Perturbations of stream ecosystems are often mediated by the terrestrial watershed, making the understanding of linkages between watersheds and streams essential. In this thesis I explore the connections between landscape characteristics, streamwater acidity and brown trout (Salmo trutta) distributions in Krycklan, a 67 km2 boreal stream network in northern Sweden. The study focuses on hydrochemical changes during the snowmelt-driven spring flood, a period of episodic acidity which is thought to place a restraint on acid-sensitive biota such as brown trout. pH ranged from 4.5-7.0 at different stream sites during winter baseflow, and declined by 0-2 pH units during spring flood. The magnitude of the pH drop at a given site was in large part controlled by changes in acid neutralizing capacity (ANC) and in natural organic acids associated with dissolved organic carbon (DOC). pH, ANC and DOC were all correlated with landscape characteristics such as proportion of peat wetlands, and stream hydrochemical response during spring flood could be explained by altered hydrological flowpaths through the catchment. The impact of acidity on brown trout distributions within the stream network was evaluated and compared to the apparent influence of other site and catchment-scale environmental factors. In situ bioassays demonstrated a strong relationship between spring flood pH and juvenile brown trout mortality, with a toxicity threshold at pH 4.8-5.4. In field surveys brown trout were not found at any sites which had pH <5.0 during spring flood, and were rare at sites which had pH <5.5 during spring flood, suggesting limitation by acidity for some streams. However, over the whole of the Krycklan stream network brown trout were more consistently associated with alluvial sediment deposits than with high pH or low inorganic aluminum concentrations. Acidity thus apparently influences trout distributions by setting a maximum potential distribution; within that potential distribution, actual dispersal is influenced by other factors, notably presence of physical substrate suitable for feeding and spawning habitat. Fulfilling chemical thresholds is therefore necessary but not sufficient for sustaining brown trout populations. In the context of environmental monitoring or stream restoration, consideration of physical habitat together with chemical conditions is advised
