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Blooms of centric diatoms are a common feature in the Bilbao estuary during summer when river flow is at its lowest and water temperature is above 20ºC. To gain insight into the specific composition of these diatom blooms, net samples and cultures of estuarine isolates were analysed under the scanning electron microscope (SEM) and by molecular analyses of the Internal Transcribed Spacers 1 and 2 plus the coding region 5.8S (ITS region) and the 28S rRNA gene. Seven species of solitary centric diatoms belonging to four genera were found in the estuary including: Conticribra weissflogii, Cyclotella atomus var. atomus, Cyclotella cryptica, Cyclotella marina, Cyclotella meneghiniana, Discostella pseudostelligera and Thalassiosira pseudonana. Dominant species during blooms were C. meneghiniana and Co. weissflogii in the upper estuary and D. pseudostelligera and T. pseudonana in the middle estuary. The morphological traits used to differentiate between species pairs of similar morphology (C. meneghiniana/C. cryptica or D. pseudostelligera/D. woltereckii) were observed to vary with environmental conditions, denoting a great deal of phenotypic plasticity which would hinder accurate identification of the species when using morphological approaches alone.

Benthic dinoflagellates of the genera Coolia, Ostreopsis and Prorocentrum isolated from coastal waters of the south-eastern Bay of Biscay were identified morphologically by means of light microscopy (LM) including epifluorescence microscopy, and scanning electron microscopy (SEM). To identify the strains to species level, molecular phylogenetic analyses using the nuclear large subunit rDNA (LSU) were performed for 16 strains of the three genera. These morphological and phylogenetic analyses revealed the presence of the following species: Coolia canariensis S. Fraga, Coolia monotis Meunier, Ostreopsis cf. siamensis Schmidt, Prorocentrum emarginatun Fukuyo, P. lima (Ehrenberg) Dodge, P. rhathymum Loeblich III, Sherley & Schmidt, and two as yet unidentified species, which in the phylogenetic tree were grouped with different strains of Prorocentrum emarginatun and P. fukuyoi Murray et Nagahama from GenBank. A strain from Minorca (Balearic Islands, western Mediterranean Sea) analysed in this study for comparat...