Doctoral Degrees (Earth Sciences)

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Browsing Doctoral Degrees (Earth Sciences) by Author "Cloete, Ryan"

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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.