Browsing by Author "Thomalla, Sandy J"

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    A novel approach to investigating chlorophyll-a fluorescence quantum yield variability in the Southern Ocean
    (2019) Bone, Emma Lewis; Vichi, Marcello; Thomalla, Sandy J; Bernard, Stewart; Smith, Marié E; Ryan-Keogh, Thomas. J
    The apparent fluorescence quantum yield of chlorophyll-a (ΦF ), i.e. the ratio of photons emitted as chlorophyll-a fluorescence to those absorbed by phytoplankton, serves as a first order measure of photosynthetic efficiency and a photophysiological indicator of the resident phytoplankton community. Drivers of ΦF variability, including taxonomy, nutrient availability, and light history, differ in magnitude of influence across various biogeographic provinces and seasons. A Multi-Exciter Fluorometer (MFL, JFE Advantech Co., Ltd.) was selected for use in in situ ΦF derivation and underwent an extensive radiometric calibration for this purpose. Wavelength-specific ΦF was determined for 66 in situ field stations, sampled in the Atlantic Southern Ocean during the austral winter of 2012 and summer of 2013/ 2014. Phytoplankton pigments, macronutrient concentrations, and light levels were simultaneously measured to investigate their influence on ΦF . While no relationship was observed between macronutrient levels and ΦF , an inverse relationship between light and ΦF was apparent. This was likely due to the influence of speciesspecific fluorescence quenching mechanisms employed by local populations. ΦF derived from ocean colour products (Φsat) from the Moderate Resolution Imaging Spectroradiometer (MODIS) were compared to in situ ΦF to assess the performance of three existing Φsat algorithms. Results indicate that accounting for chlorophyll-a fluorescence reabsorption, the inherent optical properties of the surrounding water column, and the sensor angle of observation, is crucial to reducing Φsat uncertainty. A hybrid combination of two of the algorithms performed best, and was used to derive Φsat for stations co-located to in situ iron measurements in the Atlantic Southern Ocean. A significant negative relationship was observed, indicative of the effects of iron availability on quantum yield and its potential as a proxy for iron limitation. However, separating the individual contributions of light, taxonomy, and iron limitation to Φsat variability remains a challenge. A time series analysis of Φsat was also undertaken, which revealed a prominent Φsat seasonal cycle. Ultimately, increased in situ sampling would expedite the development of improved Φsat algorithms; the routine retrieval of Φsat would offer insight into phytoplankton dynamics in undersampled regions such as the climate relevant Southern Ocean.
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    Does a ballast effect occur in the surface ocean?
    (2010) Sanders, Richard; Morris, Paul J; Poulton, Alex J; Stinchcombe, Mark C; Charalampopoulou, Anastasia; Lucas, Mike I; Thomalla, Sandy J
    The oceanic biological carbon pump (BCP), a large (10 GT C yr−1) component of the global carbon cycle, is dominated by the sinking (export) of particulate organic carbon (POC) from surface waters. In the deep ocean, strong correlations between downward fluxes of biominerals and POC (the so-called ‘ballast effect’) suggest a potential causal relationship, the nature of which remains uncertain. We show that similar correlations occur in the upper ocean with high rates of export only occurring when biominerals are also exported. Exported particles are generally biomineral rich relative to the upper ocean standing stock, due either to: (1) exported material being formed from the aggregation of a biomineral rich subset of upper ocean particles; or (2) the unfractionated aggregation of the upper ocean particulate pool with respiration then selectively removing POC relative to biominerals until particles are dense enough to sink.
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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.