Includes bibliographical references. / The Southern Ocean is an important “sink” for anthropogenic CO2, but it requires a detailed understanding of the sensitivity of the biological carbon pump to variability in physical forcing mechanisms in order to predict its continuing role. However, due to the remote and tempestuous nature of this region, in situ measurements of phytoplankton variability are scarce. Consequently, satellites, autonomous floats and gliders are increasingly being utilized as platforms for observing biogeochemical variability over broad spatial and temporal scales, through satellite ocean colour radiometry linked to inherent optical properties (IOPs) of the upper water column. In this study, the variability of in situ IOPs was investigated together with phytoplankton biomass, cell size, species composition and chlorophyll to carbon (Chl:C) ratios to isolate and understand the relationships between IOPs and biogeochemistry.

This work examines SW Indian Ocean tropical cyclone (TC) structure as a function of movement using ECMWF gridded data. Analyses of the mean summer climate for the period 1987­ 1992 shown that conditions conducive to the development ofTCs are present between the latitudes 5° and 15°S. Six westward moving TCs are selected to form the westward moving composite TC and four are used in the recurving TC composite from 3-days before the maximum surface intensity to 1­ day after. The westward moving composite was found to have a steady TC signature. Comparisons were made with TC Firinga which passed near Mauritius on 29 January 1989. The recurving TC composite was found to exhibit rapid growth with sharp parameters tendencies. Interaction with a sub­ tropical trough is seen in both TC composites. Upper level westerly winds create effective uplift which strengthens the recurving TC on the poleward side. Spectral analysis of a rainfall index compiled from Mauritius data demonstrates spectral peaks at the seasonal and intra-seasonal scale. Three peaks were identified. one within the 40-60 day Madden­ Julian Oscillation. one at 23 days and one with a 16 day oscillation. The 23 day oscillation was found to have a highest spectral power. The results of TC and rainfall analyses offer useful insights to the climate and weather of the SW Indian Ocean.

Includes bibliographical references (leaves 59-63). / From 1963 until the present, loggerhead (Caretta caretta) and leatherback (Dermochelys coriacea) female turtles nesting on a 56-km stretch of beach in Maputaland, on the northeast coast of KwaZulu-Natal, South Africa have been tagged and monitored. In this study, population trends, size, preference for timing of nesting and nesting sites, and seasonality in nesting activity and nesting area were analysed and the effectiveness and efficiency of the monitoring programme assessed. Since the programme's inception, approximately 46893 loggerhead and 11509 leatherback females have nested on the Maputaland beaches. A non-linear relationship was found between the numbers of nesting females of both species over time, with three distinct phases. large interannual variations in nesting numbers were evident for both species, as were synchronized seasons of elevated or depressed nesting activity. While Maputaland leatherbacks appear to be approximately the same size as in populations elsewhere, the loggerhead females were significantly smaller compared to other populations. Peak nesting activity occurred from November to January and was associated with warmer sea temperatures, as was the subsequent emergence of hatchlings. Marginal temporal shifts in the nesting numbers of both species were evident during peak nesting-activity seasons, with the peak nesting period beginning slightly earlier than usual. Peak nesting-activity seasons appeared to follow EI Nino events with a lag period of between 0 and 2 years. Separate 'preferred' nesting areas were found for each species, largely associated with the offshore seabed topography. The loggerheads exhibited a fairly narrow nesting-area distribution in the north, while the leatherbacks utilised a far wider area in the southern region. The length of the 'preferred' nesting area (km) was strongly correlated with the total number of nesting loggerhead and leatherback females. No major nesting-area shifts were evident over the programme's duration. However, distinct spatial shifts, with increases in the length of the 'preferred' nesting area, were evident for both species during peak nesting-activity seasons, and was possibly related to the increased influx of nesting females. Interspecific competition appeared to be of little importance in Maputaland, as both populations displayed strong synchrony in nesting declines and increases over time, and while both populations have increased, no major shifts in the 'preferred' nesting area of either species have occurred. No correlations were found between the Southern Oscillation Index (SOl) values and nesting activity or changes in nesting area. However, a negative correlation was found between the average curved carapace lengths (CCl) of both species and the SOl values, most likely related to sea surface temperatures and the resultant changes in food availability that occur during EI Nino events. Disturbance incurred during monitoring caused, at most, only a 5% reduction in the nesting rates of both species. To date, the conservation efforts of the monitoring programme appear to have come to fruition, with both the loggerhead and leatherback nesting populations healthy and in a more-or-Iess stable state. The importance and value of continuing the current monitoring programme was emphasized, and several management and sampling strategies proposed for future seasons.

Marion Island Oceanographic Survey IV, April-May 1999 / Bibliography: leaves 86-116.

