Browsing by Author "du Plessis, M"

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    The influence the oceanographic variability has on the top predators of the sub-Antarctic domain
    (2021) Evans, Sean; du Plessis, M; Wege, M; Ansorge, I; Lowther, A; de Bruyn, N
    Historically, a lack of small-scale physical oceanographic (hours to days, 1-10 km) and behavioural (<10 sec, ~1 m) observations in the relatively inaccessible Southern Ocean has led to poor quantification of marine mammal foraging behaviour and the physical upper ocean processes that may influence them. In situ temperature and depth profiles from 2009-2015 were obtained from devices fitted to 39 adult Subantarctic fur seal (SAFS) (Arctocephalus tropicalis) females inhabiting the Prince Edward Islands (PEI). This provided a unique opportunity to study the fine-scale effects of thermal water column structure and upper ocean submesoscale processes on the diving behaviour and vertical foraging effort of SAFS. Seasonal and diel trends of foraging effort were investigated and compared to upper ocean thermal structure. Dives were distinguished using the Clustering for Large Applications algorithm according to vertical movements made by the seals. Shallow, high effort dives differentiated from deep, low effort dives, primarily based on bottom effort. High effort dives, associated with high vertical foraging effort, bottom effort, and dive efficiency, were more numerous when the seals were in well-mixed water columns. Generalised additive mixed-effects models showed that thermal water column structure plays a significant role in modulating dive types made by seals. The probability of high effort dives decreased with increasing stratification, while the relationship with the stability and mean temperature of the water column was complex, yet significant. Overall, seals are predicted to enhance vertical foraging effort and dive shallower in well-mixed, warmer water columns, with a strong association to diel and seasonal trends in mixing found. However, seals do not appear to align their distribution of foraging depth with the MLD in either season. Investigation into what upper ocean processes may be driving variation in thermal water column structure surrounding the islands led to investigations into the downstream effects of the small-scale topography of the islands on ocean variability. Results show that the PEI act as a solid obstruction to the relatively laminar flow of the Antarctic Circumpolar Current and coherent eddy structures moving through the Archipelago region. The topographic influence of the PEI on multiple scales of local ocean variability in the region, from mesoscale (10-100 km, days to weeks) to submesoscale (1-10 km, hours to days) and vertical mixing (<1m, <10 sec), is illustrated. Downstream enhanced squeezing and stretching of mesoscale and submesoscale gradients intensify restratification and stability in the lee of the Islands with the potential to enhance biological productivity. Seals do not appear to adjust their fine-scale foraging behaviour to these downstream processes. By improving our understanding of SAFS habitat use, we can more accurately predict how regional and global change may affect populations in the future, linking to more effective conservation management and policy. Furthermore, this study emphasizes how concurrent measurements of oceanographic and behavioural data collected from diving samplers can be used to study the downstream effects on both physical oceanography and foraging ecology surrounding small islands.
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    Understanding the interannual variability of pCO2 in the sea-ice impacted Southern Ocean
    (2024) Jojo, Bongiwe; Vichi, Marcello; Nicholson, S; du Plessis, M
    Sea-ice is permeable and plays an active role in the marine carbon cycle via biological and physio-chemical processes. The carbon cycle in seasonally sea-ice-covered waters needs to be better understood due to a lack of observational data and the system's complexity. To characterize the interannual variability of oceanic pCO2 in the sea-ice-impacted Southern Ocean and identify their potential primary drivers, this thesis combines in situ observations with remotely sensed data and reanalysis models during the austral summer months. The region of focus is divided into three sections: the Southern Ocean, three ocean basins, and the Goodhope line transect. Averaged over the Southern Ocean, the range of year-to-year variability of pCO2 between 2000 to 2018 was between 290 atm (2004) and 355 atm (2003). It is also noted that the interannual variability in pCO2 does not correspond to that of the Southern Annular Mode (SAM) index; however, there are some indications that the SAM may be an essential driver on longer time scales. Noticeably, the year 2016 stands out as one of the warmest and has the smallest Antarctic sea-ice extent (SIE) recorded since 1979 in the Southern Hemisphere. This SIE reduction has been attributed to positive sea surface temperature anomalies, the zonal wave pattern 3, and a SAM negative phase. pCO2 decreased in response to this ice loss event highlighting its sensitivity to rapid changes in sea ice. Overall, salinity obtains the highest correlation to the annually averaged pCO2 in the Southern Ocean and various basins. Along the Goodhope Line, variability of pCO2 indicated a higher magnitude and interannual variability of pCO2 during early summer than late summer. Non-thermal drivers primarily explain the variability of pCO2. These results suggest that the leading causes of the interannual variability of pCO2 in the sea ice-impacted Southern Ocean are those associated with non-thermal drivers of pCO2.