Influence of sedimentological and hydrological processes on the distribution of the Spartina maritima salt marsh in the Keurbooms Estuary, Western Cape By Athi Nkosibonile Mfikili Submitted in fulfilment of the requirements for the degree Magister Scientiae in the Faculty of Science, Department of Botany, Nelson Mandela Metropolitan University Port Elizabeth April 2017 Supervisor: Dr Thomas G. Bornman Co-supervisor: Dr Derek R. du Preez I dedicate this thesis to my family, especially my daughter Lukho Lunathi, this one is for you baby. Also to my late grandmother Nomnceba Mfikili, mother Vuyokazi Mfikili and cousin sister Zukiswa Mfikili, you will always remain in my heart and I will never forget you. Declaration In accordance with Rule G5.6.3 [of the Nelson Mandela Metropolitan University], I, Athi Nkosibonile Mfikili (student number: 208016013), hereby declare that the above-mentioned treatise/ dissertation/ thesis is my own work and that it has not previously been submitted for assessment or completion of any postgraduate qualification to another University or for another qualification. …………………………. (Signature) Athi Nkosibonile Mfikili i Summary Salt marshes are some of the most productive ecosystems in the world and have been the centre of attention over the past few decades, due to their decline as a result of global climate change and anthropogenic impacts. The growth of salt marshes is determined by substrate type, soil conductivity and elevation. The permanently open Keurbooms Estuary along the south-east coast of South Africa is subjected to occasional fluvial flooding and its intertidal area lacks well developed salt marshes, with Spartina maritima restricted to the lower reaches of the Bitou tributary and a few sections of the Keurbooms tributary. Presumeably because of fine sediment habitat in the confluence and lower Bitou tributary. The salinity of the estuarine water ranges between 0.1 – 26.9 and 3.2 – 35.3 in the Bitou and Keurbooms tributaries respectively. A typical salt wedge salinity pattern is common in the Keurbooms tributary where saline water often intrudes underneath the freshwater, especially during high river flows. The following hypotheses were developed and tested in this study: The limited spatial distribution of S. maritima in the Keurbooms Estuary is due to limited availability of fine sediment habitat; and the source of the fine sediment in the estuary is the Bitou tributary rather than the Keurbooms tributary or the sea. It was further postulated that after sediment characteristics, floods are the major hydrological driver determining the distribution of S. maritima in the Keurbooms Estuary. The results of the surveys of the estuarine channel bottom sediments showed that the Keurbooms tributary was mostly characterized by the sand-size sediment fraction derived from the feldspathic and sandstone with evidence of fine sediment fractions restricted to the upper reaches at the confluence with Whiskey Creek. The Bitou was almost always composed of coarse sized sediments in the upper reaches, fine sediment deposits in the middle and lower reaches and medium sorted sand with almost no clay or calcium carbonate in the estuarine component below the confluence of the tributaries. These findings were further supported by the surface sediment deposited within the S. maritima intertidal salt marsh, which showed finer sediment deposits in the Bitou marsh compared to the Keurbooms marsh surface. Similar results were also found in the sediment cores, with the Keurbooms marsh sediment becoming finer with increasing depth whereas fine sediments reduced with depth in the Bitou marsh. The results of the sediment mineralogy indicated that the increased concentrations of clay minerals in the S. maritima surface ii sediments are derived from the Bokkeveld shale, siltstone and clay slate exposed above the N2 Bridge in the Keurbooms Estuary. GIS mapping shows that S. maritima has been declining over the past two decades, with rapid decreases especially evident after big flooding events. The GIS mapping also indicates that the patches of the S. maritima in the Keurbooms tributary are more exposed to big floods than the Bitou marsh. Despite showing an overall decline, S. maritima area coverage remained more consistent in the lower reaches of the Bitou tributary than in the Keurbooms tributary. Despite the larger and more persistent area cover, the S. maritima plants were shorter and less dense than the plants growing in the sandy substrate. The black/grey colouration of soil with increasing depth in the Bitou tributary was an indication of the reduced state of the soil caused by prolonged waterlogged conditions. The roots of S. maritima in both tributaries were mostly restricted to the sub-surface substrate