The Holocene evolution of Lake St Lucia, Africa's largest estuary: Geological implications for contemporary management

Andrew Green, Marc Humphries, Kate Strachan, Andrew Cooper, N. Gomes, Nonkululeko Dladla

Research output: Contribution to journalArticlepeer-review

Abstract

The Holocene evolution of one of Africa's largest estuaries, Lake St Lucia on the east coast of South Africa, is examined and juxtaposed with previous and contemporary management practices aimed at fixing the estuary state at a single point along the system's overall evolutionary pathway. The estuary has been heavily altered over the course of the last century, mostly through mouth management activities aimed at maintaining a direct connection with the ocean. Based on a high-resolution seismic reflection dataset, LiDAR and bathymetry data of the lake system, together with palaeontological (foraminifera and diatoms) and sulfur isotope geochemical data from three long (14–16 m) piston cores, we investigate the system-wide and coeval evolution of the three different sub-basins (False Bay, North Lake and South Lake) that comprise the current estuarine lake system. The northern and southern sections of the system evolved independently of each other since the Last Glacial Maximum, with each area responding differently to early sea-level forcings. In North Lake, the lagoon evolved from fully estuarine (8300 cal. BP) to a restricted lagoon with occasional marine incursions, steadily decreasing tidal prism and the development of marginal spits by wind-driven lagoonal waves. The tidal inlet closed approximately 6200 cal. BP and fluvial conditions became more dominant as main back-barrier conditions became attuned to fluvial supply. Similar conditions prevailed in the adjacent False Bay basin, albeit with an earlier onset due to sheltering by prominent bedrock peninsulas. In contrast, South Lake evolved from a fully estuarine system (9500 cal. BP) with a persistent ocean connection, to a segmented system with a diminished tidal connection by 5500 cal. BP. The inlet sealed at this point and an isolated back barrier system developed. This prompted the system-wide impoundment of waters causing back-barrier flooding and in situ drowning of bayhead deltas and segmenting spits. By 2000 cal. BP, these areas had flooded sufficiently to connect over a bedrock high that had diverted the initial palaeo-valleys to the north and south. A new, single lagoon, representative of the modern-day system formed, with a new marine connection to the modern day Mfolozi River and contemporary estuary. Periodic desiccation of the lake sub-basins since this point has been apparent. Given the complex management plans implemented to stop excess sedimentation, hypersaline events, and the continuation of “normal” lake levels, this is at odds with the natural system's evolution to a pan or wetland-type state. This geological background should be key to informing management plans that work within the natural evolutionary trajectory of the system, as opposed to attempting to maintain an unattainable status quo.

Original languageEnglish
Article number107745
JournalEstuarine Coastal and Shelf Science
Volume266
Early online date17 Jan 2022
DOIs
Publication statusPublished - 5 Mar 2022

Bibliographical note

Funding Information:
The cores and seismic data analysed in this study were collected under a project funded by the National Research Foundation of South Africa (Grant 87654 ) and the Water Research Commission (Project K5/2336 ) K.L.S. acknowledges support from the German Federal Ministry for Education and Research (BMBF, Bonn, Germany) within the project “Regional Archives for Integrated Investigation (RAiN)”. Caldin Higgs, Keegan Benallack, Jemma Finch, Letitia Pillay, Trevor Hill, Errol Wiles and Ander De Lecea assisted with field work. The iSimangaliso Wetland Park Authority and Ezemvelo KZN Wildlife kindly granted us permission to work at St Lucia. Elevation data were supplied by iSimangaliso Wetland Park Authority funded by the GEF Trust Fund and the World Bank . This paper is a contribution to IGCP639 “Sea-level change, from minutes to millennia”. Lastly, we acknowledge the editor, Steve Mitchell and the four anonymous reviewers who had many thoughtful and insightful comments which benefited the paper.

Funding Information:
The cores and seismic data analysed in this study were collected under a project funded by the National Research Foundation of South Africa (Grant 87654) and the Water Research Commission (Project K5/2336) K.L.S. acknowledges support from the German Federal Ministry for Education and Research (BMBF, Bonn, Germany) within the project ?Regional Archives for Integrated Investigation (RAiN)?. Caldin Higgs, Keegan Benallack, Jemma Finch, Letitia Pillay, Trevor Hill, Errol Wiles and Ander De Lecea assisted with field work. The iSimangaliso Wetland Park Authority and Ezemvelo KZN Wildlife kindly granted us permission to work at St Lucia. Elevation data were supplied by iSimangaliso Wetland Park Authority funded by the GEF Trust Fund and the World Bank. This paper is a contribution to IGCP639 ?Sea-level change, from minutes to millennia?. Lastly, we acknowledge the editor, Steve Mitchell and the four anonymous reviewers who had many thoughtful and insightful comments which benefited the paper.

Publisher Copyright:
© 2022 Elsevier Ltd

Keywords

  • Lagoon evolution
  • Back-barrier
  • Foraminifers
  • Lake St Lucia
  • Estuary management

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