Spatial variability of pCO2 and carbonate system members in two Brazilian estuaries under different tidal regimes

Estuaries are ecosystems under great influence of tidal regime on the distribution of the hydrochemical properties. The biogeochemical cycles are constantly changing, also influenced by climate global change, being that the dynamic of CO₂ in the air-water interface can evidence both the waters as source or sinking of CO₂. This study was performed in surface waters along the salinity gradient in two Brazilian estuaries included as RAMSAR sites aiming to verify the tidal regimes effects on the pCO₂ distribution during dry period. The parameters of carbonate system were estimated using CO₂Calc^® software, using pH and TA as inputs, considering pressure, salinity, wind speed and dissolved nutrients. Cananeia-Iguape Estuarine-Lagoon Complex (CIELC), with microtidal regime (max. height of 0.7 m), presented pCO₂ varying from 245.03 to 1334.08 μatm. Raposa-Paciência Estuary (RPE), under macrotidal regime (max. height of 3.7 m), presented variation from 217.06 to 4722.25 μatm. Both in CIELC and RPE, pCO₂ showed negative correlations with pH (r=-0.99 and -0.97, respectively) and salinity (r=-0.93 and -0.99, respectively). In the CIELC, a positive correlation between pCO₂ and DO (r=0.58) was observed, while in the RPE there was a strong negative correlation (r=-0.90). In CIELC and RPE stations with greater marine influence acted as CO₂ sinks, while the mixing zone and the inner part of estuaries acted as sources of CO₂ to the atmosphere. While the CIELC shows pCO₂ distribution more related photosynthetic pigments i.e. biological productive processes and RPE apparently, shows such values more related to organic matter degradation processes with dissolved oxygen depletion. These results are related to the hydrodynamic effect on biogeochemical cycling in regions with so distinct tidal regime.