Storm-flow disturbances are frequent during the wet season of Australian tropical savannas. We examined benthic algal resistance and resilience in open-canopy streams in the Daly River watershed. Storm flows occurred every 2 to 3 d at 1st- to 4th-order sites, with sharp rises and falls and relatively long periods of shallow, low-turbidity base flow. At a 5th-order site, storm-flow duration was longer and base flows were deeper and more turbid. We hypothesized that: 1) storm flow would dislodge benthic algal biomass, 2) baseflow biomass would be low, 3) taxon richness would be low, and 4) algal composition would be dominated by resistant algae with prostrate or erect growth forms or by fast growing colonizing algae. Hypothesis 1 was supported. Storm flows dislodged ~93% of epilithic biomass. Support for hypothesis 2 was equivocal. At the 5th-order site, sand mobility prevented establishment of benthic algae until seasonal flows receded. At the other sites, epilithon net growth rates were much greater than rates in some temperate streams. Benthic biomass was typical of temperate oligotrophic streams, but maximum biomass was typical of mesotrophic streams. We attributed the relatively rapid growth and high biomass to warm water temperatures (mean = 29°C, maximum = 36°C), high incident light, rapid algal nutrient uptake, loss of grazing invertebrates caused by storm flows, and physical impediments to fish access. Hypothesis 3 was not supported. Mean taxon richness was high because of the occurrence of rare taxa. Hypothesis 4 was not supported. Epilithic algal biomass was dominated by resistant filamentous chlorophytes. Epilithic algal resistance was similar to resistance in higher latitude streams, but resilience was greater. Epilithic algae potentially could supply autochthonous C to the Daly River and other tropical aquatic food webs.
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Vol. 33 • No. 4