Dryland rivers are characterised by highly pulsed and unpredictable flow, and support a diverse biota. However, these biota are under pressure from water diversion and regulation to supply human needs, and are vulnerable to climate change impacts on the flow regime. This study examined the contribution of floodplain sources to the productivity of a disconnected dryland river, i.e. a waterhole, after a major overland flood event. Rate measures of productivity were combined with stable isotope

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... biomass data on the food web in the waterhole and floodplain. Our study estimated that 50% of the fish carbon in the waterhole post-flooding was derived from floodplain food sources. Based on stable isotope data, this study suggests that fish were feeding on the same aquatic carbon source on the floodplain as in the waterhole. In the few months after retraction of the river to isolated waterholes, the large biomass of fish concentrated from the flooding decreased by 80%, most likely due to starvation. Based on the development of a carbon budget for the waterhole, mass mortality is hypothesized to be the cause of the high rates of heterotrophic production in the waterhole. Export of fish, via bird predation cannot be discounted, but large numbers of piscivorous birds were not observed during the study. This study suggests that floodplain inputs are important for fuelling short-term production in the waterholes, but via an unconventional pathway, i.e. fish mortality. In dryland river systems, flooding is infrequent and within-waterhole carbon sources also sustain production during dry periods. The episodic nature of flooding in dryland rivers means that changes in flow regimes, such as water regulation or abstraction, will reduce flooding and hence floodplain subsidies to the river. This is likely to have significant impacts on river productivity. Running header: Carbon in an arid floodplain river 3