AUTHOR=Cotovicz Luiz C. , Marins Rozane V. , da Silva Agda Raquel Facundo 

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

JOURNAL=Frontiers in Marine Science

VOLUME=Volume 9 - 2022

YEAR=2022

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2022.767632

DOI=10.3389/fmars.2022.767632

ISSN=2296-7745

ABSTRACT=This study presents high-resolution data on diel variations of carbonate chemistry in a semi-arid estuary (Jaguaribe River) NE Brazil, which witnessed decreasing annual rainfall and freshwater inputs due to climate change and river damming. Additionally, the estuary suffers increasing discharges from shrimp farms and urban effluents. We monitored surface water and atmospheric CO2 partial pressure (pCO2), temperature, salinity, and wind speed with continuous real-time measurements during two eulerian surveys in October 2017 (33 hours) and September 2018 (44 hours), during spring tides in the dry season. Total alkalinity (TA), pH, dissolved inorganic carbon (DIC), carbonate (CO32-), and saturation state of calcite (Ωcal) and aragonite (Ωara) were monitored hourly. Salinity (> 38) during ebb tides confirmed the negative estuarine circulation. TA and DIC were higher than in adjacent coastal ocean, showing positive correlation with salinity and negative with tidal height. TA and DIC were slightly higher than those calculated by the conservative evaporation model, suggesting their production in the estuary by aerobic and anaerobic processes. CO32-, Ωcal and Ωara showed a clear semi-diurnal (tidal-driven) and diel (24 hours; 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. pCO2 was oversaturated relative to the atmosphere (512 to 860 μatm) and the estuary was a source of CO2, 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 marine-dominated estuaries. The diel variability allowed to calculate a net heterotrophic metabolism averaging 5.17 ± 7.39 mmol C m-2 hr-1. Eutrophication amplifies the diurnal variability of the CO2 system generating large differences between daytime and night-time. The results highlight the importance of considering diurnal variability when estimating CO2 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.