Harmful microalgal episodes in Greek coastal waters GEORGIOS NIKOLAIDIS*, KONSTANTINOS KOUKARAS, KATERINA ALIGIZAKI, ANDRIANI HERACLEOUS, ELENI KALOPESA, KIMONAS MOSCHANDREOU, ELEFTHERIA TSOLAKI and APOSTOLIS MANTOUDIS Department of Botany, School of Biology, Aristotle University, 54124 Thessaloniki, Greece Abstract In the frame of the harmful algae monitoring program conducted in the main fishing and shellfish growing areas of Greece during the period 2000-2004, two types of harmful algae were recorded: 1) microalgae that produce water discoloration because of their massive growth and 2) microalgae related to toxin production. The majority of algal blooms associated with water discoloration were observed in Thermaikos Bay, where species of the genus Prorocentrum (P. redfeldii, P. micans, P. triestinum and P. obtusidens), the heterotrophic Noctiluca scintillans, the raphidiophyte Chattonella cf. globoda and a naked dinoflagellate Gymnodinium sp. reached high cell densities. Algal blooms causing water discoloration were also observed in Amvrakikos and Elefsis Bays, where the causative organisms were Alexandrium insuetum and Prorocentrum minimum. The main harmful dinoflagellates, which so far cause the greatest economic losses in Greece, due to the shellfish intoxication, belong to the genus Dinophysis. Dinophysis species were mainly observed during late winter and spring, causing recurrent diarrhetic shellfish intoxication outbreaks in the last five years in Thermaikos Bay. Amvrakikos Bay is also affected by toxic microalgae, which cause diarrhetic and paralytic shellfish intoxication. Pseudo-nitzschia species a further group of harmful algae related with amnesic shellfish intoxication episodes were found also in all monitored areas.   < back .