Phytoplankton invasions: Comments on the validity of categorizing the non-indigenous dinoflagellates and diatoms in European Seas

The validity of categorizing the diatoms and dinoflagellates reported in the literature as non-indigenous phytoplankton in the European Seas was investigated. Species that are synonymous are often included as separate species (Gessnerium mochimaensis = Alexandrium monilatum, Gymnodinium nagasakiense = Karenia mikimotoi, Pleurosigma simonsenii = P. planctonicum), while other species names are synonyms of cosmopolitan taxa (Prorocentrum redfieldii = P. triestinum, Pseliodinium vaubanii = Gyrodinium falcatum, Gonyaulax grindleyi=Protoceratium reticulatum, Asterionella japonica = Asterionellopsis glacialis). Epithets of an exotic etymology (i.e. japonica, sinensis, indica) imply that a cosmopolitan species may be non-indigenous, and several taxa are even considered as non-indigenous in their type locality (Alexandrium tamarense and A. pseudogoniaulax). The records of Alexandrium monilatum, A. leei and Corethron criophilum are doubtful. Cold or warm-water species expand their geographical ranges or increase their abundances to detectable levels during cooling (Coscinodiscus wailesii) or warming periods (Chaetoceros coarctatus, Proboscia indica, Pyrodinium bahamense). These are a few examples of marginal dispersal associated with climatic events instead of species introductions from remote areas. The number of non-indigenous phytoplankton species in European Seas has thus been excessively inflated.