Spatio-temporal variation of dinoflagellates in a tropical anthropized estuary in northeastern Brazil

This study aimed to identify dinoflagellates, addressing the ecological aspects of their distribution along the estuarine gradient of the Paraguaçu River, in northeastern Brazil, based on bimonthly collections (March 2018 to March 2019) carried out at 12 sampling stations. At each station, water samples were collected for analysis of species composition and cell density of dinoflagellates and preserved with formaldehyde at a final concentration of 4%. Twenty-one species were identified, most diversity and cell densities were recorded in Paraguaçu Channel and Iguape Bay. The highest densities of dinoflagellates were found in the rainy season in the regions of Iguape Bay and Paraguaçu Channel (Iguape Bay: dry season: 2.3 × 103 cells L−1, rainy season: 5.2 × 103 cells L−1; Paraguaçu Channel: dry season: 2.8 × 103 cells L−1; rainy season: 8.9 × 103 cells L−1). Scrippsiella cf. acuminata was the species with the highest density, forming a bloom in March 2018 (1.78 × 106 cells L−1) in the lower course of the river, in addition to high cell density in November 2018 (1.7 × 105 cells L−1) for the same region. The influence of the Pedra do Cavalo dam on the dynamics of the Paraguaçu system is notable; during the rainy season, higher nutrients concentrations and higher flows controls the dynamics of dinoflagellates both spatial and temporally, enhancing richness and density of species in areas of the Paraguaçu Channel and Iguape Bay. This study provides information on the occurrence of dinoflagellate species in an anthropogenic tropical estuary and the effect of environmental factors on their distribution in this environment, mainly regarding rainfall and water flow from the Pedra do Cavalo dam, and their influence on the estuary, providing changes in environmental conditions and, consequently, in the diversity and abundance of these organisms.