The role of hydrodynamic and biogeochemistry on CO₂ flux and pCO₂ at the Amazon River mouth

Abstract. Recent estimates indicate that the lower Amazon River outgasses significant amounts of carbon dioxide (CO₂) that was not previously accounted for the global inland water carbon budget. Detailed evaluation of seasonal variability and controlling mechanisms behind the CO₂ fluxes in this large and complex area remains incomplete. Previous observations throughout the Amazon basin showed that higher CO₂ fluxes (FCO₂) and partial pressure of CO₂ (pCO₂) occur during high water and higher wind intensity seasons. The influence of wind and water speed, depth of water column, as well as respiration of allochthonous and autochthonous organic matter, are frequently assigned as the main control variables. Here, we assess the influence of a set of biogeochemical and hydrodynamic parameters on the seasonal variation of FCO₂ and pCO₂ near the Amazon River mouth. FCO₂, pCO₂ and biogeochemical and hydrologic analyses were carried out from 2010 to 2016 during four different hydrological periods per year (N = 25) in the North Channel of the Amazon River mouth. FCO₂ and pCO₂ were used as independent variables and analyzed against 33 biogeochemical, hydrodynamic and meteorological parameters along the hydrological seasons. The highest FCO₂ and pCO₂ was obtained at high discharge season (11.28 ± 7.82 μmol m⁻² s⁻¹ and (4575 ± 429 μatm, respectively) when most of these parameters tend to be higher. Among the 33 parameters analyzed, the significant correlations with FCO₂ and pCO₂ (p < 0.05) observed were for water and air temperatures, dissolved oxygen, dissolved organic carbon, nitrate, dissolved inorganic nitrogen and pH. These variables could be considered suitable predictors for estimating pCO₂ and FCO₂ in the Amazon River mouth area. For a better estimation and understanding of carbon budgets in tropical rivers it is still required to verify and to quantify more deeply the relationship among CO₂ evasion and others hydrodynamic, meteorological and biogeochemical variables.