Fig. S1. Image of rocks collected from the streams. Biofilm growth is mostly limited to the equator of the rocks. Green biofilms are located on the upper side.


Fig. S2. Example of flow velocities measured in situ above a cobble at a vertical distance of 2 cm. Panel (A) shows all three cartesian components of the velocity vector (z-component corresponds to the vertical direction), as well as the vector magnitude, as a function of time. Panel (B) shows the projections of the velocity vector in the vertical plane (the time axis is collapsed).


Fig. S3. Salinity distributions above the surface of rocks immersed in hypersaline water (300 g L−1) and exposed to freshwater flow from below at a vertical flow velocity of 7 cm s−1. Shown are examples for a rock without a biofilm and for two rocks partially covered with biofilms. Colour bar values are in g L−1.


Video Clip S1. An in situ colour injection experiment showing the freshwater flow around a rock at the edge of a spring. The main flow of the spring is not visible in this close-up video.

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