| Summary: | Thesis (MSc)--Stellenbosch University, 2015. ENGLISH ABSTRACT: The Southern Ocean (SO) is of significant interest in the understanding of the global carbon cycle and therefore many studies have been conducted to determine the limiting factors controlling the biological pump within the region. During photosynthesis phytoplankton require various nutrients such as NO3, PO4, inorganic carbon and the micronutrient Fe. The SO is a High-Nutrient Low-Chlorophyll region, therefore no macronutrient limitation is experienced by resident phytoplankton but instead the micronutrient Fe is a significant limiting factor within these waters due to limited inputs. Due to deep mixed layer depths, ice cover, low sun angles and cloud cover throughout parts of the year, light is also considered a limiting factor in the SO. Fe and light limitation cause a decrease in photosynthetic efficiency and therefore a decrease in carbon fixation capabilities. During this study we conducted five bioassay shipboard incubation experiments during two cruises along the Greenwich meridian between South Africa and the ice edge, SOSCEx during March and SAFePool during January to February, in which we varied Fe concentrations and light levels to determine the effects of Fe and light limitation or co-limitation within resident phytoplankton. Spatial and temporal variations in phytoplankton response were studied to determine varying effects of limitation across water masses and different stages of bloom decline within the study area. The combined addition of Fe and light gave the largest increase in biomass, photosynthetic capacity and nutrient uptake. In support of the hypotheses tested changes in the photosynthetic apparatus led to changes in the photosynthetic efficiency and growth of the SO phytoplankton, as a result of variations in Fe and light availability. Variability was also observed in the response of phytoplankton to Fe and light amendments due to spatial and temporal variation in resident phytoplankton communities. It was therefore ...
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