Browsing by Author "Cloete, Ryan"

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    Links between the phytoplankton community composition and trace metal distribution in summer surface waters of the Atlantic Southern Ocean
    (Frontiers Media, 2019-06-06) Viljoen, Johannes J.; Weir, Ian; Fietz, Susanne; Cloete, Ryan; Loock, Jean; Philibert, Raissa; Roychoudhury, Alakendra N.
    This study assessed changes in the phytoplankton community related to macronutrient and bioactive trace metal distribution in surface waters of the Atlantic sector of the Southern Ocean, between Cape Town and Antarctica along the GEOTRACES GIPY_05 (mainly along prime meridian) transect in summer 2014–2015. Several general community structure features were reaffirmed, such as the restriction of cyanobacteria to the northern Subtropical Zone, while haptophytes, such as Phaeocystis, along with diatoms, dominate the community north of the Polar Front, and diatoms clearly dominate south of the Polar Front. These community structure changes were often linked with macro- and micro-nutrient composition changes. For example, the concentration of diatoms increased southwards with the availability of silica whereas the cyanobacterial contribution in the northern Subtropical Zone appeared to be linked with labile cobalt depletion. However, these links were not consistent along the entire transect, i.e., no individual nutrient, such as silica or iron, was linked to community composition changes across all water masses. Each station showed a rather unique combination of nutrient and community compositions. Our findings also indicated impacts on the phytoplankton community through trace metal distributions that could be related to a deep mixing event at ∼54∘S and to ice melt at ∼65 and 68∘S. The timing of sampling after such trace metal fluxes proved to be an important consideration, particularly where iron appeared to be preferentially depleted to near-limiting concentrations, possibly driving utilization of other metals. The study highlights the importance of considering a suite of trace metals when assessing controls of phytoplankton variability in the open ocean and emphasizes the need for higher resolution trace metal sampling and multi-element incubation studies to further study the complex relationships between phytoplankton and nutrients.
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    Measurement of bioactive trace metals (Cu and Zn) in the Southern Ocean : validation of sampling protocol and ICP-MS based analytical method
    (Stellenbosch : Stellenbosch University, 2017-03) Cloete, Ryan; Roychoudhury, Alakendra N.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: A method comprising an improved seawater collection protocol and subsequent Inductively Coupled Plasma Mass Spectrometry (ICP-MS) based analytical technique was validated through an intercalibration exercise performed with the University of Plymouth (UK), multiple cross-over stations and analyses of certified reference materials (SAFe, GEOTRACES and NASS-5). The commercially available seaFAST-pico preconcentration module was employed for the simultaneous extraction of a suite of trace metals (Mn, Fe, Ni, Cu, Zn, Co, Cd and Pb) from their seawater matrix prior to ICP-MS analysis. Extremely low detection limits (< 0.228 nmol/kg) combined with low blank values ensured quantitive recovery on ICP-MS and minimal interferences arising from alkali and alkaline earth metals (Na, K, Mg and Ca) present in the saline matrix. The results of the certified reference materials were in excellent agreement with their corresponding consensus values and validated the methods precision and accuracy. During ICP-MS analysis, repeatability and reproducibility were monitored through analysis of an internal Stellenbosch University (SU) TM4 control and various commercially available quality controls, the results of which further confirmed a high level of precision. The distribution of Dissolved Copper (DCu) and Dissolved Zinc (DZn) was investigated in the Atlantic sector of the Southern Ocean. DCu displayed typical nutrient type behaviour reflected by sub-nanomolar surface concentrations increasing steadily until maximum observed concentrations of 2 – 3 nmol/kg in the Antarctic Bottom Waters (AABW). DZn concentrations ranged between approximately 1 and 12 nmol/kg and exhibited characteristic nutrient-type behaviour although intermediate and deepwater distributions were more conservative compared to DCu. Local subsurface minima coincided with elevated levels of chlorophyll-a (chl-a) indicating biological utilisation by phytoplankton in the euphotic zone. Remineralisation of sinking organic matter, predominantly diatom frustules, from Antarctic Surface Water (AASW) resulted in deeper sub-surface maxima for DZn. The dominant supply of trace metals to surface waters south of the Antarctic Polar Front (APF) was advective upwelling of nutrient rich Upper Circumpolar Deep Water (UCDW) and AABW. Atmospheric inputs and melting ice accounted for minor surface influxes where there was a poor DCu/salinity correlation. Both trace elements displayed significant correlations with the macronutrient silica, evidence of their role in the biological cycle. An overall Cu:Si relationship of Cu (nM) = 0.011 Si (μM) + 0.851 (R2 = 0.85, n=98) was obtained for this study while the corresponding Zn:Si relationship was Zn (nM) = 0.043 Si (μM) + 1.021 (R2 = 0.80, n=98). The APF exerted a strong control over nutrient distributions separating low nutrient low chlorophyll (LNLC) subtropical waters to the north from high nutrient low chlororphyll (HNLC) waters to the south. Keywords: Southern Ocean, Dissolved Copper, Dissolved Zinc, GEOTRACES, seaFAST, ICP-MS
