Diatom immigration drives biofilm recovery after chronic copper exposure

1. The impact of immigration on the recovery of diatom assemblages after chronic exposure to copper was investigated in laboratory microcosms. 2. We examined the recovery trajectories of copper‐contaminated biofilms after reducing copper stress and with or without the possibility of immigration from unimpaired communities. 3. The biofilms mixed with unimpaired communities returned to a ‘control’ assemblage structure within 6 weeks, with recovery patterns depending on the endpoint considered (i.e. 2 weeks for relative abundances of diatom species but 6 weeks for total diatom biomass). In contrast, no recovery was observed in assemblages placed under control conditions without external immigrants. 4. Immigration has important effects on the recovery of quantitative and qualitative characteristics of biofilms.