Compositions and fluxes of particulate organic material in the Amazon River’

Lignin, elemental, and stable carbon isotope compositions are reported for local plants and for coarse (>63 pm) and fine (~63 pm) suspended particulate materials collected along a 1,950-km reach of the lower Amazon River during four contrasting stages of the 1982-1983 hydrograph. Fluxes of chemically recognizable lignin in the two size classes generally parallel each other along the mainstem with the fine fraction usually predominating. Particulate organic matter transported in the coarse size fraction of the mainstem and its major tributaries is composed of recently formed and well preserved tree leaf debris along with some wood. Organic matter in the fine size fraction is comparatively old, degraded, and rich in immobilized nitrogen and derives primarily from soils. C-4 grasses, which are abundant in the mainstem floodplain (varzea), are not major components of either the coarse or fine particulate material in the river. Particulate organic matter in both size fractions is introduced largely from upstream sources within the Rio Solimoes and Rio Madeira drainage basins. Most of this organic matter is unreactive and is transported conservatively with mineral particles along the Amazon mainstem. However, some downstream compositional trends are seen in both size fractions which reflect the addition or exchange of highly degraded, 13C-depleted, and lignin-poor organic materials from lower basin sources. The composition of river water is a complex function of diverse physical, chemical, and biological processes occurring in the drainage basin and the river itself. Processes controlling the compositions and fluxes of organic substances in river water are difficult to define because of the many molecular forms and sources that exist and the high temporal variability in production and removal rates characteristic of biological materials (Vannote et al. 1980; Cummins et al. 1983). Increasing attention is now being given to the forms, sources, and dynamics of organic materials in rivers (e.g. Degens 1982; Degens et al. 1983). This increased ’ This research was supported by NSF grant DEB 8 l-07522. Contribution 1604 from the School of Oceanography, University of Washington, and contribution 13 from the CAMREX Project. 2 To whom correspondence should bc addressed. emphasis results from the recognition that riverine organic substances influence water quality, fisheries production, and the global carbon budget (Likens et al. 198 1; Meybeck 1982) and provide a detailed, integrated recording of natural and anthropogenic activities within the drainage basin. Most studies of biogenic materials in rivers have been confined to bulk measurements of the concentrations of dissolved organic carbon (DOC) and particulate organic carbon (POC), traditionally defined by separation with a m 0.5-pm filter (Schlesinger and Melack 198 1). POC concentrations range from about 1 to 30 mg liter-l with a world average of about 5 mg liter’ (Meybeck 1982). POC : DOC ratios vary widely, with a world average of about 0.8. In most rivers POC concentrations vary directly with discharge (Meybeck 1982) and, at a given site and discharge, are typically higher dur-