Trophic characteristics of aquatic habitats with different flooding regimes in the Okavango Delta, Botswana Thesis submitted for the degree of Doctor of Philosophy by Richard Mazebedi University College London, Department of Geography March 2019 1 Declaration I, Richard Mazebedi, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis 2 Abstract In periodically flooded wetland systems, the seasonal flooding regime is the main driver of ecosystem functions. The relationship between a wetlands’ hydrology and ecosystem properties is therefore crucial to understand. Food web structure is an important ecosystem property that determines the stability of aquatic populations and hence the resilience of ecosystems to potential threats. While theoretical concepts predicting energy flow and food web dynamics in wetland ecosystems exist, there is still need for empirical research to validate the predictions to better inform local wetland management. My thesis examined the variability of algal primary productivity and, using stable isotope ratios of carbon and nitrogen (δ13C and δ15N respectively), the aquatic food web structure of differently flooded aquatic habitats within the Okavango Delta, in Botswana. The results from in-situ algal productivity incubations, showed that primary productivity rate is higher at Lake Ngami compared with Nxaraga lagoon and Phelo floodplain due to greater levels of dissolved nutrients at the lake. The relative importance of basal carbon sources for fish biomass production varied across study sites, but overall, algal sources were the principal basal carbon sources for fish across the study sites. The relative importance of carbon from macrophytes was greatest at Phelo floodplain during high flood. Based on Layman’s matrices of food web structure, fish from Lake Ngami had isotopically diverse carbon sources, greater food chain length and greater trophic niche diversity compared to fish from Nxaraga lagoon and Phelo floodplain. There was high degree of trophic niche overlap between different fish feeding guilds at all the study sites indicating opportunistic feeding behaviour among fish in the Delta. Together, the results demonstrate that the diversity of aquatic habitats within the Delta support different pathways of energy flow and different aquatic food web structures, which may be crucial for maintaining diverse ecosystem functions of the Delta. 3 Impact statement The flood pulse that periodically floods the Okavango Delta is the main driver of biological production cycles and ecosystem functions of the Delta. However, natural flooding regime is faced with threats such as large-scale water withdrawals and climate change effects. The ability to predict the effects of the hydrological threats on ecosystem properties or to mitigate the effects of the threats will, however, rely on existing knowledge of the relationship between ecosystem properties and hydrological regimes. My thesis has improved the understanding of ecosystem properties in relation to hydrology within the Okavango Delta by quantifying primary productivity and food web structure in differently flooded habitats in the Delta. The results of Chapter 5, which is based on primary production measurements, are already published as a chapter in a book based on analyses of ecosystem services at Lake Ngami (Kgathi et al. 2018). Chapter 6, on the relative importance of basal carbon sources to fish, elucidated important spatial variations of δ13C and δ15N rations, which is going to benefit the design of future stable isotope-based studies in the Delta. The results of Chapter 6 showed that the relative importance of carbon sources to fish production vary between fish feeding guilds depending on the aquatic habitat and flood season, these findings are expected to benefit fisheries management in the Delta. Chapter 7 showed that there are high trophic niche overlaps between fish feeding groups, indicating opportunistic feeding strategies, which ensure maximum utilisation of diverse food resources that are availed by seasonal floods. This finding has management implications because it stresses the need for maintaining the natural seasonal flooding regime, hence resources pulses, for which the fishes of the Delta are adapted. 4 Acknowledgements I am grateful to my family, relatives and friends for the support they gave me during my PhD endeavour. My PhD training was generously funded by the Government of Botswana, through the Department of Tertiary Education Funding. I very grateful for the sponsorship and would like to acknowledge the friendliness and support of the scholarship administrators at the Botswana embassy in London, especially my education attaché: Neha Ali. I am extremely grateful to my principal supervisor, Prof. Anson Mackay, for his support and guidance throughout my PhD training; I am forever indebted by his generous support. My special thanks also go to Prof. Vivienne Jones my co-supervisor for her insightful guidance and constructive criticism of my PhD work. I also thank Dr. Mangaliso Gondwe, my co-supervisor who is based at Okavango Research Institute (ORI), firstly for helping with logistical preparations during my fieldwork trips to the Okavango Delta and secondly for his valuable advices on field and laboratory methods for stable isotope data. I thank my reliable fieldwork team at ORI, namely, Thebe Kemosedile, Ineelo Mosie, Onias Kudakwashe and Kuatji Katjarava. 5 Table of Contents Declaration ................................................................................................................................. 2 Abstract ...................................................................................................................................... 3 Impact statement ........................................................................................................................ 4 Acknowledgements .................................................................................................................... 5 List of Figures ............................................................................................................................ 9 List of Tables ........................................................................................................................... 12 GLOSSARY OF KEY TERMS AND ABBREVIATIONS .................................................... 13 CHAPTER ONE: Wetland ecosystems: Their importance, functioning and threats............... 15 1.1. What are wetlands? ................................................................................................... 15 1.2. The importance of wetlands to humans..................................................................... 16 Ecosystem services from wetlands ................................................................................... 16 1.3. Global extent of wetlands .......................................................................................... 19 1.4. Extent and distribution of wetlands in Africa ........................................................... 20 1.5. Wetland classifications .............................................................................................. 21 1.6. Biodiversity in inland wetlands ................................................................................. 24 1.7. Threats to African wetlands ...................................................................................... 25 Overexploitation ............................................................................................................... 26 Introduced invasive species .............................................................................................. 26 Changes in flooding regimes ............................................................................................ 27 1.8. Conclusions ............................................................................................................... 29 CHAPTER TWO: Wetland food webs and the use of stable isotope analyses (SIA) in freshwater food web studies..................................................................................................... 30 2.1. Flow of organic matter in wetlands ............................................................................... 30 2.2. Wetland food webs ........................................................................................................ 34 2.3. The SIA approach to studying FWS ............................................................................. 36 What are stable isotopes ................................................................................................... 36 Use of stable isotope in ecological studies ....................................................................... 37 Controls of δ13C values in freshwater ............................................................................... 39 Variability of δ13C values in organic matter ..................................................................... 45 Controls of δ15N values in fresh water ............................................................................. 47 2.4 Stable isotope mixing models ........................................................................................ 55 What are mixing models ................................................................................................... 55 Assumptions of mixing models .......................................................................................