High-resolution spatial distribution of carbon dioxide in a tropical oligotrophic coastal water (Paraty Mirim Bay, Southeastern Brazil)

Coastal waters have been considered global hotspots of aquatic carbon cycling and carbon dioxide (CO₂) fluxes to the atmosphere. However, the spatial variability of the surface partial pressure of CO₂ (pCO_2sw) especially in oligotrophic preserved marine ecosystems is still poorly understood. Here, we determined the high resolution spatial distribution of surface pCO₂ (~2m depth) in a tropical (oligotrophic) bay in SE Brazil (Paraty Mirim Bay - PMB) during austral winter (4-8 Aug 2023), using a continuous, non-turbulent pump in closed system with an infrared gas analyzer (EGM-4® PPSystems) coupled to an air equilibrator and a multi-parameter probe (temperature, salinity, dissolved oxygen and pH, YSI Pro ODO). PMB was divided into 4 areas: Cotia (SC), Mamanguá (SM), Paraty-Mirim (PM), and outer-bay (EP), where median surface pCO_2sw remained above median atmospheric pCO₂, and dissolved oxygen saturation (DO%) was close to saturation, with little spatial variability for these two parameters, as well as surface temperature and salinity. In sector SC, we performed a 25-hour stationary sampling experiment, when pCO_2sw varied between 420-600 μatm and DO% between 87-110%. Preliminary results show that during the 25h-experiment, SC area alternates between net respiration and temperature control on pCO_2sw saturation. Our results also reinforce the importance of high-frequency measurements along coastal ecosystems to fully understand the controls on sea-air CO2 exchange.