Browsing by Author "Fawcett, Sarah"

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    Biogeochemical controls on ammonium accumulation in the surface layer of the TD: Southern Ocean
    (2022) Smith, Shantelle; Altieri, Katye; Fawcett, Sarah
    The production and assimilation of ammonium (NH₄⁺) are essential upper-ocean nitrogen (N) cycle pathways. However, in the Southern Ocean where the alternation between biological nitrate drawdown in summer and physical nitrate resupply in winter is central for setting atmospheric CO2, the active cycling of NH₄⁺ in the seasonally-varying mixed layer remains poorly understood. On a cruise from Cape Town (33.9°S) to the Marginal Ice Zone (MIZ; 61.4°S) in winter 2017, surface samples were collected and analysed for nutrient concentrations, planktonic community composition, size-fractionated rates of net primary production and N (as NH₄⁺, urea, and nitrate) uptake, and rates of NH₄⁺ oxidation. NH₄⁺ concentrations, measured every four hours, were five-fold higher than is typical for summer, and lower north than south of the Subantarctic Front (SAF; 0.01–0.26 µM versus 0.19–0.70 µM). Thus, showing that NH₄⁺ accumulates in the Southern Ocean's winter mixed layer, particularly in polar waters. NH₄⁺ uptake rates were highest near the Polar Front (PF; 12.9 ± 0.4 nM day-1 ) and in the Subantarctic Zone (10.0 ± 1.5 nM day-1), decreasing towards the MIZ (3.0 ± 0.8 nM day-1) despite the high ambient NH₄⁺ concentrations, likely due to the low temperatures and limited light. By contrast, rates of NH₄⁺ oxidation were higher south than north of the PF (16.0 ± 0.8 versus 11.1 ± 0.5 nM day-1), perhaps due to the lower light and higher iron concentrations characteristic of polar waters. Additional NH₄⁺ concentration measurements spanning the 2018/2019 annual cycle suggest that mixed-layer NH₄⁺ accumulation south of the SAF is due to sustained heterotrophic NH₄⁺ production in late summer through winter that outpaces NH₄⁺ removal by temperature-, light, and iron-limited microorganisms. The contribution by heterotrophic prokaryotes is supported by observations from winter 2017, where lower ratios of photosynthetic-to-heterotrophic cells were associated with maxima in NH₄⁺ concentrations. These observations imply that the Southern Ocean 27 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. High wintertime surface NH4 + concentrations, and the drivers of biological NH4 + cycling, may also have implications for nitrate uptake, through inhibition, and for the air-sea flux of ammonia gas, with the latter influencing the formation of aerosols, clouds, and climate.
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    Drivers of short-term variability in phytoplankton production in an embayment of the southern Benguela upwelling system: an observational and modelling study
    (2018) Burger, Jessica; Fawcett, Sarah; Moloney, Coleen
    In the southern Benguela upwelling system (SBUS), the wind-driven supply of nutrient-rich water from depth sustains elevated levels of primary productivity. St Helena Bay (SHB), a coastal embayment in the SBUS positioned north of an upwelling centre, is an area of water mass retention. In addition to supporting 40-50% of total SBUS productivity, SHB often experiences harmful algal blooms (HABs) and hypoxic conditions that are difficult to predict given the high sub-seasonal variability that characterises this region. To better understand this variability, net primary production (NPP), nitrate and ammonium uptake, and phytoplankton community composition were measured for ten days during the upwelling season at an anchor station in SHB. A period of active upwelling (days 1-5) was followed by one of relaxation (day 6-10), together constituting an “upwelling cycle”. During upwelling, the mixed layer was deeper than the euphotic zone and phytoplankton were light-limited, evidenced by high ambient nutrient concentrations and relatively low rates of NPP and nitrate uptake. During relaxation, water column stratification increased, restricting phytoplankton production to a shallow, well-lit surface layer in which nitrate was exhausted after three days. The subsequent decline in NPP and nitrate uptake rates confirms that nutrient availability succeeded light as the ultimate control on productivity during the relaxation phase. Of the three phytoplankton size classes investigated (0.7-2.7 µm, 2.7-10 µm, >10 µm), the 2.7-10 µm fraction contributed most to the measured increases in biomass and nutrient uptake rates. This was unexpected given that large (>10 µm) diatoms typically dominate in upwelling systems; however, the 2.7-10 µm size fraction achieved a faster growth rate and sustained it for longer than the other size classes. The success of this size fraction may be partly due to a capacity for luxury nitrate uptake, evidenced by a low biomass C:N ratio and a nitrate uptake rate that was decoupled from NPP. Throughout the experiment, the phytoplankton community comprised mainly Chaetoceros spp. and Skeletonema costatum. These diatoms occupy a large size range (2-80 µm), although it is likely that they mainly occurred in the 2.7-10 µm size class during the experiment. They also produce resting spores that may provide a selective advantage during seeding in highly variable upwelling systems, increasing their chances of proliferating when conditions become favourable. Once the water column stratified, the phytoplankton community diversified, with dinoflagellates and the large diatom, Coscinodiscus gigas (200-500 µm), becoming more abundant. The contribution of C. gigas to biomass and productivity was not fully accounted for in the measurements because collected seawater was screened (200 µm mesh) prior to incubation. However, a simple N₃P₃ ecological model parameterized with the observations suggests that their contribution would have been minimal. The hydrographic data indicate that another upwelling cycle commenced by day 10 of the experiment. This likely prevented the further proliferation of dinoflagellates, some of which are HAB species, that may have succeeded the small diatoms given a longer period of quiescence. One implication of this is that understanding the rapid cycling between light and nutrient limitation, as induced by an actively upwelling versus stratified water column, may advance our capacity to predict the occurrence of HABs in SHB.