layer (i.e. 0 – 0.25 m), although the Bitou populations showed more vegetative propagation than the Keurbooms populations. This mechanism of reproduction was also demonstrated during the transplant experiment which showed a greater number of new stem production in the fine sediment substrates compared to the sandy silt substrates. Although accretion rates were not determined in this study, the short-term sediment deposition rates revealed that sedimentation is active in the marshes of the Keurbooms Estuary. Therefore, in spite of showing a decline in area cover, the production of viable seed and observed vegetative propagation suggest that the S. maritima is likely to colonize open stable intertidal mudflats / sandflats, thus maintaining its distribution as an intertidal species in the salt marshes of the Keurbooms Estuary. Keywords: Keurbooms Estuary; sediment dynamics; flooding; salt marshes; Spartina maritima. iii Acknowledgements I would like to thank the Almighty God who gave me strength day-by-day to complete this work, Glory and praise to Him. This has been a multi-disciplinary study and very challenging experience for me. I was fortunate enough to collaborate with various scientists from different disciplines and I believe my scientific skills and thoughts have through this process improved immensely. I would like to give special thanks to my supervisor and mentor Dr Tommy Bornman for all the support and input. I am also grateful to Dr Derek du Preez (my co-supervisor) and Prof Maarten de Wit (collaborator/funder) for all their assistance and input. This work could not have been possible if it was not for the financial support from SAEON Elwandle Node, AEON – ESSRI (Inkaba Ye-Africa) and NMMU Postgraduate scholarship. Thank you very much. Special thanks to SAEON Elwandle Node staff for all their input, encouragement as well as assisting me on my intensive field trips, especially Tommy, Mfundo Bizani, Sean Bailey and Shaun Deyzel. I was fortunate enough to design the sampling sites and conduct all the field trips with your assistance, thank you very much. Special thanks to Arlene Cobb for all the administrative work required for all the field trips. Throughout my time at SAEON, whether in good times or tough times, I have always had the full support of the entire group. I want to acknowledge this superb group of people for always being there and for supporting me wholeheartedly. I would also like to thank SAIAB staff for allowing me to use their equipment, especially Francois Lamont and Ryan Palmer. To the Department of Botany staff and fellow students, I am grateful for all the assistance you gave me. Special mention to Andy Smith, Chan Visagie and Cynthia Tobias, thanks guys. Special thanks to Dr Paul-Pierre Steyn and Prof Janine Adams for all your input and encouragement. Postgraduate research studies could be a very long and lonely journey, however it was not the case for me, thanks to my AEON friends. Special mention to Lucian, Thomas, Vhuhwavhohau, Abiel and Dr Bastien Linol, thanks guys for all your advice and encouragement and those nice braais. iv I am also grateful to Willie (Technician at Department of Geosciences) for all his assistance. Special thanks to Mrs Megan Purchase from the Department Geology in the University of Free State for assisting me with mineralogy analysis. I am also indebted to the Department of Water and Sanitation for providing the flow data of the Keurbooms Estuary, special mention to Nolu Jafta. I am also grateful to the South African Weather Service for providing the weather data. I would also like to thank SANParks Garden Route for providing the Satellite aerial image of the Keurbooms Estuary. To Mfundo and Phumlile thanks chaps for being there for me, I am grateful for all your assistance and encouragement. My fellow Postgraduate Students Villagers especially Kakuhle, Chinidu, and Ngcali thank you for your input and continued encouragement, you have also made my stay very fun in the village and to the PGSV Football Club, you guys really kept me going, thanks. To my family, especially my special woman, my mother Sis Ndumi, you have been so patient with me I could not thank you enough, I owe you big time. I thank you for all your support and prayers. Lastly but not least, to my love Bomikazi and our daughter, Lukho thanks for your patience. v Table of Contents Declaration….................................................................................................................................. i Summary….. .................................................................................................................................. ii Acknowledgements .....................................................................................................................