Dinoflagellates display great diversity in tropical regions and play an important role in the complex microbial food webs of marine and brackish environments. The goal of this study is to identify planktonic dinoflagellates and their distribution in the estuary of the Maraú River, Camamu Bay, state of Bahia, in a region with increasing use of shellfish farming. Samples were carried out monthly from August 2006 to July 2007 at four stations along the estuary. Plankton was sampled with a 20 μm mesh net. We identified 20 dinoflagellate species. The greatest species richness was observed in the genera Protoperidinium (five spp.), Tripos (four spp.), and Prorocentrum (three spp.). Based on literature, six species were classified as potentially harmful: Akashiwo sanguinea, Dinophysis caudata, Gonyaulax spinifera, Prorocentrum micans, Scrippsiella cf. acuminata, and Tripos furca. Protoperidinium venustum was recorded for the first time in coastal waters of Bahia.

In fall 1995, during a survey in Abrolhos coral reef system (southwestern Atlantic, Brazil), significant densities (143-6174 cells L-1) of small thecate dinoflagellates were detected. Analysis of this material in scanning and transmission electron microscopy confirmed the presence of four taxa assigned to the potentially toxic genus Azadinium: A. dexteroporum Percopo et Zingone, A. luciferelloides Tillmann et Akselman, A. cf. polongum Tillmann and Azadinium sp. The latter taxon showed external morphological features quite distinct from any Azadinium taxon yet described, but its formal description as a new species depends on more detailed analysis. Species of Azadinium have never been confirmed in Brazilian waters until now, although the toxins produced by these dinoflagellates, the azaspiracids, have been detected in Brazilian southern coast without recognition of their producing organisms. The highest densities of Azadinium spp. occurred at stations south of and over the Abrolhos Bank, which receive higher nutrient concentrations due to upwelling of deep and nutrient-rich water masses.

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