Includes bibliographical references. / Operational oceanography aims to accurately hindcast and forecast the state of the ocean. An international initiative, the Global Ocean Data Assimilation Experiment (GODAE), developed and increased the capacity for global operational oceanography. However, the products from the global initiatives were regionally inapplicable due to low spatial resolutions, and have recently improved through GODAE OceanView. A number of local operational oceanographic initiatives have been setup over the southern African regional ocean, but proved to be unsustainable and ended. Recently, the aim to develop a regional ocean prediction system has arisen, and initial steps have been taken. This thesis aims to address the lack of local capacity in operational oceanography, and contribute to a crucial process in developing a regional ocean prediction system. Here, we validate and investigate the differences between three global reanalysis products, namely MyOcean (GLORYS2V1), HYCOM (U.S Naval Research Laboratory) and BlueLINK (OFAM3). These reanalysis products are validated and investigated over the greater Agulhas Current System, which is a crucial system in Southern African regional ocean. The salient oceanographic features represented in the reanalysis products are initially compared to historical literature of the region and followed by available unassimilated observations (i.e. independent). The results show that the reanalysis products from MyOcean, and the U.S Naval Research Laboratory satisfactorily simulate the major salient oceanographic features of the Agulhas Current System. Bluelink does not correctly portray the structure of the source and retroflection regions, and therefore has limited use over the Agulhas Current System. The differences between the three products indicates that the data assimilate does not sufficiently constrain the models in order for their solutions over the Agulhas System to converge. The evaluation of these global ocean reanalysis products is a critical step toward a regional ocean prediction system over Southern Africa, and building toward the local capacity to accomplish this goal.

The Agulhas Current is unique as a western boundary current in having as its source of surface water currents that are linked to the variable current regions of the North Indian Ocean. In which way the Agulhas Current derives its supply from these currents and how these currents interrelate, is very poorly understood. In an attempt to make a contribution to the understanding of this flow an isentropic analysis in depth on seven σt-surfaces for the whole South West Indian Ocean during the Northeast Monsoon season was carried out. This analysis was augmented by the calculation of the velocities according to the Witte-Margules equation on each σt-surface at ten selected vertical sections. In addition the detailed volume transport and the velocity structure for each vertical section were calculated. The results pointed to a significant variation in transport and velocity structure from year to year. The large scale circulation in depth of the ocean is described. It is found that the Agulhas Current derives its supply of water from different sources at different depths and that at depth a large measure of recirculation of Agulhas Current Water takes place. It is not thought that this has been reported before and these findings might be of importance to the understanding of the current system as a whole. A few innovations in the standard oceanographic methods are described and two computer programs to aid analyses are presented.

In the southern Benguela upwelling system (SBUS), the wind-driven supply of nutrient-rich water from depth sustains elevated levels of primary productivity. St Helena Bay (SHB), a coastal embayment in the SBUS positioned north of an upwelling centre, is an area of water mass retention. In addition to supporting 40-50% of total SBUS productivity, SHB often experiences harmful algal blooms (HABs) and hypoxic conditions that are difficult to predict given the high sub-seasonal variability that characterises this region. To better understand this variability, net primary production (NPP), nitrate and ammonium uptake, and phytoplankton community composition were measured for ten days during the upwelling season at an anchor station in SHB. A period of active upwelling (days 1-5) was followed by one of relaxation (day 6-10), together constituting an “upwelling cycle”. During upwelling, the mixed layer was deeper than the euphotic zone and phytoplankton were light-limited, evidenced by high ambient nutrient concentrations and relatively low rates of NPP and nitrate uptake. During relaxation, water column stratification increased, restricting phytoplankton production to a shallow, well-lit surface layer in which nitrate was exhausted after three days. The subsequent decline in NPP and nitrate uptake rates confirms that nutrient availability succeeded light as the ultimate control on productivity during the relaxation phase. Of the three phytoplankton size classes investigated (0.7-2.7 µm, 2.7-10 µm, >10 µm), the 2.7-10 µm fraction contributed most to the measured increases in biomass and nutrient uptake rates. This was unexpected given that large (>10 µm) diatoms typically dominate in upwelling systems; however, the 2.7-10 µm size fraction achieved a faster growth rate and sustained it for longer than the other size classes. The success of this size fraction may be partly due to a capacity for luxury nitrate uptake, evidenced by a low biomass C:N ratio and a nitrate uptake rate that was decoupled from NPP. Throughout the experiment, the phytoplankton community comprised mainly Chaetoceros spp. and Skeletonema costatum. These diatoms occupy a large size range (2-80 µm), although it is likely that they mainly occurred in the 2.7-10 µm size class during the experiment. They also produce resting spores that may provide a selective advantage during seeding in highly variable upwelling systems, increasing their chances of proliferating when conditions become favourable. Once the water column stratified, the phytoplankton community diversified, with dinoflagellates and the large diatom, Coscinodiscus gigas (200-500 µm), becoming more abundant. The contribution of C. gigas to biomass and productivity was not fully accounted for in the measurements because collected seawater was screened (200 µm mesh) prior to incubation. However, a simple N₃P₃ ecological model parameterized with the observations suggests that their contribution would have been minimal. The hydrographic data indicate that another upwelling cycle commenced by day 10 of the experiment. This likely prevented the further proliferation of dinoflagellates, some of which are HAB species, that may have succeeded the small diatoms given a longer period of quiescence. One implication of this is that understanding the rapid cycling between light and nutrient limitation, as induced by an actively upwelling versus stratified water column, may advance our capacity to predict the occurrence of HABs in SHB.

