Nutrient budgets in the Piratininga-Itaipu lagoon system (southeastern Brazil): effects of sea-exchange management

Abstract

The Piratininga-Itaipu Lagoon System and its drainage basin, located on the southeastern coast of Brazil, Niterói (RJ), has been the target of an intense process of urbanization over the last four decades. As a result of this process, both the lagoon and the adjacent sea area have shown signs of eutrophication due to the release of large quantities of domestic sewage semi (or not) treated. In an attempt to minimize these effects, in 2008 the government re-established a connection between the Piratininga lagoon and the sea. To evaluate changes and variations in the balance of dissolved inorganic nutrients, before and after interventions, the geochemical model of two stoichiometric mass balance boxes of the "LOICZ Program” (Land-Ocean Interactions in the Coastal Zone) was used. The salt and water balance showed different water residence times in the Piratininga lagoon for the two periods: 2005-2006 (before the opening of the connection between the Piratininga lagoon and the sea) and 2009-2010 (after the opening of the connection with the sea). The first period showed a value about two times higher than that obtained for 2009-2010, τ = 83 and 39 days respectively. The water residence time of the Itaipu lagoon did not show great variations between the two periods (τ = 9 and 8 days). For the period of 2005-2006, before the opening of the connection with the sea, both the lagoons were autotrophic (∆DIP > 0) and nitrogen loss was predominant (∆DIN < 0). The second period (2009-2010) was characterized by changes only in the trophic state of the Piratininga lagoon, which became heterotrophic (∆DIP > 0).