Factors driving the biogeochemical budget of the Amazon River and its statistical modelling (2011)

The seasonal and interannual fluctuations of the biogeochemical budget (solutes, suspended matter, isotopes) of the Amazon River basin were analyzed, with a special focus on 44 physicochemical parameters monitored over the period 19821984 during the Carbon in the AMazon River Experiment (CAMREX) project. The relevant factors driving this variability were identified and sorted through the implementation of a statistical-regressive model coupled to variance analysis. Basically, the compositional fluctuations in the Amazon River are related (1) to the variable contribution of major tributaries (variable regional source) to the river flow but also (2) to the variable contribution of hydrological sources, (3) to river processes, i.e. in-stream diagenesis and sediment dynamics and (4) to the hydrological budget of the floodplains. Their respective contributions to the variability of chemical signals observed in the stream waters depend on which parameter was investigated but their combination explains on average 85% of the observed variability. The variability related to regional sources was captured by the compared measures of flow discharge and biogeochemical fluxes at the outlet of the major tributaries. The variability of hydrological sources was described by the variable contribution of three runoffs of distinct but constant composition: forwarded direct runoff, delayed floodplain runoff and baseflow. Several methods were tested to depict the seasonal and interannual variations of their individual discharges. Biologically-mediated processes were related to a hydrobiological index IBIO=[O2][CO2] which allows tracking the nature of the dominant ecological regime (autotrophy vs. heterotrophy). The alteration of chemical signals related to the intermittent discharge of the floodplains (where specific processes occur such as: gas exchanges at the airwater interface, sorption of dissolved organic matter, chemical weathering, deposition vs. remobilization of sediments, etc.) was simulated by taking into account the default of hydrological balance between inflows and outflows, used as a marker of floodplains discharge. This analysis shows that the chemical baseline observed in the waters of the Amazon River is mostly acquired upstream from the junction of major tributaries with the Amazon main reach.
Resume
Les fluctuations saisonnieres et interannuelles du bilan biogeochimique (solutes, matiere particulaire, isotopes) du fleuve Amazone ont ete analysees, avec une attention particuliere apportee a 44 parametres physicochimiques mesures entre 1982 et 1984 dans le cadre du programme CAMREX. Les facteurs exercant une influence significative sur cette variabilite ont ete identifies et hierarchises par le biais dun modele statistique couple a une analyse de variance. Les variations de composition des eaux de lAmazone sont fondamentalement associees (1) a la contribution variable de ses affluents majeurs (source regionale variable) au debit total, mais aussi (2) a la contribution variable des poles de melange hydrologiques, (3) a des processus fluviaux correspondant au regime hydrobiologique et a la dynamique sedimentaire et (4) au bilan hydrologique des plaines dinondation. Leurs contributions respectives a la variabilite des signaux chimiques observes dans les eaux du fleuve dependent du parametre considere, mais leur combinaison explique en moyenne 85 % de la variabilite observee. La variabilite associee aux contributions regionales variables est apprehendee en procedant aux bilans entreessorties des debits et flux biogeochimiques. La variabilite de contribution des poles de melange est decrite par la contribution variable de trois ecoulements de compositions distinctes mais constantes : lecoulement direct a expression precoce, lecoulement local de vidange alluviale a expression differee et lecoulement de base. Douze methodes ont ete testees afin de decrire les variations saisonnieres et interannuelles des debits individuels de chaque pole de melange. Les processus controles par le vivant sont apprehendes a partir dun indice hydrobiologique IBIO=[O2][CO2] qui permet de determiner la nature du regime hydro-ecologique dominant (autotrophe vs. heterotrophe). Lalteration des signaux chimiques generee par la vidange intermittente des plaines dinondation (au niveau desquelles ont lieu des processus specifiques : echanges gazeux, sorption de matiere organique dissoute, erosion chimique, depot vs. remise en suspension de sediments, etc.) est simulee en prenant en compte le defaut de bilan hydrologique entreessorties utilise comme marqueur de debit des plaines dinondation. Cette analyse montre que le bruit de fond chimique observe dans les eaux du fleuve Amazone est principalement acquis en amont des confluences entre le troncon fluvial etudie et les principaux affluents qui lalimentent.

Research areas: Ecosystems