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

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1. We examined the effects of different ecological processes on larval blackfly (Diptera: Simuliidae) community structure at a regional scale in 51 rapids in unregulated and regulated rivers in northern Sweden (200 000 km2). 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) modelling, we developed predictive models based on environmental variables at unregulated sites. Sites with high species richness and abundance were characterized by large numbers of small suspended particles, deep water colour, high total phosphorus and nitrogen concentrations, high proportions of forest in the catchment, low frequencies of large flow increments, extended forest 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 abundance at regulated sites with reduced flow (high disturbance) and with unreduced flow (low disturbance). The residuals, i.e. differences between observed and predicted values, were used to evaluate impact strength of flow regulation.

4. A significant impact of flow disturbance on blackfly larvae was detected at regulated sites with reduced flow. Simuliid species richness and the total abundance at these sites were 25% and 50% higher, respectively, than predicted. At the disturbed sites, the abundance of blackfly predators decreased by 35%, and those of assumed competitors, grazers and net-spinning caddis larvae, by 22% and 19%, respectively.

5. The particle concentrations were not significantly different between unregulated and regulated sites and they were positively related to blackfly species richness and abundance.

6. Our results indicate that changes in water flow (hydraulic disturbance) greatly influence blackfly larvae. Predation pressure and competition is reduced, and recolonization after disturbance is rapid. Simuliid communities are a feature of disturbed sites and may be a useful indicator for evaluating the impact of flow regulation on river ecosystems. The approach taken in this study may have wider applicability in impact studies in conservation ecology.
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Keywords: PLS modelling; Simuliidae; biotic interaction; flow regulation; suspended particles

Document Type: Original Article

Publication date: October 1, 1998

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