Cargas de Fósforo Total e Material em Suspensão em um Reservatório do Semi-Árido Brasileiro

Abstract

Normal 0 21 false false false PT-BR X-NONE X-NONE SUSPENDED SOLIDS AND PHOSPHORUS LOAD IN A BRAZILIAN SEMI-ARID RESERVOIR. Semi-arid reservoirs are highly vulnerable to eutrophication and siltation, as they have a large catchment: surface area ratio (AD: AL), and high water residence time, which underlies their potential for high retention of nutrients and sediments exported from the watershed . Nevertheless, the external loads of nutrients and suspended solids to these reservoirs and their retention coefficients are unknown. In order to quantify the loads of total phosphorus and suspended solids exported from the watershed of Cruzeta (RN) reservoir, and to describe the patterns of temporal variation in total phosphorus, suspended solids and chlorophyll a concentrations in the water, five sampling points in the reservoir were monitored between March 2007 and May 2008 (n = 26). The annual exportation coefficients of total phosphorus and suspended solids were respectively, 0.03g and 7.08g per square meter of watershed. The annual loads of total phosphorus and suspended solids were respectively, 4.8g and 1160.7g per square meter of reservoir . The mass balance results shows that the reservoir retained 94% of the incoming suspended solids and 50% of the incoming total phosphorus load. The median concentrations of total phosphorus and chlorophyll a were 0,12mg L -1 and 12?g L -1 respectively, over the study period. The TN:TP ratio (atomic) were often below 16:1 suggesting nitrogen limitation. The observed negative correlation (r²=0.40; p 0.001) between fixed suspended solids and transparency (secchi depth) indicates that the major source of turbidity was abiogenic and suggests potential light limitation of primary production. The likely light and/or nitrogen limitation might explain the lower than expected chlorophyll a per total phosphorus concentrations observed in the reservoir . Keywords: Water balance; mass balance; eutrophication; reservoir; semi-arid.