| Summary: | Estuaries are prone to increasing acidification due to growing population and urbanization in addition to global climate change. Acidification is largely studied through measuring or calculating carbonate chemistry parameters (dissolved inorganic carbon, pCO2, pH, total alkalinity), therefore a robust understanding of site carbonate chemistry is key to properly assessing habitat vulnerability to instances of acidification. A challenge in doing ocean acidification (OA) work in estuaries is that in contrast to offshore marine settings, carbonate chemistry in estuaries is more dynamic, varying both spatially and temporally. Carbonate chemistry in estuaries is also more compositionally complex, because of relatively high levels of organic alkalinity (AORG). AORG is normally deemed negligible in marine settings, but it is higher in nearshore environments due to dissolved organic matter inputs from sources such as intertidal salt marshes and terrestrial runoff. Challenges associated with quantifying AORG, and the inherent molecular complexity of AORG, have resulted in very little data in existence and a lot remains unknown pertaining to its prevalence in estuaries. To address this knowledge gap, we conducted a first-order investigation of AORG within shallow habitats of the San Francisco Estuary (SFE) to document how AORG varies spatially and temporally with pH and total alkalinity (TA). In four distinct sites (deep main channel, shallow eelgrass embayment, mudflat, and tidal creek) AORG ranged from non-detectable to 189 µmol/kg, which are comparable to AORG values reported for similar sites in the United States North and Southeast. Calculating pCO2 and saturation states of aragonite and calcite by assuming that AORG is absent resulted in an overestimation of these values as AORG (contributing to TA) ranges ~10%. Our findings show that AORG should be taken into consideration to make accurate carbonate chemistry calculations in estuarine settings. https://doi.org/10.46569/6h4411181
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