Resource stress alters hydrological disturbance effects in a stream periphyton community

Scouring reduces stream periphyton biomass during hoods, and levels of light and nutrient resource supply can influence growth and community physiology between Roods. We hypothesised that for a given taxonomic structure, resource stressed communities would have lower resistance and resilience to scour disturbance than resource replete communities. We investigated this in experimental streams with four combinations of light and nutrients. Communities in all treatments were dominated by similar diatoms. Resource supply significantly influenced the resistance to scouring of some components of the communities. In particular, high nutrient stress resulted in low resistance of chlorophyll a biomass (up to 94% loss), whereas moderate light stress and low nutrient stress resulted in the most resistant communities (43% loss). Resilience of chlorophyll a was negatively correlated, and resilience of taxonomic composition was positively correlated, with resource stress. In the treatments with high nutrient supply, chlorophyll a returned to pre-disturbance levels within 6 d, but regeneration of the taxonomic structure took >9 d. For the treatments with low nutrient supply, chlorophyll a took >18 d to recover, but taxonomic composition took only a few days to recover reflecting the dominance of immigration processes (and lack of in situ growth). In all treatments the scour disturbance caused a reduction in growth rates of the regenerating communities relative to the undisturbed control communities. Overall. the disturbance had a longer-term influence on community persistence under moderate to high resource stress than under low resource stress.