BLOOMS OF Ceratium furcoides (LEVANDER) LANGHANS AND Cylindrospermopsis raciboskii (WOLOSZYNSKA) SEENAYYA & SUBBA RAJU IN LOWER SÃO FRANCISCO (NORTHEAST OF BRAZIL)

Abstract

The study aimed to record blooming episodes of the dinoflagellate Ceratium furcoides (Levander) Langhans 1925 and the cyanobacterium Cylindrospermopsis raciboskii (Woloszynska) Seenayya & Subba Raju in some areas of the São Francisco River Basin, Northeast region of Brazil. Water collections for determination of nutrients, chlorophyll a and phytoplankton density were carried out in the surface layer were collected from May/2015 to December/2016, in seven stations located in the counties of Delmiro Gouveia, Piranhas (Alagoas) and Paulo Afonso (Bahia). The determination of density (cell.mL-1) was performed in an inverted microscope (Zeiss Axiovert). During the study, the average water temperature varied between 26,38 ºC and 30.70 ºC, the pH remained alkaline, the dissolved oxygen contents were between 3.99mg.L-1 and 17.57 mg.L-1, the turbidity varied between 0.21UNT and 26.4 UNT. The maximum value for nitrate was 509.16 µg.L-1 and for total phosphorus was 92.46 µm.L-1. In May 2015, dark stains were visibly observed in some parts of the São Francisco River. These spots were caused by the accumulation of the dinoflagellate Ceratium furcoides. The bloom started in May 2015 with a maximum density of 5,600 cel.mL-1 (Biovolume 112.80µm3.L-1), decreasing considerably in the subsequent months (June, July and November) with densities below 260 cel.mL-1 ( Biovolume 5.24µm3.L-1). In February 2016 it reached another peak with a maximum density of 5,470 cell.mL-1 (Biovolume 113.97µm3.L-1) declining again in June, August and December/2016, with densities below 140 cell.mL-1 (Biovolume 2.92µm3.L-1). The highest density C. raciboskii was observed in June/2016, with 198,000 cell.mL-1 (Biovolume 884.31µm3.L-1). C. furcoides blooms are normally not toxic, whereas C. raciboskii is considered toxic. These microalgae have harmful effects on the entire aquatic community, affecting the balance of the ecosystem. The record of blooms of this species is relevant, as it provides important information about its distribution and dispersion in Brazilian aquatic systems.