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This study presents high-resolution data on diel variations of carbonate chemistry in a semi-arid estuary (Jaguaribe River) in NE Brazil, which has witnessed decreasing annual rainfall and freshwater inputs due to climate change and river damming. In addition, the estuary has been suffering with increasing discharges from shrimp farm and urban effluents. We monitored surface water and atmospheric CO 2  partial pressure ( p CO 2 ), temperature, salinity, and wind speed with continuous real-time measurements during two eulerian surveys in October 2017 (33 h) and September 2018 (44 h), during spring tides in the dry season. Additionally, pH, total alkalinity (TA), dissolved inorganic carbon (DIC), carbonate (CO 3  2– ), and saturation state of calcite (Ω cal ) and aragonite (Ω ara ) were monitored hourly. Higher salinity (&amp;gt;38) during ebb tides confirmed the hypersalinity and negative estuarine circulation. TA and DIC concentrations in the estuary were higher than in the adjacent coastal ocean due to evaporation, showing positive correlation with salinity and negative correlation with tidal height. Measured TA and DIC concentrations were slightly higher than those calculated by the conservative evaporation model, suggesting their production in the estuary by aerobic and anaerobic processes. CO 3  2– , Ω cal , and Ω ara  showed a clear semi-diurnal (tidal-driven) and diel (24 h; biological-driven) patterns: lowest values occurred at flood tide during night-time (respectively, 185 μmol kg –1 , 4.3 and 2.8), whereas highest occurred during ebb tide and daytime (respectively, 251 μmol kg –1 , 5.7 and 3.8). DIC/TA ratios were higher at night-time supporting a diel control (linked to solar irradiance) of the carbonate buffering capacity.  p CO 2  was oversaturated comparing to the atmosphere (512–860 μatm) and the estuary was a source of CO 2 , with fluxes ranging from 2.2 to 200.0 mmol C m –2  d –1  (51.9 ± 26.7 mmol C m –2  d –1 ), which are higher than emissions normally found in low-inflow, marine-dominated estuaries. The diel variability of DIC indicated a net heterotrophic metabolism averaging −5.17 ± 7.39 mmol C m –2  h –1 . Eutrophication amplifies the diel variability of the CO 2  system generating large differences between daytime and night-time. The results highlight the importance of considering diel variability when estimating CO 2  fluxes and carbonate chemistry in eutrophic, semi-arid, and tidally dominated estuaries under rapid environmental changes, and may represent future conditions in estuaries worldwide experiencing warming, increasing aridity and eutrophication.

Eutrophication Amplifies the Diel Variability of Carbonate Chemistry in an Equatorial, Semi-Arid, and Negative Estuary