Aquatic food web dynamics on a floodplain in the Okavango delta, Botswana

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The study presents the succession of food web compartments during the annual flood of a floodplain of the great inland delta, Okavango, Botswana, and emphasizes how the floodplains serve as key recruitment areas for fish. By onset of the flood, the rather nutrient poor water from the main river becomes strongly enriched by inorganic nutrients and organic debris as it flushes over the savannah. At the chosen site, peak concentrations of nitrogen and phosphorus were 4 mg and 560 μg l−1, respectively. The high concentrations of nutrients yielded an immediate burst in primary production and phytoplankton standing stock, being almost 300 μg C l−1 d−1 and 24 μg Chl. a l−1 respectively at the maximum. When submersed by water, there was apparently a succession in hatching of zooplankton resting eggs present as a `egg-bank'. Peak concentrations of zooplankton reached 10 mg DW l−1, mostly of cladocerans Moina and Ceriodaphnia, with extreme near-shore concentrations of ∼90 g DW l−1. The zooplankton was probably nutritionally subsidized by detritus. During the flood a number of fish species moved in and spawned in the shallow waters. The fish fry fed on a variety of zooplankton species, before the water level again receded two months later. Thus the annual flood creates a highly dynamic shallow water system with high productivity that supports fish recruitment in the delta. The study also emphasizes the strong links between aquatic and terrestrial production in the delta. Keywords: Okavango, flood dynamics, nutrients, plankton, fish, food web, floodplain, planktivory, detritus.

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Journal Article

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