Surface circulation in the KwaZulu-Natal Bight and its impact on the connectivity of marine protected areas

Master Thesis


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The KwaZulu-Natal Bight is a small, coastal region along South Africa's north-east coast. It stretches from Richards Bay to Durban and has a wide shelf compared to the surrounding coastline. As a result, the Agulhas Current is forced offshore, allowing the formation of complex circulation features on the KwaZulu-Natal Bight's shelf that assist with recruitment and retention of marine organisms in this region. This study aims to gain a deeper understanding of the surface circulation within the KwaZulu-Natal Bight and its impact on the connectivity between several surrounding Marine Protected Areas. These include iSimangaliso, uThukela Banks, Aliwal Shoal and Protea Banks and the information about their connectivity contributes to the CAPTOR (Connectivity And disPersal beTween prOtected aReas) project. The aim of this study is met by using high-resolution CROCO model output over a 10-year period, in combination with particle tracking tools, wind and surface drifter data. According to the model's mean circulation, the KwaZulu-Natal Bight's surface currents have a strong south-westward flow on the continental shelf slope where the effects of the Agulhas Current are strongly felt, butare weak and variable on the shelf. Observed variabilities of the mean flow have no distinct seasonal pattern and include a north- eastward current that repeatedly dominates the shelf. It is referred to as the Natal Bight Coastal Counter Current, which originates within the semi-permanent Durban Eddy in the southern KwaZulu- Natal Bight, where it extends throughout the water column. The Natal Bight Coastal Counter Current stretches along the mid-shelf into the northern KwaZulu-Natal Bight, gradually becoming shallower, weaker and narrower. When anticyclonic eddies offshore of the Agulhas Current pass this region, they occasionally replace the Durban Eddy and its associated Natal Bight Coastal Counter Current with a southward flow on the KwaZulu-Natal Bight's shelf. Therefore, the circulation in the KwaZulu-Natal Bight appears to be primarily driven by perturbations at the Agulhas Current front. However, there is also some indication of a direct wind-driven influence in coastal waters inshore of the 50 m isobath. To investigate the impact of the KwaZulu-Natal Bight's circulation on the connectivity between the above-mentioned Marine Protected Areas, particle tracking tools are used. Virtual particles are released in each Marine Protected Area within the model, during multiple northward and southward KwaZulu-Natal Bight surface circulation events. Their pathways are tracked for 30 days and reveal an overall strong southward Marine Protected Area connectivity, which is driven by the Agulhas Current, while a northward connection is less commonly observed. The northward flow of the Natal Bight Coastal Counter Current increases the water retention within uThukela Banks, but it does not extend into iSimangaliso to establish a northward Marine Protected Area connection. However, when the Natal Bight Coastal Counter Current originates within Aliwal Shoal, it may result in a northward Marine Protected Area connection between Aliwal Shoal and uThukela Banks. In this study, the virtual particles represent passively drifting larvae that are buoyant. To make these simulations more realistic, the virtual particles should be able to sink and appropriate swimming behaviours could be considered. However, swimming abilities will likely be overpowered by the surrounding circulation and observations on these behaviours are difficult to make. Therefore, the passive dispersion used in this study to mimic their trajectories may be sufficient and provides valuable insight into the impact of the KwaZulu-Natal Bight's surface circulation on Marine Protected Area connectivity and larval dispersion. The virtual particle tracking tools used in this study are not limited to biological applications. Future studies could use them to investigate the path and accumulation regions of virtual pollutants, such as microplastics, to determine the regions in which clean-ups would be most effective.