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    Exploring South Africa’s southern frontier: A 20-year vision for polar research through the South African National Antarctic Programme
    (CrossMark, 2017-06) Ansorge, Isabelle J; Skelton, Paul; Bekker, Annie; de Bruyn, P J Nico; Butterworth, Doug S; Cilliers, Pierre; Cooper, John; Cowan, Don A; Dorrington, Rosemary; Fawcett, Sarah; Fietz, Susanne; Findlay, Ken P; Froneman, P William; Grantham, Geoff H; Greve, Michelle; Hedding, David; Hofmeyr, G J Greg; Kosch, Michael; le Roux, Peter; Lucas, Mike; MacHutcho, Keith; Meiklejohn, Ian; Nel, Werner; Pistorius, Pierre; Ryan, Peter; Stander, Johan; Swart, Sebastiaan; Treasure, Anne; Vichi, Marcello; Jansen van Vuuren, Bettine
    Antarctica, the sub-Antarctic islands and surrounding Southern Ocean are regarded as one of the planet’s last remaining wildernesses, ‘insulated from threat by [their] remoteness and protection under the Antarctic Treaty System’1 . Antarctica encompasses some of the coldest, windiest and driest habitats on earth. Within the Southern Ocean, sub-Antarctic islands are found between the Sub-Antarctic Front to the north and the Polar Front to the south. Lying in a transition zone between warmer subtropical and cooler Antarctic waters, these islands are important sentinels from which to study climate change.2 A growing body of evidence3,4 now suggests that climatically driven changes in the latitudinal boundaries of these two fronts define the islands’ short- and long-term atmospheric and oceanic circulation patterns. Consequently, sub-Antarctic islands and their associated terrestrial and marine ecosystems offer ideal natural laboratories for studying ecosystem response to change.5 For example, a recent study6 indicates that the shift in the geographical position of the oceanic fronts has disrupted inshore marine ecosystems, with a possible impact on top predators. Importantly, biotic responses are variable as indicated by different population trends of these top predators.7,8 When studied collectively, these variations in species’ demographic patterns point to complex spatial and temporal changes within the broader sub-Antarctic ecosystem, and invite further examination of the interplay between extrinsic and intrinsic drivers.
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    Investigating the biogeochemistry of the Mozambique Channel thermocline using an optimum multiparameter analysis approach
    (2022) Harris, Eesaa; Fawcett, Sarah; Marshall, Tanya
    The ocean's thermocline represents the transition zone between the surface and deep ocean. Its formation is linked to vertical and horizontal diffusion, and it constitutes part of the ocean's wind driven circulation, playing a critical role in the lateral advection and vertical supply of nutrients. In the Indian Ocean, the topography, winds, and inter-ocean exchange make the basin's thermocline unique. Particularly complex thermocline circulation occurs in the Mozambique Channel, the highly dynamic region between the coast of south eastern Africa and Madagascar, owing to the confluence of tropical and subtropical regimes in the southwestern Indian Ocean. Our current understanding of the Mozambique Channel thermocline is largely derived from hydrographical analyses (i.e., conservative property relationships such as temperature-salinity), which do not resolve the influence of mesoscale features such eddies on the regional biogeochemistry. Additionally, the Mozambique Channel is one of the three source regions to the Agulhas Current, such that its biogeochemistry may influence the waters of this western boundary current. Here, the thermocline array approach within the optimum multiparameter (OMP) analysis framework was used to determine the contributions of different source waters to the thermocline across four transects sampled in the southwestern Indian Ocean, one at either end of the Mozambique Channel and two across the upper Agulhas Current. Three thermocline source regions proximate to the Mozambique Channel were identified (equatorial, tropical and subtropical) and used to initialise the OMP analysis. This localised approach was validated by the low standard deviations (average <20%) associated with perturbing the model parameters in a Monte Carlo analysis and the low residuals (average < 5%) associated with the model solution, allowing for an evaluation of relative biogeochemical changes across the region. A decline in the upper thermocline nutrient concentrations between the source regions and the transects can be explained by mixing with nutrient-deplete surface waters. By contrast, an increase in the nutrient concentrations of the thermocline evinces in situ remineralization, presumably following primary productivity in Mozambique Channel surface waters. The presence of tropical waters across the two Agulhas transects confirms a supply of tropical nutrients to the current, with the coincidence of significant tropical water contributions (up to 60%) and mesoscale eddies suggesting that these features represent a mechanism by which tropical thermocline water is supplied to the Agulhas Current region. A rise in the thermocline nitrate-to-phosphate ratios across the Agulhas transects but not in the Mozambique Channel transects indicates that nitrogen is added to the Agulhas Current region by local N2 fixation occurring in the subtropical waters south of the Mozambique Channel (25°S). This finding contradicts recent suggestions that the channel itself is a hotspot for N2 fixation. The fact that the OMP analysis applied at the regional scale captures the complexity and heterogeneity of the southwest Indian Ocean indicates that this approach can be used to quantitatively assess thermocline contributions to shallow nutrient cycling in hydrodynamically complex regions.
