The temporal phytoplankton biomass variation at two Neotropical reservoirs during an extreme drought season were analyzed. Here we sought to evaluate the main abiotic factors involved in dynamics of phytoplankton during this drought period. The main difference between the reservoirs was the intensive fish and shrimp farming in one of the reservoirs. For quantitative analysis, sampling with bottles were carried out at an average depth of 0.5m. Water temperature, pH and electrical conductivity parameters were measured in situ and water samples were collected for dissolved inorganic nitrogen and soluble reactive phosphorus analyses. Aquaculture was probably one among the causes for the reservoirs were so different in the physical and chemical variables, as shown by the principal components analysis. The results showed specific groups dominance in both reservoirs. In the Cachoeira II reservoir, an invasive dinoflagellate, Ceratium furcoides, was present in all analyzed months, while, in the Saco I reservoir, cyanobacteria group represented more than 50% of phytoplankton biomass, mainly Microcystis aeruginosa and Dolichospermum sp. In two reservoirs precipitation, soluble reactive phosphorus and electrical conductivity were positively related with phytoplankton. Phytoplankton biomass was considerably larger in the Cachoeira II reservoir, due to the greater size and biovolume of the dominant dinoflagellate. These findings suggest that species dominance in extreme drought events may be favored.

The growing concern about the shortage of fossil fuels drives the search for new sources of sustainable energy. Microalgae are considered promising in this area, since they are sources of third generation biofuels and protein. The present study aimed at describing and isolating species from the native phytoplankton communities of four reservoirs in the Pernambuco semiarid, making them available for experiments that can evaluate the biotechnological potential and meet the demands of the region. The collections occurred in water bodies located in the Pernambuco semiarid and the identification of the taxa occurred based on morphological characters using specific literature. A total of twenty-one taxa were identified, eight of them Cyanophyceae (Anabaena sp., Aphanocapsa elachista, Chroococcus dispersus, Dolichospermum sp., Merismopedia sp., Microcystis aeruginosa, Pseudanabaena limnetica, Synechococcus sp.), ten Chlorophyceae (Chlorella sp., Cosmarium bioculatum, Desmodesmus spp., Dictyosphaerium sp., Pediastrum duplex, Pediastrum simplex, Pseudokirchineriella subcapitata, Scenedesmus sp., Staurastrum leptocladum, Trochiscia sp) and only one of the following classes Bacillariophyceae (Aulacoseira granulata), Euglenophyceae (Trachelomonas volvocida) and Dinophyceae (Ceratium furcoides). The bioprospection of microalgae in the semiarid region of Pernambuco has made possible the formation of a bank of strains, important for fictional studies and obtaining biomass of species with biotechnological potential.

Ceratium furcoides is an invasive species that has caused ecological imbalance in several reservoirs in Brazil. This study investigates the main factors that may favor the occurrence of Ceratium furcoides blooms in a tropical reservoir from the Northeast Brazil, during an extreme drought season. Samples containing phytoplankton were collected monthly from February to September 2017. Quantitative analysis of C. furcoides was performed and the cell volume was estimated using geometric formulas. Mean biovolume of C. furcoides showed significant differences, ranging from 0.78 mm3 L-1 to 11.29 mm3 L-1 reported in March and September, respectively. Environmental parameters presented low oscillation throughout the study, except the conductivity. Significant relationships among the C. furcoides biovolume, water temperature and soluble reactive phosphate were observed. The findings reported here suggest that adverse conditions caused by a drought season did not negatively affect this species.