Fluid-mediated dispersal in streams: models of settlement from the drift

I propose proximal mechanisms that help explain, unify, and expand the pred ictions of widely accepted empirical models of settlement in streams. I sep arated the process that leads to settlement of a drifting particle into thr ee stages: (1) initial contact with a substrate, (2) attachment, and (3) se ttlement sensu stricto. I used physical principles (height above the bed, s inking rate, current speed profile) to predict time until contact (stage 1) . I compared these predictions with empirical measurements of settlement of individual black fly larvae (Simulium vittatum) in a laboratory flume. I d eveloped models from empirical data for stages 2 and 3. Each of these model s is individual-based and predicts the fate of a single individual. To obta in a population level prediction, models for the three stages were combined and used to simulate the settlement of a group of black fly larvae. The pr edictions of this simulation were qualitatively similar to population level data from the literature particularly after the incorporation of channel-w ide spatial heterogeneity in current speed. The effect of flow heterogeneit y on the model agrees with previous work on the lateral transport of stream invertebrates during drift events showing that many organisms settle prefe rentially in slower areas. By using proximal principles, the approach used in this study brings into focus basic parameters and processes that influen ce settlement at the scale of the organisms. It also provides a null hypoth esis against which to study the effect of local flow heterogeneity on the s ettlement of stream invertebrates and the capacity of organisms to actively influence settlement. Water currents in streams and rivers commonly transp ort large numbers of organisms. Consequently, hydrodynamic factors that fav or or hamper the settlement of these organisms can potentially influence di stributions and abundance. Moreover, if settlement probabilities vary with flow characteristics, this can in turn influence foraging strategies that r ely on fluid-mediated dispersal.