Origin and nature of humic substances in the waters of the Amazon River Basin

Abstract

Humic substances dissolved in "black water, white water, and clear water" rivers of the Amazon River hãsin were compared were humic substances found in podzolic (spodic) and latosolic (oxic) soils of the Amaon region. Humic-type organic solutes in black water originated from organic matter decomposition and ground water drainage from podzolic soils; clear water drains from latosols because colored humic solutes are adsorbed on clay minerais contained in latosols . Compari sons of organic solute distributlons in t ropical surface waters versus temperate surface wate1s by dissolved organic carbon fractionation analysls chow that the only signiflcant difference was a lower hydrophilic neutra! content o f tropical surface waters . This finding is indicativa of more rapid decay processes in tropical waters. Black-water rivers of the Amazon contained ondy one-third to one-half the organic solute content r.ontained in hlack-water rivers of the southeastern United States . About 50 percent of the organic carbon In black waters of the Am azon consisted of colorless r.on-humi c organic solutes. Humic solutes contained in Amazon black water differed from humic solutes found in the Suwannee River in the southeastern United States by having a higher E 4 / E 6 ratio in the visible absorbance spectrum which resulted in water of a more reddish coloration. Both the high E 4 / E 6 ratio and low-angle x-ray scattering data suggest relatlvely small molecular d1mensions for humic solut es in Amazon black water. Organic elemental analyses and lnfrared analysis of humic substances In Amazon waters and soils were typical for humic substances except for the low content of nitrogen for humic se lutes in black water.

Dissolved humic substances of the Amazon River system1

“…Concurrent with this is a significant increase in C : N ratios and a decrease in H : C ratios from those in other humic and fulvic acids, especially from temperate regions (Thurman 198 5). The podzol soils of the Rio Negro region and their basesoluble humic extracts also are depleted in organic nitrogen (C : N = 30 and 25; Klinge and Ohle 1964;Leenheer 1980) relative to the oxisols in the rest of the Amazon drainage basin (C : N = 13; Klinge and Ohle 1964). These results suggest that the high degree of biodegradation of the Rio Negro humic substances mostly likely occurs within the aerobic soil layers and, by analogy, that the site of formation of all the Amazon aquatic humic and fulvic acids is likewise the aerobic soil horizons.…”

“…Sandy soils, occurring mainly in the caatinga and igapo (flooded) forests of the Rio Negro basin, are podzols (3% of the total area: Sanchez et al 1982) which have a surface humus layer, a bleached quartz A horizon up to several meters thick, and an underlying aluminous clay lens cemented with humic materials (Klinge 1965). These soils are highly acidic, generally wet, and anaerobic, all of which result in the slow decay of the organic matter and allow continual leaching of humic material to yield the characteristic local blackwater streams (Leenheer 1980). …”

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.…”

On the feeding ecology of the catfish Hypophthalmus fimbriatus in the blackwater Rio Negro of the Amazon Basin

“…2) . Rio Negro waters are stained by organic compounds derived from low-lying forest growing on podzolic soils in some of the tributary catchments basins (Klinge, 1967;Leenheer, 1980) . These organic compounds, in conjunction with Guiana Shield headwaters draining granitic and gneissic formations, render the Rio Negro highly acidic, with pH values usually bellow 4.8.…”