Spatial Patterns of In-Situ Production and Respiration within a Turbid Tropical River: Implications for Amazonian Carbon Cycling

Monday, 15 December 2014
William Gagne-Maynard1, Nicholas D Ward2, Henrique O Sawakuchi3, Vania Neu4, Alan C Cunha5, Rodrigo da Silva6, Daimio C Brito5, Aline de Matos7, Richard G Keil1, Alex V Krusche8 and Jeffrey E Richey1, (1)University of Washington Seattle Campus, Seattle, WA, United States, (2)University of Florida, Geological Sciences, Ft Walton Beach, FL, United States, (3)USP University of Sao Paulo - CENA, Piracicaba, Brazil, (4)Universidade Federal Rural da Amazônia, Belem, Brazil, (5)Universidade Federal do Amapá, Macapa, Brazil, (6)Universidade Federal do Oeste do Pará, Santarem, Brazil, (7)INPE National Institute for Space Research, Sao Jose dos Campos, Brazil, (8)CENA Center for Nuclear Energy in Agriculture, Piracicaba, Brazil
Rivers worldwide are net processors of organic matter(OM), driving the evasive flux of CO2 from inland waters. The Amazon River outgases nearly 0.5PgC/yr, much of which is thought to be driven by the in-situ respiration of terrestrially-derived OM. However, the substrates fueling this respiration and the processes governing it remain fairly unconstrained.

Here, we chose to examine the roles of in-situ algal and floodplain macrophyte production in fueling respiration in the lower Amazon River. Contrary to the previous assumptions of turbid, tropical rivers, δ18O-O2 data revealed the presence of photosynthetic O2 throughout the main stem of the lower Amazon. This data was used in a steady-state model to estimate the Respiration to Production(R:P) ratio at these sites. This model reveals a low R:P, even at the mouth of the Amazon. Diel O2measurements were made to test this steady-state assumption, revealing little variation over a 24 hour cycle.

δ18O-O2 and δ13C-DIC data was combined with in-situ, continuous measurements of CO2 and O2 to show spatial trends in respiration and production. The floodplains and adjacent river margins had elevated δ18O(indicating higher photosynthetic input) and δ13C(indicating respiration of C4 macrophytes). Continuous CO2 and O2 measurements revealed that these sites were “hot spots” for respiration, with low O2 saturation and elevated pCO2 fueled by the input of macrophytic OM . Results reveal the importance of spatial dynamics in understanding respiration within large, tidal rivers.