• compensation irradiance;
  • growth rates;
  • light;
  • loss rates;
  • Pseudochattonella ;
  • salinity;
  • saturating irradiance;
  • temperature

The ichthyotoxic flagellate Pseudochattonella has formed recurrent blooms in the North Sea, Skagerrak and Kattegat since 1998. Five strains of Pseudochattonella farcimen and two strains of P. verruculosa were examined in an assay comparing the light response of specific growth rates over a range of temperatures and salinities to get further knowledge on the autecology of members of this genus. Temperature optima were lower in P. farcimen (9°C–15°C) than in P. verruculosa (12°C–20°C). P. farcimen also showed a somewhat lower salinity optimum (18–26) than P. verruculosa (20–32). All strains showed light-dependent growth responses reaching saturation between 18 and 52 μmol · photons · m−2 · s−1 at optimal temperature and salinity conditions. Compensation point estimates ranged from 4.2 to 15 μmol · photons · m−2 · s−1. Loss rates increased with temperature and were lowest at salinities close to optimal growth conditions. Blooms of P. farcimen have been recorded in nature under conditions more similar to those minimizing loss rates rather than those maximizing growth rates in our culture study.