Nutrients in surface sediments of Mirim lagoon, Brazil-Uruguay border

SANTOS¹, I.R.; BAISCH¹, P.; LIMA¹, G.T.N.P. & SILVA FILHO², E.V.

¹ Departamento de Geociências, Laboratório de Oceanografia Geológica, Fundação Universidade Federal do Rio Grande - CP 474 - 96201-900, Rio Grande - RS. e-mail: isaacrsantos@yahoo.com.br

² Departamento de Geoquímica, Universidade Federal Fluminense, 24020-007, Niterói - RJ.

ABSTRACT: Despite its ecological, economics and geopolitical importance, Mirim lagoon is still poorly known. In this paper we present the first data about organic composition and nutrient levels in lagoon sediments. Total Organic Carbon, N-total, and P-total had an average content of 1.26%, 0.163% e 0.162%, respectively. The southern part of the lagoon is characterized by higher concentrations of these elements. This is due to the proximity of the main fluvial discharges. Mirim lagoon sediments are poor in carbon and nitrogen, which is attributed to the organic input dilution, high occurrence of sediment resuspension processes, and recent deposition of lagoon substratum. The N:P ratio is low in comparison to other aquatic environments. This is likely to be associated to the intense use of phosphate fertilizers and the high frequency of sediment resuspension.

Key-words: nutrients, organic matter, coastal lagoon, sediments.