Mazebedi, Richard; (2019) Trophic characteristics of aquatic habitats with different flooding regimes in the Okavango Delta, Botswana. Doctoral thesis (Ph.D), UCL (University College London).

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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.

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