A multi-scale assessment of aquatic ecosystem processes across the land-ocean-atmosphere continuum: from headlands to the sea

Abstract

Freshwater systems provide a unique cross-section of earth processes, linking terrestrial, aquatic, marine, and atmospheric reservoirs. Once thought to be passive pipes connecting land to sea, rivers are increasingly recognized as dynamic players in global carbon cycling. Rainfall mobilizes terrestrially-derived components from the landscape and as these components travel downstream they are greatly altered within the aquatic system. The transformation and remineralization of terrestrial organic matter (OM) fuels a state of net heterotrophy and CO2 supersaturaton in the world's rivers, resulting in a globally-relevant flux of CO2 to the atmosphere. I examined the complex sequence of processes resulting in fluvial carbon fluxes at various spatiotemporal scales in a range of latitudinal settings, from small streams in the Pacific Northwest and coastal Brazil to the mouth of the world's largest river, the Amazon. High-resolution (hourly) measurements of dissolved and particulate geochemical parameters in the Pacific Northwest and the South Brazilian coast revealed a persistent positive correlation between river discharge and the concentration of carbon, nutrients, and terrestrial biomarkers. Discreet pools of soil carbon and nutrients with unique accumulation/mobilization dynamics were identified with the analysis of lignin phenols and elemental stable isotopic composition. Compositional parameters in the Pacific Northwest revealed patterns of constituent mobilization from deep and shallow soil pools, whereas compositional variability in Espirito Santo revealed functionalization of unique landscape units throughout the course of a storm. Seasonal measurements of particulate and dissolved organic matter made along the lower Amazon River revealed significant degradation of OM between the mouth and the historic upstream gauging station, Obidos. The relative abundance of both dissolved and particulate OM derived from the highland terrestrial environment, determined by isotopic and compound specific end member mixing models, significantly decreased between Obidos and each respective channel near the mouth. Highland-derived DOM was on average less than 10% of the DOC near the mouth, whereas highland POC was, on average, roughly 50% of total POC near the mouth. The concentration of POC decreased by roughly 70% between Obidos and the mouth, implying that the river contains both refractory and highly labile POC fractions. The flux of total organic carbon from the river mouth was 90% dissolved and 10% particulate. Through a series of incubation experiments it was estimated that, on average, the degradation of terrestrially derived macromolecules (e.g. lignin phenols) supports 30-50% of bulk respiration rates in the river. It is estimated that 40% of the vascular plant-derived organic carbon sequestered by the terrestrial biosphere is degraded within soils, 55% is degraded along the river continuum, and less than 5% is either stored in the basin or delivered to the ocean.