The harmful algae blooms are very weil known and characterized phenomena in some parts of the worid, due to the negative impact that they cause in the ecosystems, the adverse effects in the human health together with economic and social problems associated to the decrease in the revenues of the residents of the affected regions. Costa Rica is not an exception to these algae events. From last decade an increase in frequency, and duration of the microorganisms able to originate HAB has been observed. The dynamics and causative factors of these events are not weil known in our country. In this study we collected samples of the water in diverse sectors of the Gulf of Nicoya (long. 85ºW and lat. 10ºN) on the surface and 5 meters deep. We found 33 microalgae species: 24 dinoflagellates species, 8 diatoms and 1 cyanobacteria. They were analyzed, characterized and describes by light microscopy and scanning electron microscopy comparing them with public databases and bibliographical sources. We also describes the possible toxic or harmful potential of each specie.

Harmful algae blooms; phytoplankton; marine toxins; dinofiagellates; microalgae; cyanobacteria