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    Mixed layer nitrogen cycling in the Southern Ocean: seasonality, kinetics, and biogeochemical implications
    (2021) Mdutyana, Mhlangabezi; Fawcett, Sarah; Thomalla, Sandy J
    The alternation between summertime nitrate drawdown and wintertime nitrate recharge is central to the role of the Southern Ocean in setting atmospheric CO2. However, active cycling of nitrogen (N) in the seasonally-varying mixed layer – including the production of ammonium and its subsequent removal via phytoplankton uptake and nitrification (i.e., the oxidation of ammonium to nitrite and then nitrate) – remains poorly understood. Following the “new production paradigm”, phytoplankton production fueled by ammonium (“regenerated production”) results in no net drawdown of CO2 to the deep ocean, while growth supported by nitrate (“new production”) can be equated to CO2 removal provided that mixed-layer nitrification is negligible. While non-zero mixed-layer nitrification has been measured in many ocean regions, very few data exist for the Southern Ocean. This thesis presents new N cycle data collected across the Southern Ocean south of Africa in winter and summer that emphasize the integral role of mixed-layer N transformations in Southern Ocean productivity and biological CO2 drawdown. To evaluate the new production paradigm as a framework for quantifying Southern Ocean carbon export potential, rates of net primary production (NPP), N uptake (as ammonium and nitrate) and nitrification (ammonium and nitrite oxidation) were measured across the Atlantic sector in winter and summer. Winter mixed-layer NPP and total N (i.e., ammonium + nitrate) uptake were strongly decoupled, likely due to elevated heterotrophic bacterial consumption of ammonium. In summer, NPP and total N were generally well-coupled, although dissolved organic N apparently supported more than a third of NPP at some stations. Nitrification accounted for >100% of the nitrate consumed by phytoplankton in winter, rendering the new production paradigm ill-suited for quantifying carbon export in this season. By contrast, of the >50% of summertime NPP fueled by nitrate, < 4% on average derived from mixed-layer nitrification. While the near-zero mixed-layer nitrification rates measured in summer could be taken as confirmation that nitrate uptake is a good proxy for Southern Ocean carbon export potential, a portion of the nitrate consumed in the summertime euphotic zone was produced in the winter mixed layer and will thus not support net carbon dioxide removal on an annual basis. Despite the high rates of ammonium uptake and oxidation measured in winter Southern Ocean surface waters, mixed-layer ammonium concentrations remain fairly high, indicating an imbalance between ammonium production and consumption. Kinetics experiments conducted across the Indian sector (37-62ºS) reveal a seasonal switch from a phytoplankton community with a high affinity for ammonium in summer to one with a lower affinity in winter, even though phytoplankton at similar latitudes achieved a comparable maximum specific ammonium uptake rate in summer and winter. Rates of ammonium oxidation showed a Michaelis-Menten response to substrate availability only when the ambient ammonium concentration was ≤90 nM. This, coupled with half-saturation constants (Km values) of 28-137 nM (i.e., indicating a very high affinity for ammonium) suggest a dominant role for ammonia oxidizing archaea in mixed-layer nitrification. The maximum rate of ammonium oxidation was near-constant across the transect (37-62ºS), despite a significant gradient in sea surface temperature, light availability and ammonium concentration, perhaps due to iron limitation of ammonium oxidation, which has been hypothesized from culture experiments but not yet shown in the environment. It is possible that iron depletion in the surface Southern Ocean may limit the role of winter mixed-layer nitrification in offsetting phytoplankton CO2 drawdown annually. To better understand the controls on nitrifier ecology in the surface Southern Ocean, a series of nitrite oxidation kinetics experiments were conducted across the Indian sector (37- 62ºS) in winter. All experiments yielded a Michaelis-Menten relationship with substrate concentration, yet the nitrite oxidation rates only increased significantly at nitrite concentrations >115-245 nM, suggesting that nitrite oxidizers require a minimum (i.e., “threshold”) nitrite concentration to produce nitrate. Low derived Km values (134-403 nM) that increased with increasing ambient nitrite indicate a high affinity of nitrite oxidizers for substrate, in contrast to results from culture experiments. Throughout the Southern Ocean mixed layer, ambient nitrite concentrations are rarely less than 150 nM, regardless of season. Coincident measurements of ammonium and nitrite oxidation in the mixed layer suggest that nitrite oxidation is the rate-limiting step for nitrification in the winter Southern Ocean. This, combined with a possible nitrite concentration threshold for nitrite oxidation, may explain the perennial non-zero mixed-layer nitrite. A possible explanation for the apparent threshold nitrite requirement of nitrite oxidizers is undersaturation of the hemerich nitrite oxidoreductase enzyme, perhaps driven by the limited availability of iron in Southern Ocean surface waters. The findings described in this thesis yield new insights into the active cycling of N within the Southern Ocean's mixed layer, and particularly emphasize the need for seasonally-resolved parallel N- and iron cycle investigations to fully understand the role of nitrification in biological CO2 removal and N supply. Climate change-driven warming and acidification of Southern Ocean surface waters is already driving changes in microbial community composition, nutrient supply, and primary productivity. If we are to better predict the Southern Ocean's future role in CO2 sequestration and global ocean fertility, an improved understanding of the controls on mixed layer N cycling, particularly nitrification, is essential.
