Spatial variability of partial pressure of CO2 in two subtropical estuarine channels with distinct levels of eutrophication (São Paulo, Brazil)

Recent studies have concluded that the CO₂ degassing flux from estuarine waters can be significant, but the human influence on the high pCO₂ in these waters even is not fully known. Studies have been carried out for years on the environmental status of the Santos estuary, which is one of the most polluted estuaries in Brazil, whereas the comparatively clean Cananéia estuary nearby is not impacted by any significant human activities and thus have been considered as a reference site. Thus, water samples were obtained from 5 points along two estuarine channels (with similar salinity gradient), Ararapira (Cananéia estuary) and Bertioga (Santos estuary), on August and September of 2018, to evaluate the trophic state and pCO₂. It is important to highlight that both sampling periods presented similar meteorological conditions such as rainfall index, wind force, and cloud cover. Overall, the estuarine channels got an increase in alkalinity and pH, and a decrease in nutrients and pCO2 toward the lower zone. However, Bertioga channel presented nutrient values from 5 to 50 times higher than those found in the Ararapira channel, providing heterotrophic metabolism in the most of the first, and autotrophic metabolism in the most of the second. Due to the distinct trophic states reported, the higher pCO₂ (466 - 821 µatm) in the Bertioga channel provided high degassing fluxes (4.8 - 12.5 mmol m^-2 d^-1), whereas the lower pCO₂ (297 - 502 µatm) in the Ararapira channel provided negligible degassing fluxes (0.002 - 0.006 mmol m^-2 d^-1). Our results demonstrated human activities in estuarine watersheds can play a great influence on the increase of degassing flux areas along the estuary.