Epipelic diatoms blooming in Isahaya Tidal Flat in the Ariake Sea, Japan, before the drainage following the Isahaya-Bay Reclamation Project

A blooming diatom sample from Isahaya Tidal Flat in the Ariake Sea, south‐west Japan, before the bay was closed and drained following the Isahaya‐Bay Reclamation Project, is taxonomically examined. When the sample was collected on 25 May 1996, the area was a muddy tidal flat. Altogether, 103 diatom species were observed within 1000 counts of frustules. Both the dominant and subdominant diatom species were large raphid species. Light microscopy and scanning electron microscopy observation revealed that the dominant species was Haslea nipkowii (F.Meister) Poulin et G. Masse, which was originally described as a species of Gymsigma. So far, a report of correctly identified H. nipkowii has been presented only from China, because H. nipkowii sensu Poulin et al. from USA and France probably belongs to the other taxon, Gymsigma pallidum Riznyk. The subdominant species is described as a new species, Nitzschia gyrosigmasp. nov. This new species is similar to Nitzschia sigmoidea under light microscopy, but scanning electron microscopy observation reveals that their fine structures are quite different. Nitzschia granulata var. hyalina is transferred to the genus of Tryblionella and raised to an independent species, Tryblionella hyalina (Amosse) comb. nov.