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    Modelling waves and near-shore circulation around the Cape Peninsula: towards enhanced predictions for South African coastal activities
    (2022) de Vos, Marc; Vichi, Marcello; Fawcett, Sarah
    South Africa's coastal oceans support a multitude of human and ecological interests. My work at the confluence of academia, industry and government highlighted a gap in knowledge and capacity in the marine environmental information space. Broad research questions were developed and addressed through a combination of statistical review, data mining and numerical modelling: how do marine weather and oceanography impinge on South African coastal activities, what are the main circulation characteristics in a critical coastal region, and which environmental data are needed for predictions? Hazard modelling of environmental conditions and coastal user safety in South Africa revealed a wide sensitivity to weather-ocean hazards among different users and places. The Cape Peninsula sub-region exhibited the highest incident frequency related to increasing hazard severity. I implemented a coupled numerical ocean model (Delft3D FLOW and SWAN), with 700 m horizontal resolution to simulate waves and currents in this area. The model is driven by realistic forcing, striking a previously lacking balance between geographical coverage, spatial granularity, and process complexity. It has the potential to be deployed in forecast mode in future. Modelling confirmed the importance of wind in establishing broad circulation patterns and inducing nearshore upwelling. However, bay-scale cyclonic and anticyclonic flow in False Bay has low-medium predictability. These patterns developed as expected at times but failed to do so at others. The model's inclusion of two-way wave-current interactions enabled the identification of a novel cyclonic gyre in Table Bay during large wave events. Investigation revealed the gyre to be driven by wave induced radiation stress gradients. Model experiments with and without wave-coupling demonstrated that the gyre causes material differences in virtual drift trajectories and affects the monthly mean general circulation. This highlights its potential impact on coastal users and its implications for understanding Table Bay's circulation, where the importance of waves beyond the nearshore may be crucial for predictability. Enhanced velocity measurements, sufficient to resolve the alternating rotational circulation in False Bay and wave-driven gyre in Table Bay, are recommended. It is further suggested that appropriate wave forcing be included in any further circulation modelling in the study area.
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    Nitrogen cycling in the South Atlantic and South Indian Oceans investigated using nitrate isotopes: implications for nutrient supply, ocean fertility, carbon export, and climate
    (2023) Marshall, Tanya; Fawcett, Sarah
    Bioavailable nitrogen (N) limits phytoplankton growth across much of the (sub)tropical ocean, thereby modulating ocean fertility and climate. Dinitrogen (N2) fixation is the dominant source of new N to the ocean and is thought to occur mainly in well-lit, warm, oligotrophic waters. The under-sampled South Atlantic and South Indian Ocean basins are predicted by models to host widespread N2 fixation; for the South Atlantic, this predication contradicts the limited available observations and for the South Indian, is yet to be confirmed by measurements. In this thesis, four new nitrate isotope datasets from the South Atlantic and South Indian Oceans are presented alongside coincident nutrient and hydrographic data, and other published nitrate isotope datasets. Combined, these data provide a means of quantifying the rate and distribution of N2 fixation, along with characterizing additional co-occurring N cycle processes, mechanisms of subsurface nutrient supply, and water mass circulation. Measurements of nitrate N isotope ratios (15N) and nutrient stoichiometry (i.e., nitrate to phosphate ratios; N:P) from a zonal transect of the tropical South Atlantic (at ~12S) and a meridional transect along the Angola margin (at ~12E) reveal an N2 fixation hotspot in the eastern tropical Angola Gyre. Here, thermocline nitrate 15N is low and N:P is high relative to the underlying source water and the western tropical basin thermocline. The N2 fixation rate estimated from the Angola Gyre nitrate 15N data of 1.4-5.4 Tg N.a-1 accounts for 28-108% of the rate predicted for the South Atlantic basin. These findings contradict recent model diagnoses of N2 fixation, which predict high rates in the western tropical basin and none to the east. The overlapping biogeography of a basin-wide P excess relative to N and bioavailable iron supplied locally from the Angola margin likely control N2 fixation in the Angola Gyre. Analogous conditions elsewhere in the ocean, such as in other eastern boundary shadow zones and retentive near-coast subtropical systems, should also favour N2 fixation. The western boundary current of the South Indian Ocean, the Agulhas Current, is the strongest boundary current on Earth, yet nutrient cycling in this subtropical system remains largely uncharacterized. Measurements of the dual isotope ratios (N and oxygen) of nitrate from within and upstream of the greater Agulhas region provide insights into regional circulation and N cycle dynamics. The nitrate isotopes reveal both local and remote signals of Indian Ocean N cycling such as denitrification in the Arabian Sea and partial nitrate assimilation in Southern Ocean surface waters, as well as evidence of local N2 fixation and coupled partial nitrate assimilation and nitrification. Using a one-box model to simulate the newly-fixed nitrate flux, the local N2 fixation rate for the greater Agulhas region is estimated to Thesis abstract be 7-25 Tg N.a-1; this value is the first observation-based N2 fixation rate estimate for the South Indian Ocean. Local N cycling imprints an isotopic signal on Indian Ocean nitrate that can be tracked beyond the Indian Ocean because it persists in Agulhas eddies that “leak” into the South Atlantic at the Agulhas Retroflection. If this signal is retained in plankton that sink to the seafloor, it could be used to reconstruct past Agulhas leakage, yielding quantitative insights into the strength of the Atlantic Meridional Overturning Circulation in the past. The Agulhas Current system, like other western boundary current systems, is characterised by high energy and turbulence. A novel application of the dual isotopes of nitrate reveals the occurrence of three (sub)mesoscale mechanisms of upward nitrate supply; entrainment at the edges of a mesoscale anticyclonic eddy, inshore upwelling likely driven by a frontal eddy, and overturning at the offshore edge of the current core likely driven by coupled mesoscalesubmesoscale instabilities. The intensity and (sub)surface expression of these nutrient supply events are not always apparent in the hydrographic data, highlighting the utility of the nitrate isotopes for exploring physical ocean processes. The conditions driving the nitrate supply mechanisms in the Agulhas region are common to western boundary currents, implying that the (sub)mesoscale vertical nitrate supply is quantitatively significant at the global scale. Additionally, these events of upward nitrate supply likely increase regional fertility in all western boundary current systems, with implications for the sustenance of higher trophic levels. Finally, increasing turbulence observed along mid-latitude western boundaries may enhance the upward nutrient supply to subtropical surface waters, and possibly compensate for the diminished productivity predicted as a result of increasing subtropical gyre stratification. Collectively, the work detailed in this thesis reveals the strong regionality of N cycling in the historically under-studied South Atlantic and South Indian Oceans, as well as the importance of interpreting biogeochemical data in the context of ocean dynamics across various scales. Improved predictions of N fluxes at the basin- and global scale, which are critical for estimating the ocean's CO2 sink and fertility, will require careful consideration of these southern basins so as not to mischaracterise their functioning, as has occurred in the past.