Includes bibliographical references. / The primary objective of this thesis is to identify and investigate the most prominent circulation features of the Angola Basin from the output parameters of the OPAITOTEM Ocean Parallelisel Trois Oceans Tropicaux) tropical circulation model, focusing particularly on the so-called Angola Gyre and the Angola Dome. Analyses of the effect of windstress, windstress curl and Ekman pumping, all computed from ERS satellite-derived wind speeds, were conducted. The OPAITOTEM model does not resolve the 'Angola Gyre', but it does resolve a large-scale (1000-2000km) dome-like feature, which has been defined as the 'Model Dome' within this study. The most conspicuous feature of the Angola Basin discerned in the thermohaline output of the OPAITOTEM model is a ridge-like structure of the thermocline (the Model Dome), which results in a cool feature that 'outcrops' most distinctly at a depth of 45m. At greater depths, a permanent upward doming of the isotherms beneath the Angola Benguela Frontal Zone (ABFZ) is observed. In January, February, September, October and November the 'outcropping' of the thermocline-ridge is deepest and appears as a distinctly isolated cool feature at 4Sm. No clear cydonic circulation is associated with the Model Dome. However, the northern limb is unequivocally coincident with the South Equatorial Undercurrent (SEUC) at 2-5"S and the South Equatorial Countercurrent (SECC) at 10·S. Upon reaching the African coast, the SEUC and SECC bend poleward to form the southward Angola Current, which constitutes the eastern limb of the Model Dome. The southern and eastern limbs of the dome are not as dearly defined and are associated with a weak westward flow regime. The shallow portion of the Model Dome has a distinct semi-annual signal whereby it migrates southward between September-November and again between January-April. This signal is in accordance with the magnitude and southward displacement of the core of the SEUC. The SEUC is most intense in January, February, September and October reaching velocities of about O.14m.s·1 and is weakest in June and July (O.02-O.06m.s-) The deep portion of the Model Dome shows little seasonal variability, other than a slight northward tilt of its vertical axis when the thermocline-ridge is furthest south. Similarly, the SECC, which constitutes the northern limb of the deeper cool feature, is fairly consistent throughout the year, in both position and magnitude (-0.01 m.s-)

Bibliography: leaves 72-76. / With the limitations of in situ observations of the ocean, numerical modeling of the ocean can easily provide clues to the structures and processes that lie therein. This project is an example of the examination of numerical model output. In this case, the area of interest is an area of ocean off Namaqualand which displays a reaction to local wind forcing. A transect was performed off the coast using output from ROMS (Regional Ocean Modeling System), forced under realistic winds obtained from weekly averaged ERS data. Temporal and spatial analysis yield information on the climatological characteristics of this region.

This study analyses the physical mechanisms that impact Lagrangian pathways and transport in the southern Benguela upwelling system (SBUS),an environment in which currents are key components of many important ecological processes, including the dispersal of marine larvae. Physical advection by currents is an important mechanism for egg and larvae transport success in the SBUS since the spawning areas and recruitment areas are separated by a long distance. High-resolution numerical model simulations of the SBUS coupled with particle tracking experiments are used to investigate Lagrangian pathways between the Cape Peninsula (34◦S) and St Helena Bay(32◦S) and how they are linked to the oceanic circulation. Transport success, given by the ratio of the number of particles that reach St Helena Bay over the total number of particles released, is used quantify the alongshore connectivity between the two regions. We have identified and quantified the following physical drivers: (i) Benguela Jet, (ii) offshore Ekman transport,(iii) inner shelf poleward current, (iv) mesoscale eddies to be responsible for the spatial and temporal variability of the alongshore connectivity. The Benguela Jet was found to be the dominant driver of the connectivity at both seasonal and interannual timescales. Moreover, the presence of anti-cyclonic eddies near the shelf-edge negatively impact transport success by advecting particles into the open ocean. The opposite occurs with shelf-edge eddies as they transport particles onshore onto the shelf and the Benguela Jet contributing to positive transport success anomalies. These findings will provide a valuable information for the future studies on the role of the physical drivers that impact transport of larvae and eggs.