Oak leaves and larch and spruce needles in fine-mesh (0.3 mm) and coarse-mesh (3 mm) bags were exposed in two hard-water streams in the Swiss Jura and two soft-water streams in the Black Forest. Periodically, conidium production in samples was determined under defined conditions. Generally, conidium production per unit weight was highest in oak, followed by larch and spruce. In oak and larch, an early peak was followed by a gradual decline; in spruce, conidium production reached an early plateau in the Jura streams but increased throughout the experimental period in the Black Forest streams. Percentage similarity between substrate spora and stream spora was highest for oak, followed by larch and spruce in three of the streams with deciduous riparian vegetation. The reverse sequence occurred in the remaining stream of a coniferous forest. On oak and larch, more fungal species were recovered in fine-mesh than in coarse-mesh bags; however, these additional species were numerically unimportant. Heliscus lugdunensis was the dominant species in early stages of spruce decay. It was replaced at later stages by Alatospora acuminata in the Jura streams and by two other species in the Black Forest streams. On oak and larch, the four or five most common species remained dominant throughout the experimental period.