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    Nitrogen cycling in the subtropical southeast Atlantic and southwest Indian Oceans as recorded by the nitrogen isotopes of modern planktic foraminifera
    (2023) Granger, Robyn; Fawcett, Sarah
    Despite the importance of nitrogen (N) for ocean productivity, and the long history of using fossil foraminifera to reconstruct past ocean conditions, it is only in recent years, due to methodological advances, that the nitrogen isotope ratio (δ15N) of foraminifera has become a viable proxy for past marine nutrient cycling. Organic N trapped within planktic foraminifer shells is protected from bacterial degradation, with its δ15N recording the processes acting on the upper- ocean N pool. This thesis examines the relationship between local biogeochemi- cal cycling and foraminifera tissue- and shell-bound δ15N in the greater Agulhas Current system and southeast Atlantic Ocean, focusing on the implications for reconstructing Agulhas leakage (i.e., the transfer of Indian Ocean waters into the Atlantic). Past fluctuations in this important component of the Atlantic Meridional Overturning Circulation, whereby warm, saline Agulhas waters are transported to the North Atlantic along its upper limb, have been tied to global glacial-interglacial cycles, highlighting the region's sensitivity to large-scale cli- mate change. The work detailed in this thesis includes the first foraminifer- bound δ15N ground-truthing studies from the southeast Atlantic and the Agul- has Current regions and examines the extent to which the unique δ15N signature of Indian Ocean nitrate is preserved in the tissue and shells of foraminifera living in Agulhas leakage features (e.g., eddies). The isotopes of several forms of N, including nitrate, particulate organic N, size-fractionated zooplankton biomass, living foraminifera tissue and shell N, and fossil foraminifera, were measured and interpreted in the context of coincident hydrographic measurements to deter- mine the controls on the δ15N of foraminifera and their potential food sources. The data presented here reveal that mixed layer nitrate δ15N was noticeably lower within an Agulhas eddy than it was for the surrounding Cape Basin wa- ters, a characteristic that was likely inherited from low thermocline nitrate δ15N produced in the region of leakage origin, the Agulhas Current System. Simi- larly, the δ15N of foraminifera inhabiting the Agulhas eddy was found to be low relative to foraminifera under background southeast Atlantic conditions, despite foraminifera in the Agulhas Current System displaying on average a higher δ15N than was recorded by foraminifera inhabiting the eddy. The data therefore sug- gest that anticyclonic eddies “leaking” into the region from the Indian Ocean maintain a low-δ15N environment that sustains the growth of foraminifera for several months, and that N2 fixation and/or recycling of low-δ15N ammonium within the eddy environment likely contributed to lowering of foraminifer-δ15N. That foraminifer-δ15N is on average 2-3‰ lower in Agulhas leakage than in the southeast Atlantic suggests that enduring periods of increased leakage could result in relatively low-δ15N material being transferred to the sediment and recorded. A comparison of data from the southeast Atlantic and Agulhas regions to previous ground-truthing studies from the Sargasso Sea and Southern Ocean reveals similarities in both foraminifer tissue-shell δ15N relationships and inter- species δ15N differences. For instance, symbiont-hosting foraminifera are consis- tently lower in δ15N than deeper-dwelling, symbiont-barren individuals at the same location due to the symbiont's ability to recycle low-δ15N ammonium. Also consistent with previous studies is the positive correlation observed be- tween fossil foraminifera from core tops and modern shell- and biomass δ15N in the Atlantic, despite sediment being derived from multiple locations within the Cape Basin. This study adds to burgeoning efforts to ground-truth the foraminifer-δ15N palaeo-proxy and supports the argument that the δ15N of liv- ing foraminifera, which is set by both the local N supply and N-cycling processes, can be deduced from foraminifera shell-bound δ15N in the sediment record. Fur- thermore, the work detailed in this thesis examines how the unique δ15N of the nitrate and biological community of a particular water mass might be leveraged to reconstruct past variations in Agulhas leakage.
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    On-shelf nutrient trapping enhances the fertility of the southern Benguela upwelling system
    (2019) Flynn, Raquel; Fawcett, Sarah; Granger, Julie
    The southern Benguela upwelling system (SBUS), located off the southwest coast of Africa, supports high rates of primary productivity that sustain important commercial fisheries. The exceptional fertility of this system is reportedly fuelled not only by upwelled nutrients, but also by nutrients regenerated on the broad and shallow continental shelf. We present the first nitrate nitrogen (N) and oxygen (O) isotope data (δ15N and δ18O, respectively) from the SBUS, generated for samples collected along four hydrographic lines in February (summer) and May (early winter) of 2017. During summer upwelling, a decrease in nitrate δ 18O on the shelf reveals that on average, 30% of the subsurface nutrients derive from in situ remineralization of sinking phytoplankton biomass. In the more quiescence winter, an average of 35% of the on-shelf nitrate is regenerated, with the signal propagating further westward along the mid-shelf region such that the total regenerated nitrate burden is greater during this season. In both seasons, a shoreward increase in subsurface nitrate δ 15N and decrease in N* (i.e., total dissolved nitrogen - 16 x phosphate + 2.9) suggests N loss to benthic denitrification coincident with the on-shelf remineralization, which implies that an even higher quantity of nitrate is regenerated than we calculate. Our data show that remineralized nutrients get trapped on the SBUS shelf in summer and early winter, enhancing the nutrient pool that can be upwelled to support surface productivity and decreasing bottom water oxygen concentrations. The proposed mechanism for this “nutrient trapping” involves upwelled nutrients being removed from surface waters and converted into organic biomass that is sequestered and remineralized on the shelf while the now nutrient-deplete surface waters are advected offshore by Ekman transport. This process is aided by a number of equatorward-flowing fronts that impede the lateral exchange of waters in the upper 200 m of the water column, increasing their residence time on the shelf. The extent to which remineralized nutrients are trapped on the SBUS shelf has implications for bottom water hypoxia. Trapped nutrients will be supplied to the surface during upwelling, supporting high rates of primary productivity and a large sinking biomass flux. The subsequent on-shelf remineralization of this organic matter has the potential to further decrease already-low bottom water oxygen concentrations.
