Balanço de fósforo em uma laguna tropical rasa (Lagoa de Jacarepaguá – RJ): implicações para o gerenciamento da eutrofização

Abstract

The eutrophication of aquatic systems is characterized by increased concentrations of nutrients, mainly phosphorus (P). One of its main consequences is the excessive growth of cyanobacteria, resulting in a series of social, economic, biological, and health problems. The success of the recovery of degraded water bodies depends on a system analysis and the application of an efficient and specific management measures for the system. Mass balance is an important tool in system analysis, as it allows the identification of the main sources and quantities of nutrients in the system. In this study, we monitor the meteorological and limnological characteristics of the Jacarepaguá Lagoon and use the Land Ocean Interactions in Coastal Zone (LOICZ) model to determine the main sources and outputs of P for cyanobacterial blooms in the Jacarepaguá Lagoon. Our studies indicated the air temperature average of 25 °C and rainfall pattern typical of tropical environments. The Jacarepaguá Lagoon was characterized as a shallow system (average depth of sampling stations 1.4 m), with warm waters (26 °C), turbid (euphotic zone corresponds to 57% of the water column), brackish (3.7), alkaline (pH 8.1), with low oxygen concentration in the hypolimnion (3 mg L-1), long residence time (77 days), with a high concentration of P in the water column (1260 µg L-1) and in the sediment (751 µg P g- 1) and frequent blooms of cyanobacteria, which would classify this system as hypertrophic, according Nürnberg, 1996. The balance estimated by the LOICZ model showed a negative P balance (-979 mol d-1), indicating a sequestration of the difference between the sources (3515.8 mol d-1) and the P outputs (-2545 mol d-1) ), which results in a high production of organic matter (103779.8 mol C d-1). The main source of P for eutrophication of the Jacarepaguá Lagoon is of external origin, entering in the lagoon through tributaries (3056.6 mol d-1). The internal load (468 mol d-1) showed a lower value than that of external sources, but enough to enhance the trophic problems caused by the latter. The internal load can delay the recovery of this system, even after reducing the entry of phosphorus by tributaries. Based on the limnological diagnosis and the P balance, efficient control measures to mitigate eutrophication and cyanobacterial blooms were evaluated and discussed, focusing both on the control of internal and external sources of nutrients.