Carbon Pathways through Tropical River Systems: From Small Streams Carbon Pathways through Tropical River Systems: From Small Streams to the Oceans (Lessons from the Amazon and Mekong)to the Oceans (Lessons from the Amazon and Mekong)

Jeffrey E. Richey, School of Oceanography, University of Washington, jrichey@u.washington.edu1. The land-water interface provides the “initial

conditions,” for the input of C species and nut- rients to fluvial systems. As a parcel of water travels downstream, it is augmented by many other parcels, such that by the time it reaches the sea, initial signatures have been transform-ed beyond recognition. • How do processes of land-water interactions scale up to generate

regional patterns? • What are the effects of climate variability and human forcing? • Does the “last” gauging measuring station represent the *true*

inputs to the sea?• What is the river influence on the ocean?

3. The Rede Beija Rio sampling scheme tracked C at sites across the Amazon, from streams to sub- ( , ) to major tributaries ( ) to the mainstem ( ); exhibiting a strong “coherence” between pCO2 and the hydrograph, pH, and DOC. In situ processes (R, PPr, photo-oxidation, mediated by weathering conditions). Supersaturated pCO2 throughout the river system leads to high outgassing, ~.5Gt/y (Richey 2002).

4. Estimates of outgassing

need to be re-vised upward, to include streams and better measurements

7. The Mekong exhibits ~ same seasonal dynamics as the Amazon, with pCO2 relative to pH and other species (buffered by a more intense weathering regime.

pCO2

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Costa-Cabral et al AGU’07

Synergy

LAND USE CHANGE

CLIMATE CHANGE

DAMS

8. Mekong hydrology is subject to progressive impacts, from landuse, climate, and, increasingly, dams; with likely complex outcomes impacting timing and magnitude of discharge to the plume.

9. The Mekong influences production in the coastal and oligotrophic SCS via direct injection of nutrients and buoyancy through the river plume. The plume also appears to stimulate N2-fixation (and new production – equivalent to the Amazon?) in offshore waters, (J. Montoya, pers. comm.). Would changes in the Mekong flow regime alter these patterns…..?

Development of the Mekong plume over the Monsoon season (JJAS) with max loading at the end of August/Sept (from Navy 1/12 Global HYCOM) (courtesy J.P. Montoya and A. Bracco)

10. Do the plumes of other large tropical rivers ~ behave as the Amazon, and likely, the Mekong, with river nutrients/N2-fixation fueled C sinks? Evidence from the Ocean Science Meeting 2008 suggests “most likely.” How will *all* be affected by change?

Orinoco Congo Bay of Bengal

11. In summary, large tropical river/ocean systems should be thought of as coupled ecosystems, where the landside fluvial sink is a source of CO2 to the atmosphere, while the associated plume is a sink.

NASA SeaWIFS

2. Outgassing from emergent supersaturated groundwater w/terrestrially- respired CO2 (±35,000 uatm) yields ~100 Tg C/y over the Amazon basin, which is decoupled from and in addition to outgassing from larger rivers and floodplains (M.S. Johnson et al in review)

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6. New production fueled by allochthonous sources of nutrients and N2 fixation (by diazotrophs) enables the biologically mediated transfer of C from the atmosphere to the ocean. C uptake by the Amazon plume is ~ 15±6 TgC yr-1, in a region that is otherwise a source of CO2 to the atmosphere. (Cooley et al 2007).

Support: Amazon – NASA LBA, NSF Ecosystems; Mekong NASA IDS (Ocean), NSF Carbon

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5. Óbidos, the “end member”, misses ~30% of Q to the Atlantic …consequences?