Dynamics of Fluvial Systems

Fluvial systems integrate hydrological and biogeochemical cycles, over scales from small streams to regional and ultimately to continental basins. The key issue here is the quantity of carbon removed from the atmosphere relative to the amount returned to the atmosphere, along the overall fluvial pathways. This sequence of processes can be summarized in the form of a box model (Figure 17.1). Briefly, three primary forms of carbon of atmospheric origin are transported through fluvial systems. Particulate organic carbon (POC) enters rivers from the erosion of soils (typically older materials) and as leaf litter (typically newly produced). Dissolved organic carbon (DOC) is produced through solubilization of soil organic carbon and enters streams via groundwater. Total dissolved inorganic carbon (DIC) is produced via weathering, as the dissolution of carbonate and silicate rocks. This process sequesters atmospheric CO2, establishes the alkalinity, and influences the pH of water, which governs the subsequent partitioning of DIC between pCO2, bicarbonate, and carbonate ions. The dynamics of carbon in fluvial systems are not defined solely by the export fluxes of bulk C. Rather, they are defined as a complex interplay of multiple C fractions; each exhibits distinct dynamics and compositional traits that hold over very broad ranges of geological, hydrological, and climatic conditions (Hedges et al. 1994).

Evaluating the model of Figure 17.1 is a challenge. The dynamics are complex, and multiple time constants are involved. Data are scarce, particularly in many of the most anthropogenically affected systems. The distribution of the constituent processes varies dramatically across the face of the globe (with some of the most important regions being the least measured).

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