Respostas espécie-específicas de cianobactérias frente ao enriquecimento nutricional em reservatórios tropicais semiáridos

Abstract

Enrichment by nutrients, especially nitrogen and phosphorus, represents one of the main causes of degradation of water bodies. Identifying the nutrient considered to be limiting the growth of cyanobacteria is essential in order to develop strategies to maintain water quality, particularly in environments where the population lives with the challenge of water scarcity. The present dissertation work sought to determine, experimentally, the nutrient that limits the growth of cyanobacteria in semi-arid tropical reservoirs. Four reservoirs (Serrinha II, Jazigo, Cachoeira II and Barra do Juá), belonging to the Pajeú river basin, state of Pernambuco, semi-arid region of Brazil, were selected for data capture during the region's dry season (June 2022), To obtain the data, 5 points were selected in each reservoir. The samples directed to the setup of the experiment were composed of 25L of water, 5L of which were collected at each sampling site in an integrated manner at the three depths: subsurface, 1% luminosity and aphotic zone. From these 25L collected, four samples were separated before starting the experiment, in order to understand the Tinitial. These were destined to the quantitative analysis of the phytoplankton groups and chemical analysis of the water. In the experimental stage, four treatments were established, containing 4 replicates each: control (no addition of nutrients), +N (addition of 14 mL of nitrate/NaNO3), +P (addition of 1.4 ml of phosphate/Na2PO3) and +NP (combined at the mentioned concentrations) in order to simulate eutrophic conditions. After 24 hours of duration of the bioassay, samples were taken from each replica for quantitative analysis and chemical analysis of the water. In the reservoir considered oligotrophic (Cachoeira II), a significant difference was observed between the control treatment and the other bioassay treatments (p<0.05), while in the two reservoirs considered mesotrophic and in the eutrophic reservoir (Serrinha II, Jazigo and Barra do Juá, respectively) no significant differences were observed between treatments (p>0.05). However, although it was not verified that the addition of N and P significantly influences the increase in total biomass in most of the environments studied, it was observed that the response of cyanobacteria to this nutrient input is species-specific.