NO3- was always less than 70 ‰, indicating that daytime processes involving OH are the dominant NOx oxidation pathway during summer. Unusually low δ18O–NO3- values (less than 31 ‰) were observed at the western edge of the Weddell Sea. The air mass history of these samples indicates extensive interaction with sea-ice-covered ocean, which is known to enhance peroxy radical production. The observed low δ18O–NO3- is therefore attributed to increased exchange of NO with peroxy radicals, which have a low δ18O, relative to ozone, which has a high δ18O. This study reveals that the mid- and high-latitude surface ocean may serve as a more important NOx source than previously thought and that the ice-covered surface ocean impacts the reactive nitrogen budget as well as the oxidative capacity of the marine boundary layer.

In South Africa, the Highveld region and the Johannesburg-Pretoria megacity are known as global NOx (NOx = NO + NO2) “hotspots” identified by satellite-based instruments. The ultimate sink for atmospheric NOx is conversion to aerosol nitrate. However, measurements of aerosol nitrate concentrations do not provide information on which NOx sources served as nitrate precursors at that location. This complicates efforts to reduce concentrations of particulate matter (PM) in these air quality priority areas. Here, we measured the nitrogen stable isotopic composition of nitrate from daily wintertime collections of coarse mode PM2.5-10 (PM ≤ 10 and >2.5 µm in diameter) at three air quality monitoring stations located in the Vaal Triangle Air-Shed Priority Area (VTAPA). The overall aim of this case study was to evaluate the use of the distinct stable isotopic signatures of various NOx sources to identify their relative contribution to aerosol nitrate across the Highveld. The nitrogen isotopic ratios of aerosol nitrate were similar across the three sites, with greater day-to-day variability than site to site variability. Air mass history was the main driver of the variability in the nitrogen isotopic ratios of aerosol nitrate, with significantly higher isotopic ratios observed for air masses originating from the southwest. Using an isotope mixing model we determined that NOx from coal-burning is the dominant contributor to aerosol nitrate (66%), followed by biomass burning (16%), vehicles (12%), and soil emissions (6%).

The production and removal of ammonium (NH4+) are essential upper-ocean nitrogen cycle pathways, yet in the Southern Ocean where NH4+ has been observed to accumulate in surface waters, its mixed-layer cycling remains poorly understood. For surface seawater samples collected between Cape Town and the Marginal Ice Zone in winter 2017, we found that NH4+ concentrations were 5-fold higher than is typical for summer and lower north than south of the Subantarctic Front (0.01–0.26 µM versus 0.19–0.70 µM). Our observations confirm that NH4+ accumulates in the Southern Ocean's winter mixed layer, particularly in polar waters. NH4+ assimilation rates were highest near the Polar Front (12.9 ± 0.4 nM d−1) and in the Subantarctic Zone (10.0 ± 1.5 nM d−1), decreasing towards the Marginal Ice Zone (3.0 ± 0.8 nM d−1) despite the high ambient NH4+ concentrations in these southernmost waters, likely due to the low temperatures and limited light availability. By contrast, rates of NH4+ oxidation were higher south than north of the Polar Front (16.0 ± 0.8 versus 11.1 ± 0.5 nM d−1), perhaps due to the lower-light and higher-iron conditions characteristic of polar waters. NH4+ concentrations were also measured along five transects of the Southern Ocean (Subtropical Zone to Marginal Ice Zone) spanning the 2018/19 annual cycle. These measurements reveal that mixed-layer NH4+ accumulation south of the Subantarctic Front derives from sustained heterotrophic NH4+ production in late summer through winter that, in net, outpaces NH4+ removal by temperature-, light-, and iron-limited microorganisms. Our observations thus imply that the Southern Ocean becomes a biological source of CO2 to the atmosphere in autumn and winter not only because nitrate drawdown is weak but also because the ambient conditions favour net heterotrophy and NH4+ accumulation.

Central to the Southern Ocean's role in setting atmospheric CO2 is the seasonal alternation between upward mixing of nutrients and their subsequent consumption by phytoplankton. Active nutrient cycling within the mixed layer, including the release of ammonium (NH4+) and its removal by phytoplankton and nitrifiers, also affects Southern Ocean CO2 drawdown, yet remains poorly understood. We conducted kinetics experiments across the Southern Ocean south of Africa to investigate the dependence of NH4+ uptake (summer, winter) and oxidation (winter) on NH4+ concentration. NH4+ uptake followed a Michaelis-Menten function in both seasons, with the maximum rate (V-max) decreasing poleward, apparently controlled mainly by light in winter and temperature in summer. The half-saturation constant (K-m) increased poleward with increasing ambient NH4+ ([NH4+](amb)) and was threefold higher in winter (150-405 nM) than in summer (41-115 nM), suggesting that summertime phytoplankton are adapted to low-NH4+ conditions while winter communities typically receive a higher NH4+ supply. NH4+ oxidation showed a high affinity for NH4+ (K-m = 28-137 nM), suggesting a dominant role for ammonia-oxidizing archaea, and followed a Michaelis-Menten curve only when [NH4+](amb) was <= 90 nM. V-max was near-constant across the region regardless of [NH4+](amb), temperature, or light. From coincident mixed-layer NH4+ oxidation and iron measurements, we hypothesize that iron availability may (co-)limit the V-max of NH4+ oxidation. If verified, this suggestion has implications for models that parameterize nitrification as a linear function of [NH4+](amb). Additionally, iron depletion may limit the role of mixed-layer nitrification, which is dominant in the winter Southern Ocean, in offsetting phytoplankton CO2 drawdown annually.

