Summertime net community production (NCP) via underway measurements of O2/Ar and its control on CO2 flux in the northern Gulf of Mexico

This study provides a comprehensive evaluation of metabolic status in the northern Gulf of Mexico (nGOM) using high-resolution O₂/Ar-derived net community production (NCP) and air-sea CO₂ flux data from the Gulf of Mexico Ecosystems and Carbon (GOMECC) expedition in summer 2021. We found that the entire nGOM is a weak CO₂ source to the atmosphere and net heterotrophic during the summertime. Focusing on both coastal and basin areas, with a particular emphasis on the diverse coastal ecosystems of the nGOM, we identified the Mississippi–Atchafalaya River Plume as an autotrophic hotspot, exhibiting interquartile NCP values between 28.0 and 65.1 mmol C m⁻² d⁻¹. In contrast, the East Texas region and the Dry Tortugas were predominantly heterotrophic, with the latter's CO₂ production mechanisms influenced by its unique shallow reef system compared to a watershed environment. Near Panama City, a near-trophic equilibrium was observed, likely resulting from the interplay between coastal waters and the Apalachicola River outflow. In addition, this study emphasizes the significant influence of harmful algal blooms (HABs) in West Florida, demonstrating their transformative impact on biogeochemical processes and CO₂ dynamics. The study also investigates the dynamic interactions between biological oxygen supersaturation and the changes in dissolved inorganic carbon (DIC), offering insights into the nGOM's role as a slight summertime CO₂ source despite regional variations in biogeochemical processes. Furthermore, the integration of satellite chlorophyll-a (Chl-a) analysis with NCP measurements provides a broader perspective on the contribution of phytoplankton biomass to the area's carbon budget, emphasizing the importance of nutrient-rich river plumes in supporting marine productivity and carbon uptake. This study not only enriches our understanding of the nGOM's ecological dynamics but also contributes to the broader discourse on coastal oceans' role in the global carbon cycle, providing a critical foundation for future research in similar ecosystems.