Colonization dynamics of biofilm-associated ciliate morphotypes at different flow velocities

The impact of flow velocity on initial ciliate colonization dynamics on surfaces were studied in the third order Ilm stream (Thuringia, Germany) at a slow flowing site (0.09 m s−1) and two faster flowing sites (0.31 m s−1) and in flow channels at 0.05, 0.4, and 0.8 m s−1. At the slow flowing stream site, surfaces were rapidly colonized by ciliates with up to 60 cells cm−2 after 24 h. In flow channels, the majority of suspended ciliates and inorganic matter accumulated at the surface within 4.5 h at 0.05 m s−1. At 0.4 m s−1 the increase in ciliate abundance in the biofilm was highest between 72 and 168 h at about 3 cells cm−2 h−1. Faster flow velocities were tolerated by vagile flattened ciliates that live in close contact to the surface. Vagile flattened and round filter feeders preferred biofilms at slow flow velocities. Addition of inorganic particles (0, 0.6, and 7.3 mg cm−2) did not affect ciliate abundance in flow channel biofilms, but small ciliate species dominated and number of species was lowest (16 species cm−2) in biofilms at high sediment content. Although different morphotypes dominated the communities at contrasting flow velocities, all functional groups contributed to initial biofilm communities implementing all trophic links within the microbial loop.