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    Phytoplankton's role in the biological pump during the growth season across the Atlantic Southern Ocean
    (2023) Flynn, Raquel; Fawcett, Sarah
    Southern Ocean phytoplankton growth is seasonally co-limited by light and iron availability. In winter, iron and macronutrients such as nitrate are supplied to surface waters during deep mixing events. Sea ice melt and increased solar radiation in spring drive rapid stratification of surface waters, alleviating phytoplankton from light limitation and allowing them to consume the nutrients supplied in winter. In the framework of the “new production paradigm”, phytoplankton growth fuelled by upwelled nitrate (“new production”) can be equated to atmospheric CO2 removal, while growth fuelled by ammonium and urea (“regenerated production”) results in no net CO2 drawdown. As such, once phytoplankton begin assimilating the nitrate supplied from the subsurface, the upper Southern Ocean ecosystem starts removing atmospheric CO2. As the spring/summer growth season progresses, mixed-layer iron concentrations decline, leading to phytoplankton growth being predominantly fuelled by regenerated nutrients, with a concomitant decrease in CO2 removal. The role of phytoplankton in the Southern Ocean's CO2 sink remains poorly understood, particularly early in the growth season (i.e., spring) and in regions where thick sea-ice conditions persist year-round (e.g., Larsen C Ice Shelf; LCIS). For the research described in this thesis, field sampling was undertaken near the LCIS during summer 2018/2019 and across the Atlantic Southern Ocean in spring 2019. Rates of net primary production (NPP) and nitrogen (N; as nitrate, ammonium, and urea) uptake were measured in both seasons to determine the dominant N source supporting phytoplankton growth and to quantify carbon export potential. In spring, coincident rates of iron uptake were also measured, allowing for an assessment of the iron requirements of different phytoplankton size classes. In contrast to expectations that large diatoms dominate the open Southern Ocean spring bloom, the biomass and rates of NPP and N uptake were dominated by nanoplankton (2.7-20 µm), particularly the diatom Chaetoceros spp., which employs a “boom-and-bust” life-cycle. It appears that Chaetoceros were able to grow so rapidly because of their low iron and light requirements, as well as their ability to form long spiney chains that reduce grazing pressure. The Chaetoceros bloom was relatively short-lived (a few weeks) and its senescence, driven by iron limitation and increased grazing pressure, drove a massive export event that accounted for roughly a third of the carbon exported over the entire spring/summer growth season. In contrast to spring, the late-summer phytoplankton community in the Weddell Sea relied strongly on regenerated N, with ammonium and urea consumption fuelling 53 ± 8% of their growth at LCIS. These high rates of regenerated production coincided with an elevated ammonium supply rather than low iron availability as sea-ice melt led to non-limiting iron concentrations in surface waters. The high ammonium concentrations partially inhibited nitrate uptake and thus, biological carbon export, particularly near LCIS. Here, the phytoplankton community shifted from Phaeocystis Antarctica- to diatom-dominated as the growth season progressed, coincident with an increase in upper water-column stratification. This shift may have further decreased carbon export as P. Antarctica can fix up to 50% more carbon per mole of phosphate consumed than diatoms. To further investigate the role of small (<20 µm) phytoplankton in nutrient and carbon cycling in the summertime Weddell Sea, the 15N/14N of four groups sorted by flow cytometry (Synechococcus, picoeukaryotes, nanoeukaryotes, and cryptophytes) was measured. Phytoplankton growth fuelled by subsurface nitrate produces particulate organic N (PON) that is high in 15N/14N relative to growth fuelled by ammonium. Using biomass 15N/14N as a measure of new- versus regenerated N dependence, the contribution of each phytoplankton group to carbon export potential can be estimated. Synechococcus generally relied on nitrate less than the other phytoplankton groups although interestingly, its nitrate dependence was highest at LCIS and increased with increasing seawater temperatures and ammonium concentrations. Conversely, the high LCIS ammonium concentrations may have partially inhibited nitrate consumption by the picoeukaryotes, nanoeukaryotes, and cryptophytes. These groups relied most heavily on nitrate at the Antarctic Peninsula and in the Weddell Gyre even though total productivity was significantly lower in these regions than at LCIS. The flow cytometry-15N/14N data yield a higher (although still overlapping) mean estimate of new- relative to total production (i.e., the proportion of exportable carbon) than the 15N-tracer-derived uptake rates (56 ± 18% versus 47 ± 15% of productivity fuelled by nitrate). This discrepancy is likely due to the different time-scales captured by the two methods, with PON isotopes integrating over days to weeks and the rate experiments quantifying N uptake at the time of sampling only. Additionally, flow cytometric sorting may have excluded certain phytoplankton groups that relied more heavily on regenerated N (e.g., P. Antarctica). It is thus important that the results of either method be interpreted in the appropriate context. The work presented in this thesis shows how seasonal shifts in the phytoplankton community drive changes in biological carbon export in the Southern Ocean. In contrast to previous suggestions, the novel measurements described herein indicate that although summer is the period of maximum biomass accumulation in the Southern Ocean, it may not be the season of highest carbon export. This finding can be explained by the increasing reliance of phytoplankton on regenerated N as the season progresses. In ice-adjacent waters, this shift is largely due to elevated ammonium availability rather than iron limitation, which may (partially) inhibit nitrate uptake. By contrast, in the open Southern Ocean, iron limitation drives the seasonal shift towards regenerated production because of the high iron requirement of nitrate uptake. The work detailed in this thesis also indicates that different phytoplankton groups are better adapted to different physicochemical conditions (e.g., P. Antarctica to deep mixed-layers versus diatoms to recently-stratified waters), with the dominance of one group over another acting to strengthen or weaken the Southern Ocean's biological pump. Understanding the drivers of phytoplankton community composition is essential if we are to predict how phytoplankton will respond to a changing climate, and the implications of their response for the Southern Ocean's biological pump.
