Organic Alkalinity modulates pH from the Sea-Surface Microlayer during a mesocosm study

Abstract. The ocean plays a central role in climate regulation by exchanging carbon dioxide (CO₂) with the atmosphere. This exchange depends on the transfer efficiency across the air-sea boundary layer, the sea-surface microlayer (SML) known to be an organic-rich boundary with a thickness of less than 1 mm. The parameters dissolved inorganic carbon (DIC) and total alkalinity (TA) describe the state of the marine carbon system (MCS). However, organic alkalinity (OA), which arises from weak acid-base functional groups in dissolved organic matter, remains poorly constrained. It is known to modulate pH in organic-rich environments. Yet, to our knowledge, it has not been quantified directly in SML before. Here, we show that the enrichment of OA in the SML modulates pH and that its effect propagates further down into the underlying water (ULW). We track the evolution of the MCS during a 35-day mesocosm study where we induced a phytoplankton bloom. Three distinct bloom phases were identifyed by different biological processes dominating within the system. Dissolution dominated during the pre-bloom phase; photosynthesis and calcification prevailed during the bloom; and CO₂ invasion, together with respiration, was most pronounced in the SML during the transition to the post-bloom phase. These processes provided the context for the observed variability in OA. We measured OA directly by differential potentiometric back-titration as a second titration on the same titrated TA samples. OA in the SML was persistently enriched (Enrichment Factor (EF) > 1) and reached concentrations up to 264 µmol kg⁻¹. On average, it contributed 8.4 % of TA, compared to 3.1 % in the ULW. Concurrently, the vertical pH differences between SML and ULW decreased towards zero as the bloom began and occasionally became negative. Over the study period, OA EF and ΔpH were negatively correlated (Spearman ρ = -0.75, p = 0.024), indicating that stronger OA EF dampens the pH rise associated with the bloom onset and its effect propagates further down to the ULW. Recognising that OA enrichment modulates pH in both the SML and the ULW, routine inclusion of OA in near-surface measurements and a three layer air-SML-ULW framework should guide future evaluations of air-sea CO₂ exchange.