Abstract
We tested the role of immigration from the plankton on the structure of local surface‐associated communities of ciliates and heterotrophic flagellates (HF) in flow cells fed with fresh riverine water (Rhine, Germany). By applying size fractionation, we reduced the immigration potential of HF from the plankton in mature, pregrown biofilms by 93%, and observed no significant effects on the abundance and taxonomic composition of HF. Compared with a treatment with natural plankton density, a 100% reduction in the number of planktonic cells flowing over mature biofilms resulted in a reduced total abundance of ciliates, although only slight effects on the relative composition of present morphospecies were detectable. When starting with sterile flow cells, we found an initial linear increase in HF abundance proportional to their planktonic abundance; additionally supplemented planktonic bacteria showed that this initial colonization rate was resource independent. The initial phase was followed by an exponential increase, which was also independent from the resource level and strongest when initial colonization was low. In contrast to the early succession stages, the final abundances reached were independent of the plankton abundance but strongly dependent upon the local resource level. Immigration is an important factor controlling the initial substrate colonization and early growth, and ciliate and HF communities in biofilms become increasingly independent of immigration with maturation, being then controlled by local factors such as resources.