Dissolved humic substances of the Amazon River system1

Abstract

Aquatic humic and fulvic acids from nine mainstem and seven major tributary sites in the Amazon River Basin are characterized by their elemental and lignin phenol compositions. Combined humic substances represent 60% of the riverine dissolved organic carbon (DOC), with fulvic to humic acid (FA : HA) ratios in the mainstem averaging 4.7 -t 1 .O. All dissolved humic and fulvic acids have clearly recognizable lignin components at levels (8 and 3% of the carbon) suggesting a predominantly allochthonous source. Lignin compositional characteristics are dominated by diagenetic transformations, which include preferential loss of methoxylated structural units and oxidative degradation of lignin side chains. Fulvic acids have consistently lower lignin levels, lower lignin phenol methoxylation, higher acid : aldehyde ratios, and higher C : N ratios than coexisting humic acids, all indicative of greater aerobic degradation of the fulvic acid fraction. An extreme example of lignin degradation is seen in the Rio Negro humic and fulvic acids, which differ significantly in lignin and elemental compositions from other blackwater tributary and mainstem samples.Over most of the mainstem humic and fulvic acids behave conservatively. However, downstream of the Rio Negro confluence, humic acids appear to be selectively adsorbed onto fine suspended particles. Approximately 25 and 40% of the total fluxes of lignin and of carbon are represented in the dissolved humic substances. The annual flux of dissolved, chemically recognizable lignin in the Amazon River at 6bidos is calculated to be 1.2 x 10" g yr-*.

A contribution to the chemical characterization of rivers in the Rio Negro Basin, Brazil

“…This result is similar to the discharge ratios of 2.7:1 3 and 3.5:1 6 , obtained from the literature. Sample RS5 seems to be a linear combination of samples RN5 and RS4, based on the discharge ratio (see Table 4).…”

“…Since the fifties, Amazonian rivers have been the object of many chemical and geochemical studies (for example, Sioli 4 , Stallard and Edmond 5 and Ertel et al 6 ). As a consequence of the limited infrastructure available and the large distances involved, most studies were restricted to the main stem of the Amazon, and a large number were based on samples collected only at Manaus or near the river mouth.…”

“…As a consequence of the limited infrastructure available and the large distances involved, most studies were restricted to the main stem of the Amazon, and a large number were based on samples collected only at Manaus or near the river mouth. Studies on blackwater rivers have been rare, and generally limited to a small number of chemical compounds 2,6,7 . Following previous work on the association of metals and humic compounds of the Negro and Solimões rivers near Manaus 8 , this work reports on the concentration of minor and trace elements in the Negro Basin and makes a comparison with the Solimões.…”

The influence of seasonalness on the structural characteristics of aquatic humic substances extracted from Negro River (Amazon State) waters: interactions with Hg(II)

“…The samples indicate carbon atoms having more aliphatic characteristics when compared with the data of Leenheer 36 and Ertel et al 37 However, in relation to the data obtained by Sargentini et al, 40 the samples present more aromatic characteristics.…”

“…The H/C, C/N, C/O atomic ratios obtained from element analysis (Table 2) have been utilized to aid the characterization of HS and the comparison of HS from different matrices can indicate the degree of similarity between them. Utilizing these ratios, Leenheer 36 and Ertel et al 37 verified structural similarities of AHS in comparison with HS from soil samples from the central-Amazonian-basin region indicating that the AHS from black-water rivers are originally from podzol-type soils.The H/C and C/N atomic ratios give an indication of the structure and molecular shape of the HS. 38 The hydrogen content is related to the degree of saturation.…”