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    Plankton dynamics of the open Southern Ocean and surrounding the (Sub)Antarctic islands
    (2023) Stirnimann, Luca; Fawcett, Sarah; Bornman, Thomas G; Verheye, Sir Hans M.
    The Southern Ocean is a high-nutrient, low-chlorophyll region where primary productivity is limited mainly by iron and light availability, yet it accounts for ~30-40% of global ocean CO2 absorption annually. Marine plankton play a major role in the Southern Ocean CO2 sink as they fix dissolved atmospheric CO2 into organic carbon biomass, much of which supports the ocean food web and a portion of which sinks into the ocean interior, thereby removing atmospheric CO2 on decadal to centennial timescales (i.e., the biological carbon pump). The importance of plankton diversity and dynamics in modulating carbon production and export remains poorly understood, particularly around the many (Sub)Antarctic islands where physical and biogeochemical variability is high. The major motivation for the work presented in this thesis is an improved understanding of the role of the plankton system in Southern Ocean fertility and carbon export, and relatedly, the response of the plankton to environmental forcing such as changes in nutrient dynamics driven by hydrography and island mass effects. To that end, I investigated plankton community diversity and ecological dynamics in the context of nutrient cycling, primary production, and carbon export potential in the open Southern Ocean and in the vicinity of its many island systems. Specifically, I used carbon and nitrogen stable isotope ratios as a tool to quantify carbon export potential and food web dynamics across all major hydrographic zones and basins of the Southern Ocean. Five main findings emerged. Firstly, I developed insights into the major drivers of spatial and temporal variability in the carbon and nitrogen isotope ratios (δ13C and δ15N) of the Southern Ocean's plankton system using circum-Antarctic carbon and nitrogen isoscapes. Along with the drivers commonly invoked by previous studies, I further determined a relationship between the δ13C and δ15N of suspended particulate matter (SPM) and phytoplankton community composition, with diatoms exerting a particularly strong influence on the δ13C and δ15N of the SPM, which is subsequently transferred to the zooplankton. Secondly, I observed that the (Sub)Antarctic islands tend to increase the δ13C and δ 15N of phytoplankton and zooplankton relative to the open Southern Ocean. This trend can be explained by the input of terrestrially-derived iron and other nutrients (e.g., ammonium and/or urea from birds and seals) into the surface layer, which stimulate diatom growth on nitrate and/or exogenous reduced nitrogen sources that are high in δ15N. Thirdly, I applied a new approach using the δ15N of seawater nitrate and SPM to quantify carbon export potential across the summertime Southern Ocean. I found that carbon export potential is highest near the islands and melting sea ice, driven by the input of limiting nutrients (i.e., iron) and by the dominance of diatoms. Fourthly, I found that the δ15N of SPM is a reliable baseline for trophic analysis of the zooplankton system over a large spatial extent of the Southern Ocean (i.e., circum-Antarctic). Since the collection and analysis of SPM samples for δ15N is relatively straightforward, this result should be welcomed by researchers who use such data to reconstruct trophic flows through plankton food webs, as well as the movements and dietary histories of zooplankton in the Southern Ocean. Finally, my new zooplankton δ13C and δ15N isoscapes reveal that during the summer, the primary zooplankton consumers in the Subantarctic waters of the Southern Ocean occupy a low trophic position akin to herbivores, implying that the Subantarctic food web may act to retain organic carbon within the euphotic zone instead of exporting it to depth. By contrast, the primary consumers in Antarctic waters occupy a higher trophic position that suggests they are omnivores and carnivores, which potentially indicates a shorter food chain and thus a stronger biological pump. The work detailed in this thesis suggests new methodological approaches for studying the Southern Ocean plankton system and offers an improved understanding of plankton dynamics and their relationship(s) with the biogeochemical processes that govern the different zones of the Southern Ocean.
