Compositions and fluxes of particulate organic material in the Amazon River1

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Abstract
Lignin, elemental, and stable carbon isotope compositions are reported for local plants and for coarse (>63 µm) and fine (µm) 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 (várzea), 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 Solimões 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.