Carbon is transported from the land to the oceans via rivers and groundwater. The transfer of organic matter from the land to the oceans via fluvial systems is a key link in the global carbon cycle. Rivers also provide a key link in the geological-scale carbon cycle by moving weathering products to the ocean (Colorplate 1). The conventional perspective is that rivers are simply a conduit for transporting carbon to the ocean. Reevaluation of this model suggests that the overall transfer of terrestrial organic matter through fluvial systems may be more complex (Richey, Chapter 17, this volume). A robust evaluation of the role of rivers, however, is complicated by both the diverse dynamics and multiple time constants involved and by the fact that data are scarce, particularly in many of the most affected systems.
Humans have had a significant impact on the concentrations of carbon and nutrients in river systems. Intensifying agriculture has led to extensive erosion, mobilizing perhaps 10—100 times more sediment, and its associated organic carbon, than undisturbed systems. Sewage, fertilizers, and organic waste from domestic animals also contribute to the carbon and nutrient loads of rivers. Not all of these materials make it to the ocean. Much can be deposited near and along river channels. The retention of par-ticulate material in aquatic systems has increased since preindustrial times because of the proliferation of dams (primarily in the 30—50°N regions), which has increased the average residence time of waters in rivers. If the carbon mobilized via erosion were subsequently replaced by newly fixed carbon in agriculture, then a sink on the order of 0.5 — 1.0 PgC y-1 would be created. But organic carbon does not move passively through river systems; even very old and presumably recalcitrant soil carbon may be at least partially remineralized in aquatic systems. Remineralization of organic carbon during transport leads to elevated levels of dissolved CO2 in rivers, lakes, and estuaries worldwide. These high concentrations subsequently lead to outgassing to the atmosphere on the order of ~1 PgC y-1, with the majority in the humid tropics (Richey, Chapter 17, this volume).
Despite the increased particulate retention in rivers, a significant amount of carbon escapes to the ocean. DIC, particulate inorganic carbon (PIC), dissolved organic carbon (DOC), and particulate organic carbon (POC) exported from rivers to the coastal ocean are 0.4, 0.2, 0.3, and 0.2 PgC y-1, respectively (Chen, Chapter 18, this volume). These fluxes include a pronounced, but difficult to quantify, anthropogenic component. These values are poorly constrained by direct measurements and may represent minimum esti mates of inputs to the coastal ocean (Richey, Chapter 17, this volume). Groundwater discharges, which make up about 10 percent of the surface flow to the ocean, also contribute poorly known contributions of carbon and nutrients to the coastal oceans.
The final step in the land-to-ocean pathway is the marine fate of fluvial carbon. Previous estimates have suggested that the marginal seas are net heterotrophic. Chen (Chapter 18, this volume) has suggested that continental shelves are in fact net autotrophic, mostly because of production from coastal upwelling of nutrient-rich waters. If this is correct, biological production in the coastal zone may decrease the thermodynamic drive to outgas the terrestrial carbon delivered by rivers, resulting in greater preservation in the marine environment.
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