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    On the distribution and biogeochemical cycling of bioactive trace metals in the Southern Ocean
    (Stellenbosch : Stellenbosch University, 2020-12) Cloete, Ryan; Roychoudhury, Alakendra N.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: Bioactive trace metals, including copper (Cu), zinc (Zn), nickel (Ni) and cadmium (Cd), are essential micronutrients to marine phytoplankton and their availability in the surface ocean has been shown to influence phytoplankton community composition and abundance. Through photosynthesis, phytoplankton are the primary drivers of the marine carbon cycle therefore constraining trace metal – phytoplankton dynamics, as well as other biogeochemical processes controlling trace metal distributions, is critical to understanding the greater carbon cycle. Owing to logistical constraints, less attention has been directed toward trace metal cycling on a seasonal basis. This is particularly important in the high latitude ocean regions where large seasonal fluctuations in environmental conditions e.g. light, wind and temperature, are likely to impact trace metal distributions directly or indirectly. To this end, this study focuses on the data scarce Southern Ocean and investigates the distribution and biogeochemical cycling of Cu, Zn, Ni and Cd in summer and in winter, two seasons with contrasting environmental conditions for phytoplankton growth. This framework provided a unique opportunity to characterise the Southern Ocean winter reset period and to assess the role of deep winter mixing as a potential in-situ physical trace metal supply mechanism to aid surface productivity. In order to address these questions, research cruises were conducted in summer and winter in the Atlantic sector of the Southern Ocean (0 - 8°E) while a third cruise took place in winter in the Indian Sector of the Southern Ocean (30°E). In the Atlantic sector, first winter measurements of dissolved (0.2 µm filtered seawater) Cu (dCu), Zn (dZn) and Ni (dNi) were compared with corresponding summer measurements from the same locations. Differences in trace metal distributions were most evident in the surface mixed layer where winter concentrations were consistently greater compared to summer. Variations in trace metal seasonality were also linked to latitude whereby seasonal variations observed near the Sub-Antarctic Front (SAF; ~ 46°S) were negligible for all three trace metals and largest variations, up to 2.0 nmol kg⁻¹ for dCu, 1.2 nmol kg⁻¹ for dZn and 0.3 nmol kg⁻¹ for dNi, were observed near the southern extent of the Antarctic Circumpolar Current (ACC; ~ 54°S). The primary driver of these variations was through differences in biological activity with lower trace metal utilisation during winter. Sub-optimal growth conditions experienced by phytoplankton during the winter were further confirmed through calculated trace metal uptake ratios and estimations of phytoplankton productivity. Our results further suggest that deep winter mixing, i.e. the mixing of depleted summer surface waters with nutrient enriched sub-surface waters, constitutes a potentially significant surface water source of dCu, dZn and dNi with implications for phytoplankton productivity over the subsequent spring and summer seasons. In the Indian sector of the Southern Ocean (30°E longitude), winter measurements of dZn and dissolved cadmium (dCd) were coupled with particulate (> 0.45 µm filter) zinc (pZn) and cadmium (pCd) in an effort to investigate winter trace metal cycling in more detail. Distinct changes in dissolved and particulate Cd and Zn cycling were observed between the various frontal regions encountered and were related to changes in phytoplankton community composition and physical circulation patterns. Our data suggests diatoms are major drivers of the observed trace metal gradients through their preferential consumption of dCd and dZn, relative to the major nutrient phosphate (PO4), in the Antarctic Zone (AAZ). Here, high dCd/PO4 and dZn/PO4 uptake ratios set the high ratios of pCd and pZn to phosphorous (P; pCd/P; pZn/P) observed in surface waters. Ultimately, the uptake characteristics of diatoms at higher latitudes influences Cd and Zn cycling at lower latitudes as a result of the northward flow of surface waters depleted in dCd and dZn. In addition, because diatoms require silicic acid (Si(OH)4), the export of their cells below the winter mixed layer provides additional insights as to the observed deep water coupling of dZn and Si(OH)4. Below the surface, Cd and Zn cycling is predominantly controlled by remineralisation, vertical mixing and upwelling. We conclude that winter Southern Ocean surface waters are not biologically dormant and that trace metal cycling is influenced by biological productivity during winter.