Browsing by Author "Viljoen, Johannes Jacobus"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    CHEMTAX determination of Southern Ocean phytoplankton distribution and adaption: An observational and experimental study assessing the co-limitation of Light, Iron and other Trace Metals on phytoplankton productivity and community composition.
    (Stellenbosch : Stellenbosch University, 2018-12) Viljoen, Johannes Jacobus; Fietz, Susanne; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: This multi-manuscript study reports an elucidated understanding of the consequences key environmental changes has on phytoplankton community structure, productivity and their acclimation abilities for the Atlantic Sector of the Southern Ocean. All three manuscripts are based on data gathered on board the RV SA Agulhas II on a north-south transect along the Good Hope monitoring between Cape Town and the Antarctic Fimbul ice shelf (0° longitude). This transect crossed multiple oceanic fronts and zones from which a multi-parameter in-situ observational study of the links between trace metals and phytoplankton dynamics could be carried out. In addition, two stations were used to conduct bulk-community bottle incubation experiments to assess the response of phytoplankton in growth, photophysiology and community structure to iron and light variability in the Polar Frontal Zone and Antarctic waters. The results from the incubation studies revealed that the response of phytoplankton to changes in iron and light availability are very region-specific. In the Polar Frontal Zone irradiance is limiting significant phytoplankton growth and in the Antarctic Zone, iron addition resulted in growth stimulation even at low light levels. By combining data from the surface and the un-amended incubation bottles sampled from the respective depth of fluorescence maximum an assessment of how representative the surface and chla-max communities are of their region and, considering logistical constraints, if we can only use one to understand phytoplankton dynamics. Due to additional changes in irradiance and availability of iron with vertical depth, which is region specific, the acclimation of phytoplankton, which affects their photochemical efficiency (Fv/Fm) and productivity, can vary with depth which is exemplified by regional differences in water column structure and dynamics. The possibility of sampling a niche community when only one depth is sampled, which could lead to results of a very depth-specific acclimated phytoplankton community and not that of a regional representative one, was also investigated. The multi-parameter in-situ observational study that includes parallel trace metal and phytoplankton community structure data showed that there are certain changes in phytoplankton community structure that can only be explained by the addition of a suite of trace metals as additional variables. This led to the observation that when these essential micronutrients are incorporated into studies, the timing of phytoplankton sampling relative to the time of an event like the release of trace metals from hydrothermal vents or melting ice can be very important. For the Atlantic Southern Ocean, it can be shown that the present and future state of phytoplankton dynamics greatly depends on key environmental changes such as irradiance, varying degrees of iron-light co-limitation and the proven strong influence of trace metal availability. Keywords: pigments; acclimation; multi-parameter; incubation; GEOTRACES
  • Thumbnail Image
    Item
    Phytoplankton and trace metal dynamics in the Southern Ocean
    (Stellenbosch : Stellenbosch University, 2023-03) Viljoen, Johannes Jacobus; Fietz, Susanne; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: Marine phytoplankton in the Southern Ocean are essential for the Antarctic food web and help regulate the global ocean biogeochemistry, thereby mediating the warming effect of carbon dioxide (CO2). Yet, spatially and temporally, phytoplankton distribution and their controlling factors are still poorly studied in the Southern Ocean. Owing to low sampling resolution, features such as frontal and island regions have been less sampled. Moreover, the dynamics of trace metals which serve as essential micronutrients has been the least studied in the Southern Ocean. Most studies focussed on iron (Fe) and total chlorophyll-a, few on extended suites of metals and phytoplankton groups. There are still uncertainties how the trace metals Fe, copper (Cu), zinc (Zn), nickel (Ni), cobalt (Co), manganese (Mn), and cadmium (Cd), will affect phytoplankton abundance and community composition, especially in the understudied Southern Ocean during winter. In this thesis Southern Ocean phytoplankton dynamics and the complex interplay with various parameters were elucidated. Through three research papers, phytoplankton distribution was studied utilizing multi-parameter datasets, collected during two research cruises to the Southern Ocean one during summer in the Atlantic sector (0 - 8°E) and another during winter in the Indian sector (30°E). Summer phytoplankton distribution were characterised in the Atlantic Southern Ocean at high resolution across the major zones, within fronts, in sea ice influenced zones (e.g., polynya) and close to Subantarctic islands using a suite of HPLC phytoplankton pigments. Cyanobacteria dominated the Subtropical zone; haptophytes the Subantarctic and Polar Frontal zones while diatoms and haptophytes dominated the Antarctic zone south of the Polar Front. Additionally, distinct communities were observed in frontal, sea ice and near-island