Razão n/p, cianobactérias e índice de estado trófico em três reservatórios sob estiagem prolongada no Nordeste

Abstract

The degree of eutrophication was analyzed in three reservoirs at the Brazilian semi‐arid tropic, in Ceará State. The study covered a period of one decade and the water sampling was quarterly. The analysis took into account total nitrogen (N), total phosphorus (P), chlorophyll a (Cla) and total cyanobacteria numbers (CB). These quality parameters were complemented with rainfall data (Pp) and percentage of available water volume in the reservoirs (AV%). The percentages of compliance of the Cla concentrations with the environmental standard (CONAMA 357/05) were 60, 6 and 100% in the Banabuiú, Cipoada and Capitão Mor reservoirs, respectively. For total phosphorus, the percentages were 10, 63 and 12%, respectively. For the total count of cyanobacteria, the compliance results were 23, 6 and 86%. The results showed that the degree of eutrophication in Cipoada was higher than in Banabuiú, followed by Captain Mor. The environmental standard does not directly address total nitrogen, but this nutrient correlated positively with P and Cla. Concentrations of total nitrogen may have an equivalent impact to phosphorus. In the study, N: P > 38 ratios predominated in Cipoada and Banabuiú. In Captain Mor the range from 22 to 38 was predominant for the N: P ratio. The TSI values were high, indicating hypertrophy in the Cipoada and Banabuiú, and eutrophic in Captain Mor. Reduction of AV% contributes to the process of eutrophication, with increased nutrients and cyanobacteria counts.