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    Toward an improved understanding of the Southern Ocean's biological pump: phytoplankton group-specific contributions to nitrogen and carbon cycling across the Subantarctic Indian Ocean
    (2021) Forrer, Heather; Fawcett, Sarah
    Iron (and silicate) (co-)limitation of phytoplankton is considered a primary cause of the Southern Ocean's inefficient biological pump. However, the role of phytoplankton community structure and response to nutrient cycling remains poorly understood. In a mass balance sense, phytoplankton consumption of new nitrogen (N; e.g., allochthonous nitrate) is proportional to net carbon (C) export, while growth fueled by recycled N (e.g., ammonium) yields no net C flux. The N isotope ratio (δ15N) of surface biomass has long been used as an integrative tracer of new versus regenerated uptake. This approach is rendered more accurate by coupling either fluorescence-activated cell sorting (FACS; of nano- and picophytoplankton; 0.4-20 μm) or microscopy (for microphytoplankton; >20 um) with groupspecific δ15N measurements. Samples were collected for the analysis of nutrients and nitrate-, FACS-, and microscopy-δ15N on a mid-summer transect of the Subantarctic Indian basin during the 2016/17 Antarctic Circumnavigation Expedition (ACE) cruise. The data show that all phytoplankton populations preferentially utilize nitrate (≥55%) across the Indian Sector of the Subantarctic, potentially driving higher C export potential than previously estimated. Indeed, near the Subantarctic islands, 72% of microand >80% of nano- and picophytoplankton growth is supported by nitrate. This is likely due to the partial alleviation of phytoplankton iron and silicate stress, largely as a result of bathymetric upwelling, which constitutes a manifestation of the island mass effect. C export potential is lower in the open ocean region away from the islands where iron stress has been shown to be higher; here, nitrate supports >55% of micro- and picophytoplankton and 7 to 79% of nanophytoplankton growth. In terms of relative abundance (RA), the open Subantarctic is dominated by picoeukaryotes (64%), although there exists a large disconnect between relative abundance and potential contribution to C export. The three largest surface-ocean phytoplankton populations included in this study – microphytoplankton, cryptophytes, and nanoeukaryotes – each contribute ~30% to the total C export potential across the Subantarctic Indian sector while picophytoplankton contribute ~5%. Thus, as has been concluded previously, the larger phytoplankton size classes are disproportionately important drivers of the Subantarctic biological pump. Other interesting ecological findings include diatom-dominated microphytoplankton populations apparently fueled by a significant fraction of regenerated N, even in areas of iron supply, and Synechococcus relying near-exclusively on new N, in contrast to subtropical observations. Additionally, the abundance of Synechococcus appears to be controlled by the availability of iron across the Subantarctic, with silicate and temperature playing a supporting role.
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    Towards high fidelity mapping of global inland water quality using earth observation data
    (2021) Kravitz, Jeremy; Matthews, Mark; Bernard, Stewart; Fawcett, Sarah
    This body of work aims to contribute advancements towards developing globally applicable water quality retrieval models using Earth Observation data for freshwater systems. Eutrophication and increasing prevalence of potentially toxic algal blooms among global inland water bodies have become a major ecological concersn and require direct attention. There is now a growing necessity to develop pragmatic approaches that allow timely and effective extrapolation of local processes, to spatially resolved global products. This study provides one of the first assessments of the state-ofthe-art for trophic status (chlorophyll-a) retrievals for small water bodies using Sentinel-3 Ocean and Land Color Imager (OLCI). Multiple fieldwork campaigns were undertaken for the collection of common aquatic biogeophysical and bio-optical parameters that were used to validate current atmospheric correction and chlorophyll-a retrieval algorithms. The study highlighted the difficulties of obtaining robust retrieval estimates from a coarse spatial resolution sensor from highly variable eutrophic water bodies. Atmospheric correction remains a difficult challenge to operational freshwater monitoring, however, the study further validated previous work confirming applicability of simple, empirically derived retrieval algorithms using top-of-atmosphere data. The apparent scarcity of paired in-situ optical and biogeophysical data for productive inland waters also hinders our capability to develop and validate robust retrieval algorithms. Radiative transfer modeling was used to fill this gap through the development of a novel synthetic dataset of top-of-atmosphere and bottom-of-atmosphere reflectances, which attempts to encompass the immense natural optical variability present in inland waters. Novel aspects of the synthetic dataset include: 1) physics-based, two-layered, size and type specific phytoplankton IOPs for mixed eukaryotic/cyanobacteria 6 assemblages, 2) calculations of mixed assemblage chl-a fluorescence, 3) modeled phycocyanin concentration derived from assemblage based phycocyanin absorption, 4) and paired sensor-specific TOA reflectances which include optically extreme cases and contribution of green vegetation adjacency. The synthetic bottom-of-atmosphere reflectance spectra were compiled into 13 distinct optical water types similar to those discovered using in-situ data. Inspection showed similar relationships and ranges of concentrations and inherent optical properties of natural waters. This dataset was used to calculate typical surviving water-leaving signal at top-of-atmosphere, as well as first order calculations of the signal-to-noise-ratio (SNR) for the various optical water types, a first for productive inland waters, as well as conduct a sensitivity analysis of cyanobacteria detection from top-of-atmosphere. Finally, the synthetic dataset was used to train and test four state-of-the-art machine learning architectures for multi-parameter retrieval and cross-sensor capability. Initial results provide reliable estimates of water quality parameters and inherent optical properties over a highly dynamic range of water types, at various spectral and spatial sensor resolutions. It is hoped the results of this work incrementally improves inland water Earth observation on multiple aspects of the forward and inverse modelling process, and provides an improvement in our capabilities for routine, global monitoring of inland water quality.
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    Wintertime nitrate isotope dynamics in the Atlantic sector of the Southern Ocean
    (2014) Smart, Sandi; Sigman, Daniel; Fawcett, Sarah; Thomalla, Sandy; Reason, Chris
    We provide the first data on wintertime patterns of the nitrogen (N) and oxygen (O) isotopes of seawater nitrate for the region south of Africa. Water column profile and underway surface samples collected in July 2012 span a range of latitudes from the subtropics to 57.8°S, just beyond the Antarctic winter sea-ice edge (56.7°S). The data are used in the context of simple models of nitrate consumption (including the Rayleigh model) to estimate the isotope effect (the degree of isotope discrimination) associated with the assimilation of nitrate by phytoplankton. We focus on the Antarctic region (south of 50.3°S), where application of the Rayleigh model to depth profile N isotope data yields considerably lower isotope effect estimates (1.6-3.3‰) than commonly observed in the summertime Antarctic