Understanding the transport and accumulation of microplastics is useful to determine the relative risk they pose to global biodiversity. The exact contribution of microplastic sources is hard to elucidate; therefore, investigating the Antarctic Weddell Sea, an area known for its remoteness and little human presence (i.e. limited pollution sources), will help us to better understand microplastic transportation. Here, we investigate the presence of microplastics in a range of Antarctic sample media including air, seawater, and sediment. We hypothesised that multiple transportation processes including atmospheric and oceanic vectors determine the presence of microplastics in the Antarctic. Using techniques including Polarised Light Microscopy and Raman Spectrometry, we identified mostly fibres and categorised them based on their optical and chemical properties. A total of 47 individual microplastic categories (45 of which were fibres) were identified in the air, seawater, and sediment samples. The majority of categories did not overlap multiple media (42/47); however, four fibre categories were present in both air and water samples, and another fibre category was found in all three media (category 27). We suggest that the large variety of fibres identified and the overlap of fibre categories among media indicates that the pollution may result from multiple diffuse sources and transportation pathways. Additionally, our Air Mass Back Trajectory analyses demonstrates that microplastic fibres are being transported by air masses or wind, and strongly suggests that they are transported to the Antarctic from southern South America. We also propose that fibres may be transported into the Antarctic in subsurface waters, and as pollution was identified in our sediment and additional sea ice samples, we suggest that the coastal and Antarctic deep sea may be a sink for microplastic fibres. The results shown here from a remote, near-pristine system, further highlight the need for a global response to the plastic pollution crisis.

1 Department of Oceanography, University of Cape Town, Private Bag X3, Rondebosch, 6 Cape Town, South Africa 7 2 Southern Ocean Carbon and Climate Observatory (SOCCO), CSIR, Rosebank, Cape Town, 8 South Africa 9 3 Department of Conservation and Marine Sciences, Cape Peninsula University of 10 Technology, Cape Town, South Africa 11 4 Marine and Antarctic Research centre for Innovation and Sustainability (MARIS), 12 University of Cape Town, Cape Town, South Africa 13 14 * Corresponding author: smtsha023@myuct.ac.za 15 16

The Weddell Sea represents a point of origin in the Southern Ocean where globally important water masses form. Biological activities in Weddell Sea surface waters thus affect large-scale ocean biogeochemistry. During January–February 2019, we measured net primary production (NPP), nitrogen (nitrate, ammonium, urea) uptake, and nitrification in the western Weddell Sea at the Antarctic Peninsula (AP) and Larsen C Ice Shelf (LCIS), in the southwestern Weddell Gyre (WG), and at Fimbul Ice Shelf (FIS) in the south-eastern Weddell Sea. The highest average rates of NPP and greatest nutrient drawdown occurred at LCIS. Here, the phytoplankton community was dominated by colonial Phaeocystis antarctica, with diatoms increasing in abundance later in the season as sea ice melted. At the other stations, NPP was variable, and diatoms known to enhance carbon export (e.g. Thalassiosira spp.) were dominant. Euphotic zone nitrification was always below detection, such that nitrate uptake could be used as a proxy for carbon export potential, which was highest in absolute terms at LCIS and the AP. Surprisingly, the highest f ratios occurred near FIS rather than LCIS (average of 0.73±0.09 versus 0.47±0.08). We attribute this unexpected result to partial ammonium inhibition of nitrate uptake at LCIS (where ammonium concentrations were 0.6±0.4 µM, versus 0.05±0.1 µM at FIS), with elevated ammonium resulting from increased heterotrophy following the accumulation of nitrate-fuelled phytoplankton biomass in early summer. Across the Weddell Sea, carbon export appears to be controlled by a combination of physical, chemical, and biological factors, with the highest potential export flux occurring at the ice shelves and lowest in the central WG.

