Regional-scale linkages of terrestrial and lotic ecosystems in the Amazon basin: a conceptual model for organic matter
McClain, M. E.; Richey, J. E.
Terrigenous organic matter originating at the margins of streams in headwater regions of the Amazon basin accounts for a large part of the organic matter carried by the major tributaries of the basin. The purpose of this paper is to articulate our present understanding of terrestrial to lotic transfers in these headwater regions in the form of a conceptual model. On a regional scale, the headwaters of the Amazon basin may be subdivided into four dominant terrain types: (1) terra firme forests developed on oxisols and ultisols, (2) campina forest developed on spodosols, (3) savanna developed on oxisols, ultisols, and alfisols, and (4) montane forest developed on ultisols and inceptisols. Within these terrains terrigenous organic matter is transferred to streams via direct litterfall and blow-in, groundwater baseflow, stormflow, and seepage from fringing wetlands. Based on the limited available data and data from other systems, our current conceptual model is as follows. Direct litterfall contributions from overhanging canopies are similar across the basin and on the order of 0.7 kg/m2/year. Blow-in contributions are probably on the order of 20% of direct litterfall fluxes. Groundwater baseflow contributions of organic matter (OM) are strongly correlated with soil type and fall into two distinct classes, one draining the campina terrain which is characterized by high dissolved organic carbon (DOC) concentrations (> 20 mg/l) and another draining terra firme, savanna, and montane terrains which is characterized by lower DOC concentrations (< 5 mg/l). Groundwater OM contributions are also compositionally distinct, with elevated proportions of hydrophobic organic molecules in groundwater draining campina. Stormflow contributions across the basin are dominated by saturation overland flow originating in riparian areas, and transferred OM consists primarily of litter washed in from the surrounding forest floor and material flushed from fringing wetlands. In montane terrains, however, there may also be a significant erosive input of soil OM. Contributions of OM from fringing wetlands are most prevalent in lowland terrains where broad, flat riparian zones provide ample sites for wetland development. Both stormflow and seepage from fringing wetlands transfer a wide spectrum of OM composition, ranging from freshly fallen leaves to heavily decomposed and refractory dissolved molecular forms.