Longitudinal changes in the abundance of filter feeders and zooplankton inlake-outlet streams in northern Sweden

The Sheldon & Oswood (1977) model, that predicts a linear relationship betw een log filter-feeder density and log distance from a lake and assumes prop ortionality between filter-feeder density and seston concentration, was tes ted on data from four lake-outlet streams. In addition, longitudinal change s in species composition and body size in zooplankton were investigated. Mo st filter feeders deviated from the model and a quadratic, rather than line ar, decrease in density was a common pattern. Total filter feeders and blac kfly larvae showed a hump-shaped distribution pattern. Lower current veloci ty at sites close to lakes is suggested as an explanation. No general propo rtional relationship between net-spinning caddis larvae and zooplankton was found and several reasons for this result are discussed. Zooplankton bioma ss and mean zooplankton body length decreased with distance from lakes. The abundance of cyclopoid adults, copepodites, and nauplii declined at a high er rate than Bosmina spp. This result indicates that factors such as select ive predation by filter-feeders and/or taxon-specific sedimentation rates, are acting to reduce zooplankton abundance in these lake-outlet streams.