Spatial and Temporal Variability of pCO2, 13C-CO2, and [O2] in the Tidal Amazon River.

Abstract:

River systems play an important role in both transporting and altering organic carbon fixed in terrestrial systems. However, there is a key gap in our understanding of riverine fluxes due to the lack of systematic measurements made in the lower, tidally-influenced reaches of large river systems. For example, the traditional end-member for the Amazon River is located at Óbidos, which is 900km from the mouth of the river. Below this point, tides produce semi-diurnal fluxes to and from floodplains and channels, resulting in complete flow reversal without salinity intrusion.

The lower Amazon remains a critical study area because (1) outgassing rates in tropical rivers are extremely high, typically exceeding temperate counterparts and (2) the Amazon’s discharge represents a significant proportion of global freshwater input to the ocean(~20%). Furthermore, a lack of measurements in this area due to sampling difficulties means that the processes governing biogeochemical dynamics in this region remain unconstrained.

In this study, we implemented a continuous, in-situ equilibrator system for the real-time measurement of pCO₂, ¹³C-CO₂, and [O₂]. Measurements were collected along various river transects, at floodplain margins and river confluences, and during transit up and down the river. Cruises were planned to coincide with various stages of the hydrograph, with measurements collected at high water, low water and falling water.

High-resolution measurements showed both temporal changes in pCO₂ and ¹³C-CO₂, and also allowed us to identify “hotspots” of increased pCO₂. Within the lower river, several of these hotspots coincided with river confluences and floodplain margins. Measurements over the course of tidal cycles revealed little change in dissolved gas measurements within the mainstem. This approach shows the importance of considering spatial variability in large, dynamic systems influenced by tidal fluxes.