Abstract
Floodplain aquatic ecosystems experience temporal changes in basal production sources and inputs from allochthonous sources that influence energy flow. We analyzed stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) from 449 fish muscles representing 18 species to characterize energy pathways in two rivers of the lower Okavango Delta, Botswana. We sampled fish during wet and dry seasons and used mixing models to estimate proportions of major basal production sources assimilated by fishes. We estimated trophic position (TP) for each fish population and assessed the association between TP and the degree that fish biomass was supported by a single production source. During the wet season, fishes in the Boteti River assimilated material through food chains originating mainly from aquatic macrophytes. During the dry season, terrestrial C3 plants and terrestrial C4 grasses assumed greater importance. In the Boro River, terrestrial C3 plants were inferred to be an important production source supporting fishes during the wet season, with aquatic macrophytes becoming more important during the dry season. In both rivers and seasons, the degree of dependence on a single source was negatively related to TP, supporting the hypothesis that organisms positioned higher in the food web tend to be supported by multiple food chains and basal sources, whereas organisms lower in the food web may be supported by many or few food chains. We showed that consumers in river food webs shift foraging habits in response to seasonal patterns of connectivity and habitat availability, thus promoting long-term population and community stability.
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Acknowledgements
We would like to thank Choto J. Choto and Aaron Samoxa of the Department of Wildlife and National Parks (DWNP) in Maun for logistical support and assistance in the field during the first sampling in August 2017. Ketlhatlogile Mosepele and Ineelo Mosie of the Okavango Research Institute assisted with gill nets and a boat. The anti-poaching unit of the DWNP provided an outboard engine that could operate in relatively shallow water.
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This study was financially supported by the Botswana International University of Science and Technology, through a graduate fellowship awarded to Thethela Bokhutlo.
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This study complied with the requirements of the Texas A&M Animal Ethics Committee (TAMU AUP IACUC 2017–0069). Data collection was permitted through a director’s permit no. 00000654A, which was obtained from the DWNP.
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Bokhutlo, T., Keppeler, F.W. & Winemiller, K.O. Seasonal hydrology influences energy channels in food webs of rivers in the lower Okavango Delta. Environ Biol Fish 104, 1303–1319 (2021). https://doi.org/10.1007/s10641-021-01156-x
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DOI: https://doi.org/10.1007/s10641-021-01156-x