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Diversity and Dominance in phytoplankton community during nycthemeral cycles at different thermal patterns in water body in semiarid region of Brazil

The vertical distribution of phytoplanktonic species in tropical regions responds quickly to the physicochemical factors of water with ecophysiological changes and biomass. Abiotic factors are responsible for the changes in the dominance of the different groups. The study analyzed the diversity and dominance of the phytoplankton community in a eutrophic reservoir localized in the semiarid region of Brazil. The samplings were conducted in nycthemeral cycles in different thermal regimes along a vertical profile. The biomass was determined by density and specific biovolume. Patterns of richness, diversity and specific dominance index were analyzed. The significant differences between the biotic data were analyzed by the ANOVA two-way and t-test. Similarity matrices were plotted using the Jaccard and Bray-Curtis indices with subsequent data ordination. 33 taxa were identified during the study. Differences in thermal patterns that are occurring in nycthemeral cycles showed changes in the structure of the phytoplankton community. The atelomictic event contributed to changing of the algal structure, especially in biomass, dominance, and diversity that shows significant differences between periods. The lowest diversity reported (<1.03 bits.L-1) were influenced by monospecific dominance of the cyanobacteria C. raciborskii, thus recording greater biomasses, forming blooms throughout the study period. In the tropical eutrophic reservoirs, an algal bloom of cyanobacteria has persisted throughout the year, regardless of variations in the thermal regime, thereby reducing the phytoplankton diversity. 