St Helena Bay (SHB), a retention zone in the southern Benguela upwelling system, supports 40–50% of the region's primary productivity. It also experiences harmful algal blooms (HABs) and hypoxic conditions that are difficult to predict given the high sub-seasonal variability. To better understand this variability, net primary production (NPP), nitrate and ammonium uptake, and phytoplankton community composition were monitored for ten days in SHB during summer, the season of enhanced upwelling. A period of active upwelling (days 1–5) was followed by one of relaxation (days 6–10). During upwelling, the mixed layer was deeper than the euphotic zone and phytoplankton were light-limited, evidenced by high ambient nitrate concentrations and low rates of NPP and nitrate uptake. During relaxation, stratification increased, restricting phytoplankton production to a shallow euphotic zone in which nitrate was exhausted after three days; the subsequent decline in NPP and nitrate uptake rates confirm that nutrient availability rapidly succeeded light as the dominant control on productivity. Nanophytoplankton (mainly the small diatoms, Chaetoceros spp. and Skeletonema costatum) dominated the biomass, NPP, and nitrate uptake throughout the experiment. We attribute this to their ability to respond quickly to newly-upwelled nitrate and to sustain elevated nitrate uptake rates for longer than pico- and microphytoplankton. They may also engage in luxury nitrate uptake, storing nitrate intracellularly under high-nutrient conditions and assimilating it later when nutrients are depleted. Additionally, Chaetoceros spp. and S. costatum are chain-formers, benefitting from the advantages of being small (i.e., as individual cells) and large (i.e., when aggregated into chains). A weakening of surface stratification late in the experiment may have prevented dinoflagellates, some of which are HAB species, from succeeding the diatoms. One implication of this is that understanding the rapid cycling between light and nutrient limitation of SHB phytoplankton, induced by an actively-upwelling versus stratified water column, may improve our capacity to anticipate HABs and the associated hypoxic events.

Net primary production (NPP) is indicative of the energy available to an ecosystem, which is central to ecological functioning and biological carbon cycling. The Southern Ocean’s Weddell Sea (WS) represents a point of origin where water masses form and exchange with the atmosphere, thereby setting the physical and chemical conditions of much of the global ocean. The WS is particularly understudied near Larsen C Ice Shelf (LCIS) where harsh sea-ice conditions persist year-round. We measured size-fractionated rates of NPP, nitrogen (N; as nitrate, ammonium, and urea) uptake, and nitrification, and characterized the phytoplankton community at 19 stations in summer 2018/2019, mainly near LCIS, with a few stations in the open Weddell Gyre (WG) and at Fimbul Ice Shelf (FIS). Throughout the study region, NPP and N uptake were dominated by nanophytoplankton (3-20 μm), with microphytoplankton (>20 μm) becoming more abundant later in the season, particularly at FIS. Here, we observed high phytoplankton biomass and diversity, and the community was dominated by diatoms known to enhance carbon export (e.g., Thalassiosira spp.). At LCIS, by contrast, the community comprised mainly Phaeocystis Antarctica. In the open WG, a population of small and weakly-silicified diatoms of the genus Corethron dominated the phytoplankton community. Here, euphotic zone-integrated uptake rates were similar to those at LCIS even though the depth-specific rates were lower. Mixed-layer nitrification was below detection at all stations such that nitrate uptake can be used as a proxy for carbon export potential sensu the new production paradigm – this was highest near FIS in late summer. Our observations can be explained by melting sea ice near the ice shelves that supplies iron and enhances water column stratification, thus alleviating iron and/or light limitation of phytoplankton and allowing them to consume the abundant surface macronutrients. That the sea ice melted completely at FIS but not LCIS may explain why late-summer productivity and carbon export potential were highest near FIS, more than double the rates measured in early summer and near LCIS. The early-to-late summer progression near the ice shelves contrasts that of the open Southern Ocean where iron is depleted by late summer, driving a shift towards smaller phytoplankton that facilitate less carbon export.

The data submitted here is published in the manuscript entitled "On-shelf nutrient trapping enhances fertility of the southern Benguela upwelling system". Our data show that regenerated nutrients get “trapped” on the shelf of the southern Benguela upwelling system (SBUS), increasing the on-shelf nutrient pool available for upwelling. Nutrient trapping occurs when phytoplankton consume upwelled nutrients, sequestering them in their biomass, then sink and are decomposed on the shallow continental shelf, releasing nutrients to bottom waters. The nutrient-deplete surface waters subsequently flow offshore. SBUS nutrient trapping appears to be assisted by hydrographic fronts that limit the offshore transport of phytoplankton, such that their sinking and subsequent decomposition occurs on-shelf. Decomposition consumes oxygen, which means that enhanced nutrient trapping may increase oxygen depletion in the SBUS, with ecosystem-wide deleterious effects.

The elevated levels of primary productivity associated with eastern boundary currents are driven by nutrient- rich waters upwelled from depth, such that these regions are typically characterised by high rates of nitrate-fuelled phytoplankton growth. Production studies from the southern Benguela upwelling system (SBUS) tend to be biased towards the summer upwelling season, yet winter data are required to compute annual budgets and understand seasonal variability. Net primary production (NPP) and nitrate and ammonium uptake were measured concurrently at six stations in the SBUS in early winter. While euphotic zone NPP was highest at the stations nearest to the coast and declined with distance from the shore, a greater proportion was potentially exportable from open-ocean surface waters, as indicated by the higher specific nitrate uptake rates and f-ratios (ratio of nitrate uptake to total nitrogen consumption) at the stations located off the continental shelf. Near the coast, phytoplankton growth was predominantly supported by ammonium despite the high ambient nitrate concentrations. Along with ammonium concentrations as high as 3.6 µmol l–1, this strongly suggests that nitrate uptake in the inshore SBUS, and by extension carbon drawdown, is inhibited by ammonium, at least in winter, although this has also been hypothesised for the summer.