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Phosphorus (P) is the main nutrient responsible for the harmful effects caused by the enrichment of aquatic systems, and submerged macrophytes play an important role in this process, since they can both remove and release this nutrient in environmental compartments. The present study aimed to evaluate the influence of submerged macrophytes on P in the water, sediment, and water-sediment interface in a eutrophic reservoir in a semiarid region and to evaluate the relationship between the concentration of this nutrient in macrophyte tissue and that available in the different compartments. Were performed ten collection campaigns, in three reservoir locations: at the entrance of the Paraíba River; in the intermediate area between the river entrance and the dam and at the dam. We observed a difference in the P concentration inside and outside macrophyte banks, and this difference was determined by the abundance and intensity of macrophyte growth and decomposition. In sites with extensive vegetation banks and where decomposition was more intense, macrophytes released P to the water-sediment interface and sediment compartments. By contrast, in sites with smaller vegetation banks and where macrophytes did not show reduced abundance, P was removed from these compartments. The entry of new water originating from river water transfer was an important modifying factor of the physical and chemical characteristics and macrophyte abundance. The zone where the river enters the reservoir was the area most affected by the water transfer. The macrophyte decomposition in this zone resulted in the highest P concentration in the water-sediment interface and sediment compartments, which demonstrates the importance of macrophytes in the fertilization of water bodies and, consequently, in the eutrophication process. In turn, in the dam zone, where the macrophyte banks were more stable and without large variations in abundance, P was removed from the compartments. A relationship between P in macrophyte tissue and that available in the environment was observed, particularly at the water-sediment interface, indicating that this compartment was the main P source for these plants, which demonstrated that these plants store higher amounts of P in nutrient-rich sediment and water, functioning as an indicator of the nutritional status of a reservoir.

Cyanobacteria harmful algal blooms (CyanoHABs) have become increasingly frequent and intense in public supply reservoirs as a result of eutrophication and global climate change. The semi-arid region of Brazil has a well documented history of CyanoHABs but the underlying factors that control the excessive proliferation of these organisms and the production of their bioactive secondary metabolites are not comprehensively understood. This study aimed to identify the environmental factors that explain the spatial and temporal variations in the abundance of cyanobacteria and the concentration of cyanotoxins (microcystins, saxitoxins, and cylindrospermopsin) in semi-arid reservoirs. The following hypotheses were tested: (a) the largest biovolumes of potential toxin producing cyanobacteria occur when cyanotoxin concentrations are highest; and (b) the environmental factors that explain variations in biovolume of cyanobacteria also explain changes in cyanotoxins concentrations. Samples were taken from four reservoirs located in the Northeast region of Brazil, over a three-month period (October 2016 and February and June 2017). Of the 24 species of cyanobacteria identified, 13 were potentially toxin-producing. Physicochemical variables such as water volume of the reservoir, water transparency, soluble reactive phosphorus, and total phosphorus explained the abundance of cyanobacteria and the levels of cyanotoxins. These results corroborate the hypothesis that similar physicochemical conditions influence the abundance and diversity of cyanobacteria and cyanotoxins. Cyanobacterial blooms composed of more than one potential toxin producing species were observed in the studied reservoirs, where potential microcystin-producing species were the most common. Microcystins and saxitoxins were detected in all the reservoirs studied, while cylindrospermopsin and the cyanobacterium Cylindrospermopsis raciborskii were simultaneously recorded in only one reservoir (Camalaú Reservoir). Cylindrospermopsin was only detected in a reservoir for the first time in the State of Paraíba. Canonical redundancy analysis showed that the cyanotoxins were related to potential toxin producing species. These results corroborate the proposed hypothesis that there is a correlation between cyanotoxins and the biomass of potential producers. Also, there were situations where cyanotoxins were detected without the presence of potential producers. These results demonstrate the need for reassessment of potential toxin producing species of cyanobacteria in semi-arid reservoirs. This may lead to the identification and characterization of novel producers of these bioactive secondary metabolites.