Physical and biogeochemical controls on O2, CO2 and pH in the northern Gulf of Mexico: A synthesis of field research between 2006 and 2018

Nutrient biogeochemistry and carbon cycle in the northern Gulf of Mexico (nGOM) are strongly influenced by river discharge, eutrophication, air-sea gas exchange and coastal currents. In the surface water, the spatial distributions of oxygen (O₂), partial pressure of CO₂ (pCO₂), dissolved inorganic carbon (DIC), and pH are closely associated with the trajectory of the river plumes and in situ biological activity driven by riverine nutrients. In the plume region the highest O₂ and pH values and lowest pCO₂ and DIC values were observed at intermediate salinities where light and nutrient were both favorable for phytoplankton production. Net community production (NCP) rates in the surface mixed layer estimated by several methods including high resolution O₂/Ar ratio data, pCO₂/DIC and nutrient balances, and bottle incubation generally agree with each other but also show interesting contrast and even conflicting features. We will synthesize multiple-year geochemical tracer data assisted by remote sensing data and numerical model efforts.

Low O₂, high DIC and low pH values were observed in hypoxic bottom waters under and downstream of the productive surface plumes. The subsurface pH has a close correlation with excess DIC and apparent oxygen utilization (AOU), confirming that decomposition of organic matter is the dominant factor regulating pH variability. Both observation and model results show that bottom water hypoxia and acidification can occur rapidly after storm disturbances. Our data also suggest that bottom water hypoxia and acidification appear to grow more severe in recent years. Our analysis suggests that benthic flux may have played an important role in leading to an additional acidification in extremely hypoxic and anoxic bottom waters.