Ecological processes affecting community structure of blackfly larvae in regulated and unregulated rivers: a regional study

1, We examined the effects of different ecological processes on larval blac kfly (Diptera: Simuliidae) community structure at a regional scale in 51 ra pids in unregulated and regulated rivers in northern Sweden (200 000 km(2)) . These processes were flow disturbance, biotic interactions (predation and competition) and the supply of food resources (suspended particles) to the larvae. 2, Using partial least squares projection to latent structures (PLS) modell ing, we developed predictive models based on environmental variables at unr egulated sites. Sites with high species richness and abundance were charact erized by large numbers of small suspended particles, deep water colour, hi gh total phosphorus and nitrogen concentrations, high proportions of forest in the catchment, low frequencies of large flow increments, extended fores t growth period, low cover of filamentous algae on the substratum, and low altitude. 3, These PLS models were used to predict blackfly species richness and abun dance at regulated sites with reduced flow thigh disturbance) and with unre duced flow (low disturbance). The residuals, i.e, differences between obser ved and predicted values, were used to evaluate impact strength of flow reg ulation. 4. A significant impact of flow disturbance on blackfly larvae was detected at regulated sites with reduced flow. Simuliid species richness and the to tal abundance at these sites were 25% and 50% higher, respectively, than pr edicted. At the disturbed sites, the abundance of blackfly predators decrea sed by 35%, and those of assumed competitors, grazers and net-spinning cadd is larvae, by 22% and 19%, respectively. 5, The particle concentrations were not significantly different between unr egulated and regulated sites and they were positively related to blackfly s pecies richness and abundance. 6. Our results indicate that changes in water flow (hydraulic disturbance) greatly influence blackfly larvae. Predation pressure and competition is re duced, and recolonization after disturbance is rapid. Simuliid communities are a feature of disturbed sites and may be a useful indicator for evaluati ng the impact of flow regulation on river ecosystems. The approach taken in this study may have wider applicability in impact studies in conservation ecology.