regions. For example, phytoplankton abundance in fronts were up to 10-fold higher than in the major zones while haptophytes dominated communities close to the Subtropical Front. Diatoms dominated in the areas affected by recent sea ice melt while Phaeocystis and coccolithophores dominated post-bloom communities. Close to Subantarctic islands, cryptophyte blooms were observed whereas diatoms were dominant further downstream. Results suggest that a combination of nutrient, including trace element supply and mixing regime are essential in controlling the magnitude and composition of blooms close to fronts, sea ice and islands, and in turn, affect Southern Ocean food web activity and potential carbon export. Having noticed the importance of zonal changes and small-scale features, as well as the likely importance of trace element supply, the second and third study included these dynamics. The relationship between phytoplankton dynamics (abundance and community structure) and micronutrients (i.e., trace metals) were investigated for the first time during winter in the Indian sector of the Southern Ocean. This was achieved through two studies using a unique dataset of HPLC chlorophyll-a and accessory pigment concentrations with parallel sampled macronutrients and a suite of dissolved and particulate trace metals and phosphorous concentrations. Results suggest phytoplankton were still active and a dominant contributor to the uptake and remineralisation of trace metals even though their abundance were lower than during summer. Through a suite of metal* calculations, based on the macro- and micronutrient concentrations and the estimated requirements of Southern Ocean phytoplankton, varying degrees of deficiency and potential for co-limiting conditions were proposed. Accordingly, micronutrients are suggested as a major driver of winter phytoplankton abundance and community structure across the Southern Ocean. Some trace metals had stronger relationships with specific phytoplankton groups compared to total phytoplankton abundance. In turn, specific groups, such as diatoms, were confirmed to be major drivers of trace metal dynamics across the transect through preferential uptake. For example, cyanobacteria, a group not considered by previous trace metal studies in this region, was suggested to be dependent and responsible for major uptake of Co and Mn while diatoms were strongly associated with Zn. Ultimately, the preferential uptake by specific phytoplankton groups in deficient conditions, such as diatoms, can aggravate limiting conditions and lead to a change in composition. Therefore, the strong association of specific phytoplankton groups, with different remineralisation lengths, to specific micronutrients during winter would affect the release of these micronutrients for the utilisation by phytoplankton during the following spring and summer seasons. Hence, these two studies yielded new knowledge on phytoplankton-micronutrient dynamics that contributes critical seasonal information for biogeochemical models. Collectively, the research in this thesis demonstrates the importance of understanding not only phytoplankton abundance, but also its community composition and how small- and large-scale changes in the chemical environment, including the availability of trace metals, can influence phytoplankton dynamics.
  • Thumbnail Image
    Item
    Die psigososiale ontwikkeling van graad 8-seuns volgens Erikson se psigososiale ontwikkelingsmodel
    (Stellenbosch : Stellenbosch University, 2002-12) Viljoen, Johannes Jacobus; Ackermann, C. J.; Stellenbosch University. Faculty of Education. Dept. of Educational Psychology .
    ENGLISH ABSTRACT: The primary goal with this research project was to investigate the psychosocial development of a group grade 8 boys before adolescence in terms of the four developmental crisis according to Eriksson's' theory. The study proceeded through two phases. A questionnaire investigation with a larger study group (N=221) was done in phase 1, and this (quantitative) data was analysed. Phase 2 comprised a further investigation, which took place 2 years later. Based on the results of the questionnaire investigation, a small group (N=ll) experiencing a potencial low self-consciousness was identified and focus group interviews were conducted with them. The researcher made the assumption that individuals' experiences of the first four stages of Erikson's psychosocial developmental model form the basis for later identity development during adolescence, as well as continuing personality development. Current research indicates that the total scale and two of the sub-scales ('Industry vs Inferiority' and 'Autonomy vs Shame') show sufficient reliability (Alpha-coefficients are greater than 0.70). The distributions of the sub-scales were in all cases skew to the left, which indicate that most of the scores for each sub-scale are closer to the maximum value than the minimum value. A negative measure of skewness is reported. All the sub-scales relate significantly to each other (p < 0.01). Therefore, one can conclude with relative certainty that this scale is valid. Age does not correlate significantly with any of the sub-scales (p > 0.05 in all cases), but a significant correlation was not expected given the small age variance. From the qualitative responses of phase 2 it appeared that the present level of expectations, which learners experience, is not appropriate. They would benefit from more positive feedback on their successes. The responses reflect that feedback is not always constructive, nor given in a positive, non-judgemental spirit.