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

Abstract:

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.