USING PREDICTIVE MODELS TO ESTIMATE EFFECTS OF FLOW REGULATION ON NET-SPINNING CADDIS LARVAE IN NORTH SWEDISH RIVERS

1. Effects of flow regulation on abundance and richness of net-spinnin g caddis larvae (Hydropsychoidea; Trichoptera) were examined in fifty- two North Swedish rivers. Models based on conditions at unregulated si tes were used to predict densities at impacted sites and the residuals , i.e. the differences between observed and predicted numbers, were us ed as measures of the effects. 2. Two broad categories of impacted sit es were identified. Sites with regulated but unreduced flow often have reduced seasonal flow variation and slightly enhanced shortterm varia tion, while those with reduced flow usually have long periods of low a nd stable flow occasionally interrupted by short periods of violently fluctuating flow. 3. The abundance of net-spinning caddis larvae was s ignificantly lower than predicted at both types of sites (reduction 30 %), while richness was significantly reduced only at sites with reduce d now (reduction 20%). 4. A subsequent analysis of the relationship be tween the effects and variables altered by flow regulations, e.g. flow magnitude and flow variation, indicated that species richness was pri marily affected by the occurrence of periods with zero flow, while neg ative effects of abundance were associated with high flow variability. 5. Significant negative effects on the abundance of individual taxa w ere observed for Cheumatopsyche lepida and Hydropsyche pellucidula at sites with regulated but unreduced flow. 6. Analyses of the relationsh ips between the effects and regulation-related variables suggest that H. siltalai was significantly negatively affected by the occurrence of zero flow and that negative effects on Arctopsyche ladogensis were as sociated with flow reduction and high flow variability.