Aldose, amino acid, and elemental compositions were determined for flux-weighted samples of coarse (> 63 pm) and fine (< 63 pm) particulate organic material and ultrafiltered (> 1,000 Daltons) dissolved organic matter collected at three sites along the Brazilian Amazon River and six of its major tributaries. Concentrations of total organic C (TOC) were relatively uniform (55Ok 100 PM) at all sites, with DOC comprising the major (50-100%) component. An average of 77% of the total DOC was isolated by ultrafiltration.The greatest compositional differences observed in the Amazon River system were among the coarse, fine, and dissolved organic fractions. All coarse particulate fractions were nitrogen-poor (atomic C : N = 21) and exhibited stable carbon isotope, aldose, and amino acid compositions similar to those of angiosperm tree leaves. Coarse particulate organic materials, although the least degraded of the three fractions, had lost appreciable carbohydrate and had immobilized excess nitrogen of apparent bacterial origin. Fine particulate materials were more nitrogen-rich (C : N = 9) than coarse counterparts and had lower total aldose yields and glucose percentages. Fine particles gave greater total yields of amino acids, characterized by high ratios of basic vs. acidic components. Coexisting dissolved organic materials recovered by ultrafiltration were nitrogen-poor (C: N = 27-52) and yielded the lowest amounts of aldoses, among which deoxy sugars were concentrated. Dissolved fractions gave extremely low yields of amino acids in mixtures that were enriched in nonprotein components and in acidic vs. basic molecules. These yield and composition patterns are consistent with a "regional chromatography" model in which highly degraded leaf material is solubilized and then partitioned between soil minerals and water during transport to the river, resulting in suspended fine particulate organic materials of soil origin that are nitrogen-rich and coexisting dissolved organic substances that are nitrogen-poor. manuscript.

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Amino acid analyses were performed on a suite of potential organic matter sources to coastal marine environments. They yielded no source-indicati.ve compositional parameters, but microbes produced markedly higher carbon-normalized yields than did vascular plant tissues.Sediment trap samples collected monthly for a year at 30 and 60 m in Dabob Bay, Washington, were also analyzed, as were subsamples from a sediment core taken at the same site. Amino acid yields from the trap samples exhibited spring and fall maxima that coincided with blooms, and a winter minimum that resembled yields in underlying sediments. Amino acids in all samples were primarily of marine origin and low yields in winter traps resulted from low planktonic production, selective amino acid loss, and resuspension of amino acid-depleted sediments. Nonprotein amino acid levels and the fraction of total N represented by amino acids are indices of diagenetic alteration. Amino acids accounted for 13-37% of the total 0rgani.c C and 30-8 1% of the total N in the 30-m samples, and, on average, 10 and 37% of sedimentary C and N.Midwater amino acid fluxes also displayed a winter minimum and spring and fall maxima; they were more pronounced than fluxes of bulk particles or C, due to a lesser influence of resuspension. On average, -80% of the total midwater particulate amino acid flux was lost prior to incorporation in surface sediments. Amino acids were degraded selectively and represented 35 and 7 1% of the C and N remineralized at the benthic interface. The labile fraction closely resembled fresh plankton in its amino acid composition and carbon-normalized yield.Although trapping of poisoned zooplankton may have caused some artificially high amino acid yields, diagenesis was apparently the primary control on calculated reactivities. Among the amino acids, reactivity patterns indicated relative preservation of diatom cell-wall protein. Within the sediments, amino acids were again degraded preferentially relative to organic C and N, but no downcorc compositional changes were observed. Midw.ater and sediment burial fluxes ofplanktonic amino acids represented, respectively, -13 and 2.5% of annual mean amino acid production by primary producers.Coastal marine environments are sites of a major fraction of global marine primary production, and 80-90% of global organic C (OC) burial has been estimated to occur I Present address: Department of Oceanography, Universitv of British Columbia, Vancouver V6T 124.2 To whom reprint requests should be addressed. Acknowledgments cThis research was supported by NSF grant OCE 87-16481.Contribution 1933 from the School of Oceanography, University of Washington.We acknowledge M. Goiii, B. Bergamaschi, M. Peterson, and P. Coble for reviews of this manuscript. The manuscript also benefited from the comments of L. Mayer and an anonymous reviewer. We thank J. Stem for many of the elemental analyses, M. Orellana and M. Talbot for assistance with phytoplankton cultures. G. Drury and P. Crawford were boat captains. M. Gofii is...

Abstract13 C tracer experiments were conducted at sites spanning the steep oxygen, organic matter, and biological community gradients across the Arabian Sea oxygen minimum zone, in order to quantify the role that benthic fauna play in the short-term processing of organic matter (OM) and to determine how this varies among different environments. Metazoan macrofauna and macrofauna-sized foraminiferans took up as much as 56 6 13 mg of added C m 22 (685 mg C m 22 added) over 2-5 d, and at some sites this uptake was similar in magnitude to bacterial uptake and/or total respiration. Bottom-water dissolved oxygen concentrations exerted a strong control over metazoan macrofaunal OM processing. At oxygen concentrations .7 mmol L 21 (0.16 ml L 21 ), metazoan macrofauna were able to take advantage of abundant OM and to dominate OM uptake, while OM processing at O 2 concentrations of 5.0 mmol L 21 (0.11 ml L 21 ) was dominated instead by (macrofaunal) foraminiferans. This led us to propose the hypothesis that oxygen controls the relative dominance of metazoan macrofauna and foraminifera in a threshold manner, with the threshold lying between 5 and 7 mmol L 21 (0.11 to 0.16 ml L 21 ). Large metazoan macrofaunal biomass and high natural concentrations of OM were also associated with rapid processing of fresh OM by the benthic community. Where they were present, the polychaete Linopherus sp. and the calcareous foraminiferan Uvigerina ex gr. semiornata, dominated the uptake of OM above and below, respectively, the proposed threshold concentrations of bottom-water oxygen.1 Present address: Danish Meteorological Institute, Lyngbyvej 100, 2100 Copenhagen, Denmark. AcknowledgmentsThe experimental work in this study was conducted aboard the RRS Charles Darwin. We thank Oli Peppe and Eric Breuer for running the lander and megacorer. We also thank two anonymous reviewers for their constructive feedback.

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