Fluorescence and UV-VIS techniques were employed for the investigation of natural organic matter (NOM) of a tropical lake. The relationships of absorbance/dissolved organic carbon (A/DOC), fluorescence intensity/dissolved organic carbon (FI/DOC), fluorescence ratio (FR), and peak wavelength with the highest intensity (PW) were used to distinguish the pedogenic or aquagenic origin of NOM. The values of FR, PW and A 285 /DOC of high waters (HW) or flooded period samples and of low waters (LW) period samples of the dry season, except for September 2002, confirm the predominance of pedogenic material. The spectra of water were similar to the standard fulvic acid (FA), and the spectra of FA from the lake were similar to the nearby soils, indicative of pedogenic predominance. The results confirm that the dissolved NOM of Patos Lagoon -MS (Brazil), in all sampling periods, predominantly consisted of humic substances (FA) of pedogenic origin.

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The sediment of Lagoa dos Patos-MS, Brazil, was investigated to verify the influence of metal sulfides and oxyhydroxides of Mn and Fe on the heavy metal availability. The spectrophotometric method of methylene blue was used, with 8 interlinked tubes containing the samples with SnCl 2 , where N 2 was introduced to release the H 2 S extracted with 6.0 mol L -1 HCl, and trapped in 0.05 M NaOH solution. The influence of SO 4 2-, Mn(IV) and Fe(III) oxyhydroxides was investigated with samples constituted by a mixture of MnO 2 ; Na 2 SO 4 ; FeCl 3 , and sediments. The presence of SnCl 2 was very important to avoid the interferences of iron and manganese oxyhydroxides. The method of standard addition was applied and the efficiency was (100.8 € 9.4)%. The ratio among the quantities of metals potentially available and the acid volatile sulfide (AVS) indicate that the system presents small metals availability to the benthic community, by the existence of sulfide capable to immobilize the metal as insoluble sulfide.