Components and Mechanisms of Total Alkalinity Variability in an Intertidal Salt Marsh

Under increased ocean acidification, assessing natural diurnal variation in coastal ocean acidity requires renewed evaluation of carbonate system measurements. A more detailed understanding of these measurements will be useful when considering the growth and settlement of calcifying organisms, and will help assess future effects of acidification on natural variability in coastal acidity. This study used an original CO2-free titration system based on an existing method (Cai et al., 1998) to assess components of total alkalinity (TA) variability within the Sage Lot Pond intertidal salt marsh on Cape Cod, MA in July and October of 2015. Additionally, the isotopic signature of the dissolved inorganic carbon (DIC) pool was fit into an isotope mixing model to further explore variability in TA. In the marsh, non-carbonate alkalinity (NCA) contributed a measurable amount of alkalinity to TA, with the majority of NCA constituted by “organic/undefined” alkalinity. Average TA in the marsh varied between 1812-2097 µmol L-1 and average NCA varied between 85-149 µmol L-1. Throughout the days studied, NCA made up 4-8 % of TA and thus was considered a “baseline” of alkalinity contributing to TA. The consistent occurrence of “organic/undefined” alkalinity within the marsh has implications for calculations within the carbonate system that involve TA measurement, where the use of TA rather than carbonate alkalinity (CA) results in a 20-30 µatm overestimation of pCO2 and a 0.05-0.3 overestimation of ΩA. Variations in carbonate alkalinity (CA) were determined to result in the measured TA variability. Most clearly during the July sampling days, TA variability was reflected by changes in the porewater-derived DIC produced from respiration within the marsh sediments, which may indicate processes responsible for net alkalinity generation such as iron and sulfate reduction (paired with organic carbon oxidation) or inorganic CaCO3 dissolution. This porewater-derived DIC component (DICPW) made up between 0.1-7.5% of the DIC pool on the dates studied. Although results show a possible relationship between tidal stage and DICPW and TA variability, it is uncertain whether the measured DICPW andTA variability is caused by tidally-driven porewater exchange, shifts in local metabolism as a result of cycles of light availability, or other factors. Future work should address identifying whether these and other factors are drivers of TA variability, and should if possible gain samples at higher daily resolution in order to more fully grasp diurnal changes in TA and other parameters. These efforts would crucially advance our understanding of the magnitude and variability of processes affecting carbonate chemistry in coastal regions